diff options
194 files changed, 17 insertions, 11532 deletions
diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..d7b82bc --- /dev/null +++ b/.gitattributes @@ -0,0 +1,4 @@ +*.txt text eol=lf +*.htm text eol=lf +*.html text eol=lf +*.md text eol=lf diff --git a/LICENSE.txt b/LICENSE.txt new file mode 100644 index 0000000..6312041 --- /dev/null +++ b/LICENSE.txt @@ -0,0 +1,11 @@ +This eBook, including all associated images, markup, improvements, +metadata, and any other content or labor, has been confirmed to be +in the PUBLIC DOMAIN IN THE UNITED STATES. + +Procedures for determining public domain status are described in +the "Copyright How-To" at https://www.gutenberg.org. + +No investigation has been made concerning possible copyrights in +jurisdictions other than the United States. Anyone seeking to utilize +this eBook outside of the United States should confirm copyright +status under the laws that apply to them. diff --git a/README.md b/README.md new file mode 100644 index 0000000..358fdd9 --- /dev/null +++ b/README.md @@ -0,0 +1,2 @@ +Project Gutenberg (https://www.gutenberg.org) public repository for +eBook #55126 (https://www.gutenberg.org/ebooks/55126) diff --git a/old/55126-0.txt b/old/55126-0.txt deleted file mode 100644 index fa241b7..0000000 --- a/old/55126-0.txt +++ /dev/null @@ -1,4761 +0,0 @@ -The Project Gutenberg EBook of Cloud Studies, by Arthur W. Clayden - -This eBook is for the use of anyone anywhere in the United States and most -other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms of -the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - -Title: Cloud Studies - -Author: Arthur W. Clayden - -Release Date: July 16, 2017 [EBook #55126] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK CLOUD STUDIES *** - - - - -Produced by Cindy Horton, deaurider, and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - - - - - - - -CLOUD STUDIES - -[Illustration: A SUNSET SKY. - - _Frontispiece._] - - - - - CLOUD STUDIES - - By ARTHUR W. CLAYDEN, M.A. - - PRINCIPAL OF THE - ROYAL ALBERT MEMORIAL COLLEGE, EXETER - - LONDON - JOHN MURRAY, ALBEMARLE STREET - 1905 - - - - - PRINTED BY - WILLIAM CLOWES AND SONS, LIMITED, - LONDON AND BECCLES. - - - - -PREFACE - - -To the meteorologist I hope the following pages may prove not only of -some interest, but of practical value as a small step towards that -greater exactness of language which is essential before we can attempt -to explain all the details of cloud structure, or even interchange -our ideas and observations with adequate precision. The varieties -depicted and described have been selected from many hundreds, as those -which seem to me to show such differences of form as to imply distinct -differences in the conditions to which they are due. I have not -attempted to deal with the physical causes of condensation except in a -general way, being unwilling to introduce diagrams of isothermals and -adiabatics and such purely scientific methods into a work also intended -for a wider public. For those who wish to pursue this part of the -subject I have appended a list of papers from the _Quarterly Journal -of the Royal Meteorological Society_ and other sources, which may -serve as references. I also hope that some more votaries of the science -may be induced to realize that meteorology does not consist solely of -the tabulation of long columns of records, but includes subjects for -investigation as much more beautiful as they are more difficult. - -To the artist I trust they may also be of some use, by calling -attention to the variety and exquisite beauty of the sky. Nothing is -more extraordinary in art than the general negligence of cloud-forms. -Many of them are quite as worthy of careful drawing as the leaves of a -tree, the flowers of a field, the ripples on a stream, or the texture -of a carpet, or a marble pavement. Yet it is the common rule to find -pictures, which are otherwise marvellous examples of skill and care, -disfigured by impossible skies with vague, shapeless clouds, as untrue -to nature as it would be possible to make them. Grace of outline, -delicacy of detail and texture, richness of contrast, beauty of form -and light and colour, all are present in the skies, and combine to make -a whole well worthy of the best that art can give. The illustrations I -offer are not selected for pictorial effect; they are chosen from a -purely scientific point of view; but they are enough to indicate what -could be done if the facts of nature were treated with high artistic -skill. - -In addition to the meteorologist and the artist, there are a much -larger number who follow neither profession, but who love Nature in -all her moods; and to them also I hope these pages may be of interest. -Indeed, if only a few of them should be stimulated to take up a branch -of nature study which has given me many an hour of quiet enjoyment, the -labour of bringing these notes together will not have been in vain. - - ARTHUR W. CLAYDEN. - - St. John’s, - Exeter. - - - - -CONTENTS - - - CHAPTER PAGE - - I. Introductory 1 - - II. Cirrus 21 - - III. Cirro-stratus and Cirro-cumulus 45 - - IV. Alto Clouds 59 - - V. Lower Clouds 71 - - VI. Cumulus 84 - - VII. Cumulo-nimbus 105 - - VIII. Wave Clouds 119 - - IX. Cloud Altitudes 137 - - X. Cloud Nomenclature 154 - - XI. Cloud Photography 165 - - References 181 - - Index 183 - - - - -LIST OF ILLUSTRATIONS - - - PLATE PAGE - - A Sunset Sky _Frontispiece_ - - 1. Part of a Great Halo 22 - - 2. Part of a Solar Halo 23 - - 3. Cirro-nebula changing to Cirro-stratus 24 - - 4. Cirro-nebula changing to Cirro-cumulus 27 - - 5. High Cirrus. (_Cirrus Excelsus_) 31 - - 6. Windy Cirrus. (_Cirrus Ventosus_) 32 - - 7. Thread Cirrus. (_Cirro-filum_) 34 - - 8. Tailed Cirrus. (_Cirrus Caudatus_) 35 - - 9. Hazy Cirrus. (_Cirrus Nebulosus_) 36 - - 10. Change Cirrus. (_Cirrus Inconstans_) 37 - - 11. Common Cirrus. (_Cirrus Communis_) 40 - - 12. Band Cirrus. (_Cirrus Vittatus_) 41 - - 13. Band Cirrus. (_Cirrus Vittatus_) 42 - - 14. Hazy Cirro-stratus. (_Cirro-stratus Nebulosus_) 46 - - 15. Cirro-stratus 47 - - 16. Cirro-stratus. (_Cirro-stratus Communis_) 48 - - 17. Flocculent Cirro-stratus. (_Cirro-stratus Cumulosus_) 49 - - 18. Cirro-stratus and Cirro-cumulus 50 - - 19. Cirro-cumulus 50 - - 20. Hazy Cirro-cumulus. (_Cirro-cumulus Nebulosus_) 51 - - 21. Hazy Cirro-cumulus. (_Cirro-cumulus Nebulosus_) 51 - - 22. A Sunset Sky 52 - - 23. Speckle Cloud (Ley). (_Cirro-macula_) 53 - - 24. Cirrus Caudatus and Cirro-macula 55 - - 25. Alto-cumulus Informis 64 - - 26. Hazy Alto-cumulus. (_Alto-cumulus Nebulosus_) 65 - - 27. Flat Alto-cumulus. (_Alto-cumulus Stratiformis_) 65 - - 28. High Turreted Cloud. (_Alto-cumulus Castellatus_) 66 - - 29. High Ball Cumulus. (_Alto-cumulus Glomeratus_) 67 - - 30. Mackerel Sky. (_Alto-stratus Maculosus_) 68 - - 31. Mackerel Sky. (_Alto-stratus Maculosus_) 69 - - 32. Alto-strato-cumulus 70 - - 33. Sunset. (_Alto-cumulus Castellatus Fractus_) 70 - - 34. Three Layers of Stratiform Cloud after Rain 73 - - 35. Rain-Cloud. (_Nimbus_) 75 - - 36. Rain-Cloud. (_Nimbus_) 75 - - 37. Stratus Communis 77 - - 38. Strato-cumulus 77 - - 39. Strato-cumulus 78 - - 40. Stratus Maculosus 78 - - 41. Common Stratus. (_Stratus Communis_) 79 - - 42. Roller Cloud. (_Stratus Radius_) 80 - - 43. Small Cumulus. (_Cumulus Minor_) 94 - - 44. Cumulus 95 - - 45. Large Cumulus. (_Cumulus Major_) 96 - - 46. Fracto-cumulus 97 - - 47. Fall Cloud. (_Stratus Lenticularis_) 98 - - 48. Thunder-clouds forming 109 - - 49. Thunder-clouds. (_Cumulo-nimbus_) 110 - - 50. Thunder-clouds. (_Cumulo-nimbus_) 111 - - 51. Thunder-cloud. (_Cumulo-nimbus_) 111 - - 52. Thunder-cloud. (_Cumulo-nimbus_) 111 - - 53. The Flank of a Great Storm 112 - - 54. Crested Alto Waves. (_Alto-cumulus Undatus_) 120 - - 55. Alto Waves. (_Alto-stratus Undatus_) 121 - - 56. Cirro Ripples. (_Cirro-cumulus Undatus_) 122 - - 57. Waved Cirro-stratus. (_Cirro-stratus Undatus_) 136 - - 58. Camera for measuring Altitudes 141 - - 59. Print from a Negative used for measuring Altitude 144 - - 60. Pair of Prints showing the Displacement of the Cloud 145 - - 61. Cloud Camera for Studies 171 - - - - -CLOUD STUDIES - - - - -CHAPTER I - -INTRODUCTORY - - -ALL who have the faculties proper to man must have been to some extent -students of cloud form. Go where we will, do what we will, we cannot -easily escape from the sky, or avoid noticing some of its features -and coupling them with the varying conditions of weather. We all -sometimes want to know if it is likely to rain, or whether some other -change is probable; and experience soon shows us that the clouds give -the simplest and most obvious indication of what we may expect. It is -almost impossible to avoid noticing that certain types of cloud, or the -simultaneous appearance of certain types, is the usual accompaniment -of definite kinds of weather or of particular changes. Thus it is that -most people acquire some small measure of weather wisdom before their -schooldays are over. - -Generation after generation, through all human history, the same causes -must have led to the same conclusions; and the study of clouds must, -therefore, be one of the oldest of all branches of scientific inquiry. -Yet, old as it is, it is still in its infancy, having made very little -advance indeed towards the precision of an exact science. - -There are many reasons for this want of growth, and so far as -the theoretical aspects of the subject are concerned it is easy -enough to understand. Clouds are among the most inaccessible of -terrestrial objects. Except by balloon ascents, by sending up kites -bearing recording instruments, or by making observations among the -mountain-tops, we have no means of getting at them to study the -conditions under which they exist. Temperature, pressure, humidity, -have generally to be guessed at, those guesses being based on the -scanty data which have been laboriously obtained by one or another of -these cumbrous methods. Moreover, many clouds have such vast dimensions -that it is very difficult to grasp all that goes on in such a space. - -Besides the difficulty of attacking the problems presented by cloud -formation, it is probable that even if we could have got among the -clouds at will, we should have understood little more than we do, from -a want of sufficient certainty on many of the purely physical questions -involved. It is not many years since Mr. J. Aitken discovered the -necessity for material nuclei as a first step in the formation of cloud -particles, and not many months have elapsed since Mr. C. T. R. Wilson -showed that those particles can be formed by the action of radiation -on the air itself. There is nothing surprising, therefore, in the fact -that our theoretical knowledge of the why and wherefore of the facts -revealed by a study of clouds is limited to general principles, and -quite fails to say exactly why each special form should be assumed. The -matter for surprise is quite different. - -Theoretical explanations are not the first step in the working out -of a branch of science. It begins with the acquisition, by diligent -and painstaking observation, of a great mass of facts. This may go on -for centuries, the accumulation growing greater and greater, until -at last some one comes who examines the records, classifies them -carefully, and finally makes a summary in the form of a number of -generalizations, which are announced under the name of Laws. - -Two examples of such “Laws” will suffice. Astronomers for centuries had -observed the movements of the planets, always with increasing accuracy, -until Tycho Brahe made his famous series of observations on the planet -Mars. These materials fell into the hands of Kepler, and the result of -his work was the announcement of Kepler’s Laws, which state the rules -which govern the movements of the planets in their orbits. He found -that the records could not be accounted for unless the planets moved -in a certain way, but he knew nothing of the reasons for a method and -order which clearly existed. - -Kepler’s Laws, in fact, rest upon another set, namely, Newton’s Laws of -Gravitation, and these are themselves a second example. They are the -summary of accumulated experience, and even at the present day we know -nothing certain as to why two bodies should attract each other, and -nothing as to why that mutual attraction should act as it was found to -act by Newton. - -The observational part of cloud study, however, is still in its -infancy, in spite of the fact that it has been going on for such -countless ages. We are still in the condition of the humble observers -engaged in the comparatively humdrum task of gathering facts for -future arrangement and interpretation. Cloud observers, in all ages, -have suffered from a peculiar difficulty. They have had no common -language, no code of signs by which they could benefit from the work -of those who had gone before them, no means of transmitting their own -experience to each other, or to those who would come after them. No -progress would be possible in any study under such conditions, for -each person would begin where the previous generation began, instead -of taking up the task where others had left it. In all languages there -is an extraordinary scarcity of cloud names, and such as do exist are -frequently applied to quite different forms by different people. So -pronounced is this lack of terms, that any one who tries to describe -a sky without using any of the modern scientific names, finds himself -obliged to rely on long detailed descriptions, backed with references -to well-known objects, whose outlines or structures resemble the -clouds more or less vaguely; and even then he has to be a word-painter -of singular skill if his description calls up in the mind of the reader -a picture much like the original. - -It was to meet this want of a common tongue that Luke Howard, in 1803, -proposed his scheme of cloud names. He recognized three main types of -cloud architecture, which he named Cirrus, Stratus, and Cumulus. Cirrus -included all forms which are built up of delicate threads, like the -fibres in a fragment of wool; Stratus was applied to all clouds which -lie in level sheets; and Cumulus was the lumpy form. - -By combinations of these terms other clouds were described. Thus, a -quantity of cirrus arranged in a sheet was called cirro-stratus, while -high, thin clouds like cirrus, but made up of detached rounded balls, -was cirro-cumulus. Many cumulus clouds, arranged in a sheet with little -space between them, became cumulo-stratus, while the great clouds from -which our heavy rains descend partake, to some extent, of all three -types, and were therefore distinguished by a special name--Nimbus. - -This system had much to recommend it. The three fundamental types -were obvious to all. Their names were descriptive, and were derived -from a dead language, so that no living international jealousies were -raised. It was sufficiently detailed to serve the purposes of the time, -when accurate observation was in its infancy. Hence it was universally -adopted, and will pretty certainly hold its own as the broad basis upon -which any more detailed system must necessarily rest. - -It has done excellent service; but although observation of clouds in -a general way is far from complete, attention is now being given to -much smaller details and much more minute differences of form, and our -vocabulary must be amplified. Precision of description is the first -essential of a satisfactory system, and the question is, what sort of -edifice can we build on Luke Howard’s foundation. - -The great difficulty is the infinite variety of clouds. Certain forms -may be arbitrarily selected as types, and names may be given to them; -but however well they are chosen, a very short period of observation -will show that there are all manner of intermediate forms, which make -a perfect gradation from one type to another. This fact should never -be forgotten. There is always a danger that the use of any system -of names based on types shall lead to the neglect of everything not -typical. A curious illustration is afforded by the well-known fact, -that in arranging collections of fossil shells, it is frequently found -that some specimens do not exactly match the type examples to which -names have been assigned. In former days it was the custom to throw -aside such “bad specimens,” as they did not show plainly the specific -characters. It is now realized that they have a value of their own, -in that they are the links in the evolutionary chain, once supposed -to be missing. Indeed, it is not unfrequent nowadays to see carefully -selected series, showing the gradual change whereby one species -passed into another, displayed in the place of honour, while the type -specimens are relegated to humbler places in the general collection. - -Types there must be, no doubt, and where the series is continuous, -some one must make the selection. With clouds the series is absolutely -continuous. The task is like choosing typical links from a long chain -in which each link is almost exactly like its neighbours, yet no -two are alike, and the greater the distance between them the less -their likeness. Clearly any system put forward must be accompanied -by illustrations, so that all may know exactly which links have been -chosen. - -Many attempts have been made to meet the want; some of the systems -proposed being based on the forms assumed by the clouds, some on -their supposed mode of origin, and some on their altitudes. Those -which were not founded on Luke Howard’s types had no chance of being -accepted, while knowledge was not yet sufficiently far advanced to make -classifications based on origin of form at all possible. But the great -reason why none of the proposed schemes could come into general use was -that they were put forward without adequate illustration, so that none -but their authors knew exactly what they meant.[1] - -Matters came to a head in 1891, when an International Meteorological -Conference met at Munich. One object of this gathering was to promote -inquiries into the forms and motions of clouds, by means of concerted -observations at the various institutes and observatories of the globe. -Luke Howard’s system was not enough for the purpose in view, and the -addition of more detailed terms had to be settled before work could be -begun. - -Professor Hildebrandsson, of Upsala, and the Hon. Ralph Abercromby -jointly submitted a revised scheme, the main feature of which was the -introduction of a new class of clouds, to be distinguished by the -prefix alto-before the other name. Such alto clouds were less lofty and -denser than cirrus. This scheme was the best before the Conference, and -without waiting to discuss, and possibly improve it, it was formally -adopted, and a committee appointed to arrange and publish an atlas -showing pictures of the type-forms. This atlas did not appear until -1896, and in the mean time the Rev. W. Clement Ley had published -proposals of his own, some of which had much to recommend them. But -he was too late. The International Committee had come to a decision, -and, although it may be far from ideal, the system backed by such an -authority must be regarded as the standard until some similar gathering -has given worldwide sanction to a change, and even then it would be -better to modify by addition rather than by substitution. - -The subjects of the following pages are named in general accordance -with this International Code, but they are by no means restricted to -types. Their object is not to attempt any repetition of the work which -has already been well done by the Atlas Committee, but rather to show -the chief varieties within a type. It will, however, become abundantly -evident that the standard system is far from complete, and that any -minute and detailed study of cloud-form must take note of the precise -variety. - -This at once raises the question whether many of these varieties -are not sufficiently distinct to be given definite names. If a -meteorologist is told that cirrus clouds were seen on a particular -occasion, he instinctively asks--What sort of cirrus? and is utterly -unable to form any mental picture of the clouds until the question -has been answered by a detailed description. A glance at a few of the -plates further on will show the difficulty plainly, and it occurs with -other forms of cloud as well as cirrus. - -Is it not time that the International names were regarded as those of -the cloud genera, and to add specific names for those varieties which -seem to imply some difference in kind in the conditions which have led -to their formation? This has been here attempted by translating into -Latin the ordinary English term by which the variety would naturally -be described. More extended observation will probably show that other -species should be introduced, and possibly some of those suggested -in these pages may have to be subdivided. Whatever the names may be, -specific distinction of some sort is an essential preliminary to -detailed study of the why and wherefore of the particular forms. - -The International system is as follows:-- - - A. Upper clouds. - (_a_) Cirrus. - (_b_) Cirro-stratus. - B. Intermediate clouds. - (_a_) Cirro-cumulus and alto-cumulus. - (_b_) Alto-stratus. - C. Lower clouds. - (_a_) Strato-cumulus. - (_b_) Nimbus. - D. Clouds of diurnal ascending currents. - (_a_) Cumulus and cumulo-nimbus. - E. High fogs. - (_b_) Stratus. - -In this tabulation the forms marked (_a_) are detached and occur in dry -weather, while those marked (_b_) are widely extended. The original -scheme also gives the mean heights of the various types, but these -values have been omitted here because they are extremely variable, -and impossible to ascertain with any approach to accuracy by mere eye -estimates. They vary also with the season, and probably also with the -locality. Moreover, the altitude is no guide to the name, except that -on the whole the types occur in the order given, taking group A as the -highest and group E as the lowest. In the chapter on cloud altitudes -this subject will be further considered, and under the descriptions -of cloud-forms their average height or actual measurements for the -particular specimen figured will be given whenever possible. - -Before coming to the description of individual forms, it may not be out -of place to give brief consideration to the best means of observing -them in nature. For eye observation, of course, no directions are -needed when we are dealing with the lower and denser varieties; but -when we come to the highest groups it sometimes becomes necessary to -protect the eye from the brilliant glare which may make it impossible -to detect the real structure. Smoked glass, neutral-tinted spectacles, -or yellow glass all have something to recommend them; but by far the -most convenient means is to look, not at the clouds themselves, but -at their images formed in a black mirror. A lantern cover glass, or a -thin piece of plate-glass, blacked on the back with some black paint, -serves admirably. But all black paints are not equally good. The best -are oil paints which dry with a glossy surface, the so-called enamels. -They have the advantage that the varnish with which they are mixed has -an index of refraction not very different from that of the glass. The -consequence is that so little light is reflected from the blackened -back, compared with that which is reflected from the front surface of -the glass, that the second image can only be detected with difficulty. -If the mirror is a piece of black or deeply coloured glass all trace of -the second image is lost. - -With this simple appliance it is easy to study the details of the -thinnest clouds right up to the sun, and even the image of the sun -itself may be glanced at without serious discomfort. Nor is the general -diminution of brightness the only gain. If the glass is so held that -the light from the cloud makes an angle of about 33 degrees with the -surface, some of the blue light from the sky is suppressed altogether, -while that from the cloud is practically unaffected. The exact fraction -suppressed depends upon the part of the sky relative to the sun, and -also on the position of the mirror, but a few minutes’ trial will show -when the maximum effect has been reached. - -It is astonishing to see for the first time how the delicate filaments -of cirrus or the beautiful structures of cirro-cumulus stand out -shining white on the deep blue background; and the use of the black -mirror is a revelation to most. It also has one indirect advantage, -which is really more important than it seems. By gazing down into a -mirror long-continued observations can be made, and one form of cloud -may be watched changing into another, and possibly back again into -its original shape, without any danger of incurring that unpleasant -result of much looking upwards which is sometimes known as exhibition -headache. Such a mirror may be quite small, so that it can be carried -in a pocket-book, a point of some moment, as many of the forms of -cirrus are exceedingly transient, coming and going in a few minutes, -while others are in a state of continuous change. This is particularly -often the case with the exquisite ripple clouds, and the delicate -lacework of the higher kinds of cirrus. - -Still another advantage possessed by the mirror is that it makes it -easy to see the solar halos formed on the verge of a cyclone, and -to detect their iridescent colouring in a way which is quite beyond -the reach of the naked eye or any protective spectacles. Every one -is familiar with the faint halos formed round the moon, but the -corresponding solar phenomenon is comparatively little known, though it -is far commoner, much more brilliant, and often glows with colour. Its -very brightness, and that of the background on which it is projected, -hides it from the eye, except on those rare occasions when the sun is -conveniently hidden by some thicker cloud. - -If some permanent record is desired, much can be done with a few light -strokes of a pencil, but more ambitious pictures are best secured -by the use of soft pastels, aided by a liberal use of the finger or -leather stump. Ordinary paints, whether oil or water-colour, are of -little use for actual study of cloud detail, except in the hands of a -highly skilled artist who knows how to get the effect he wants in the -minimum of time. - -But no sketching or drawing can make records of cirrus or alto clouds -with the speed and accuracy necessary for careful study. Photography -is really the only way in which the amazing wealth of detail can be -truthfully portrayed. Yet even the camera has its limitations. It -does not record colour, and completely fails to delineate the forms -of alto-stratus, stratus, or nimbus, if they are present in the most -typical condition, that is to say, when they cover the whole sky with -a uniform tint. It is only when these forms are more or less broken up -that a photograph, or anything other than a carefully coloured picture, -will represent them at all. - -Cloud photography, even of the most delicate and brilliant varieties, -is easy enough when the right methods are followed; but these are not -the same as those which are right for portraiture or landscape work -of the usual kind. The background of blue sky produces almost the same -effect on the plate as the image of the cloud itself, and the whole art -consists in an adequate exaggeration of the minute difference so as to -reveal the details of form and structure. - -A slow plate--the accompanying illustrations have all been taken -on Mawson and Swan’s photo-mechanical plates--extremely cautious -development, and sometimes intensification of the image, are all that -is necessary; but the process becomes easier if, instead of pointing -the camera to the cloud, it is directed to the image formed in a -properly constructed black mirror. Many of the following studies have -been taken by this method, and details of the camera and processes -employed will be found in a later chapter, for the convenience of any -one who may be inspired to take up a fascinating branch of photography. - -It has been said that reference will be made to the average altitudes -of the different types of cloud, and to the actual altitude of some of -the varieties shown. The question will, no doubt, have occurred to -some as to how those altitudes have been measured. The methods are all -more or less complicated, involving rather laborious calculation. They -generally depend upon simultaneous observations made from two stations -at opposite ends of a measured base line. Sometimes the observations -are made directly by pointing an instrument at each station to some -agreed point of the cloud. It is obvious that the two directions must -converge to this point. If the convergence is measured, the exact -distance from either station can be calculated, and if the angle -between the cloud-point and the horizon beneath it is noted, it is a -simple matter to deduce the actual altitude of the cloud. At other -places the observers have relied upon the comparison of photographs -simultaneously taken from the two stations. In this method it is -necessary to know the exact direction in which the camera is pointed, -and the position of the image upon the plate then gives the direction -of the cloud as seen from that particular station, and the subsequent -calculations are the same. - -Measurements by one or the other of the above methods have been made at -several places, the most extensive series being those which have been -compiled at Upsala, and at the Blue Hill Observatory in Massachusetts. -The method employed by the writer at Exeter has been rather different, -and a description will be found later on in the chapter on Cloud -Altitudes, the fuller consideration of which comes naturally after the -different forms have been described and compared. - -FOOTNOTE: - -[1] See reference No. 2 on p. 181. - - - - -CHAPTER II - -CIRRUS - - -A CLOUD is sometimes defined as any visible mass composed of small -particles of ice or water suspended in the air, and formed by -condensation from the state of vapour. As a general rule this is exact -enough, but under certain circumstances it is possible to have the -particles so small, and so thinly scattered, that it is not fully -satisfied. The resulting mass may not be actually visible. The presence -of the condensed particles may be indicated by nothing more than a -slight whitening of the blue sky, or by the formation about the sun or -moon of bright circles of light known as halos. If such a halo appears, -it is generally a phenomenon of brief duration. Sometimes the circle -breaks and becomes incomplete by the passing away of the thin patch of -cloud, sometimes the cloud increases in density until the rings are -destroyed. - -The thinnest variety of this halo-producing structure is quite -invisible to the eye. It is so thin as to have no distinctly noticeable -effect upon the colour of the sky, but the optical results of its -presence may be very remarkable. Highly complicated systems of rings -are sometimes produced, the rings, as a rule, falling into two groups. -The commonest form has the sun (or moon) in the centre, and a circle -of pale light at a distance of about 22 degrees. Larger rings are seen -less frequently, which have an angular radius of about 46 degrees, and -as a rule have the sun situated on the ring itself. In Plate 1 we have -a part of such a great halo. The camera was directed towards the east, -and tilted upwards at an angle of about 40 degrees. The sun was behind -the camera, in the south-west, and the ring could be traced right up to -it on either side. - -At the same time the sun was surrounded partially by a halo of the -more ordinary type, which was brightly coloured, making an effective -contrast to the dull white of the greater ring. The phenomenon did -not last more than half an hour, and the changes in its appearance -coincided with a growing density of cloud. When first noticed the -great ring was alone, and the sky was of a full blue, but a silvery -film came gradually up from the south-west, and the smaller and -brighter halo flashed out as the delicate curtain came near the sun. -Slowly the cloud spread to the north-east, gathering density from the -opposite point of the compass; and by the time the ordinary halo was at -its best, the great white ring had completely vanished. - -[Illustration: Plate 1. - -PART OF A GREAT HALO.] - -[Illustration: Plate 2. - -PART OF A SOLAR HALO.] - -These circles, and the bright spots called mock-suns or mock-moons -which often accompany them, can all be explained on the assumption -that their cause is the passage of light through a veil composed of -hexagonal crystals of ice. The simple halo of 22 degrees radius is -common in most parts of the world, being very generally formed by the -film of high cloud which marks the advancing edge of a cyclonic cloud -system. A portion of one is shown in Plate 2, in which the rudimentary -fibrous structure of the sheet of cloud is distinctly seen. Halos of -this sort are frequently coloured, often most brilliantly so; but the -tints are seldom noticed unless a black mirror is used. They are -sometimes quite as bright as those of an ordinary rainbow, but instead -of being projected upon a background of dark rain-clouds, they are -seen against a part of the sky which is near the sun, and therefore -exceptionally bright. - -The red is always on the inside of the ring, the violet outside, -thereby distinguishing them at once from the so-called coronæ, which -are formed around the sun or moon when shining through a sheet of alto -or other lower cloud made up of liquid particles. In these the radius -of the rings is much less, and the red is on the outside, the violet -actually touching the central luminary. - -The cloud which produces halos is called cirro-nebula. It is much -thinner, and on an average higher than cirro-stratus. Mr. Ley named -it cirro-velum (or cirro-veil), but cirro-nebula has now got to be -fairly well understood. It sometimes appears and disappears in a -curious manner, showing that it occurs in patches, which drift about or -which keep forming and melting away, only to repeat the process. If, -however, it forms part of an advancing cyclone fringe, then the sky -gets whiter and whiter, until it is covered with a sheet of undoubted -cirro-stratus. This process of growing density is shown in progress in -Plate 3. - -[Illustration: Plate 3. - -CIRRO-NEBULA CHANGING TO CIRRO-STRATUS.] - -Cirro-nebula, as we shall call it, floats at very great altitudes in -temperate regions; but in polar latitudes, where the optical phenomena -peculiar to it are most brilliant and diversified, it seems probable -that the ice dust is much lower down, even in actual proximity to the -ground. In England its height varies greatly with the time of year, and -other circumstances, but mounts up in summer to such altitudes as nine -miles or more; the greatest height yet recorded being 9·6 miles, or -about 15,500 metres, at Exeter. - -The change from cirro-nebula to cirro-stratus is generally accompanied -by the formation of a distinct fibrous structure, easily observable by -the black mirror. This is not really a new feature, but only a further -development of a structure already existing, but too minute to be -easily seen. True halo-producing cirro-nebula may usually be shown to -possess more or less of a fibrous texture in an indirect way, which is -worth a brief description. - -In order to observe the spots on the sun and other features of the -solar surface, it is a common practice to hold a white screen, say, -about a foot from the eyepiece of a telescope, while the instrument -is pointed to the sun. An image, considerably magnified, is thus -projected on to the screen, and the solar details can be studied with -ease and safety. If thin clouds drift before the sun, their images are -similarly projected as they pass across its disc, and it is possible -thus to detect not only the fibrous texture but also the movement of -cirro-nebula.[2] - -The change into cirro-stratus is also attended by a marked fall in -altitude, but whether this is due to an actual descent of the cloud -particles, or to a downward spread of the conditions which give rise -to them, cannot at present be definitely settled. The balance of -probability points very strongly towards the downward spread of the -conditions. It is likely that the clouds, particularly the cyclonic -specimens, are wedge-shaped, and that as they pass overhead we see -first the thin edge, and later on the thicker parts, which project -much lower down. This is just one of those many minor problems in -cloud mechanics which we are not able to solve from the scanty data on -record. - -Occasionally cirro-nebula breaks up into little detached -semi-transparent cloudlets, all of them exceedingly thin, and showing -a complicated mottling, resembling, on a minute scale, the ripple -clouds of much lower altitudes. Such a sky is depicted in Plate 4, but -no reproduction can possibly do justice to the minute and delicate -features of the real thing. The arrangement of the faint markings was -in a state of continual flux, curiously similar to the ever-changing -aspect of the sun’s photosphere when seen under adequate power. Some -parts of the cloud stratum would at one moment break up into distinct -granules arranged in complicated patterns, other parts would assume a -fibrous texture, and yet other places would show a continuous smooth -sheet. In a minute or two all would be changed--the smooth part -granulated, the fibres vanished, and the granules fused together, and -so on, no two of a series of photographs representing the same details. - -[Illustration: Plate 4. - -CIRRO-NEBULA CHANGING TO CIRRO-CUMULUS.] - -These changes of form continued until the whole was hidden from view -by a veil of much lower stratiform cloud, one advance portion of which -is shown. Plate 4 does not represent a type or a distinct variety of -cloud. It is an intermediate form, or a temporary condition, showing -cirro-nebula in the act of changing into cirro-cumulus, or possibly -cirro-stratus. - -Cirro-nebula itself, in its simpler form, is, however, a distinct type. -It is true that it never persists over one locality for more than an -hour or two without passing into some denser form, but while it lasts -its features are so distinctive, and the optical phenomena to which it -gives rise are so striking and significant, that it is a matter for -surprise that it should in the International system have been relegated -to the position of a subordinate variety of cirrus. It is more nearly -related to cirro-stratus, but is sufficiently distinct from that to -deserve at least specific rank. - -True typical cirrus must have a plainly shown fibrous structure. The -fibres may cross and interlace, they may radiate in fan-like manner, -or they may curl and twist like a well-trimmed ostrich feather. The -clouds so formed must not be arranged in a continuous level sheet, or -they at once become cirro-stratus, and it is impossible to invent a -definition which will mark the exact limits of either type. Typical -cirrus consists of detached clouds. They cast no shadows on the -landscape, for the simple reasons that they are semi-transparent and -their component parts too narrow. If the sun is shining down obliquely -through the naked boughs of a tall tree, it will be seen that the -lowest twigs cast fairly sharp shadows on the ground, but that even -these are bordered by a fading rim; the twigs further up cast no sharp -shadows, but broader faint bands of shade; while the topmost boughs -cast no shadows which can clearly be identified. In other words, the -more distant the narrow twig is from the ground the narrower the real -shadow or umbra, and the broader the penumbra becomes, until when the -distance is sufficient the shadow is all penumbra. Cirrus filaments -throw nothing but a faint penumbra. Indeed, it is only when they lie -in the earth’s shadow, and stand against the background of a faintly -lighted sky, that they show any sign of shadow even on themselves. - -There is no doubt that they are composed of particles of ice. They are -formed at altitudes where the thermometer must be many degrees below -freezing-point, and not a few of the thinner examples show fragmentary -halos like those of cirro-nebula. - -Their actual altitudes are very variable, being greater in summer than -in winter, and reaching a maximum for any given station after a long -spell of hot weather. Exact measurements have not yet been made in -tropical latitudes or in polar regions, but there is every reason to -expect that the upper limit of cirrus for equatorial districts will -be found to be much higher than in the temperate zones where actual -observations have been made. In places nearer to the Arctic Circle it -is also almost certain that the altitudes will be less. - -In the New England states, as shown by the Blue Hill observations, the -maximum altitude for summer was found to be little under 15,000 metres. -At Upsala, in Sweden, it was 13,300 metres. The average altitudes at -the same observatories were, respectively, about 9900 and 8800 metres. -At Exeter the writer’s own measurements give an average for the summer -months of 10,200 metres, with a minimum rather lower than was the case -in America or Sweden, and with a maximum far above the foreign values. -In winter cirrus certainly comes much lower down, but the number of -observations is fewer. - -[Illustration: Plate 5. - -HIGH CIRRUS. - -(_Cirrus Excelsus._)] - -The loftiest variety of cirrus appears in the afternoon in very hot -weather, sometimes quite late in the evening; and in autumn it is by no -means a rare event for it to suddenly form just when the sunset colours -are fading, or even after they have paled into twilight. Under such -circumstances it stands out of a shining silvery grey colour against -the background of the darkening sky. A specimen of it is shown in Plate -5, which shows its extreme slightness of form and delicacy of texture. -Sometimes it remains visible so long after the stars have begun to -show as to give the idea that it is self luminous, and the illusion -is certainly very strong. The writer has noted several instances in -which it was plainly visible, like a silvery curtain, though the sky as -a whole was so dark that stars like the five brightest points of the -Great Bear could be seen through the cloud, and much smaller stars down -to the third and fourth magnitude were plainly visible in the clear -intervals. It has sometimes been called luminous cloud, and Mr. Ley -estimated its altitude at upwards of 90,000 metres; but if we think of -it as reflecting the light of the distant colourless twilight there is -no need to regard it as anything fundamentally different from other -clouds, or to assume a greater altitude than we know to have been the -case. The specimen figured occurred in the early afternoon on June -12, 1899, at Exeter, and careful measurements of its altitude were -made. This worked out as 17·02 miles, or more than 27,000 metres, a -value so much greater than all other measurements of the kind that it -was only after most careful verification and reference to duplicate -records that it could be accepted. It differs in several ways from the -lower varieties, being thinner, more glistening, and in every way more -delicate. A suitable distinctive name would be high cirrus, or cirrus -excelsus. - -[Illustration: Plate 6. - -WINDY CIRRUS. - -(_Cirrus Ventosus._)] - -Lower down by thousands of metres come the feathery masses of typical -windy cirrus, such as are shown in Plate 6. Indeed, in cold winter -weather they occur within three or four thousand metres of the ground. -In the instance figured the wind was blowing from left to right, and -the clouds were travelling swiftly. The upper filaments appeared to be -repeatedly torn away from the main masses, while the long faint streaks -which trail below and behind are evidently due to streams of fine -particles falling from the main centres of condensation into a less -rapidly moving stratum below. There is no room for doubt that these -clouds, like others of a similar order, are formed by a direct passage -from the vapour to the solid, or that the fibres are made of minute -snowflakes. The condensation is evidently attended by rapid movements, -which draw out the cloud, as fast as it is formed, into long curving -lines which mark lines of motion. The variety is always, therefore, an -indication of strong winds and rapid eddying movements in the region -in which it occurs. Such strong disturbances overhead almost always -accompany similar but less intense movements at the ground-level, and -when they do not accompany them they precede them. The cloud is well -named windy cirrus, which may be converted into a specific name, cirrus -ventosus. - -The next variety we come to (Plate 7) is in some ways rather similar. -It is, however, thinner, more delicate, and is entirely composed -of fine threads, which are more systematically arranged. Generally -there is a bundle, or several bundles, of long parallel fibres, which -form, so to say, the quill of the feather, with numbers of shorter -threads branching out from them at various angles. Cirrus ventosus was -indicative of irregular movements in various directions; this variety -points also to complicated movements, but executed in accordance with -some sort of system, strangely complex and wonderfully ordered. The -specimen figured is the type of what Mr. Ley called cirro-filum, or -thread cirrus, and his name can hardly be bettered. It is a cloud of -summer, and occurs rather high up in the cirrus zone, but no actual -measurements can be quoted. It is fairly common, but not nearly so -frequent as the last. - -[Illustration: Plate 7. - -THREAD CIRRUS. - -(_Cirro-filum._)] - -A somewhat more familiar variety is shown in Plate 8. Little irregular -feathers of cirrus, from which long tapering streamers point downwards -in graceful curves, or else lag behind in the direction from which -the clouds have travelled. If clouds of this type are carefully -watched, it will soon be seen that each feathery head is a centre of -condensation, and that the tails or streamers are nothing else than -falling particles, which dwindle slowly away by evaporation, and which -gradually sink below the level of the heads. It is usual, in dealing -with cloud-forms like these, to speak of air-currents of different -velocities almost as if the winds at different levels were as clearly -separated as oil and water, or even air and water. This can hardly -be the case, for if such a thing should occur as an air-current of -one velocity flowing over another of less speed, or of a current in -one direction over another moving in a different course, the two must -inevitably mix at their junction, and in a very short time the passage -from the lower current to the upper one would be quite gradual. No -doubt we can often observe two, three, or more layers of cloud moving -in different directions; but if we were to send up a balloon, it would -be rare indeed to find its direction of horizontal movement changed in -a few metres of ascent. Different and distinct air-currents are often -invoked to explain cloud-forms quite unnecessarily. It is far more -likely that the differential movements involved in the explanation of -the features of these cirrus varieties are due to increased velocity -with greater altitude, to progressive change of direction, to irregular -eddies, or to the interaction of ascending and descending convection -currents. Indeed, it is probable that careful study of the growth and -decay of these clouds will, in time, lead to a clearer understanding -of atmospheric movements, and so enable us to say more precisely why -they are as we see them to be. The variety shown in Plate 8 is rare -except in combination with other forms. It might well be termed tailed -cirrus or cirrus caudatus. - -[Illustration: Plate 8. - -TAILED CIRRUS. - -(_Cirrus Caudatus._)] - -The form of cirrus shown in Plate 9 is far more frequently seen than -either of those which have been described. In this the fibrous texture -is very imperfect, and the cloudlets show a tendency to arrange -themselves in a kind of ribbed structure in two directions almost at -right angles to each other. But this last is an accidental feature -of the particular example, and not in any way a specific character -of the cloud. The reason for regarding it as a distinct variety is -the total absence of sharply defined lines, not only the heads of -condensation, but even the long streamers attached to them being -uniformly hazy and ill-defined. It is a form of cirrus which comes at -all seasons, but most frequently in summer; it moves always with great -slowness, indicating a quiet atmosphere free from disturbance of any -kind. The conditions necessary for its appearance are a nearly uniform -distribution of pressure over a considerable area, chequered by little -shallow depressions of some trifling fraction of an inch. In hot -weather these are the conditions under which thunder-storms develop, -and this hazy cirrus, or cirrus nebulosus, may be taken as a certain -sign of such an atmospheric state. - -[Illustration: Plate 9. - -HAZY CIRRUS. - -(_Cirrus Nebulosus._)] - -So far as permanency of form is considered, hazy cirrus is one of the -most persistent, and affords a marked contrast to the species shown in -Plate 10, which represents the most fugitive. Five minutes before the -photograph was taken the same part of the sky was a deep, clear blue, -without any trace of cloud. Suddenly a few short curling wisps made -their appearance. These rapidly increased in number, until a delicate -filmy network extended over the greater part of the field of view. But -while the camera was being adjusted for an exposure, part of the net -had broken up into the granular structure shown in the lower part of -the photograph. The granulation rapidly spread through the net, almost -as if the fibres had been curdled, and five minutes later the whole had -been converted into a patch of cirro-cumulus which soon fused into a -uniform sheet. Meanwhile the same series of phenomena were taking place -in other parts of the sky. - -[Illustration: Plate 10. - -CHANGE CIRRUS. - -(_Cirrus Inconstans._)] - -On other occasions exactly the same set of events have been seen to -follow each other in the inverse order. Beginning with a fairly even -sheet, this broke up into granules, and they in turn seemed to be -frayed out into short hazy and wavy fibres which slowly melted away. - -Clearly we have here to do, not with a distinct type of cloud, but -rather with the first step towards the formation of one, or the last -stage in the life of one which is drying up. But sometimes the life of -the cloud is so short that it never passes beyond this first stage; -and it is by no means a universal rule for a growing sheet of cirrus -to pass through this stage at all. It therefore represents a peculiar -state of instability, and requires a name of its own. Sometimes patches -of it will come and go in an apparently capricious manner for an -hour or more before permanent condensation is effected or before the -sky finally clears. But this is a rare event, since the slow change -of conditions which has brought the stratum of air to the unstable -condition is generally progressive, and instead of stopping at the -critical point, goes beyond it, with the result that the condensation -grows or the cloud disappears entirely. Change cirrus, or cirrus -inconstans, would be an appropriate name for a kind of cloud which is -so plainly indicative of instability. - -The critical condition referred to is, of course, that in which a -particular stratum of air is just saturated, or is just on the point -of forming visible cloud. If any cause is brought to bear on such a -stratum which brings about even slight cooling, cloud must be produced; -and, conversely, anything which results in the slightest heating must -cause it to disappear. The shortness and haziness of the fibres, and -the fact that they gather themselves into granules, shows that the -cloud is formed in a stratum of air which is either still, or is moving -as a whole, without any of those differential movements which seem to -be necessary for the longer fibrous details. - -The causes which may bring about the local cooling and heating are easy -to understand when we remember how the air will be affected by the -uneven contours of the ground. As it passes over hill and valley the -up-and-down movements of the lower layers, or even the disturbances -caused by passing over a wood or clump of trees, all must be propagated -upwards. Each disturbance must slowly spread laterally and diminish -vertically, so that it will reach the cirrus zone as a broad and -gentle dome-like oscillation. Suppose now a series of such slight -upheavals to reach the critical level. The passage of the waves will -mean alternate expansion and compression. Expansion means cooling, and -therefore cloud-production; compression means heating, and therefore -the destruction of cloud. - -From the most transient form of cirrus we pass, in Plate 11, to the -most persistent and probably the most frequent. It occurs in detached -masses which have very variable forms but are wholly fibrous, with the -details arranged in a very irregular manner. The example figured was -taken in the evening during a long spell of fine weather. If such a -cloud is watched, its permanence of detail is very striking, and must -be due to a persistence of slow eddying movements and to a continual -renewal and waste of the component particles of each wisp. This is -the kind of cirrus selected generally as the type of cirrus, and the -selection is a good one. Common cirrus, or cirrus communis, it should -be called. Settled conditions and fine weather are its usual attendants. - -[Illustration: Plate 11. - -COMMON CIRRUS. - -(_Cirrus Communis._)] - -We next come to a variety which is anything but a harbinger of good, -namely, the long stripes or bands of cirrus which stretch outwards -from the margin of the cloud canopy of a cyclonic storm. In some ways -these appendages to the great nimbus resemble the strips of cirriform -cloud which fringe the summit of a thunder-cloud. They look as if they -must have been formed by the blowing away, by a rapid wind, of the top -of an uprising column of vapour-charged air. Their main outline may -thus be easily accounted for, but we have only to study their detailed -structure for a few minutes to feel that they really present a problem -of a very high order. Plate 12 shows a fairly simple example, but Plate -13 represents a cloud of very great complexity. To take this last the -camera was tilted upwards at an angle of 45 degrees, so that the top of -the picture is not far from the zenith. The wonderful plume of cloud -rose from the southern horizon, and ended in a great sheaf of fibres -and films spread out like a partly opened fan whose edge was only about -50 degrees above the northern horizon. Its length as it passed overhead -lay between a point a little east of south to a little west of north; -and the broad band moved as a whole, without any marked internal -changes, from the south-west towards the north-east. The weather was -very unsettled. A long procession of cyclones had been sweeping along -our western shores, and the barometer was just beginning a fresh and -rapid fall. During the ensuing night a heavy gale burst over the south -of England. - -[Illustration: Plate 12. - -BAND CIRRUS. - -(_Cirrus Vittatus._)] - -[Illustration: Plate 13. - -BAND CIRRUS. - -(_Cirrus Vittatus._)] - -The whole phenomenon was highly characteristic. These great bands with -the divergent striation might well be known as storm bands, from their -almost invariable connection with the violent atmospheric movements -to which they are most probably due. Plate 12 shows a much less -dangerous variety of the same species, which is distinguished from it -by the comparative absence of internal detail and by the curled ends. -Clouds of this character have sometimes been called cirro-filum, but -a comparison of the plates with the typical cirro-filum of Plate 7 -will show that there is little resemblance; and the attendant weather -is also in marked contrast, both of which facts imply a fundamental -difference in the causes to which their features are due. Banded or -ribboned cirrus is the name which they immediately suggest, and this -may be rendered cirrus vittatus. - -This ends our survey of cirrus clouds. Any one who compares the plates -so far given will see that they represent forms so diverse that it is -impossible to avoid the conclusion that the conditions under which they -are produced must differ not only in degree but also in kind. What -those conditions are we have attempted here and there to suggest, but -in no case can we feel that the explanation has been at all complete. -In some cases, notably the last, we are face to face with such -complicated details that it is hopeless to attempt to explain them in -the present state of our knowledge. Fact upon fact must be accumulated -until we can give their history from their earliest beginnings; and -far more accurate and detailed knowledge of the attendant atmospheric -conditions must be acquired before we can hope to rob such elaborate -structures of their present mystery. - -This requires the co-operation of many eyes and many minds, and exact -specific names must be an essential preliminary. Those which have been -suggested in these pages are-- - - 1. Cirro-nebula, or Cirrus haze. - 2. Cirrus excelsus, or High cirrus. - 3. Cirrus ventosus, or Windy cirrus. - 4. Cirro-filum, or Thread cirrus. - 5. Cirrus nebulosus, or Hazy cirrus. - 6. Cirrus caudatus, or Tailed cirrus. - 7. Cirrus vittatus, or Band cirrus. - 8. Cirrus inconstans, or Change cirrus. - 9. Cirrus communis, or Common cirrus. - -FOOTNOTE: - -[2] The telescope with which these observations have been made is a -6·8-inch refractor equatorially mounted. - - - - -CHAPTER III - -CIRRO-STRATUS AND CIRRO-CUMULUS - - -SEVERAL of the varieties of cirrus already discussed may gather so -abundantly at some given level in the atmosphere, that the most obvious -feature comes to be this arrangement in a sheet. The cloud then -becomes cirro-stratus, and should be so named. We have described how -cirro-nebula frequently grows in density until it fails to produce halo -phenomena, and may even reduce the sun to a hazy patch of light showing -no outline. This is the most typical of all forms of cirro-stratus. -It has always a distinctly fibrous or streaky appearance, whereby it -is at once distinguished from a similar but lower cloud which will be -described later on. - -A similar sheet may be formed from the fusion together of the streaks -of cirrus-nebulosus, the bands of cirrus vittatus, or the development -of cirrus inconstans. But the general rule is that the cirro-stratus -retains more or less of the specific characters of the parent form. - -Plate 14 shows a hazy form of cirro-stratus developed from the nebulous -cirrus. Its altitude was great, being about 10,000 metres. The -processes of growth and change could be studied easily. First would -appear some faint spots and streaks; these quickly fused together -into larger patches, which again joined to their neighbours. In a few -minutes the cloud so formed would return to the mottled or streaky -appearance, and either disappear entirely or become very thin, only -to recommence the process. This went on for more than an hour, the -cloudy patches getting larger and larger, until the critical condition -was passed, and the sky was covered with a general veil of typical -cirro-stratus. - -[Illustration: Plate 14. - -HAZY CIRRO-STRATUS. - -(_Cirro-stratus Nebulosus._)] - -In Plate 15 we have an example of a cloud which is clearly -cirro-stratus, but the sheet is broken up into long bands, and each of -these is made up of common cirrus. In the upper part of the picture -the sprays of cirrus are forming, and as they come into being they -are arranged in rows. We have here to do with a phenomenon of a very -different order from the one presented by the true banded cirrus. The -arrangement into belts must here be due to some kind of wave-movement -in the air, breaking up the critical plane into long ribs transverse -to the direction of the wave-movement. The specimen shown was moving -in a direction nearly at right angles to the bands, though the surface -wind was nearly parallel to their length. The question at once -presented itself as to whether the movement of the bands was really a -drift of the cloud, or whether it was not a case of the propagation -of cloud production with the advancing wave. This was easily answered -by watching the details of a band. The advancing side was always -feathery, and careful observation showed that the edge advanced by -throwing out new threads and curls. A given thread or curl, one moment -at the edge, would in a few minutes be well in the band. Quite opposite -events were taking place in the rear of each band. The cloud was there -obviously melting away. Indeed, to sum the matter up, the cloud bands -flowed past their details just as the waves on the sea flow past the -floating foam. Evidently we have in Plate 15 the result of a plane of -commencing cirrus formation broken into a series of troughs and waves -by an undulatory movement of the air. But, as we have already said -when speaking of cirrus inconstans, the condition in which trifling -up-and-down movements can determine whether condensation shall, or -shall not, take place seldom lasts long. It is usually only a stage -in a continuous change, and in this particular instance the banded -structure was soon replaced by a fairly continuous sheet of typical -cirro-stratus. - -[Illustration: Plate 15. - -CIRRO-STRATUS.] - -The next plate, No. 16, shows a similar process. In the upper part we -have cirrus inconstans forming in patches out of a deep clear-blue sky. -Its hazy fibres grow closer and closer, betraying a slight tendency to -gather in narrow ripple-like bands, but the structure is soon lost in -the uniform white sheet of interlacing fibres, which differ from common -cirrus in little else than their number and closeness. Nevertheless, -the stratiform arrangement is quite obvious enough to warrant the use -of the term “cirro-stratus.” - -[Illustration: Plate 16. - -CIRRO-STRATUS. - -(_Cirro-stratus Communis._)] - -The change of cirro-nebula into cirro-stratus is shown in Plate 4, to -which reference has already been made. The structures are remarkably -delicate, showing in the middle a distinct irregular mottling; and -rather further towards the top right-hand corner a ripple structure -appears, and in the top left-hand corner the sheet is denser and -whiter. The altitude of this cloud was evidently great, and actual -measurement showed it to be 7·6 miles. It did not last long, and after -its change into broken patches of denser cirro-stratus, still higher -clouds were revealed through the gaps. - -Cirro-stratus often forms almost simultaneously at more than one level, -and when that happens the full stratiform appearance is generally -reached first by the lower layer. In Plate 17 we have two layers. The -fluffy bits of cirrus nebulosus, in the lower part of the picture, are -really the higher clouds. Below them, probably by many thousands of -feet, floats the denser cloud shown in the upper part of the picture. -This is an interesting link between the fibrous and the granular forms -of cirrus, and is probably best described as spotted cirro-stratus, -or cirro-stratus maculosus. It is a form very frequently met with, -but seldom showing any persistence. It is indicative of condensation -in a calm atmosphere, and not unfrequently marks either the small -irregularities of pressure which form the conditions for thunderstorms, -or the beginning of the break up of an anticyclone. - -[Illustration: Plate 17. - -FLOCCULENT CIRRO-STRATUS. - -(_Cirro-stratus Cumulosus._)] - -A coarser texture and greater density are shown in Plate 18, where we -have cirro-stratus in the lower part of the picture, and cirro-cumulus -in the upper. The altitude of this cloud was only about 4000 metres, -one of the least values recorded at Exeter for cirro-stratus of any -kind. The intimate admixture of the fibrous and granular forms is very -clearly shown. - -[Illustration: Plate 18. - -CIRRO-STRATUS AND CIRRO-CUMULUS.] - -This close relation is equally obvious in Plate 19, where the cloudlets -are arranged in loosely marshalled rows, dimly resembling the banded -structure of Plate 15. But in this case the direction of movement was -with the long lines, and the propagation of cloud production followed -the same course. Some of the little cloudlets have an opacity, and -therefore brilliancy, quite unusual for cirro-cumulus, but their -intimate association evidently in the same plane with undoubted cirrus -shows that they must fall under that general description. It is a cloud -indicative of unsettled weather, and the exceptional brilliancy is -doubtless due to an unusual quantity of vapour at the cloud plane, -which must mean that the change from the dry stratum above to the damp -one below must be much more sudden than is ordinarily the case. Clouds -of this kind might well be called cirro-stratus cumulosus. - -[Illustration: Plate 19. - -CIRRO-CUMULUS.] - -We now come to two companion pictures, Plates 20 and 21, which were -taken within half a minute of each other. In the first the camera -was directed towards the west, and in the second it was facing the -north-west. The sun was nearing the horizon, and was only just outside -the field of view in each case, so that the two photographs form -a panorama of the western sky. A solar halo had disappeared about -half an hour previously, and the cirro-nebula had changed into the -remarkable forms of cloud depicted. Plate 21 shows cirrus ripples in -the upper part, and cirro-cumulus in soft, ill-defined balls in the -lower part; but they were at the same level, and are only different -parts of the same cloud plane. In Plate 22 we see similar ball-like -cloudlets ranged in long lines which run almost at right angles to -the ripples of the companion picture. Clouds like these are rare. -They are almost unknown during the early part of the day, and, so -far as the writer’s experience goes, they are only to be found in -the afternoon towards sunset. Some of our most gorgeous sunset skies -are due to them; for their altitude is considerable, and they do not -light up with the sunset colours until the lower clouds have become -dark shadows against the glowing background. The hottest months of the -year, the still air and great evaporation which are the contributing -causes of thunderstorms, are also the conditions under which such -skies may be seen. Indeed, while these photographs were being taken, -heavy thunderstorms were in progress within less than a hundred miles. -Cirro-cumulus nebulosus, or hazy cirro-cumulus, describes the form -correctly. - -[Illustration: Plate 20. - -HAZY CIRRO-CUMULUS. - -(_Cirro-cumulus Nebulosus._)] - -[Illustration: Plate 21. - -HAZY CIRRO-CUMULUS. - -(_Cirro-cumulus Nebulosus._)] - -The next plate, No. 22, gives a view of an evening sky about half an -hour after sunset. The lower clouds, cirro-cumulus and cirro-stratus, -of a deep purple brown, standing out dark against a gold-coloured sheet -of higher cirro-stratus, which comes out white in the photograph, while -the purple-tinted sky comes dark. We have here three distinct layers, -all cirrus. First, the hazy cirro-cumulus, forming two bars across the -lower part of the picture; then long bands of cirrus or cirro-stratus, -best seen in the bottom right-hand corner; and, far above both, the -cirro-stratus which was reflecting the yellow sunlight. Such a sky -might be an indication of thunder conditions, or it might be due to an -unusual quantity of vapour in the atmosphere produced by some other -cause. The actual conditions were the gentle flow over England of -vapour-laden air from the western ocean, heralding the change from a -long spell of fine hot weather, due to a July anticyclone, to a month -of heavy rains and western gales, accompanying the passage of a long -procession of cyclones along our western shores. - -[Illustration: Plate 22. - -A SUNSET SKY.] - -Again, a marked contrast is shown in Plate 23. Here we have the highest -and thinnest form of cirro-cumulus, the one named cirro-macula by -Mr. Ley. It is rarely, if ever, seen before eleven o’clock in the -morning, and is far commoner in the afternoon. The example shown was -photographed at sunset at the close of a day which had been almost -cloudless. Cirro-macula forms here and there in a clear sky. A hazy, -whitish patch appears, which at first shows no definite structure, but -looks almost like a little bit of cirro-nebula. This suddenly splits -up by clear blue lanes running through it, and cutting the patch up -into irregular segments, which quickly round themselves off into minute -bits usually whiter on their edges and semi-transparent in the centre. -The process can be strikingly imitated by scattering on water some fine -powder which will float. If left without disturbance, the particles -draw together into numerous small groups, leaving lanes of clear water -between them. - -[Illustration: Plate 23. - -SPECKLE CLOUD (Ley). - -(_Cirro-macula._)] - -Cirro-macula frequently gives rise to the fibrous form of cirrus we -have called cirrus caudatus. The granules of the cirro-macula grow -denser, and begin to drop their frozen particles as soon as they become -large enough. Indeed, a cloudlet of cirro-macula may sometimes be seen -to turn bodily into a fine line of falling crystals, which will be a -curving line of cirrus. On the other hand, it will sometimes remain -visible for an hour or more without any trace of descending streaks or -floating fibres. Pure cirro-macula such as Plate 23 is not often seen; -it is far more frequently mixed with more solid-looking cloudlets and -descending fibres, such as are shown in Plate 24, which gives the same -point of view as 23, but a quarter of an hour later, and photographed -with a longer focus lens. These two photographs, together with 20 and -21, give excellent examples of the use of the black mirror. In none -of the four could the naked eye detect all of the cloud structures. -The whole sky was a blaze of dazzling light, but by adjusting exposure -and development the details are fully brought out without the least -difficulty. - -[Illustration: Plate 24. - -CIRRUS CAUDATUS AND CIRRO-MACULA.] - -Cirro-stratus, we see from the examples which have been considered, -hardly deserves to be treated as a distinct genus of cloud. Its -formation is identical with that of many species of cirrus, or in some -cases with that of the speckle cloud, cirro-macula, or even the coarser -kinds of cirro-cumulus. The different varieties which it shows are best -rendered by reference to the specific names of the detached forms which -have similar features. - -Cirro-cumulus, on the other hand, does present clearly marked -varieties. Cirro-macula is so distinct that it might well be given -the name awarded to it by Mr. Ley, while the term “cirro-cumulus” is -reserved for the coarser and rounder forms. The hazy, ripple-like -structures of Plate 4 and Plate 20 should also have some distinctive -appellation, as will be suggested later on when dealing with wave -clouds as a whole. - -It is difficult to find any short way of expressing the various ideas -which should be summed up in the name of a cloud. There seems no -alternative to the use of additional words, unless it be to follow -the example of chemists, and compound appalling names similar to -those which terrify the uninitiated who think they would like to read -something about, let us say, the coal-tar dyes. - -If a cloud belongs to the order cirrus, is in a level sheet, and -that sheet is composed of interlacing or curling fibres, like those -of common cirrus, we can hardly express the facts more briefly than -by calling it cirro-stratus communis, or common cirro-stratus. If -it consists of cirrus bands fused together, but still showing the -banded structure, it is cirro-stratus vittatus. Again, if it is finely -speckled, like cirro-macula, it may be described as cirro-stratus -maculosus, and if the structure is coarser it may be called -cirro-stratus cumulosus. - -As a general average, cirro-stratus lies somewhat lower in the -atmosphere than the detached forms, probably because the conditions -which give rise to the latter reach to greater altitudes in patches -than it is possible for them to reach in a continuous manner. Vapour -becomes rarer with increased height and with diminished temperature, -so that it must, on the whole, be less frequently present in -cloud-producing quantity as the height increases. At great altitudes -it will be seldom that the quantity is great enough to produce a -stratiform cloud, though it may well be enough for cirro-macula, or the -detached forms of cirrus, like cirrus excelsus. - -The production of cirro-cumulus and cirro-stratus sometimes spreads -across the sky with astonishing speed, and this rapid advance of the -edge of the cloud may lead to quite mistaken ideas as to the velocity -of the wind at that altitude. In the case of cirro-cumulus, or -cirro-macula, it is easy to fix attention on a single cloudlet. If this -has the usual ball-like form, it can only be regarded as floating in -the air and moving with it. Meanwhile new cloudlets may be forming and -growing denser, so that the cloud patch as a whole may be apparently -advancing at a much greater rate. Careless observation would then lead -to the idea that the cloud was moving much faster than it really is, -but if the attention is rigidly fixed on a particular cloudlet the -mistake is impossible. If the cloud is a variety of cirro-stratus, -it is not always easy, or even possible, to distinguish between the -advance of condensation and the movement of the whole, but it can -nearly always be done if the cloud shows any definite features upon -which attention can be fastened. Sometimes none sufficiently marked can -be seen, and when that happens it is still possible in most cases, by -watching the edge of the cloud-mass, to see whether new cloud is being -added to that edge. The wave-like forms present a special case, which -will be dealt with in a later chapter, after the general principles -of cloud formation have been discussed in connection with the great -clouds of the lower air, whose causes and conditions are far better -understood. - - - - -CHAPTER IV - -ALTO CLOUDS - - -FROM cirro-cumulus and cirro-stratus we pass through almost insensible -gradations to the denser forms classed together in the alto group. -These clouds are fundamentally different, in that they are always -composed of liquid particles, though there is no doubt, from their -great altitude, that their temperature must often be many degrees below -the ordinary freezing-point of water. When this is the case, they -are not unfrequently more or less mixed with streaks and filaments -exactly like those described under the name of cirrus, which have been -explained as due to slowly falling snowflakes. It is not immediately -obvious how such apparently contradictory statements can be reconciled. -The explanation is that minute droplets of water may be cooled many -degrees below freezing-point without changing into ice, and that such -super-cooled droplets congeal instantly if a few of them join together -to form a larger drop. Practically the same process may be watched any -day when there is a sharp frost and dense fog drifting slowly along. -The fog-particles are liquid, and produce optical effects in the -neighbourhood of any brilliant light, like an arc lamp, absolutely the -same as those which would be produced if the temperature were above -freezing-point, while there are none of the different phenomena which -might be expected if the particles were crystalline ice-dust. As these -liquid particles drift along they come in contact with branches of -trees and other obstacles, the surface stratum which surrounds them and -binds them into spheres is broken, and the drop instantly solidifies. -It is to be noted, moreover, that the drop does not freeze as such, -but merely adds some more particles to the branching crystals of hoar -frost, which grow outwards always towards the direction from which the -fog is drifting. - -Most liquids, when freed from contact with solid bodies, or when -surrounded by a smooth envelope of uniform character, can be cooled -below their normal freezing-point without solidification taking place; -but the introduction of a particle of the solid, or sometimes of any -foreign body, instantly brings about a rapid freezing of the whole. -These phenomena of surfusion, as it is called, have long been known, -and many of them are very interesting and difficult to understand. -Indeed, it is probable that we shall have to add largely to our -knowledge of the forces which bind the molecules of a body together to -form a solid, and which direct the processes of crystallization before -we shall be able to interpret with any certainty a series of facts -depending on the attributes of those very forces. - -Water is no exception. If finely divided, as by placing it in fine -capillary tubes, in the pores of wood, or in the narrow spaces of a -wick, it may be cooled several degrees below normal freezing-point. -In a cloud, or fog, all the conditions necessary for surfusion to -take place are undoubtedly present. The water is pure, the envelope -is uniform, the subdivision is exceedingly minute, and the drops are -free from most of the mechanical disturbances which bring about the -solidification of larger masses in the laboratory. - -Thus we see there is nothing at all surprising in the fact that clouds -composed of liquid particles may exist at temperatures below the -ordinary freezing-point. On the contrary, we should expect that the -solidification of the cloud particles would not take place until the -temperature was many degrees below freezing, as is certainly the case -with clouds of the cirrus order. At temperatures between this unknown, -but low value, and the normal freezing-point, the clouds will be -composed of liquid; but when the particles join together, snowflakes -will result instead of raindrops; and this will be just as true of -alto clouds as it is of the great vaporous mountains of the lower -regions of the air which bring falls of snow. The streaks often mixed -with alto-cumulus are cirrus threads, and are, no doubt, of exactly -the same nature as the tails of cirrus caudatus, or even the fibres of -cirro-filum. - -The simplest alto cloud is alto-stratus. When this is complete, so as -to cover the sky, it can be distinguished from cirro-stratus by the -absence of fibrous structure, and by the facts that it never produces -any halo or fragment of a halo, but instead surrounds the sun or -moon with a white blur, or, if it is thin enough, with a close ring -of coloured light much nearer than a halo, and with the colours in -the inverse order--that is, with the red furthest from the centre. -Some of these so-called coronæ are very beautiful when seen in the -black mirror, and some of those formed around a full moon show quite -brilliant tints to the unaided eye. Of course, these meteorological -coronæ have no relation whatever to the true solar corona; they are -simply formed by the passage of the rays of light through the veil of -small particles, and may be easily imitated. Take a piece of glass such -as a lantern-cover glass, breathe on it, and hold it close before the -eye while looking at some small source of light. If the dew deposit -is thin, bright colours are shown in a luminous ring surrounding the -light, and the thinner the deposit of dew the larger the ring will be. -Breathe heavily so as to give a thick deposit, and the light will be -seen to be the centre of a patch of white brightness without any colour. - -The phenomena are due to what is known as diffraction, and if the -other conditions are unchanged the diameter of the ring is inversely -proportional to the size of the particles. Purity of colour in these -rings is an indication of uniformity in the size of the particles. -When the moon is shining through a sheet of alto-stratus, which thins -off to one edge, very beautiful effects may often be noticed, and the -change from the colourless blur, when a thicker part of the cloud -is interposed, to the brilliant colours of the corona formed by the -thinner edges is very striking. Similar phenomena are shown almost -equally well by any of the alto clouds, but cirrus thin enough to -produce a coloured corona will generally produce a halo. - -Alto-cumulus of the kind most nearly allied to cirro-cumulus is shown -in Plate 25. The upper part of the picture shows ragged, irregular -patches, with slight indications of fibrous streaks. The lower portion -shows rounder, ball-like cloudlets, a few of the larger of which have -distinct shadows on the side away from the sun. This plate gives -alto-cumulus in a partly formed condition, but it is not a mere passage -form. Sometimes exactly such a cloud will float overhead for hours, -showing very little movement and only slow changes of detail. It is -therefore a distinct variety, and may be called alto-cumulus informis. - -[Illustration: Plate 25. - -ALTO-CUMULUS INFORMIS.] - -A less definite form is shown in Plate 26. It may also be regarded as -only partly formed, but its construction is quite different, every part -being misty and ill defined. It is a common cloud, especially in sultry -summer weather with still air. Under those circumstances, after a hazy -morning, it may be seen slowly forming during the afternoon, growing -in density as the hours go by, until it reaches a maximum about five -or six o’clock, after which it melts away, or settles down into small -patches of high stratus. Most frequent in summer, it is by no means -rare in autumn and winter, but still air is essential. From its hazy -appearance it may be called alto-cumulus nebulosus. - -[Illustration: Plate 26. - -HAZY ALTO-CUMULUS. - -(_Alto-cumulus Nebulosus._)] - -Fixity of detail and slow movement characterize both the foregoing -forms, and in that respect our next picture (Plate 27) shows a cloud -which is a great contrast. Its detached cloudlets are rather flatter -and thinner, and though the cloud as a whole will often persist for -hours, it is undergoing continual change, and is formed when the air -is far from still. Cloudlets form and gather into stratiform patches, -which soon break up again and disappear; and the process goes on -here and there, sometimes accompanied by fairly rapid movement of -the patches as a whole. This cloud may be described as alto-cumulus -stratiformis. - -[Illustration: Plate 27. - -FLAT ALTO-CUMULUS. - -(_Alto-cumulus Stratiformis._)] - -We now come, in Plate 28, to a cloud of singular beauty. It forms -rapidly in a clear sky, its first traces bearing a striking -resemblance to cirro-macula, but the floccules, instead of remaining -semi-transparent or dropping cirrus threads, rapidly become opaque -balls of cloud which lengthen upwards. This upward tendency causes -the formed cloudlets to have their longer axes vertical, which is -very characteristic. It might be named alto-cumulus castellatus, -or high-turreted cloud. Mr. Ley named it stratus castellatus, or -turret-cloud, but it certainly belongs to the cumulus section of -the alto group. Thunder weather is the invariable condition for -its production. If it is seen, at least in England, thunderstorms -are certain to be recorded not very far away. When this particular -photograph was being taken in South Devon, very destructive storms were -recorded in Brittany and in the English Midlands, and the anvil-shaped -tops of unmistakable thunder-clouds were visible above the horizon -while the exposure was being made. - -[Illustration: Plate 28. - -HIGH TURRETED CLOUD. - -(_Alto-cumulus Castellatus._)] - -Another form of almost equal beauty is shown in Plate 29. The rounded -balls make their appearance as semi-transparent spots upon the sky, and -in their general characters might easily be mistaken for cirro-macula. -But a few minutes will be enough to decide the question. The little -spots rapidly grow denser, frequently becoming ragged at the edges; -they never drop down the slender filaments which usually descend from -cirro-macula, and their edges are never denser than their central -parts, which, it will be remembered, was a frequent feature of the true -speckle cloud. The cloudlets are obviously rounded balls arranged in -patches, which may turn gradually into alto-stratus by their fusion, -or, after an existence of minutes or hours, the whole may disappear -by a disintegration of each ball, by its breaking up into a ragged -mass and melting away. The altitude at which this cloud forms is -between 5000 and 9000 metres, according to measurements made by the -writer, the actual specimen figured being about 7000. It is almost as -characteristic of thunder weather as the last, but whereas Plate 28 -shows a variety which is most often seen before 3 p.m., since it only -occurs while the cloud planes are rapidly rising, the one before us -may be formed at almost any time of day, but most frequently occurs in -the afternoon. An imperfect form of it is frequently met with about -sunset, in which the rounded balls are not usually so well defined as -when the sun is high above the horizon. Alto-cumulus glomeratus would -be a suitably descriptive name. - -[Illustration: Plate 29. - -HIGH BALL CUMULUS. - -(_Alto-cumulus Glomeratus._)] - -If it were possible to take a good typical example of the variety -just described and roll it flat, so that each cloudlet should be -reduced to a lenticular shape, we get a type which seems seldom to -appear during the heat of the day, and to be most frequent about -sundown. It consists, as shown in Plate 30, of distinct cloudlets, -with considerable spaces between them, and gives the impression of a -discontinuous level sheet. But the component cloudlets are much too -definite, and preserve their individuality far too well to suggest any -idea of a broken stratus; the spotted structure is the predominant -feature, while the stratiform arrangement is almost equally plain. -Alto-stratus maculosus would be a suitable term. It is not so high a -cloud as the glomeratus type, the one shown being at an altitude of -about 5600 metres. The plate shows the position of the setting sun, -which is partly hidden behind some dark patches of broken alto-stratus -(fracto-alto-stratus), the hazy form and boundaries of which form an -effective contrast to the shining cloudlets 2000 metres or so above -them. Many of our most beautiful sunsets are due to this form of cloud, -particularly in the late autumn. It is a cloud of calm weather, and -often floats apparently motionless, and undergoing little change, -like flakes of glowing fire against the background of a fading sky -long after the sun has disappeared. It is not indicative of thunder -conditions, and it may occur on the margins of an anticyclone. - -[Illustration: Plate 30. - -MACKEREL SKY. - -(_Alto-stratus Maculosus._)] - -A lower and coarser form of the spotted alto-stratus is shown in -Plate 31, where it is seen through the gaps in a thin sheet of broken -stratus. In this case also the sun was getting low in the sky, being -hidden by the denser bit of stratus in the bottom left-hand corner. - -[Illustration: Plate 31. - -MACKEREL SKY. - -(_Alto-stratus Maculosus._)] - -Alto-stratus does not often, if ever, grow from the fusion of the -cloudlets of the maculosus type. But it does come from alto-cumulus -glomeratus, and also from a form shown in Plate 32. Here we have -alto-stratus in process of growth. Small irregular lumps of cloud -forming on the right-hand side of the picture grow larger and more -irregular, begin to fuse together towards the centre, and on the -left-hand side the fusion is almost complete. Still, although the -sky is covered with cloud, the lumpy form is plainly visible. The -term “alto-strato-cumulus” is suitable, as it differs from the more -frequent and much lower cloud, which will be described further on as -strato-cumulus, in little else than altitude and general massiveness -of texture. This high strato-cumulus is common enough, too common, -indeed, in England, as it produces many a dull grey sky both in summer -and in winter. In the latter season it is not unfrequent with the cold -east winds of February and March. It is probably the lowest of the alto -clouds; the lowest measurement made by the writer being 1828 metres at -Exeter, but lower altitudes seem to have been recorded elsewhere. - -[Illustration: Plate 32. - -ALTO-STRATO-CUMULUS.] - -Alto-cumulus castellatus, which is breaking up and disappearing, is -shown in Plate 33. It was photographed with a long-focus lens, so that -the scale of representation is about eight times as great as that of -Plate 27. This view was taken at Exeter while a thunderstorm was in -progress at Bristol. - -[Illustration: Plate 33. - -SUNSET. - -(_Alto-cumulus Castellatus Fractus._)] - - - - -CHAPTER V - -LOWER CLOUDS - - -THE clouds of the lower portions of the atmosphere are formed in -regions where water vapour is abundant, and frequently in easy reach -of the strong ascending and descending air currents produced by the -varying temperatures and irregular surface of the ground. It is -sufficient to recite these conditions to show that these lower clouds -will be denser, larger, coarser in texture, and characterized by -greater definiteness of form than those we have, so far, considered. - -In the International system they are classified thus-- - - Group C. Lower clouds. - (_a_) Strato-cumulus. - (_b_) Nimbus. - Group D. Clouds of diurnal ascending currents. - Cumulus and cumulo-nimbus. - Group E. High fogs. - Stratus. - -This is certainly the least satisfactory part of the whole scheme, -and it is not at all easy to see upon what grounds it was adopted by -the International Committee. Group D--cumulus and cumulo-nimbus--do -show important differences from the other groups, though it is often -difficult to say whether the sky should be described as covered with -strato-cumulus or as covered with numerous small cumulus. It is the -separation of stratus--placing it in a group by itself, and making that -the lowest--which is the worst point. As a matter of fact, stratus -may exist at any altitude from sea-level up to such heights that we -should not hesitate to call it alto-stratus. Indeed, there is no -essential character of alto-stratus which distinguishes it from some -of the lower forms. Whatever its altitude, its thickness and the size -of its particles may vary in a precisely similar manner. We may have -the particles exceedingly small, when the fog will be dry, and such a -stratus may be so thin as hardly to dim the sun; or it may be so thick -as to completely hide it. On the other hand, the fog may consist of -particles easily visible to the naked eye, forming the so-called Scotch -mist, or the “dry” fog of Dartmoor, which will wet things as rapidly -and more thoroughly than a smart shower. When such a fog accumulates -to a sufficient depth, the particles in their fall pick up others, and -the result is a distinct fine rain. This may occur not only near the -ground, but at almost any level below that at which the cloud would -pass into the region of cirrus. - -Plate 34 shows three layers of stratus, in each case much broken up. -The highest layer is a good example of alto-stratus maculosus. Lower -down, by half a mile or more, come parts of a grey sheet considerably -denser and thicker. It is a matter of taste whether this should be -called high stratus or low alto-stratus. There is no test by which -the one can be distinguished from the other. Lower again come the -detached darker clouds, which are fragments of a sheet of stratus -which is breaking up and disappearing. The photograph was taken in the -afternoon, after a wet morning, and all three layers were probably -relics of the great rain-cloud system, or nimbus, which produced the -rainfall. - -[Illustration: Plate 34. - -THREE LAYERS OF STRATIFORM CLOUD AFTER RAIN.] - -We have referred to the production of fine rain from a thick fog. If -now such a thick layer of coarse-grained fog--if we may use such a -phrase--is suspended overhead at a moderate altitude, the result is -a drizzling rain underneath, and the cloud at once becomes a nimbus. -When Luke Howard adopted the term “nimbus,” he proposed to employ it, -apparently, for a vast mass of cloud such as that which forms the rainy -region of a cyclone; a huge pile of clouds containing representatives -of all his other types in some unknown but close relationship. It was, -in fact, a comprehensive term, and as such there was a good use for -it. At present it is applied to any cloud from which rain is falling, -except when the cloud can be identified as a variety of cumulus which -is called cumulo-nimbus. But we have already said that a stratus may be -a rain-cloud, and so may other varieties. Moreover, whenever a nimbus -breaks sufficiently for us to be able to see its upper surface, we -invariably find that, if it were viewed from above, we should, without -a moment’s hesitation, place it in one of the other groups. It is only -when we are underneath it we can see its rain-producing character, and -give it the orthodox name. The real fact is that nimbus should be an -adjective, meaning rain-producing, and not a substantive. - -However, it has its allotted place in the International system, and -it is better to adhere as far as possible to a defective but widely -recognized system until it can be authoritatively amended, rather than -to make an individual attempt to ignore it. The facts are sufficiently -obvious, and the days of nimbus as a type are numbered. The two plates, -Nos. 35 and 36, are fair typical representations of the clouds usually -known as nimbus; but they are both of them only the under-surfaces of -other clouds, Plate 36 showing the under-surfaces of a group of heavy -cumulo-nimbus all joined together so as to cover the sky, while Plate -35 shows a mass of dense strato-cumulus. The rain-cloud is always -a form of either stratus or cumulus, or a combination of the two, -sometimes in further combination with clouds of the alto class, or even -extending upwards to cirro-stratus and cirro-nebula. Where it consists -of a single layer, that layer differs from its rainless representative -only in greater thickness from base to summit, or in greater density; -and when there are several distinct layers of cloud, so that the -lowest is shaded by the higher, rain may fall, even though they differ -in no visible way from clouds which would be rainless if alone. Plate -33 is an example. - -[Illustration: Plate 35. - -RAIN-CLOUD. - -(_Nimbus._)] - -[Illustration: Plate 36. - -RAIN-CLOUD. - -(_Nimbus._)] - -Nimbus, indeed, is not a type-form, but is merely a typical condition, -and when used as a substantive is only a convenient way of expressing -our ignorance as to the real form of the cloud we so describe. - -The altitude of the base of a rain-cloud may vary considerably. It -may be anything from sea-level up to heights which vary with the -geographical conditions and with the conditions of temperature and -pressure, but probably in this country never greater than 7000 or 8000 -metres. - -Rain, or snow, often falls from clouds at greater altitudes than these, -but unless in its descent it passes through other lower clouds, the -drops, as a rule, will dry up and disappear. The author has often seen -quite heavy rain descending from a cloud, and disappearing completely -within a thousand feet or so of the cloud-base. On rarer occasions a -still more remarkable thing may be seen--namely, a shower falling from -an upper cloud into a lower, and none between this lower cloud and the -ground. This curious phenomenon can only be explained by supposing -that the convection currents which make the lower cloud are strong -enough to support the small raindrops. - -Pure stratus is a level sheet of cloud with little variation of -thickness, not ascending every here and there into rounded lumps. Its -most typical form covers the whole sky with a uniform grey pall, which -may or may not completely hide the sun. Such a cloud does not lend -itself to pictorial representation. A frequent form, in which the sheet -is more or less broken, is shown in Plate 37. This is a variety which -is frequent in the summer mornings, and generally breaks up and clears -away before eleven o’clock. If, however, it appears in autumn and -winter with layers of alto cloud above, it may grow denser, and turn -into a stratiform nimbus, or it may go on drifting overhead for several -days without sign of change. - -[Illustration: Plate 37. - -STRATUS COMMUNIS.] - -Break up such a sheet of cloud by numerous meandering cracks, and round -off the detached pieces so as to give them a more or less rounded -or pyramidal section, and the cloud becomes strato-cumulus, typical -representations of which are shown in Plates 38 and 39, which depict -different parts of the same sky. In Plate 38 the camera was pointed due -west, and in Plate 39 it was turned round to the north-west, so that -the two views do not quite meet. - -[Illustration: Plate 38. - -STRATO-CUMULUS.] - -[Illustration: Plate 39. - -STRATO-CUMULUS.] - -Plate 40 is a different variety. It is stratiform, each component -cloudlet being rather ragged at the edges. In some ways it resembles -cirro-macula and the speckled varieties of alto cloud, but it -is coarser in texture and obviously at no great altitude. The -International system would call it strato-cumulus, but Mr. Ley gives -a representation from another negative taken at the same time as the -type of what he calls stratus maculosus, a name which seems far more -suitable, since the cloud bears a much closer relation to stratus than -to cumulus. In the particular instance figured, the broken structure -did not last long; the spaces gradually closed in, and a complete -stratus was the result. - -[Illustration: Plate 40. - -STRATUS MACULOSUS.] - -Strato-cumulus often lasts for hours, with little or no perceptible -change, but stratus maculosus rarely persists for more than half an -hour. The first is a cloud of fairly stable conditions, the latter -is dependent for its existence upon the near approach to critical -conditions at one particular level, and, as we have said in other -cases, such a critical state is almost always soon passed, with the -result that the cloud either masses into a denser form, or else breaks -up and disappears. If the up and down currents are strong enough to -persist, the result will be strato-cumulus and not stratus maculosus. - -A kind of stratus which is frequently seen in the daytime is shown -in Plate 41. This is literally a lifted fog, having been formed -about midday, after ground fog in the early morning. It would be -called common stratus, or stratus communis. When it appears it is a -fairly persistent form, sometimes breaking up or swelling up into -strato-cumulus, but more often splitting into long rolls of cloud, -with margins like those of cumulus. This phenomenon is shown in Plate -42, which was taken in December at 11 a.m., on a day which opened -with a thick ground fog. A precisely similar cloud is frequent in the -early hours of a summer morning, as a stage in the dispersal of a -radiation ground fog. The fog first lifts from the ground, until it -reaches a height of a few hundred metres, when it splits into the long -rolls whose axes are at right angles to the direction of drift. The -consequence is very strange if you stand on a hilltop close under the -drifting mass, and look towards the horizon in the direction of drift. -The changing shadows give the impression that the clouds are actually -rolling along, though of course no such thing is really taking place. -As time goes on the rolls grow larger and the interspaces wider; then -transverse fissures appear, and gradually the rolls break up into small -detached cumulus. Cumulus radius, from the Latin for a rolling-pin, -might be a suitably descriptive name, but it should not be forgotten -that it is only an intermediate link between stratus and cumulus, -and, indeed, is more nearly related to the former, since it is never -produced except on the break up of stratus, while it may dry up and -disappear without reaching the cumulus stage at all. Stratus radius -would therefore be a better name. - -[Illustration: Plate 41. - -COMMON STRATUS. - -(_Stratus Communis._)] - -[Illustration: Plate 42. - -ROLLER CLOUD. - -(_Stratus Radius._)] - -Cumulus is closely related to another form of stratus, which Mr. Ley -has named stratus lenticularis, but this appears to be so frequently -the last stage in a disappearing cumulus that its history will come -better later on. It is mentioned here as it is, after all, one of -the commonest of all forms of stratus, the form which appears at, -or after, sunset, and is one of the few clouds which have an English -popular name--Fall cloud. Plate 47 gives a representation of it, -standing out dark against an evening sky, with a sheet of alto-stratus -far above it in the upper part of the photograph. - -To sum up, then, we have among the lower clouds of more or less -stratiform pattern-- - -Stratus communis, or Common stratus. - -Stratus lenticularis, the Fall cloud. - -Stratus radius, or Roll cloud. - -Stratus maculosus, or Mottled stratus. - -Strato-cumulus, or Sheet cumulus. - -The last leads naturally to the consideration of cumulus and -cumulo-nimbus, while the term “nimbus” does not belong to any one -type-form, but sometimes to one, sometimes to another, and generally to -a mixture of two or more. - -A good many years ago the writer made a series of measurements of -the thickness of detached clouds of the stratus and cumulus types, -such as those which may produce a shower. The conclusions reached in -consequence of those determinations have since been amply confirmed -by subsequent observations. In winter no rain will fall from a -cloud unless it reaches a minimum thickness of at least 100 metres, -while in summer it must have rather greater thickness. There is one -exception, and that is in winter, when the temperature is so low that -the drop starts on its downward journey as a flake of snow. When this -is the case, rain may fall from a layer of thin lifted fog, not quite -thick enough to hide the blue colour of the sky. But under ordinary -conditions of temperature, if the cloud has a thickness less than 2000 -feet, or 616 metres, rain is unlikely, but if it does come, the drops -will be small and the fall of rain quite trifling. - -Above this thickness the heaviness of the rain and size of the drops -increases, so that if the distance from base to summit be between -2000 and 3000 feet, or 600 to 1000 metres, the fall will be gentle. A -thickness of 4000 to 6000 feet, or 1200 to 1800 metres, gives large -drops and a fairly heavy shower, while, in summer time at least, -cold heavy rain and hail come from clouds measuring 6000 to 10,000 -feet, or in round numbers 1800 to 3000 metres or more. In winter the -necessary dimensions seem to be less, but the rule still holds equally -good, that the rain-cloud does not necessarily differ in any way from -the rainless one, except in thickness, and that when the requisite -thickness is present rain is not always the result. - - - - -CHAPTER VI - -CUMULUS - - -UNDER the general term cumulus there are grouped the most common, the -best known, and the grandest forms of cloud. Indeed, beautiful as the -cirrus and alto clouds may be, there is a solid grandeur about the -greater forms of cumulus which gives them a beauty of their own quite -comparable with the charm afforded by the delicate tracery of their -more lofty rivals. - -Cumulus can be divided into several types, which are best considered -in the order of growth. They are all formed in the lower part of the -atmosphere, their under-surfaces varying in altitude from about 600 -metres, or even less, up to 3000 metres, or slightly more. The writer’s -own measurements vary from a minimum of 584 metres to a maximum of 2286 -metres, with an average of a little more than 1000 metres. - -They are described in the International system as “clouds in a rising -current,” and there is no doubt the description is correct. Each -cumulus must be looked upon as simply the visible top of an ascending -pillar of damp air. The vapour which makes its appearance in the cloud -is present in the transparent air beneath, and the base of the cloud -is simply the level at which that vapour begins to condense into -visible liquid particles. Since cumulus clouds are caused by ascending -currents, these currents must be brought about either by the general -disturbance of the air due to a cyclonic movement, or by the local -irregularities of temperature on the ground produced by the sun’s heat. -As a matter of fact, we do get cumulus produced in great abundance in -the rear of every cyclone, and we get them also under the conditions -of still air and hot sun, which specially favour evaporation and the -development of differences of temperature. The cyclone cumulus may come -at any hour of the day or night, though comparatively rare between -midnight and the morning. Heat cumulus is generally formed during the -afternoon, and it is only under relatively uncommon conditions that it -persists during the night. If the cloud has not grown to very great -size it usually begins to break up and disappear about sunset, but -if it has grown to the enormous dimensions of a summer thunder-cloud -it may go on growing, piling mass on to mass, until it generates a -thunderstorm, even in the hours of early morning. - -In the case of some of the higher kinds of cloud, we are not able to -give any certain account of the mechanics of their production from a -study of those clouds themselves. We have already referred incidentally -to some of the speculations as to their origin and some of the facts -definitely known, but considerable light can be thrown on the genesis -of all the varieties of cirro-cumulus and alto-cumulus by a careful -study of their larger and more accessible representatives of lower -regions. - -The cyclone cumulus does not differ in any essential from the clouds -of calm weather. The only difference is that the uprising currents are -perhaps partly eddies, and the rate of fall of temperature with ascent -is often more rapid. - -Given any mass of air at a particular temperature, it can take up and -hold in the form of invisible vapour a fixed quantity of water, and no -more. When it holds the maximum possible it is said to be saturated. If -it is nearly saturated it would be called damp; if far from saturated, -dry. Now, the warmer the air the larger the quantity of vapour -necessary to saturate it, so that if a quantity is saturated at a high -temperature, and is then cooled, it will no longer be able to retain -all its moisture in the invisible form, but the surplus quantity will -make its appearance as liquid particles, that is to say, as mist or -cloud. - -Similarly, if a quantity of air is not fully saturated at its -particular temperature, and is then cooled, it will approach nearer and -nearer to saturation, and if the process is continued long enough the -result will be cloud formation. - -All clouds, without exception, are produced by exactly such cooling -of air containing water vapour, first to the temperature at which the -quantity it contains is the maximum possible, and then beyond that -point. Now, if we start with very warm air, and cool it 1 degree, we -decrease its vapour-holding power, and the decrease per degree grows -less and less as the temperature falls. Suppose, for instance, we have -air saturated at 61 degrees and cool it to 60 degrees, the quantity -of vapour condensed will be equal to the difference of holding power. -Suppose, again, we have air saturated at 31 degrees and we cool it to -30 degrees, the quantity of vapour condensed will again be equal to the -difference of holding power; but this quantity will be very greatly -less than in the former case. Cooling air saturated at 61 degrees to 60 -degrees might produce a dense cloud; but applying a similar reduction -of 1 degree to air saturated at 31 degrees, if we take the same volume -of air, will only produce a very much thinner result. Here we see one -good reason why the highest clouds are the thinnest and the alto clouds -of intermediate density. - -The necessary cooling may be brought about in several ways. Firstly, -the air is capable of radiating its heat into space, and therefore -of cooling. But we know little of the laws which govern atmospheric -radiation, and presumably, if cloud could be produced by such means, -it ought to make its appearance most frequently in the small hours of -the morning before sunrise. We are, however, unaware of any variety -of cloud which answers those conditions, unless it be the ground -fogs which so often form during the night; and these, we know, are -certainly due to the chilling of the air by contact with the ground, -which has been cooled by radiating away its heat. On the contrary, it -is well known to astronomers that the sky is, on the whole, clearer -and freer from clouds after midnight than in the earlier hours of the -night--a circumstance which is particularly unfortunate for the amateur -star-gazer, who has to be up and about at the same time as the rest of -the working world. Cooling by radiation we may then dismiss as a cause -of cloud formation of no great efficacy, and certainly one which has -little to do with the production of cumulus. - -Cooling by contact with a cold body is another and more potent cause. -We often see it in a mountain district, where a frost-bound peak -stands facing the wind with glittering snow-slopes on which the sun -is shining, while a long tongue of cloud hangs like a banner on its -leeward side. In such a case it is easy to understand how the air -sweeping by the icy mass is chilled below its saturation point; but as -it passes on, the chilled portions become mixed with the rest, and the -cloud evaporates again. It is not quite so easy to see how far this -cause is responsible for the clouds which are formed when the warm damp -air of the ocean drifts over a comparatively cold land. It is probable -that the contact chilling is in this case only part of the explanation, -and that other causes co-operate. - -The mixing of warm damp air with cold has often been adduced as a cause -of clouds. No doubt it might be, and some of the stratiform types may -possibly be formed at the junction between a warm damp stratum of air -and a cold one, but no example is certainly known. It may also be a -contributing cause in producing the sharply defined upper surfaces of -some cumulus or strato-cumulus clouds, but these are in the main most -certainly due to the chief cause of cloud production--namely, what is -known as dynamic cooling. - -If a quantity of air exists under a certain pressure and at a certain -temperature, on reducing the pressure it will expand, and in the act of -expanding it will become cooler. This may easily be illustrated with -an air-pump. Let a damp sponge or a piece of wet blotting-paper stand -under a glass receiver over an air-pump until the air has become damp. -If the apparatus is in a darkened room, and a powerful beam of light -from a lantern is sent through the receiver, the damp air will be seen -to be quite clear; but a stroke or two of the pump removes some of the -air, the remainder is chilled by its own expansion, and a dense cloud -is precipitated. If this cloud be viewed closely, it will be seen to -be composed of minute particles, which, on looking towards the light, -glow with the colours of a corona. In a few minutes the cloud will -disappear, but it can be recalled again and again by successive strokes -of the pump, getting thinner and thinner as the air gets more and more -rarefied; an illustration of a second reason why the high clouds are -thinner than the lower. - -Some years ago Mr. John Aitken showed that if the damp air used in -this experiment were carefully filtered, so as to remove all foreign -particles, no cloud was produced, and the introduction of a puff of -unfiltered air was attended by immediate condensation. The deduction -was that vapour, even below its saturation temperature, cannot produce -cloud unless nuclei of some sort are already present, presumably dust -particles. Later on it was shown by Mr. Shelford Bidwell and others -that gaseous particles, such as those produced by the burning of -sulphur, would serve the purpose, and that the brush discharge from an -electrified point was in some mysterious way particularly effective. -It has recently been shown by Mr. C. T. R. Wilson that causes such as -the radiations of radium, or the impact of ultra-violet rays, acting on -the air itself, splits up some of its particles into the smaller bodies -known as ions, and that these are efficient nuclei. These experiments -open up many most interesting questions, but, unless it is to explain -the extreme density and darkness of a thunder-cloud, they do not seem -to play any important part in determining the forms to be assumed. -Nuclei in sufficient abundance are probably always present at any -height which can be reached by enough vapour to form a cloud. - -Now, if we have a quantity of air, say at sea-level, damp but not -saturated, and it is caused to ascend, either because it is warmer and -therefore lighter than the surrounding air, or for some other reason, -as it moves upwards the pressure upon it will decrease, it will -expand, and in the act it will be steadily cooled. This cooling may -after a time bring it down to the same temperature as the rest of the -air at its particular level. If so, it will no longer be lighter, and -the ascent will come to an end. But before this state of affairs is -attained it may have reached its saturation point, and cloud production -will begin. - -It is true that the rarefaction of the air tends to enable it to retain -more vapour than it could if it were cooled without change of density. -The temperature of the air being fixed, its holding power increases -with decrease of pressure. But this increase is much less than the -diminution due to cooling, and the result in nature must be similar to -what we can see happen under the receiver of the air-pump. - -The condensation of water introduces another factor of great -importance. It has just been said that the ascending air may be cooled -so rapidly as to be reduced to the same temperature as the rest of -the air at that level, and if so the ascent will end. Clearly the -cessation or persistence of the upward motion depends upon whether the -diminution of temperature per 100 metres of ascent is most rapid in -the rising column or in the air outside it. As long as the ascending -air is warmer than that outside, but at its own level, so long will -ascent continue. Now, as long as no condensation was taking place, the -rate of cooling would follow a simple law which produces a cooling of -1 degree for about 100 metres of ascent; but as soon as water vapour -begins to pass into the liquid form, a large quantity of heat is set -free, and the rate of cooling is consequently greatly lessened. Cloud -production tends, therefore, to accelerate ascent, and the greater the -amount of condensation, the more important will this consideration -become; though, on the other hand, when once the cloud is formed, it -tends to stop the rising current by shading the air and ground beneath -it. - -On an ordinary day the rate of decrease of temperature as we ascend is -rather less than the value given above, and uprising currents are soon -checked. If they do extend far enough to reach cloud production, the -clouds will be small, forming the smallest variety of cumulus. This is -shown in Plate 43. Small irregular uprising currents have just been -able to reach far enough up to have their summits tipped with cloud. - -[Illustration: Plate 43. - -SMALL CUMULUS. - -(_Cumulus Minor._)] - -After the foregoing explanation, it is easy to see why at a given time -the floating cloudlets should have a common base level. This is the -height to which the air must attain before reaching its saturation -temperature. Each cloudlet marks an uprising current, and the intervals -show the position of the counterbalancing descending streams. - -A larger variety is shown in Plate 44. In this the level base and -generally pyramidal shape is shown, and also the hard, rounded upper -surface. The thickness of this cloud was about 500 metres. When clouds -like these are visible, they may be the beginning of larger ones, and -the only way to judge whether they are likely to develop into rain- or -shower-clouds is to watch them. If they are seen to be growing larger, -and particularly if detached fragments are developing into clouds, -further growth is almost certain, and rain is probable. - -[Illustration: Plate 44. - -CUMULUS.] - -If great towering masses are making their appearance with little dark -fragments between them, as shown in Plate 45, then smart showers may -be confidently expected. The cloud figured was a shower-cloud, and -the distance is seen through the veil of falling rain. The height -and thickness of this particular cloud were measured just after its -photograph had been taken. Its base was 1200 metres above the ground, -and its summit was 1500 metres further. Its thickness from summit to -base was, therefore, not much short of a mile, and the total contents -of the cloud were probably between one and a half and two cubic miles. -The upper contour is hard and rounded, as in the smaller cloud of Plate -44, but the whole cloud is much larger. - -[Illustration: Plate 45. - -LARGE CUMULUS. - -(_Cumulus Major._)] - -We have already explained that there seems to be a definite connection -between the thickness of such clouds and the amount of precipitation -from them. Small cumulus, less than 120 metres thick, rarely produces -rain, and nothing like a heavy shower is likely unless the thickness -exceeds 400 metres. In winter, especially in hard frost, snow crystals -may fall from the smallest cloud, even from little fragments only a -few metres thick, but the quantity of water so precipitated will, of -course, be small. - -As long as the top of the cumulus is rounded and clearly defined, -the conditions of aĂ«rial equilibrium are stable, and the growth of -the cloud has been brought to an end by a stoppage of the ascending -current. In Plate 45 the ascent has been hindered both by the -mechanical action of the falling raindrops and by the cooling of the -lower parts of the ascending column by the descent into it of the cool -drops from its colder upper part. This is probably one of the chief -reasons why a shower-cloud never maintains its activity as a rain -producer for more than a very limited period. As the cloud drifts over -the landscape, it seldom maintains its showery character for more than -ten or twenty miles, often for much less. - -Cumulus, like any of these three, is a cloud of the daytime. It -generally begins about ten or eleven o’clock in the morning, grows -larger until about four o’clock, and then begins to break up and -disappear. After the ascending currents have ceased, the component -cloud particles slowly settle down into the warmer air beneath, -until the mass has lost its proper pyramidal form, and has become -an irregular cloud, such as is shown in Plate 46. This is known as -degraded or fracto-cumulus. - -[Illustration: Plate 46. - -FRACTO-CUMULUS.] - -One consequence of the arrest of the uprising currents is the -formation of lenticular patches of stratus, called by Mr. Ley stratus -lenticularis. This is often formed about sunset, and has been named -fall cloud, from its appearance at the fall of night. The name is -appropriate in another way. The ascending currents having ceased, -the cloud particles slowly subside until they dry up in some warmer -stratum. The water vapour does not continue its descent, but slowly -diffuses in all directions, and if the fall of cloud particles is -sufficient, this stratum, which is approximately coincident with the -base of the original cumulus, soon becomes saturated, and further -particles which fall into it remain visible. This saturated zone will -slowly sink lower and lower with the descent of the particles, until it -reaches regions in which the temperature is high enough for the whole -to be evaporated without reaching saturation point. Evening stratus in -calm weather always goes through this sequence of changes. It usually -forms at, or soon after, sundown, and begins to break up and disappear -as the stars are becoming visible in the darkening sky. Plate 47 shows -a specimen of this evening stratus. - -[Illustration: Plate 47. - -FALL CLOUD. - -(_Stratus Lenticularis._)] - -A curious feature is sometimes shown on the underside of a thick cloud, -which is probably due to the upper part of the ascending column having -been carried beyond its position of equilibrium by its own inertia, -and then falling back again in the teeth of the still rising lower -part. The result is to give the base of the cloud an appearance of -a number of rounded masses hanging downwards below the cloud, very -suggestive of the idea that the cloud is upside down. Such an event -will not often occur, and when it does the conditions are quite wanting -in stability, and the consequent features will be very transient. When -the base of a cumulus or cumulo-nimbus is so affected, the cloud is -known as festooned cumulus, or cumulus mammatus. A precisely similar -structure may be seen under strato-cumulus, or even thick stratus. In -some countries it seems to be frequently observed, but in England it is -so uncommon that the writer has only noted it about a dozen times in -twenty years, and on no one of these did it last long enough to allow -of its portrait being taken. It is an indication of very disturbed -conditions, and is usually followed by heavy rain. - -When cumulus clouds are formed in air which is steadily moving as -a whole, that is to say, when there is a steady breeze, they have -a very decided tendency to follow each other in long lines. It may -often be noticed that in a particular place with a certain direction -of wind these long processions follow definite tracks in relation to -the geographical features. The phenomenon does not seem to have been -recorded except in hilly country, but has frequently been observed by -the writer. It is not the same thing as the formation of stationary -belts of cloud transverse to the wind. These cumulus float along with -the movement of the air, and the question to be answered is, why should -they follow each other so persistently, and why should the intervening -belts of sky be so continuously free from cloud. - -If we consider that the warm damp air which supplies them is drawn -from the ground, it seems that any cause which tends to direct this -warm stratum into definite channels, as it is carried on by the wind, -will be a competent cause of the whole phenomenon. This we find in -the presence of lofty hills which stand in the way of the warm -surface winds, causing them to follow more or less the general trend -of the valleys, and so delivering the rising convection currents of -cloud-producing air at the same spot. - -It is easy to conceive that other causes, such as a difference in -temperature or dampness of neighbouring tracts, resulting from whether -they are bare or wooded, marshland or sandy plain, might equally -suffice; or might, at least, powerfully co-operate with, or counteract, -the effect of hill and vale. But in any case it is plain that the -geographical conditions to the windward of the place of observation not -only may affect the occurrence and distribution of cloud, but if the -wind is steady it is difficult to see how they could avoid affecting it. - -Another puzzling phenomenon, sometimes presented by cloud and fog, is -that our instruments for detecting humidity show that the air within -them is not always fully saturated. It seems probable that this is due -to such cloud or fog having begun the process of drying up, or that -in some way not fully understood the presence of the cloud particles -after they have first come into existence may cause the withdrawal of -some of the moisture from the intervening damp air. The surface of -each minute droplet exerts a pressure on its interior similar to the -pressure exerted by the film of a soap-bubble on the air within it, -and it is conceivable that some of the uncondensed vapour from outside -may diffuse through this enclosing surface film, and be retained -there in consequence of the pressure. If this is so, and subsequent -investigation can alone decide the matter, it will follow that when -once cloud production has begun it will be continued until the air -between the cloud particles is reduced so far below its saturation -point that the tendency of the drops to evaporate, that is to say, for -the imprisoned water to escape through the confining film, balances the -retaining pressure. - -This consideration, however, is quite incompetent to affect the general -explanation of cloud formation which has been given. Its result would -be to carry condensation a little further than the exact saturation -point, and to retard equally slightly the subsequent evaporation of the -cloud particles. - -We have spoken of the typical cumulus as having a roughly pyramidal -shape, and if the horizontal movement of the air is small, the -loftiest point of the cloud will be situated approximately above -the centre of its base. But if the horizontal movement increases in -velocity, so that the top is in a more rapidly moving stratum than the -base, it will lean forward in the direction of movement. This is a very -common phenomenon, being generally shown by cumulus on a windy day. - -On much rarer occasions the converse occurs, and the top of the cloud -lags behind the base, the explanation being a lessening of the velocity -of the wind as the height above ground increases. But such conditions -rarely occur, and when they do they are due to local eddies and affect -only a limited area. Hence such clouds are isolated, and indicate a -disturbed state of the air and uncertainty of weather. The clouds which -lean forward are formed under conditions which are spread over wide -districts, such as the rear of a large cyclone, and cumulus of that -kind may follow one another across the sky for hours or even days as -long as the wind persists. - -So far we have considered only the round-topped types of cumulus--those -which mark the tops of ascending currents whose ascent has been -stopped at a comparatively early stage, or those whose ascent is still -in that early stage, though the upward movement has not yet come to an -end. The full story of the growth of a cumulus is identical with that -of the youth of a cumulo-nimbus, the later stages of which we will -consider in another chapter. - - - - -CHAPTER VII - -CUMULO-NIMBUS - - -GRANDEST of all clouds are the huge mountains of vapour which are the -parents of summer thunder-storms. They are at once distinguished from -ordinary cumulus by their upper parts, which sometimes reach beyond the -region of the alto clouds high into the realm of cirrus, and extend -outwards as a broad disc, which is occasionally indistinguishable from -the cirro-nebula and cirro-stratus which form the van of a cyclone -cloud canopy. Indeed, there seems to be no essential dividing line -between a large cumulo-nimbus and the cloud pile of a small cyclone, -and no real difference between them except their size. - -As a matter of fact, the term cumulo-nimbus would only be given to the -cloud when a large fraction of the whole can be seen at once. - -In dealing with common cumulus, it has been pointed out that the -cessation of the uprising convection currents which determines the -maximum height to which the cloud will grow is due to the rate of -cooling within the ascending column being greater than the rate of -cooling outside it. It follows that when the ascending current has -reached a certain height it will, as a whole, be just as heavy as an -equal column outside. Ascent must then cease. The equilibrium of the -air in such a case is said to be stable, and the condition of such -stability is simply that the general rate of fall of temperature per -100 metres of ascent is less than the rate of cooling dynamically -produced in an ascending current. - -If, however, the general rate of fall of temperature is greater than -that produced dynamically, the consequence will be that the upward -tendency of the rising air will increase as it moves upward, and the -taller the column becomes the greater will be the difference of weight -between the inside and outside columns. In such a case the equilibrium -is said to be unstable, and the result will be the production of -cumulo-nimbus. - -Just as cumulus may be divided into heat cumulus and the clouds of -the rear of a cyclone, so cumulo-nimbus may be divided into the same -two groups. In the case of the heat thunder-clouds the instability -of the air is effected by the rapid heating of its lower layers in -contact with the ground, those lower layers being so quickly warmed -that there is not time for them to become mixed with the overlying -air in which the rate of decrease is normal. If there is much wind we -rarely get cumulo-nimbus, because the heated air is mixed mechanically -with the overlying parts, and the rate of decrease is approximately -normal throughout. Calm air and hot sun are then one set of necessary -conditions for the production of instability. - -But it is well known that thunder-showers and lesser examples of -cumulo-nimbus are by no means infrequent in the rear of a cyclone, and -such storm clouds are usually attended by considerable wind. They are, -as a rule, much smaller than those produced by heat, but they have the -same form, and are evidently due to instability in the lower part of -the air, and the question is how can that condition be produced. In -order to find the answer it is necessary to refer to the temperature -phenomena of a cyclonic area. If a cyclone be divided into four -quadrants by two lines drawn through its centre, one in the direction -in which the system is travelling and the other at right angles to -it, then the front right-hand quadrant is the warmest and the rear -right-hand quadrant much colder. The cumulo-nimbus clouds of a cyclone -are limited to the first part of this cold quadrant, that is to say, to -the portion of the storm in which a great volume of cold air is flowing -over a district which has just been warmed and wetted by the preceding -part. The result is that, the air being warmed by contact with damp -ground at a temperature many degrees above that of the air itself, we -have produced exactly the same unstable state at a low temperature as -we have at a high temperature in the case of heat storms. The lower -temperature of the whole is enough to account for the smaller volume -of the cloud, and that in turn explains why cyclone thunderstorms are, -generally speaking, on a much smaller scale than heat storms. - -The life history of a cumulo-nimbus is easily studied on a suitable -day. The rapid heating of the lower layers of air causes them -to expand bodily, and as they do so they lift the overlying air, -frequently in broad domes or waves. The first result is the expansion -of these upper zones, which are lightened by the flowing away of -still higher layers. Expansion means chilling, and sooner or later -its effects become visible in the formation of cirro-stratus, -cirro-cumulus, or alto-cumulus. Simultaneously the heated air near the -ground begins to rise up in tall columns, while the cooler air from a -little higher descends to take its place. Soon patches of lower cloud -appear, at first hazy and indistinct, but gradually shaping themselves -into cumulus with hazy base and rounded summits. These rapidly assume -the typical pyramidal shape, with level base and sharply contoured -top, and so far there is little to distinguish them from an ordinary -cumulus (see Plate 48). But watch them carefully. Here and there some -will be growing taller than their fellows, and as they grow their rate -of growth increases until the top begins to show signs of spreading -outwards. Rapidly the bulging summit throws out long fingers of -cloud, radiating from the central column almost as if propelled by -some repulsive force. At first, these fingers are merely projecting -lumps of cloud with rounded ends, but in a few minutes they undergo a -sudden and striking change. The whole summit becomes frayed out, drawn -out into long radiating lines, which thin off against the blue sky -exactly like the edges of a sheet of cirro-stratus. False cirrus is -the name commonly given to this, but there seems no valid reason why -it should be regarded as “false.” The top of the cloud rapidly spreads -horizontally, forming a disc of cirriform cloud, which sometimes -spreads several miles ahead of the rest of the storm. Meanwhile, the -original cumulus column loses all its deep folds and convolutions, and -other round-topped cumulus arise around it until the completed system -consists of a more or less disc-shaped mass of cumulus, with a common -base, rising higher and higher towards some central point, where these -are connected, by an uprushing column of vapour, to an upper disc with -cirriform margins. - -[Illustration: Plate 48. - -THUNDER-CLOUDS FORMING.] - -In Plate 49 we have on the left hand a specimen in which the -outspreading is just beginning, and the same cloud is shown half an -hour later on the left of plate 50. A complete cumulo-nimbus in full -work is shown on the right of Plate 49, and the same appears on the -right in Plate 50. - -[Illustration: Plate 49. - -THUNDER-CLOUDS. - -(_Cumulo-nimbus._)] - -[Illustration: Plate 50. - -THUNDER-CLOUDS. - -(_Cumulo-nimbus._)] - -These clouds were thunder-clouds, the larger one being a smart -thunderstorm with heavy hail. They were photographed in the evening, -and in the second picture the sun was just below the horizon. - -But, to continue the story of a thunder-cloud, we always find that -after a time the cirriform top flattens out and gradually subsides, -and this is usually accompanied by a descent of the cloud base to a -lower level. Meanwhile, it frequently happens that the whole series of -phenomena is repeated in one of the attendant cumulus. Plates 51 and 52 -are also two views of the same cloud at different times. In Plate 51 we -have the main part of the storm on the right, while on the extreme left -a lower part of the cloud is rising rapidly into a tall dome. In Plate -52 the central top has lost its cirriform margin and has distinctly -flattened, while the left-hand dome has risen much higher and is -beginning to throw out the projecting bits. - -[Illustration: Plate 51. - -THUNDER-CLOUD. - -(_Cumulo-nimbus._)] - -[Illustration: Plate 52. - -THUNDER-CLOUD. - -(_Cumulo-nimbus._)] - -The hard-topped cumulus which fringe the lower disc, and the vast pile -of cirriform and hazy cloud which forms the centre of a cumulo-nimbus, -are shown in Plate 53, which represents part of the side of a great -thunder-cloud. In this case the diameter of the lower disc was about -15 miles, and the upper disc was rather larger. The uprising column -in the middle was about 7 miles across, and the height from base to -summit about 3 miles. The whole system contained between 100 and 150 -cubic miles of cloud. When photographed it was over the northern part -of Salisbury Plain. Lightning played repeatedly between the back of the -white cumulus and the hazy mass behind it, and the rumble of thunder -was all but continuous for nearly half an hour as the great cloud -passed by. - -[Illustration: Plate 53. - -THE FLANK OF A GREAT STORM.] - -This was an unusually large cloud for this country, but specimens of -10, 20, or 30 cubic miles are quite common. - -Now for the explanation of the series of events. To begin with, we have -the production of an ordinary cumulus, but the equilibrium is unstable, -the growth of the cloud, therefore, becomes more and more rapid, and -the rapid condensation adds to the instability until the rising -column is so much lighter than an equal column outside that a powerful -updraught is created, strong enough for a time to hold up the raindrops -or even hailstones. At length the condensation is complete, the upper -part of the cloud consisting of snow crystals exactly like those of any -other cirrus. In the mean time, the rapid ascending current necessarily -involves an indraught from around, and consequent descending currents -to supply it. The result is to set up a circulating system, moving -inwards along the ground, upwards in the central column, and outwards -in the upper disc. The downward currents are sometimes shown by a -curling over of the edges of the upper disc, but the phenomenon is not -often seen, as the descending movement is generally enough to dry up -the cloud particles. - -The rapid rising of damp air drawn from the ground brings about rapid -condensation and heavy rain. The large size of thunder-drops is almost -certainly due to the fact that it is only the larger drops which can -fall in the teeth of the strong updraught. But when these drops begin -to fall, and still more when cold hailstones begin to fall through -this ascending air, it becomes chilled from top to bottom, and the -column is broken or even stopped altogether. The frozen particles -which make up the top subside gradually, and the chilling of the air -immediately below the cloud brings the saturation level nearer the -ground, and we say the cloud base descends. - -The arrangement of a thunder-cloud into the upper and lower discs with -a connecting uprush gives to the typical cloud a shape something like -that of an anvil when seen sideways, but in the larger clouds the -disc-like form is more obvious. In any ordinary thunderstorm the great -majority of the discharges of lightning play between the two discs, and -the larger the cloud the more frequent these are. Such discharges as -pass between the cloud and the earth come exclusively from the base of -the lower disc if the cloud is large, and generally follow immediately -after or simultaneously with one between the two discs. The phenomena -of lightning are intensely interesting, but the purpose of these pages -is confined to the study of clouds and cloud forms, and it would be -going beyond our scope to discuss either lightning or hail. Both are, -however, so closely related to cumulo-nimbus that they can hardly be -passed over in silence. One thing is certain, and that is that neither -the electrical developments nor the hail has anything to do with the -growth of the cloud. On the contrary, both are consequences of the -cloud, the hail being due to the great altitude, and consequent low -temperature of the upper part of the cloud, and also to the violent -uprising currents within it; while the electrical phenomena are due -to either the enormous amount of condensation, or to friction due -to the rapid uprush, or more probably to the fact that considerable -differences of electrical condition exist in the distant parts of the -air connected by the cloud, and between which its circulating currents -move. These differences are known to exist at all times, and we cannot -here discuss their origin. - -The formation of cumulo-nimbus and cumulus is dependent upon the -presence of a large amount of water vapour. It is worth while to -consider whether the atmospheric movements which bring about the -condensation could exist without moisture. Wherever we find differences -of temperature between neighbouring places we must get currents of -hot air rising from the warmer spots, and compensating descending -currents around them. But we have pointed out that if the rate of -cooling as we ascend in the still air is less than the rate at which -an ascending current will be dynamically cooled, such a rising current -will come to rest. If, on the other hand, the rate of cooling in the -ascending current be less than in the still air, the equilibrium will -be unstable, and a violent uprush will result. - -Now, in a climate such as our own, where the lower regions of the air -contain large quantities of water vapour, any considerable rise brings -about more or less condensation, and that condensation is attended by -a liberation of very large quantities of heat, which retard cooling in -the ascending current, and so facilitate the production of instability. -But if this cause is put aside it is still possible to have a similar -circulation. When discussing the causes of instability, it has been -pointed out that the prime condition was an unusually rapid rate of -fall of temperature in still air, such as may be produced by hot -sunshine. Now, these conditions are exactly those which will give -rise to the phenomenon of the mirage, and which reach their fullest -development in great desert districts when the air is still. - -Again, it has been pointed out that the causes which bring showers and -thunderstorms to an end include the chilling of the lower parts of the -ascending column by the descent of cold rain or hail from above. We may -also add the shading of the underlying ground by the cloud itself, and -the absorption of heat in the partial evaporation of some of the rain -during the lower part of its fall. In a desert district the arising -currents are so dry that even a very great ascent does not often result -in visible cloud; and when it does, the cloud is produced at so great -a height that the air is too rarefied to produce anything much denser -than thin alto-stratus, from which no falling droplets could reach -the earth. It seems, then, that there will be no such automatic check -on the growth of the circulating system, and it will go on growing in -volume and intensity indefinitely. As a matter of fact, this is not the -case. A different check does come into operation, but not until the -indraught and updraught have become so powerful as to draw up the dust -and sand and generate a sandstorm, the weight and shade of which, in -time, destroys the circulating currents which uplifted them. - -Since, however, condensation is a considerable factor in producing -instability, we should expect that such sandstorms would be rarer than -thunderstorms are in an equally hot but well-watered district, which -is the fact. Again, since rain and cloud are checks upon such systems, -we should expect the sandstorm systems to be larger and far loftier -than thunderstorms, and to consist of far more violent atmospheric -movements. This also is the case, and when we know that some of these -disturbances have the dimensions of a cyclonic storm, it is easy to -understand how the finest dust may be raised to vast altitudes, into -the great upper currents of the air, by which it may be borne hundreds -of miles before returning to the ground. It is thus that the dust of -the African deserts is carried across the Mediterranean to Europe, and -the yellow loess from Mongolia even to the eastward of Japan. - - - - -CHAPTER VIII - -WAVE CLOUDS - - -REFERENCE has already been made on more than one occasion to the -remarkable rippled or wavy structure sometimes assumed by clouds. The -waves may be of almost any dimensions, from the broad bands into which -a sheet of cirro-stratus or of alto-stratus is sometimes divided, down -to the most minute ripples. Sometimes they are ranged in long straight -lines, sometimes they are bent into sharp angles, and sometimes curved -in very elaborate patterns; but whether they be large or small, -straight or curved, no one can see them and fail to conclude that they -must be due to an action more or less analogous to the causes which -produce waves on the sea or ripple marks upon the sand. - -Wave clouds occur at all heights where clouds are formed. The break-up -of a lifting fog into roller clouds is probably the lowest example, -but it may more frequently be seen in higher clouds of the alto or -cirrus kinds. - -A low example is given in Plate 40, which represents stratus maculosus, -and which has already been described. A higher type is shown in Plate -54, which is a wave-like arrangement of alto-cumulus. Rather higher -come the long zig-zag bands of Plate 55, in which the stratiform -arrangement is more obvious, and which would be best described as a -wave-form of alto-stratus. These two plates form striking contrasts. -The clouds shown in the first are distinctly of the cumulus order, and -a prominent feature is the way in which the right-hand side of each -wave has a clear-cut rounded contour like that of the upper edge of -a small cumulus, while the left-hand edge of each band is frayed out -into a ragged fringe. The whole cloud was moving slowly in a direction -nearly, but not quite, at right angles to the waves, and the fringed -edge formed the rear. It is evident that this peculiar structure must -be due to a series of narrow waves intersecting a plane in which the -air is just on the point of producing alto-cumulus. If there were no -such waves, the little uprising currents, with their intervening -down currents, would be irregularly distributed, and all the wave -disturbances have had to do is to arrange them. The consequence is -that as the waves pass along the stratum the air is alternately raised -and lowered. Where it is rising condensation takes place, where it is -falling evaporation results. - -[Illustration: Plate 54. - -CRESTED ALTO WAVES. - -(_Alto-cumulus Undatus._)] - -[Illustration: Plate 55. - -ALTO WAVES. - -(_Alto-stratus Undatus._)] - -The cloud, like most other wave clouds, did not retain its features for -any length of time, but the gaps closed slowly in as the cloud-bands -increased in size, until a sheet of alto-stratus was produced. Since -the time of day was the morning, it is almost certain that the plane -of saturation was rising in accordance with the general law, which is -that the planes of condensation rise steadily, until about two or three -o’clock in the afternoon, and then slowly descend. - -In Plate 55 each band is much flatter and less dense. They are just -as evidently formed by wave movements intersecting the plane of -condensation; but this was formed in the evening when the sun was -nearing the horizon, and at a time when the cloud planes are as a rule -rapidly descending. - -Among the alto clouds wave-forms sometimes persist for a fairly long -time, and in this case the bands moved steadily onward in a direction -equally inclined to their length and breadth, that is to say, from the -bottom left-hand corner of the photograph to the top right-hand corner. -As they passed across the sky new bands kept on making their appearance -at about the same spot, each band persisting with little change until -it had passed out of sight. - -Going much higher up into the region of cirrus, we meet with the -most minute and delicate ripple clouds. Some of these have already -been referred to. They are connected with either cirro-macula, -cirro-cumulus, or cirro-stratus, just as the coarser textured waves we -have been considering are connected with alto-cumulus or alto-stratus. -In Plate 56 we have an example in which we can see the stages in -the process. Nearest to the zenith we have cirro-cumulus, which is -here and there irregularly distributed, but is generally arranged in -delicate ripples, which are variously curved. Nearer the horizon the -troughs of the waves are filled in, and sheets of cirro-stratus are the -result. Here, again, the wave-form is evidently not typical. It is an -arrangement of either cirro-cumulus or cirro-stratus, produced by the -intersection of the plane of condensation by a series of wave movements. - -[Illustration: Plate 56. - -CIRRO RIPPLES. - -(_Cirro-cumulus Undatus._)] - -The arrangement is, however, so striking a feature when it is well -shown that any description of the cloud which contains no reference to -the waves is manifestly incomplete, and this would be best effected -by adding the word undatus or waved to the name of the cloud. Plate -54 will then be alto-cumulus undatus, Plate 55 alto-stratus undatus, -and Plate 56 would be described as cirro-cumulus undatus, passing into -cirro-stratus undatus and cirro-stratus. In popular language Plate 55 -might be called alto waves, Plate 54 crested alto waves, and Plate 56 -cirro ripples. - -If we are satisfied that the wave clouds are due to a wave movement -intersecting a plane of incipient cloud formation, the whole question -of their mode of production resolves itself into two parts--how is that -plane of incipient condensation produced? and how can we account for -the intersecting waves? - -The first question has by far the greater importance, since it amounts -to asking for a general explanation of the production of high clouds, -especially the forms of cirro-cumulus, cirro-stratus, cirro-macula, -and the corresponding alto varieties. There are, again, two divisions -also to this question. How does the water vapour reach the stratum -in sufficient quantity to saturate it? and when condensation takes -place, why does it so frequently assume the characteristic mottled -and granular forms like crowds of little cumulus clouds arranged in -one level? This last sentence gives the clue. They are, in truth, -little cumulus clouds, and must be formed in exactly the same way as -their vastly larger prototypes of lower regions. It has been explained -that low cumulus is the result of large upward moving air columns or -convection currents, each one being initially caused by the heating of -the vapour-laden air near the ground, and each uprising column being -supplied by cooler descending air which flows down in the intervening -spaces. It has also been explained that these movements result in -changes of temperature, which tend to check those movements and restore -the original equilibrium. Suppose this to occur, as it constantly does, -without any column reaching sufficiently high to produce a cloud. There -will be no visible effect, but, nevertheless, an important change has -taken place. Every ascending current has lifted some water vapour with -it to a higher level, and the descending drier air has come down in -contact with the ground or damper air to become equally charged with -moisture in its turn. The process will be repeated again and again, -and at one level after another, so that the water vapour travels ever -higher and higher. - -This process of interchange between ascending and descending air -has been called by Mr. Ley inversion, but the term does not seem -very suitable, and interconvection would be better. The two opposite -currents pass through each other, as if the ascending air gathered -itself into definite channels, and passed through holes in the -descending mass like the passage of water upwards through a descending -plate of perforated metal. Moreover, just as the holes in such a -descending plate might have any size, so that the ascending streams -might vary in breadth from the finest hair to a column of huge -diameter, in exactly the same way the ascending columns of air may vary -from the smallest imaginable size to the great cumulo-nimbus currents. -It is the little currents which account for the constant quiver of the -margins of any object which is viewed through a large telescope by day, -and for the haze, so characteristic of a hot day, which makes distant -objects seem ill-defined and in a state of continual tremble. The rays -of light in passing through the intersecting streams are bent a little, -now this way, now that, as the air currents sway to and fro. - -The near neighbourhood of the ground is not essential. As long as the -temperature of the air at any level is rising, so long interconvection -must occur. The process will be independent of the presence or absence -of wind. All that wind can do is to mix up the air at different levels, -breaking the system of currents and reducing it to, so to say, a finer -texture, or producing eddies, if strong enough, which direct the -currents and gather them into definite channels. The final result in -any case is that, with rising temperature, water vapour is steadily -borne upwards from the ground. - -As it ascends the air becomes cooler, and yet retains its water vapour. -When the rising currents are large they mix little with the descending -dry air, and on reaching a certain level condensation takes place, -and we have the beginning of a cumulus. If they are of a more moderate -size they will ascend less rapidly, the admixture with descending air -will bear a larger proportion to the whole, and the plane at which -condensation will begin will be higher, and then each small column -will be tipped with a ball of alto-cumulus. Make the interconvection -currents smaller still, and the cloud plane will be lifted yet higher, -and we shall have cirro-cumulus or cirro-macula. - -Now, the more even the distribution of temperature on the ground the -less the probability of coarse interconvection, and the same is true -of any higher stratum of air, provided it is free from disturbing -influences from outside. If, therefore, we have large currents near -the ground, ending, as they must, in cumulus, it has already been -explained that these clouds stop the action, and the general system of -large currents will be restricted to the region in which they occur. At -some distance above the lower clouds the only difference will be that -water vapour has been brought up to their level in great abundance. -Smaller systems of interconvection can then exist, and so we may have -the spectacle of several layers of cloud--cumulus capping the great -currents of lower regions, alto-cumulus forming the summits of the -smaller currents of intermediate regions, and cirro-cumulus floating -far above both. - -Frequently it happens that before the ascent of vapour has gone quite -far enough to produce a cloud, other causes co-operate, and the cloud -makes its appearance suddenly over considerable patches of sky. The -most potent of these is a fall of the barometric pressure, which is -brought about by some of the air far above the region of even the -highest clouds flowing away to some other district. The air at all -lower levels being thus relieved of the superincumbent pressure, -immediately expands, and is thereby cooled throughout. Consequently, if -at any level it was near its point of saturation, it will be carried -beyond that point, and cloud will rapidly make its appearance over a -large part of the sky, possibly at more than one level. Stratiform -arrangements will be the rule; but if interconvection is going on at -the time, its presence will be betrayed by a granular or cumuloid -structure. Interconvection clouds should then be most frequent, and -best formed when the air as a whole is still or moving slowly (so as -not to create great eddies), when the temperature is rising rapidly, -and when the barometer is making a sudden fall. All these conditions -are met in thunder weather, and at the time when a summer anticyclone -is giving way. It will be remembered that many of the most beautiful -forms have been described as forming under one or the other of these -very conditions. - -A second contributing cause, and one which tends to make the -condensation in patches or long broad bands ranged roughly at right -angles to the direction in which the air is moving, has been referred -to earlier. It is the passage of the air over an undulating country; -the up-and-down movements of the lower air being transmitted upwards -to great altitudes, as ever broadening and flattening waves. If the -upper air is flowing more rapidly than the lower, these broad waves -may be far ahead of their real cause, which will, therefore, quite -escape recognition, but the phenomenon is constantly to be detected -in the arrangement of the lower clouds. Two instances in the writer’s -experience will suffice. It was desired one morning to measure the -altitude of some small clouds which were passing from the north-west -at a height of probably between 2000 and 4000 metres, over a hill only -about 150 metres higher than the valley in which the apparatus was -fixed. In order to make the measurement, it was necessary for the cloud -to cross the valley and appear in the same field of view as the sun, -according to the method that will be described further on. But in order -to cross the valley the air had to descend, and so, of course, had the -cloud stratum, though to a less extent. But small as the descent was, -it was enough to dry up the clouds entirely, and for more than a couple -of hours the clouds came sailing over the hill, disappearing entirely, -and then reforming so far beyond that no measurement was possible, -since not one single fragment came near enough to the position of the -sun, which remained shining brightly through a broad clear gap between -two patches of cloud-strewn sky. - -On another occasion considerable preparations had been made for some -photographic observations during an eclipse of the sun. The observatory -stands on the eastern side of the valley of the Exe, which is flanked -on its western side by a long ridge of hills going up to 800 feet above -the sea. Beyond these hills lies the deep, narrow valley of the Teign, -and beyond that the granite ramparts of Dartmoor, 1000 feet above the -sea. The wind was blowing gently across the two valleys, and shortly -before the eclipse began a broad strip of thin cloud formed above and -rather towards the eastern side of the Exe valley, just where the sun -was, while at the same time the sky was practically clear half a mile -further east, and bright sunlight was streaming down on the ridge -between the two rivers a few miles towards the west. The cloud was -never thick enough to quite hide the sun, so that the eclipse was easy -to watch with the naked eye; but in spite of fairly rapid movement of -the cloud masses as they drifted before the sun, they kept on forming -in just the same place, and completely prevented the carrying out of -the programme planned. It is almost certain that the phenomenon was -brought about by an upward moving wave marking the place where the -level of approaching saturation was upheaved by the disturbance caused -by crossing the two valleys and intervening ridge. - -These two instances are not quoted as examples of a rare occurrence, -but as definite simple instances of a phenomenon which may be -constantly observed, and as proof that the conformation of the ground -does exercise an influence upon the distribution of cloud. - -But no irregularities of the ground will suffice to explain the minute -waves and ripples which have been described at the beginning of this -chapter. These must be due to wave disturbances in the air itself. -They have been explained as due to two different currents of air, -either a warm damp current flowing over a cold one, or _vice versâ_. -Now, such an occurrence as a warm damp current flowing over a cold -one must be very rare, though it is impossible to deny that it might -occur. The immediate contact of a cold current above a warm damp one is -equally unlikely, unless the general atmospheric condition were greatly -disturbed, which is the same thing as saying that wave clouds would not -occur. They are most frequent at just those times when interconvection -has freest play, and this is amply sufficient to account for a plane of -saturation without any necessity for a hypothesis of two layers of air -at different temperatures all but producing cloud at their junction. -No convincing evidence of cloud production by such means has yet been -adduced, and it is better to rely upon causes which we know do operate -than to call in theories as to what might possibly happen. This is -one of those points in the study of clouds which need investigation, -and until proof is forthcoming it is better to say that the admixture -of two strata of air might conceivably produce cloud, but most forms -can be accounted for by other causes of which we have more positive -evidence. - -Still, the wave clouds are due to waves, and there seems no other way -of accounting for them than the supposition of gentle differential -currents. But if such currents occur the ripples and waves will not -be limited to a definite surface, so to say, of contact, but will be -propagated upwards and downwards for considerable distances from the -level of greatest disturbance. Whether, therefore, the level at which -the natural operation of interconvection has produced saturation is -high or low in this region, the result will be the marshalling of the -ascending and descending elements of the convection system in the -characteristic waves. - -The differential currents, then, which cause the waves must not be -conceived as producing those waves at a surface of contact, nor must -the currents be thought of as separated by any definite surface, but -rather by a region of variable but usually considerable depth, in which -the air is disturbed by a series of small slow eddies and oscillatory -movements. When the waves are parallel straight lines the air currents -may be really portions of a whole, having the upper part more rapid -than the lower. In such a case the direction of movement should be -at right angles to the cloud lines. If the upper current differs in -direction as well as velocity, the direction of movement of the clouds -will be intermediate, and will resemble that of the upper or lower -current, according to their relative distances from the plane at which -the clouds are formed. - -The behaviour of the clouds will depend upon the relative shares in -their production borne by interconvection pure and simple and by the -wave oscillations. If the stratum is one in which cloud would actually -be formed independently of the up-and-down movements, all this will -be able to do will be to arrange the cloudlets at their birth, and -these will then continue to exist, drifting with the general horizontal -movement of the air like any other cloud of the same order. - -On the other hand, if the production of cloud is dependent upon the -vertical oscillations, the cloudlets or lines of cloud will move with -the air waves, and their rate of motion and direction of motion will -be determined by the rate and direction of the waves, which may be -quite different from that of the air at that stratum as a whole. The -ascending waves will be marked by lines of cloud generally rounder and -better defined on their advancing sides, while the descending troughs -will be marked by clear intervals. - -Wave movements of the necessary kind are frequently very complicated, -and it is not by any means a rare occurrence to see the wave lines in -one part of the sky at all sorts of angles with similar lines in other -parts, or even to see two or more sets of waves at different altitudes -crossing one another. Either phenomenon is always accompanied by rapid -changes in the cloud, and the rippled structure is short-lived. This -was the case with the clouds shown in Plate 54. Plate 53, on the -contrary, shows great uniformity in the wave lines, and although the -vertical oscillation is probably the main cause of condensation, the -form was unusually persistent. - -Irregular patches of wave disturbance, affecting a plane occupied by -cirro-stratus vittatus, are shown in Plate 57. In this case the wave -systems only touch the cloud plane here and there, and the places of -contact varied rapidly. It is pretty clear from this photograph that -the idea of the waves being formed at a surface of contact between two -diverse currents will not suffice. The bands of the cirro-stratus are -for the most part unbroken and unaffected; it is only here and there -that the wave region touches them. - -[Illustration: Plate 57. - -WAVED CIRRO-STRATUS. - -(_Cirro-stratus Undatus._)] - -The conclusions at which we have arrived are simple, and there is -little room for doubt as to their main correctness, but there are -numerous minute features presented by these beautiful cloud patterns -which await interpretation, and they reveal complicated oscillatory -movements in the air which are difficult to account for, whether we -seek their originating causes or the mechanics of their motions. - - - - -CHAPTER IX - -CLOUD ALTITUDES - - -DURING an extended experience of cloud photography, it was found that -it was quite possible to get pictures which showed the cloud detail -even when the sun was in the field of view. Sometimes the solar image -was reversed, but if the exposure was very short this was not the -case. In such photographs the structure of the cloud was exceedingly -clear and sharply defined quite close to the sun. Indeed, the intense -illumination seemed to reveal minute details of internal arrangement -which could not be detected in similar clouds some distance away. - -The methods which had been employed for the measurement of cloud -altitudes elsewhere have already been briefly referred to. Some of -them required two observers, who were equally responsible, each of -them having to direct his apparatus or camera to the same point of the -cloud, and to record the exact direction in which the instrument was -pointed. The instruments, if accurate, were costly, and there were many -opportunities for error in reading the graduated circles which gave the -directions. Moreover, in most of these methods the two observers were -connected by telephone, and had to agree on the exact point towards -which their instruments should be directed; either the exact point -of the cloud, or the precise direction as shown by the mounting of -the camera or other instrument. At Kew some of these sources of error -were avoided by fixing the two cameras with the axes of the lenses -and centres of the plates in a vertical position and exposing the two -plates simultaneously. The Kew observations were not long continued, -and for some years the only measurements in progress were those carried -out abroad, particularly at the Blue Hill Observatory and at Upsala. - -The experience gained in photographing clouds in order to record their -forms suggested a way in which many of the sources of error in previous -measurements of altitude could be avoided, especially by simplifying -and reducing the operations at the moment of making the observation. - -If two cameras are placed at the opposite ends of a measured base line, -whose direction is known, and if they are both pointed towards the sun, -on making the exposures by electrical means at the same moment, the -position of the image of the sun upon the plate gives the direction in -which the cameras are pointed. It will be in the same direction as seen -from both ends of the line. - -Now, if we note the time at which the exposure is made, this with the -date gives all that is required for ascertaining the sun’s position -in the sky, and is, therefore, the only exact observation which need -be made at the time of taking the photographs. Mistakes are almost -impossible, as each plate contains its own record of the sun’s -position, and even if some of the plates should get mixed the images of -the clouds will generally suffice to pair them properly. For general -measurements there is one grave defect in the method, and that is that -it can only be used when the sun and cloud can be got into the same -field of view. But with the higher varieties of cloud this is generally -possible, and it was just these higher sorts about which knowledge was -least certain, and which it was proposed to study. - -An initial difficulty was the finding of a level site, flat land being -very uncommon in Devonshire, but fortunately a suitable place was -found in some artificially levelled ground close to Exeter, belonging -to the London and South Western Railway Company. It was a stretch of -ground intended to be covered with sidings, but had not been finished, -and had become overgrown with grass, stunted sallows, and other wild -plants. Being railway ground, it was, comparatively, though by no means -entirely, free from mischievous and inquisitive people. The next point -was a suitable camera. It must have fairly long focus in order to give -a large image, and therefore large displacement; it must be capable -of being pointed in any direction and clamped there; and it must be -capable of standing considerable extremes of temperature and variations -of dampness, as it was intended that they should be kept on the spot in -wooden structures, which served for stands as well as to contain the -apparatus. - -The pattern finally decided upon is represented in Plate 58, which -shows one of the cameras pointed up to the sky and standing on one of -the stands. These cameras were to take plates of whole plate size, two -double dark slides of the ordinary pattern being attached to each. - -[Illustration: Plate 58. - -CAMERA FOR MEASURING ALTITUDES.] - -The camera looks rather complicated, but it is really simple. Its -body consists of front and back, each attached to a central part by a -short bellows and sliding on a base board, to which it can be clamped -by screws of the usual pattern. The central part carries trunnions, -such as are used for looking-glasses, which swing in sockets carried -by two upright supports, so as to give the whole free motion in a -vertical plane. In order to be able to fix it firmly at any angle, the -base board of the camera body carries on its underside a thin board -projecting beneath it and forming a segment of a circle whose centre -would be the horizontal axis through the trunnions. The board passes -between the jaws of a small wooden clamping vice in front, which is -carried by the square base to which the uprights are fixed. The whole -is firmly made of well-seasoned pine, and has stood well the hard usage -of half a dozen years. - -There is no focusing screen. Focusing was done with great care once for -all, and then a coat of hard varnish was put over all the adjusting -screws. A small view-finder is attached to one side, and it was by -this that the camera was pointed in the desired direction. - -In order to lessen risk of mistake, it was so arranged that the two -slides belonging to one camera would not fit the other. The lenses, -of 18 inches focus, and giving sharp detail all over the plate, were -carefully matched, and the focus adjusted until the images given by -them when placed side by side appeared to coincide exactly. They were -provided with iris diaphragms, which were shut down to an aperture of -a quarter of an inch, and with shutters which could be released at the -same moment by an electric current, acting through the electro-magnet -shown under the lens on the front of the camera. - -The shutters were of the kind known as the “Chronolux,” which will give -any exposure from the sixty-fourth of a second up to three seconds. But -it was found in practice that the highest speed was sufficient and gave -satisfactory results. Of course, there was no idea of adjusting matters -on each occasion so as to get the best possible negatives capable of -yielding good prints. Measurement was the object, and if the negative -showed the sun and sufficient cloud detail for the identification of -cloud points, that was all that was wanted. The shutters gave a good -deal of trouble at first. Their sliding parts were made of ebonite, and -when the cameras were left in their stands with an August sun shining -down upon them, everything inside got very hot and the ebonite warped; -but the difficulty was got over by substituting aluminium. - -The two camera stands were placed 200 yards apart, and were connected -by a line of telegraph wire carried on short poles. At each end of -the wire an insulated connecting piece was brought down to the camera -stand, and to the batteries and other apparatus. The current which was -sent through this wire by pressing a contact at one end of the line did -not directly make the exposures; but two similar relays were brought -into action, and each of these sent the current from a local battery of -LeclanchĂ© cells through the electro-magnet on the camera and made the -exposure. - -After development the two negatives showed the image of the sun, not -far from the centre of the field of view, and the cloud whose altitude -was required. Since this was taken from two different points of view, -the negatives were not alike, but the distances between the centre -of the sun’s disc and any special point of the cloud were different. -For instance, if the cloud were east of the sun, with its edge just -apparently touching the solar image as photographed from the eastern -station, then the negative taken from the western end of the base would -show an interval of clear sky between the two, which would be greater -as the cloud was lower. - -It often happened that after developing the plates the image of the -sun was lost in a black blur, but it was easy to reduce this part of -the image by local application of a reducing agent[3] by means of -a paint-brush, until the disc became clear enough. Two lines were -then drawn on the negative, one vertical and the other horizontal, -intersecting each other at the centre of the sun’s image. These lines -served as the starting-points for exactly measuring the distance from -their point of intersection to any selected point of the cloud. - -The distances could generally be determined to a fiftieth or a -hundredth part of an inch, and their difference was, of course, -dependent upon the direction of the sun relative to the base line and -the altitude of the cloud, but for low level clouds the difference -was sometimes so great that no pair of corresponding points could be -detected, while it was often as much as an inch. With higher clouds -the differences were smaller, but unless the sun was very low in the -sky, either east or west, the displacements of the cloud image were -great enough to give reliable measures. Specimen prints from pairs of -negatives are shown in Plates 59 and 60. - -[Illustration: Plate 59. - -PRINT FROM A NEGATIVE USED FOR MEASURING ALTITUDE.] - -[Illustration: Plate 60. - -PAIR OF PRINTS SHOWING THE DISPLACEMENT OF THE CLOUD.] - -The processes by which the measurements are worked out are -laborious,[4] and consist of two parts, the first being the -determination of the exact position of the sun from the date, hour, and -latitude and longitude of the place, and the second, the determination -of the position of the cloud. Two points which represent the same part -of the cloud are selected, and their respective distances from the two -lines drawn through the sun are measured as accurately as possible. -Now, a certain distance on the negative corresponds with a definite -angular displacement, and a scale can be constructed showing how much -should be added to or subtracted from the sun’s position to get the -exact position of the cloud. This being done, it is then a simple piece -of trigonometry to deduce the actual height of the cloud above the -place of observation. The work of computation, however, was greatly -lightened by the fact that many of the pairs of negatives showed more -than one layer of cloud; thus Plate 59, which is a fair specimen, -shows three layers, and, consequently, one determination of the sun’s -position sufficed for three distinct results. - -For the highest clouds the displacements were, of course, small, and -could only be made with certainty of a correct result within about -three hours of noon. Earlier than 9 a.m., or later than 3 p.m., the -sun was too nearly in a line with the two stations, or too low in the -sky, to give a sufficient displacement of image. A base line of 400 -yards instead of 200 would have been better for the high clouds. But, -on the other hand, when low level clouds are viewed from two different -spots their outlines may seem so changed that it may be impossible to -identify a pair of corresponding points, and the same difficulty may -also arise when high clouds are seen through a gap in a lower stratum. -The longer the base line the more frequent and more obtrusive would -this perspective difficulty become, so the distance of 200 yards -between the stations was adopted as a convenient mean. - -The method of making the observations was simple. Each observer was -provided with some signal flags, by which the necessary communications -were made in accordance with a simple code. Call the two observers A -and B, and suppose A directed the operations. He watched the sky until -a favourable opportunity seemed to be approaching. He then signalled -to B, and both cameras were turned to the sun, the dark slides were -inserted, the shutters set, and everything made ready. Signals were -then interchanged, to signify that preparations were complete, and when -A saw that the edge of the cloud had reached a suitable position to be -in the same field of view with the sun, the contact key was pressed and -the plates simultaneously exposed. At the moment when this was done the -time was noted. Several observations were thus made in a short time. - -Measurements were carried out as opportunity allowed over four -consecutive seasons, from the beginning of April until the end of -October. During the last of the four years, the site had become less -convenient owing to an extension of the railway work, and early in -November the series was brought to an abrupt conclusion by a heavy -gale, which snapped off all the poles carrying the connecting wire. But -by that time 423 measurements had been obtained, the great majority of -which referred to clouds of the cirrus and alto groups. - -The general results may be tabulated thus, giving heights in metres:-- - - -------------------+-------------+---------+---------+--------- - | Number of | Maximum | Minimum | Mean - |observations.|altitude.|altitude.|altitude. - +-------------+---------+---------+--------- - Cirrus | 58 | 27,413 | 4,114 | 10,230 - | | | | - Cirro-stratus | 64 | 15,503 | 3,840 | 9,540 - | | | | - „ cumulus | 63 | 11,679 | 3,657 | 8,624 - | | | | - Alto-cumulus | 83 | 9,390 | 1,828 | 5,348 - | | | | - Cumulus top | 42 | 4,582 | -- | 3,006 - | | | | - „ base | 48 | 1,959 | 584 | 1,290 - | | | | - Strato-cumulus | 27 | 6,926 | 823 | 2,248 - | | | | - Cumulo-nimbus top | 15 | 6,409 | 2,004 | 8,002 - | | | | - „ „ base | 15 | 2,286 | 766 | 1,045 - -------------------+-------------+---------+---------+--------- - -These values are not very different, on the whole, from those which -have been arrived at elsewhere, and in making a comparison it must -be borne in mind that there is always a little want of precision in -cloud nomenclature. As a whole, the Exeter maxima are greater than the -foreign ones, and this is very markedly so in the case of cirrus, for -which the American highest record is 14,930 metres, the Swedish record -is 13,376, while the Exeter value is 27,413 metres, or about 17 miles. -But this extreme measurement, and several others unusually large, -were made in one morning, a day of very hot damp weather, when cloud -formed at seven different levels: cumulus at a height of 1·9 miles, -alto-cumulus at 3·9 miles, cirro-cumulus at 4·7 miles, cirro-stratus -(No. 1) at 8 miles, cirro-stratus (No. 2) at 9·6 miles, cirrus at 11·5 -miles, and cirrus excelsus at 17 miles. By about half-past one in the -afternoon the sky was completely overcast with dull grey clouds, which -cleared off at half-past four, and at half-past five in the evening the -cirrus had fallen to 7·9 miles, and the cirro-cumulus to 4·3 miles. -If this one day’s observations had been omitted, the Exeter maximum -would only have been little more than 1000 metres above the record from -across the Atlantic, but 1000 metres is a height worth noting. - -While the Exeter maxima are all rather greater, we find the minima -for cirrus, cirro-stratus, and cirro-cumulus are rather less than at -the foreign stations; that is to say, that clouds are formed over -Devonshire both at lower and at higher levels than seems to be the case -in Massachusetts or Sweden. It seems probable that this is due to a -greater humidity on our western coasts, such as we should suppose would -be the case from their position and the prevailing winds and ocean -currents. If so, we should expect the great convection clouds to be -larger. Thus, at Exeter, out of only fifteen examples of cumulo-nimbus, -the top varied from 2004 metres to 6409, with an average base level -of 1045. At Upsala the maximum was 5970 and the minimum 1400, with an -average base level of 1400. The mean thickness of the Swedish clouds -was only 1400 metres, while that of the Devonshire specimens was more -than 2000 metres. - -Again and again, during the progress of these measurements, it was -found that the greatest altitudes and the richest development of the -higher varieties occurred towards the end of a spell of fine calm -weather, when convection had had free play day after day. A slight -fall of the barometer, only the hundredth part of an inch, would -usually, under those circumstances, bring about abundant formation -of high clouds, frequently of the undatus kind. All the cumulus -clouds, by which we mean to include alto-cumulus and cirro-cumulus, -are most frequent when the levels of condensation are rising, while -the stratiform clouds are an indication of no vertical movement or -of active descent. Pure cirrus is indicative rather of movement in a -horizontal direction, and may occur when the condensation levels are -stationary, or when they are rapidly changing either way. - -In broken weather the natural movements of the atmosphere and of its -vapour are masked and disturbed by the strong eddies brought by the -cyclonic systems. It not unfrequently happens that the region of -disturbance does not reach up to the level of the highest cirrus, or, -what is more probable, the cyclonic system leans so far forward that we -may have in its rear the upper clouds floating quietly far above the -comparatively shallow region of disturbance, while in front the upper -part of the storm system projects above undisturbed air. - -The frequent appearance of cloud almost at the same time at more than -one level is at first rather difficult to understand, but it will be -noticed that when this occurs the barometer almost invariably falls. -Now, if we suppose that the air is nearly saturated at more than one -level, and that the whole is then bodily relieved of some of the -superincumbent mass, so that the barometer falls, the mass of air will -at once swell up, being cooled from top to bottom simultaneously, and -wherever it is damp enough cloud will be formed. - -The converse is equally true. If we have cloud at several levels, and -the whole is compressed by the addition of more air above, which is the -case when the barometer rises, that compression will be accompanied -by the generation of heat and the consequent disintegration and -disappearance of the clouds. - -FOOTNOTES: - -[3] Ferricyanide of potassium and hyposulphite of soda. - -[4] From the declination of the sun corrected for variation and from -the known latitude, the meridian zenith distance is calculated. - -From the Greenwich time, the longitude, and the equation of time, the -hour angle is obtained. - -Now, if H be the hour angle, D the reduced declination, and M the -meridian zenith distance, the sun’s altitude may be calculated by the -formula-- - - log versin H + L cos lat. + L cos D-20 = log _n_, - -where _n_ is a natural number, and - - _n_ + vers M = covers alt. - -Again, to find the azimuth-- - - vers sup. (lat. + alt.)-vers polar dist. = _m_, - -where _m_ is another natural number, and - - log _m_ + L sec. lat. + L sec. alt.-20 = log vers azim., - -reckoned from the south. - -Hence the position of the sun is ascertained for both negatives. - -By actual measurements on the plates and reference to a previously -constructed scale the position of the cloud as seen from each camera -is next determined, and the angle subtended by the base line at a -point X vertically beneath the cloud is calculated. If A and B are -the stations, and _a_ and _b_ the angles from them respectively, the -distance AX is given thus-- - - log AX = L sin _b_-L sin AXB + log AB, - -and the height _h_ of the cloud above X is given by-- - - log _h_ = log AX + L tan alt.-10. - - - - -CHAPTER X - -CLOUD NOMENCLATURE - - -SINCE a considerable number of new terms have been suggested in the -foregoing pages, it may be convenient to collect them and tabulate -them, so as to show their relation to those already recognized by the -International system. - -In the atlas put forward by the committee, sixteen varieties are -recognized by distinct names, and these are drawn up in tabular form -with appropriate abbreviations for use in making records. - -The names are-- - - Cirrus. Ci. - Cirro-stratus. Ci. S. - Cirro-cumulus. Ci. Cu. - Alto-cumulus. A. Cu. - Alto-stratus. A. S. - Strato-cumulus. S. Cu. - Nimbus. N. - Cumulus. Cu. - Cumulo-nimbus. Cu. N. - Stratus. S. - Fracto-cumulus. Fr. Cu. - Fracto-nimbus. Fr. N. - Fracto-stratus. Fr. S. - Stratus-cumuliformis. S. Cf. - Nimbus-cumuliformis. N. Cf. - Mammato-cumulus. M. Cu. - -During our survey of these groups we have found that some of them -include clouds of many shapes, which must be due to very diverse -conditions. It follows that if observations are to be made on the -occurrence of these special kinds, with a view to arriving at a -thorough understanding of the circumstances to which they owe their -forms, it becomes necessary to devise a code of names and symbols -whereby an interchange of ideas and records may be rendered possible. -Specific names have been proposed as each form was considered, and -it only remains to sum them up concisely. Subsequent observation, -particularly in other climates, may show that further additions should -be made; but if the principle of specific names be once admitted, it -will be easy to fill any omission. - - -GROUP CIRRUS. - -Under the general head of cirrus we have found nine distinct forms-- - -1. _Cirro-nebula_ (Ley) (Plates 2 and 3). Cirrus veil. - -Characterized by comparative absence of structure and by the formation -of halo. Ci. Na. - -2. _Cirro-filum_ (Ley) (Plate 7). Thread cirrus. - -Built up of fine long threads, straight, curved, or crossing, but free -from hazy curling or flocculent structures. Ci. F. - -3. _Cirrus excelsus_ (Plate 5). High cirrus. - -Characterized by great altitude, thinness, irregular branching -structure. Ci. Ex. - -4. _Cirrus ventosus_ (Plate 6). Windy cirrus. - -Characterized by curving branches leaning forward in the direction of -movement, and other long curving streamers lagging behind and below. -Fluffy parts are usually present, and mark the origins of the long -curling fibres. Ci. V. - -5. _Cirrus nebulosus_ (Plate 9). Hazy cirrus. - -Characterized by the absence of sharply defined lines, fibres, or -streamers; all parts of the cloud being hazy, and suggestive of other -varieties of cirrus out of focus. Ci. Neb. - -6. _Cirrus caudatus_ (Plate 8). Tailed cirrus. - -Characterized by small hazy or fluffy heads behind or below which hang -long streamers, which taper away more or less to a point. The tails are -sharply defined, and so are the edges of the heads. Ci. Ca. - -7. _Cirrus vittatus_ (Plates 12 and 13). Ribbon cirrus. - -Characterized by formation in long bands of cloud, sometimes made of -parallel long fibres with cirrus haze linking them together, sometimes -consisting of a long bundle of fibres, from which others diverge at an -angle as shown in the plate. Ci. Vt. - -8. _Cirrus inconstans_ (Plate 10). Change cirrus. - -Characterized by a peculiar ragged, wavy appearance. It is -generally only the beginning or the end of a mass of cirro-stratus -or cirro-cumulus, but occasionally it vanishes shortly after its -appearance, without reaching the further stage. Ci. In. - -9. _Cirrus communis_ (Plate 11). Type cirrus or common cirrus. - -Characterized by short irregularly curling fibres collected together -in considerable patches. No definite arrangement into any of the forms -already described. Ci. Com. - - -GROUP CIRRO-STRATUS. - -Under this group the cloud usually shows some structure, being -apparently built up from a massing together of detached forms at a -common level. When this is so it should be described by adding the -specific name of the detached form most nearly related. - -1. _Cirro-stratus nebulosus_ (Plates 3, 4, and 14). Hazy cirro-stratus. - -Characterized by absence of visible structure. Ci. S. Neb. - -2. _Cirro-stratus communis_ (Plate 16). Common cirro-stratus. - -Characterized by the presence of short curling fibres matted together. -Ci. S. Com. - -3. _Cirro-stratus vittatus_ (Plate 57). Ribboned cirro-stratus. - -Characterized by being made up of long stripes or bands of cloud. Ci. -S. Vt. - -4. _Cirro-stratus cumulosus_ (Plate 17). Flocculent cirro-stratus. - -Characterized by an obscurely granular structure. Ci. S. Cu. - -Many forms of cirro-stratus are arranged in waves or ripples. This is -indicated by attaching the word undatus, or waved, after the ordinary -specific name, or the letter U after the abbreviation. - - -GROUP CIRRO-CUMULUS. Divisible into three species. - -1. _Cirro-macula_ (Ley) (Plate 23). Speckle cloud. - -Characterized by semi-transparency, by the fact that the particles -are frequently whiter and more opaque on their edges. A patch of -cirro-macula always looks like a thin sheet which has curdled. Ci. Ma. - -2. _Cirro-cumulus nebulosus_ (Plates 20 and 21). Hazy cirro-cumulus. - -Characterized as rounded balls of semi-transparent cloud, but -ill-defined and hazy. No shadows. Ci. Cu. Neb. - -3. _Cirro-cumulus_ (Plates 18 and 19). - -Characterized as opaque rounded balls clearly defined, but showing no -shadows on their under sides. Ci. Cu. Com. - -Wave forms again are indicated by the addition of the word undatus. - - -GROUP ALTO CLOUDS. Divisible into nine species. - -1. _Alto-stratus._ High stratus. - -A uniform veil of cloud showing no details of structure except local -variation in density in patches. Rarely dense enough to completely hide -the sun, or even the full moon. A. S. - -2. _Alto-stratus maculosus_ (Plate 30). Mackerel sky. - -Characterized as numerous nearly equal and small lenticular patches -ranged on a level and about equi-distant from each other. A. S. Mac. - -3. _Alto-stratus fractus_ (Plate 34). - -Patches and bits of cloud of irregular shape, but resembling broken -bits of a level sheet. A. S. Fr. - -4. _Alto-strato-cumulus_ (Plate 32). - -Intermediate between alto-stratus and alto-cumulus. A. S. Cu. - -5. _Alto-cumulus informis_ (Plate 25). - -Characterized as more or less rounded cloudlets interspersed with -ragged bits of cloud and occasionally with streaks of cirrus, the -cloudlets showing no clear-cut outlines, but having distinct shadows. -A. Cu. In. - -6. _Alto-cumulus nebulosus_ (Plate 26). - -Hazy alto-cumulus. A. Cu. Neb. - -7. _Alto-cumulus castellatus_ (Plate 28). Turret cloud. - -A high cloud resembling a number of tall narrow cumulus clouds on a -very diminutive scale. The cloudlets show distinct shadows, are very -opaque, and their upper margins are sharply defined. Vertical axes -longer than the horizontal ones. A. Cu. Ca. - -8. _Alto-cumulus glomeratus_ (Plate 29). - -Characterized by the roundness and regularity of the cloudlets, which -have sharp margins, cast distinct shadows, and have their axes about -equal in all directions. A. Cu. Gl. - -9. _Alto-cumulus communis._ - -Small high cumulus of the ordinary pyramidal pattern. A. Cu. Com. - -10. _Alto-cumulus stratiformis_ (Plate 27). - -Flattened cloudlets gathering into small detached sheets. A. Cu. S. - - -Lower clouds. GROUP STRATUS. - -1. _Stratus communis_ (Plates 37 and 41). - -In its most typical state, stratus consists of a sheet of cloud of -approximately uniform thickness. The most common form, however, does -vary considerably, though usually dense enough to hide the sun. -Portions of such a sheet would take the same specific name, unless the -portions are very small and ragged, which would be expressed by adding -the word fractus. S. Com. - -2. _Stratus maculosus_ (Plate 40). - -Formed either by the appearance of cloud in lumps, which are always -lenticular in shape, and ultimately join together to form a stratus, or -by the break up of the typical stratus. S. Mac. - -3. _Stratus radius_ (Plate 42). Roll cloud. - -Formed during the break up of a low stratus, which separates up into a -number of parallel lines of cloud. S. R. - -4. _Stratus lenticularis_ (Plate 47). Fall cloud. - -Formed by the collapse of cumulus or strato-cumulus. A cloud of -evening, easily recognized as lenticular patches. S. L. - -5. _Strato-cumulus_ (Plates 38 and 39). - -A term applied to either a stratus which has thickened every here and -there into cumulus, or a number of cumulus which have joined together -so as to show a nearly continuous common base. S. Cu. - - -GROUP CUMULUS. - -1. _Cumulus minor_ (Plate 43). Small cumulus. - -Cumulus clouds so small as to present the appearance of rounded lumps, -no definite pyramidal form or flattened base. Cu. Mi. - -2. _Cumulus major_ (Plates 44 and 45). Large cumulus. - -Characterized by a flattened base and rounded clear-cut upper surfaces. -Cu. Ma. - -3. _Cumulo-nimbus_ (Plates 49 to 52). Storm cloud. - -Characterized by the expanded, anvil-shaped, or disc-shaped top, -cirrifying at its edges. - - -GENERAL TERMS - -_Nimbus_, a term applied to a cloud from which rain is falling. When -the form of the cloud is visible, the term should be attached to that -belonging to the cloud. It may, however, be used as a substantive alone -when there is nothing to show from what sort of cloud, or combination -of clouds, the rain is falling (Plates 35 and 36). - -Nimbus is either heavy stratus, massive strato-cumulus, or a -combination of these with stratiform clouds above, and possibly ragged -masses of fracto-cumulus below. N. either alone or after the sign of -the cloud. - -_Fracto-_ is a term placed as a prefix before the name of a cloud to -indicate that the cloud has ragged irregular margins, as if it had been -more or less torn to pieces. It is sometimes less awkward to append the -word fractus after the name of the cloud. - -A convenient abbreviation would be to write F. after the name of the -cloud. - -_Undatus_, or waved, should always be added to the name of any cloud -which shows the arrangement so described. - - - - -CHAPTER XI - -CLOUD PHOTOGRAPHY - - -REFERENCE has been made in the first chapter to the fact that those -who wish to make a photographic study of clouds must follow a special -course of procedure. For every photographic purpose there is some -particular process or some special kind of apparatus which is better -fitted for the end in view than any other, and half the difficulty in -attaining success is to find out the best tools and the best methods. - -There is no difficulty whatever in securing excellent photographs of -heavy grey clouds, or of clouds which stand out dark against a twilight -sky. Any camera and any plate can be used, and in an experienced hand -will ensure success after a few trials, but except under these special -conditions, cirrus, in all its varieties, the alto clouds, and even -many of the lower ones, present a real difficulty due to two causes. -In the first place, they and their surroundings are so brilliant that -a very short exposure is sufficient, far shorter than would be needed -for a sunlit landscape; and in the second place, the actinic value of -the light they reflect is very little greater than that received from -the background of blue sky. When so minute a difference comes to be -represented in the monochrome of the ordinary photograph, the eye fails -to appreciate it, and all the finer details are lost. - -Now, if proper care is taken in the development of a negative, -satisfactory results may be attained even if the exposure is twice as -great, or only half as great, as it should have been to get the best -result. But if the exposure is four or more times the best duration, -the negative will generally yield but poor contrasts, if any result at -all can be coaxed out. Again, if the exposure is only a quarter or less -of the ideal time, little or no image will come out. Suppose, now, we -have a brilliant object, and the correct exposure for the plate and -aperture of lens employed should be one-fiftieth of a second; if we -make an error either in judging or in effecting the exposure, which -amounts to one twenty-fifth of a second too much, we get the negative -exposed three times as much as it should be. Suppose, again, the object -is less brilliant, and the correct exposure should be one-fifth of a -second, an equal error of one twenty-fifth will make little difference. -But in photographing cirrus and such clouds, if we used the same plates -and the same lens apertures as we employ for ordinary landscape work, -we should want exposures of the order of those given by a focal plane -shutter, and a mistake either in judging or in making the exposure, of -even the hundredth part of a second, would be fatal to good results, -and would probably completely spoil the plate. Evidently one of our -first steps must be to lengthen the correct exposure. - -There are four ways in which this can be done--by using a slow-acting -plate, by lessening the aperture of the lens, by putting some -transparent screen in front of the lens to shut off some of the light, -and, finally, by pointing the camera, not at the cloud itself, but at -its image in a black mirror. - -Of these, of course the slow plate and small aperture are the simplest -to adopt, and all the cloud studies shown in the illustrations to -these pages have been taken on plates prepared for photo-mechanical -purposes or for transparencies. There seems to be nothing to choose -between these two brands. Orthochromatic, isochromatic, double-coated, -and many other special types of plate had previously been tried, both -with coloured filters in front of the lens and without them, without -showing any marked superiority over an ordinary plate of low rapidity. -At last the photo-mechanical plates were tried, and the efforts made -to get satisfactory cloud portraits, which had previously been marked -only now and then with satisfactory results, became uniformly and -continuously successful. - -If the slow plates are exposed in the camera without either a screen or -the black mirror, the diaphragm should be reduced to a small size and -the exposure suitably adjusted. The length of exposure may generally -be judged by looking at the image on the focusing screen, and reducing -the aperture until the picture shows its detail easily. Then, regarding -the picture as that of a sunlit sea or distant landscape, judge the -necessary exposure by the brightness of the image. - -No definite rule can be given. The light varies enormously from day to -day, and hour to hour, and especially with the position occupied by -the cloud relative to the sun. Thus, working with a lens of six inches -focus and an aperture of a quarter of an inch, the exposure may vary -from the quickest snap of a Thornton-Pickard roller blind to as much as -a quarter of a second, or even more. Again, using a lens of eighteen -inches focus and an exposure of a fiftieth of a second, the necessary -aperture might vary from an eighth of an inch up to an inch and a -half. But if we suppose that we are dealing with an ordinary bright -summer sky between 9 a.m. and 5 p.m., and that the clouds are cirrus -or cirro-cumulus, an aperture of about one thirty-second of the focal -length will probably give some sort of image with a snap-shot exposure. -At first the failures will be many, but a little practice will soon -enable very respectable pictures to be taken by varying either the -diaphragm or the speed of shutter. Heavier clouds of the alto types -will need rather longer exposure or larger aperture. - -The lens may be of any kind, as long as it gives a well-defined image, -but there are many advantages in using one of the rectilinear type -provided with an iris diaphragm. A rapid lens is not needed; indeed, it -has been pointed out that slowness is a very great desideratum, and if -the camera is provided with a rapid lens it must be ruthlessly stopped -down. For general cloud purposes the best kind of lens is a wide-angle -rectilinear, but many occasions will present themselves on which a -lens of longer focus will be wanted in order to give more insight into -the details of some specially delicate clouds. If the lenses are good, -and the focusing is accurate, enlargements will go a long way towards -revealing the minuter structures, but the results can never be quite so -well defined as a direct photograph in a long camera. - -A shutter will be essential, and it should be one which opens in the -middle, or which travels across the lens. The shutters which are -ingeniously contrived to give more exposure to the lower part of the -picture than to its upper part are useless for the purpose in view. It -should have some latitude of exposure, from about one-sixtieth of a -second up to a full second or more. - -Then as to the camera. Any light-tight camera will do, and, as -the objects will all be at a great distance, it may very well be a -fixed-focus one, or may be kept set up and fixed in focus for a distant -object. If not, on setting it up it should be focused on the horizon or -most distant object possible, and not on the cloud itself. As, however, -the clouds present themselves at all heights above the horizon, even -in the zenith, it becomes necessary to have some means of pointing the -camera in such directions. To a certain extent the ordinary stand does -allow of tilting, but a special support which will allow the camera to -be fixed firmly in any position is of the greatest convenience. - -If the study is meant to be at all prolonged, the best plan is to make -a suitable camera, once for all, which can be left in fixed focus, so -as to be always ready, and which can be directed with equal ease to any -part of the sky, from the horizon to the zenith. If it is intended to -use a black mirror, then a special mount becomes almost essential. - -Many of the most delicate of the photographs reproduced here have been -taken with a camera of peculiar pattern, the structure of which is -shown in Plate 61. The lens is an ordinary rapid rectilinear, and the -stop used was generally one-sixteenth of the focal length. The shutter -is a light slip of aluminium, which can be drawn across from side to -side at any desired pace. The body of the camera is mahogany, with a -bellows part for getting correct focus, but when once this was obtained -the back was clamped to the tail-board and a little varnish brushed -over the clamping screws. - -[Illustration: Plate 61. - -CLOUD CAMERA FOR STUDIES.] - -The camera swings on a couple of screws, which act as trunnions. These -pass through two upright arms, which spring on either side from the -base board, which is attached to the ordinary camera stand. This base -board can be rotated into any horizontal position desired, and the -camera can be tilted through any vertical angle by swinging it between -the uprights, and can be clamped by tightening the two trunnion screws. -These screws are so placed on the front of the camera that the lens and -its attachments on the one side nearly balance the back part of the -camera on the other side, and so lessen the danger of slipping. - -Supported in front of the lens by light brass-work is the black mirror, -made of a very dark glass optically worked on the front face. It is -a curious fact that, although bits of plate-glass blackened on the -back seem to the naked eye to give a single image of sufficient truth, -if such a mirror is placed in front of the camera the second faint -image formed by reflection from the blackened surface is almost always -to be detected. Moreover, the lens with its large aperture at once -detects irregularities in the surface of the glass, which are quite -imperceptible through the narrow limits of the pupil. Black glass, -with a truly worked surface, is essential then, but the surface need -not be of the high order of excellence required for mirrors used for -telescopic work, since the first image is not, as a rule, intended to -be highly magnified. - -The mirror is held so that its surface makes an angle of about 33 -degrees with the axis of the lens, and the block carrying shutter and -mirror can be turned round into any position by slipping it round the -lens mount as an axis. The mirror thus always retains the correct angle. - -The action of the mirror is to a large extent due to mere diminution -of brightness, but it also partly extinguishes the blue light of the -sky without exerting any such influence on the white light from a -cloud. This is due to the fact that the blue light of the sky is partly -polarized, while that reflected from the cloud is not. Now, polarized -light which falls upon a black mirror held in a particular position is -not reflected by it. This position depends upon various circumstances, -but one condition is that the reflected ray must make an angle of about -33 degrees with the surface of the glass. The amount of the polarized -component of the blue light varies greatly, but is at a maximum at all -points 90 degrees away from the sun. This, then, is the best possible -position for photographing a cloud, as the whole of this polarized -component may be suppressed by adjusting the mirror to the proper -position, and then the most delicate cirrus fibres stand out brilliant -on an almost black background. - -The black mirror could with some advantage be replaced by a Nicol’s -prism mounted between the components of the lens, so that it could be -turned in any position; but Nicol’s prisms are expensive, and such an -arrangement would cost many times the sum sufficient for an excellent -mirror, and then would narrow down the field of view in a very -inconvenient way. - -With this apparatus exposures of a tenth to a fifth of a second were -usually required for high clouds in bright daylight, while longer -times, up to a second, might be required under less actively actinic -conditions. - -The exposure having been made, the next step is development. - -Now, every practical photographer has his own pet formula, his own -particular favourite among the numerous developing compounds now on -the market. It is, therefore, rather a thankless task to offer advice -as to which should be selected. In all probability as good results may -be got by other methods and other formulæ, and the description which -follows must be understood rather as an account of the process actually -adopted, than advice as to that which should be chosen. - -The developer used has been always pyro and ammonia, made up in -accordance with the formula-- - - Pyro 30 grains - Potassium metabisulphite 30 „ - Ammonium bromide 30 „ - Water 10 ozs. - -But if much work was anticipated the solution was made up in a more -concentrated form, and diluted to this strength of 3 grains of pyro per -ounce for actual use. - -The ammonia solution is prepared by mixing 3 drams ammonia fortiss. -with 20 ozs. of water. - -In developing it is necessary to remember that our object is to make -the most of a very small difference in effect. The plate is first -flowed over with a mixture of sufficient developer, with not more than -a quarter of its bulk of the ammonia. If the cloud should flash out in -a few seconds add more of the pyro solution, but unless the exposure -has been much overdone this will not happen. If the image begins to -appear after from thirty to forty seconds it is probable that the best -result will be reached by leaving it alone, but if there is any hanging -back of the detail another quarter bulk of ammonia should be put into -the glass, the developer mixed with it, and the whole returned to the -developing dish. - -If no image appears after about forty seconds, add more ammonia as -above described, and leave for another forty seconds, and so on, -until by this method of trial the right quantity of alkali for the -particular exposure has been ascertained. The development must never be -hurried, or the background of sky will blacken too soon, and in some -cases it may take a quarter of an hour or more to get enough density on -the cloud. But as a general rule the image is fully out in about two -minutes, and the plate is then washed and fixed in the usual way. - -If a black mirror is used there will seldom be any necessity for -intensification, but if not, it may frequently be required, especially -for the more delicate kinds of cirrus. Indeed, the image may sometimes -be so thin that the common process of intensification by mercury and -ammonia does not give density enough. If that seems at all likely to -be the case, it is wiser to use the formula known as Monckhoven’s, -since that simply adds silver to silver instead of replacing the silver -image by some other body, and the process can consequently be repeated -more than once, if sufficient density is not secured by the first -application. The formula does not seem to be very often used, so it may -be best to quote it. - - A. Potassium bromide 100 grains - Mercuric chloride 100 „ - Water 10 ozs. - - B. Potassium cyanide (pure) 100 grains - Silver nitrate 100 „ - Water 10 ozs. - -Place the washed negative in A until it has gone white, then rinse it -well and transfer to B, in which the image turns to a velvety black. -After washing, the process can be repeated. - -Intensification is, however, only a way of saving photographs which -cannot be secured again. If the first photograph of a particular -variety of cloud is not satisfactory, it ought at least to tell the -operator where he had gone wrong, and a second attempt should produce -a better result than any image built up by chemical action on an -imperfect base. - -There is nothing novel in any of these methods, and there is no doubt -that other formulæ would be as good; but the one thing essential is to -have a developer whose action can be held under control, and to apply -that developer in such a way that very considerable over-exposure -will not result in the ruin of the plate. If a number of photographs -have been taken in about the same part of the sky, and within a short -time of each other, then the correct proportions of developer and -alkali will be nearly the same for all, but the first of such a batch -will always have to be attacked in the cautious step-by-step method. -Patience and perseverance, backed by a steady refusal to be discouraged -by the failures which are at first inevitable, are as certain to be -crowned by success as they are in other studies. - -The workers are few, and there is much to be done; for it is mainly to -those who will photograph the higher clouds, and so trace the stages -of their growth and decay, that we must look for the data which will -enable us to solve the problems they present, and so enlarge the narrow -boundaries of our knowledge of some of the most beautiful things in -Nature. - - - - -REFERENCES - - - 1. “International Atlas of Clouds” (Atlas International des Nuages). - Hildebrandsson, Riggenbach, and Teisserenc de Bort. Paris. 1896. - - -This is the atlas referred to in the text. The letter-press is short, -and is repeated in English, French, and German. - - 2. “Annals of the Astronomical Observatory of Harvard College.” - Vol. XXX. Observations made at the Blue Hill Meteorological - Observatory. Part III. Measurement of Cloud Heights and - Velocities. By H. H. Clayton and S. P. Fergusson. Part IV. - Discussion of the Cloud Observations. By H. H. Clayton. - -This last gives a very concise account of all the different proposals -which have been made for the systematic naming of clouds. - - 3. “Études International des Nuages.” 1896-1897. Observations et - Mesures de la Suède. I., II. Publication de l’Observatoire - MĂ©tĂ©orologique de l’UniversitĂ© Roy. d’Upsala. H. H. Hildebrandsson. - -An account of the Upsala observations referred to in the text. - - 4. _Quarterly Journal of the Royal Meteorological Society_. - - Helm Wind. Marriott. 1886 and 1889. - - The Thickness of Shower Clouds. Clayden. 1886. - - Methods of Cloud Measurement. Ekholm. 1888. - - Cirrus Formation. Clayton. 1890. - - Nomenclature of Clouds. Hildebrandsson. 1887. - - „ „ Abercromby. 1887. - - „ „ Wilson-Barker. 1890. - - „ „ Gaster. 1893. - - „ „ Scott. 1895. - - A New Instrument for Cloud Measurement. Ekholm. 1893. - - Calculation of Photographic Cloud Measurements. Olsson. 1894. - - The Motion of Clouds. Shaw. 1895. - - 5. Reports of the British Association. Reports of the Committee on - Meteorological Photography. Clayden. 1891 to 1900. - -The reports for 1896 and 1900 refer mainly to the measurements -described in the text. - - 6. “Cloudland.” Clement Ley. - -The work in which Mr. Ley set forth his proposed scheme. - - 7. “A Popular Treatise on the Winds.” Ferrel. - -Not a “popular” work in the usual sense, but contains lucid -descriptions of the mechanics of the atmosphere. - - 8. There are many excellent text-books on meteorology, all of which - deal more or less with the movements of the atmosphere and the - formation of clouds. - - - - -INDEX - - - Abercromby, 10 - Aitken, 3, 91 - Altitude of clouds, 19, 149 - ---- of rain-clouds, 76 - Alto clouds, 59, 159 - Alto-cumulus communis, 161 - ---- castellatus, 66, 160 - ---- glomeratus, 67, 161 - ---- informis, 64, 160 - ---- nebulosus, 65, 160 - ---- stratiformis, 65, 161 - Alto-strato-cumulus, 70, 160 - Alto-stratus maculosus, 68, 160 - Ascent of vapour, 124 - Atlas, International, 11 - - - Band cirrus, 41 - Bidwell, Shelford, 92 - Black mirror, 14, 173 - Blue Hill, 20, 138 - - - Cameras, 140, 171 - Change cirrus, 37 - ---- of velocity, 35 - Cirriform top of thunder-cloud, 110 - Cirro-cumulus, 45, 159 - ---- nebulosus, 52, 159 - Cirro-filum, 33, 156 - Cirro-macula, 53, 159 - Cirro-nebula, 26, 27, 155 - Cirro-stratus, 45, 157 - ---- communis, 47, 158 - Cirro-stratus cumulosus, 48, 158 - ---- nebulosus, 45, 158 - ---- vittatus, 158 - Cirro-velum, 24 - Cirrus, 21, 155 - ---- altitudes, 30, 149 - ---- communis, 40, 157 - ---- caudatus, 34, 156 - ---- excelsus, 31, 156 - ---- inconstans, 37, 157 - ---- nebulosus, 36, 156 - ---- ripples, 51 - ---- ventosus, 32, 156 - ---- vittatus, 41, 156 - Cloud altitudes, 149 - ---- nuclei, 3 - ---- photography, 17, 165 - Condensation, 93 - Cooling by contact, 89 - ---- by expansion, 92 - ---- by mixture, 90 - ---- by radiation, 88 - - - Development, 175 - Differential currents, 133 - Diffraction, 63 - Dimensions of clouds, 112 - Dry fog, 73 - - - Equilibrium, stable and unstable, 106 - Exeter measurements, 137 - Exposure, 167 - - - Fall cloud, 81, 88 - False cirrus, 110 - Fog particles, 60 - Fracto, 164 - - - Gravitation, 4 - Great waves, 129 - - - Halos, 21, 22, 23 - Heat cumulus, 85 - ---- thunderstorms, 107 - Hildebrandsson, 10 - Hoar frost, 60 - Howard, Luke, 9, 10 - - - Intensification, 177 - Interconvection, 125 - International Code, 11 - ---- Committee, 10 - ---- System, 12, 71 - Ions as nuclei, 92 - Irregularities of ground, 39 - - - Kepler, 4 - Kew, 138 - - - Ley, Clement, 24, 34, 55 - Lightning, 114 - Lower clouds, 71 - - - Mammato-cumulus, 99 - Meteorological Conference, 9 - Methods of computing altitudes, 143 - Munich, 9 - - - Newton, 4 - Nimbus, 74, 163 - Nuclei, 92 - - - Photographic methods, 17, 165 - - - Rain-clouds, altitude, 76 - ----, thickness, 81 - Rate of fall of temperature, 94 - - - Sandstorms, 117 - Saturation, 87 - Scotch mist, 73 - Slow plates, 167 - Spread of condensation, 57 - Stable equilibrium, 106 - Strato-cumulus, 77 - Stratus, 72, 161 - ---- communis, 77, 161 - ---- lenticularis, 80, 98, 162 - ---- maculosus, 78, 162 - ---- radius, 79, 162 - Subsidence of cloud top, 111 - Surfusion, 61 - Swing stand, 171 - - - Turreted cloud, 66 - Tycho Brahe, 4 - Types, 8 - - - Umbra and penumbra, 29 - Undatus, 164 - Unsaturated cloud, 101 - Upsala, 10, 138 - - - Wave clouds, 47, 119, 133 - Wilson, C. T. R., 92 - - -THE END - - -PRINTED BY WILLIAM CLOWES AND SONS, LIMITED, LONDON AND BECCLES. - - - - - * * * * * - - -Transcriber’s Note - -Variations in hyphenation (i.e. thunderstorm and thunder-storm) have -been retained. The following apparent typographical errors were -corrected: - -Page 171, “focussed” changed to “focused.” (it should be focused on the -horizon) - -Page 173, “aperature” changed to “aperture.” (the lens with its large -aperture) - - - - - - -End of the Project Gutenberg EBook of Cloud Studies, by Arthur W. Clayden - -*** END OF THIS PROJECT GUTENBERG EBOOK CLOUD STUDIES *** - -***** This file should be named 55126-0.txt or 55126-0.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/5/5/1/2/55126/ - -Produced by Cindy Horton, deaurider, and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. Special rules, set forth in the General Terms of Use part -of this license, apply to copying and distributing Project -Gutenberg-tm electronic works to protect the PROJECT GUTENBERG-tm -concept and trademark. Project Gutenberg is a registered trademark, -and may not be used if you charge for the eBooks, unless you receive -specific permission. If you do not charge anything for copies of this -eBook, complying with the rules is very easy. You may use this eBook -for nearly any purpose such as creation of derivative works, reports, -performances and research. They may be modified and printed and given -away--you may do practically ANYTHING in the United States with eBooks -not protected by U.S. copyright law. Redistribution is subject to the -trademark license, especially commercial redistribution. - -START: FULL LICENSE - -THE FULL PROJECT GUTENBERG LICENSE -PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK - -To protect the Project Gutenberg-tm mission of promoting the free -distribution of electronic works, by using or distributing this work -(or any other work associated in any way with the phrase "Project -Gutenberg"), you agree to comply with all the terms of the Full -Project Gutenberg-tm License available with this file or online at -www.gutenberg.org/license. - -Section 1. General Terms of Use and Redistributing Project -Gutenberg-tm electronic works - -1.A. By reading or using any part of this Project Gutenberg-tm -electronic work, you indicate that you have read, understand, agree to -and accept all the terms of this license and intellectual property -(trademark/copyright) agreement. If you do not agree to abide by all -the terms of this agreement, you must cease using and return or -destroy all copies of Project Gutenberg-tm electronic works in your -possession. If you paid a fee for obtaining a copy of or access to a -Project Gutenberg-tm electronic work and you do not agree to be bound -by the terms of this agreement, you may obtain a refund from the -person or entity to whom you paid the fee as set forth in paragraph -1.E.8. - -1.B. "Project Gutenberg" is a registered trademark. It may only be -used on or associated in any way with an electronic work by people who -agree to be bound by the terms of this agreement. There are a few -things that you can do with most Project Gutenberg-tm electronic works -even without complying with the full terms of this agreement. See -paragraph 1.C below. There are a lot of things you can do with Project -Gutenberg-tm electronic works if you follow the terms of this -agreement and help preserve free future access to Project Gutenberg-tm -electronic works. See paragraph 1.E below. - -1.C. The Project Gutenberg Literary Archive Foundation ("the -Foundation" or PGLAF), owns a compilation copyright in the collection -of Project Gutenberg-tm electronic works. Nearly all the individual -works in the collection are in the public domain in the United -States. If an individual work is unprotected by copyright law in the -United States and you are located in the United States, we do not -claim a right to prevent you from copying, distributing, performing, -displaying or creating derivative works based on the work as long as -all references to Project Gutenberg are removed. Of course, we hope -that you will support the Project Gutenberg-tm mission of promoting -free access to electronic works by freely sharing Project Gutenberg-tm -works in compliance with the terms of this agreement for keeping the -Project Gutenberg-tm name associated with the work. You can easily -comply with the terms of this agreement by keeping this work in the -same format with its attached full Project Gutenberg-tm License when -you share it without charge with others. - -1.D. The copyright laws of the place where you are located also govern -what you can do with this work. Copyright laws in most countries are -in a constant state of change. If you are outside the United States, -check the laws of your country in addition to the terms of this -agreement before downloading, copying, displaying, performing, -distributing or creating derivative works based on this work or any -other Project Gutenberg-tm work. The Foundation makes no -representations concerning the copyright status of any work in any -country outside the United States. - -1.E. Unless you have removed all references to Project Gutenberg: - -1.E.1. The following sentence, with active links to, or other -immediate access to, the full Project Gutenberg-tm License must appear -prominently whenever any copy of a Project Gutenberg-tm work (any work -on which the phrase "Project Gutenberg" appears, or with which the -phrase "Project Gutenberg" is associated) is accessed, displayed, -performed, viewed, copied or distributed: - - This eBook is for the use of anyone anywhere in the United States and - most other parts of the world at no cost and with almost no - restrictions whatsoever. You may copy it, give it away or re-use it - under the terms of the Project Gutenberg License included with this - eBook or online at www.gutenberg.org. If you are not located in the - United States, you'll have to check the laws of the country where you - are located before using this ebook. - -1.E.2. If an individual Project Gutenberg-tm electronic work is -derived from texts not protected by U.S. copyright law (does not -contain a notice indicating that it is posted with permission of the -copyright holder), the work can be copied and distributed to anyone in -the United States without paying any fees or charges. If you are -redistributing or providing access to a work with the phrase "Project -Gutenberg" associated with or appearing on the work, you must comply -either with the requirements of paragraphs 1.E.1 through 1.E.7 or -obtain permission for the use of the work and the Project Gutenberg-tm -trademark as set forth in paragraphs 1.E.8 or 1.E.9. - -1.E.3. If an individual Project Gutenberg-tm electronic work is posted -with the permission of the copyright holder, your use and distribution -must comply with both paragraphs 1.E.1 through 1.E.7 and any -additional terms imposed by the copyright holder. Additional terms -will be linked to the Project Gutenberg-tm License for all works -posted with the permission of the copyright holder found at the -beginning of this work. - -1.E.4. Do not unlink or detach or remove the full Project Gutenberg-tm -License terms from this work, or any files containing a part of this -work or any other work associated with Project Gutenberg-tm. - -1.E.5. Do not copy, display, perform, distribute or redistribute this -electronic work, or any part of this electronic work, without -prominently displaying the sentence set forth in paragraph 1.E.1 with -active links or immediate access to the full terms of the Project -Gutenberg-tm License. - -1.E.6. You may convert to and distribute this work in any binary, -compressed, marked up, nonproprietary or proprietary form, including -any word processing or hypertext form. However, if you provide access -to or distribute copies of a Project Gutenberg-tm work in a format -other than "Plain Vanilla ASCII" or other format used in the official -version posted on the official Project Gutenberg-tm web site -(www.gutenberg.org), you must, at no additional cost, fee or expense -to the user, provide a copy, a means of exporting a copy, or a means -of obtaining a copy upon request, of the work in its original "Plain -Vanilla ASCII" or other form. Any alternate format must include the -full Project Gutenberg-tm License as specified in paragraph 1.E.1. - -1.E.7. Do not charge a fee for access to, viewing, displaying, -performing, copying or distributing any Project Gutenberg-tm works -unless you comply with paragraph 1.E.8 or 1.E.9. - -1.E.8. You may charge a reasonable fee for copies of or providing -access to or distributing Project Gutenberg-tm electronic works -provided that - -* You pay a royalty fee of 20% of the gross profits you derive from - the use of Project Gutenberg-tm works calculated using the method - you already use to calculate your applicable taxes. The fee is owed - to the owner of the Project Gutenberg-tm trademark, but he has - agreed to donate royalties under this paragraph to the Project - Gutenberg Literary Archive Foundation. Royalty payments must be paid - within 60 days following each date on which you prepare (or are - legally required to prepare) your periodic tax returns. Royalty - payments should be clearly marked as such and sent to the Project - Gutenberg Literary Archive Foundation at the address specified in - Section 4, "Information about donations to the Project Gutenberg - Literary Archive Foundation." - -* You provide a full refund of any money paid by a user who notifies - you in writing (or by e-mail) within 30 days of receipt that s/he - does not agree to the terms of the full Project Gutenberg-tm - License. You must require such a user to return or destroy all - copies of the works possessed in a physical medium and discontinue - all use of and all access to other copies of Project Gutenberg-tm - works. - -* You provide, in accordance with paragraph 1.F.3, a full refund of - any money paid for a work or a replacement copy, if a defect in the - electronic work is discovered and reported to you within 90 days of - receipt of the work. - -* You comply with all other terms of this agreement for free - distribution of Project Gutenberg-tm works. - -1.E.9. If you wish to charge a fee or distribute a Project -Gutenberg-tm electronic work or group of works on different terms than -are set forth in this agreement, you must obtain permission in writing -from both the Project Gutenberg Literary Archive Foundation and The -Project Gutenberg Trademark LLC, the owner of the Project Gutenberg-tm -trademark. Contact the Foundation as set forth in Section 3 below. - -1.F. - -1.F.1. Project Gutenberg volunteers and employees expend considerable -effort to identify, do copyright research on, transcribe and proofread -works not protected by U.S. copyright law in creating the Project -Gutenberg-tm collection. Despite these efforts, Project Gutenberg-tm -electronic works, and the medium on which they may be stored, may -contain "Defects," such as, but not limited to, incomplete, inaccurate -or corrupt data, transcription errors, a copyright or other -intellectual property infringement, a defective or damaged disk or -other medium, a computer virus, or computer codes that damage or -cannot be read by your equipment. - -1.F.2. LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the "Right -of Replacement or Refund" described in paragraph 1.F.3, the Project -Gutenberg Literary Archive Foundation, the owner of the Project -Gutenberg-tm trademark, and any other party distributing a Project -Gutenberg-tm electronic work under this agreement, disclaim all -liability to you for damages, costs and expenses, including legal -fees. YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT -LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE -PROVIDED IN PARAGRAPH 1.F.3. YOU AGREE THAT THE FOUNDATION, THE -TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE -LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR -INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH -DAMAGE. - -1.F.3. LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a -defect in this electronic work within 90 days of receiving it, you can -receive a refund of the money (if any) you paid for it by sending a -written explanation to the person you received the work from. If you -received the work on a physical medium, you must return the medium -with your written explanation. The person or entity that provided you -with the defective work may elect to provide a replacement copy in -lieu of a refund. If you received the work electronically, the person -or entity providing it to you may choose to give you a second -opportunity to receive the work electronically in lieu of a refund. If -the second copy is also defective, you may demand a refund in writing -without further opportunities to fix the problem. - -1.F.4. Except for the limited right of replacement or refund set forth -in paragraph 1.F.3, this work is provided to you 'AS-IS', WITH NO -OTHER WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT -LIMITED TO WARRANTIES OF MERCHANTABILITY OR FITNESS FOR ANY PURPOSE. - -1.F.5. Some states do not allow disclaimers of certain implied -warranties or the exclusion or limitation of certain types of -damages. If any disclaimer or limitation set forth in this agreement -violates the law of the state applicable to this agreement, the -agreement shall be interpreted to make the maximum disclaimer or -limitation permitted by the applicable state law. The invalidity or -unenforceability of any provision of this agreement shall not void the -remaining provisions. - -1.F.6. INDEMNITY - You agree to indemnify and hold the Foundation, the -trademark owner, any agent or employee of the Foundation, anyone -providing copies of Project Gutenberg-tm electronic works in -accordance with this agreement, and any volunteers associated with the -production, promotion and distribution of Project Gutenberg-tm -electronic works, harmless from all liability, costs and expenses, -including legal fees, that arise directly or indirectly from any of -the following which you do or cause to occur: (a) distribution of this -or any Project Gutenberg-tm work, (b) alteration, modification, or -additions or deletions to any Project Gutenberg-tm work, and (c) any -Defect you cause. - -Section 2. Information about the Mission of Project Gutenberg-tm - -Project Gutenberg-tm is synonymous with the free distribution of -electronic works in formats readable by the widest variety of -computers including obsolete, old, middle-aged and new computers. It -exists because of the efforts of hundreds of volunteers and donations -from people in all walks of life. - -Volunteers and financial support to provide volunteers with the -assistance they need are critical to reaching Project Gutenberg-tm's -goals and ensuring that the Project Gutenberg-tm collection will -remain freely available for generations to come. In 2001, the Project -Gutenberg Literary Archive Foundation was created to provide a secure -and permanent future for Project Gutenberg-tm and future -generations. To learn more about the Project Gutenberg Literary -Archive Foundation and how your efforts and donations can help, see -Sections 3 and 4 and the Foundation information page at -www.gutenberg.org - - - -Section 3. Information about the Project Gutenberg Literary Archive Foundation - -The Project Gutenberg Literary Archive Foundation is a non profit -501(c)(3) educational corporation organized under the laws of the -state of Mississippi and granted tax exempt status by the Internal -Revenue Service. The Foundation's EIN or federal tax identification -number is 64-6221541. Contributions to the Project Gutenberg Literary -Archive Foundation are tax deductible to the full extent permitted by -U.S. federal laws and your state's laws. - -The Foundation's principal office is in Fairbanks, Alaska, with the -mailing address: PO Box 750175, Fairbanks, AK 99775, but its -volunteers and employees are scattered throughout numerous -locations. Its business office is located at 809 North 1500 West, Salt -Lake City, UT 84116, (801) 596-1887. Email contact links and up to -date contact information can be found at the Foundation's web site and -official page at www.gutenberg.org/contact - -For additional contact information: - - Dr. Gregory B. Newby - Chief Executive and Director - gbnewby@pglaf.org - -Section 4. Information about Donations to the Project Gutenberg -Literary Archive Foundation - -Project Gutenberg-tm depends upon and cannot survive without wide -spread public support and donations to carry out its mission of -increasing the number of public domain and licensed works that can be -freely distributed in machine readable form accessible by the widest -array of equipment including outdated equipment. Many small donations -($1 to $5,000) are particularly important to maintaining tax exempt -status with the IRS. - -The Foundation is committed to complying with the laws regulating -charities and charitable donations in all 50 states of the United -States. Compliance requirements are not uniform and it takes a -considerable effort, much paperwork and many fees to meet and keep up -with these requirements. We do not solicit donations in locations -where we have not received written confirmation of compliance. To SEND -DONATIONS or determine the status of compliance for any particular -state visit www.gutenberg.org/donate - -While we cannot and do not solicit contributions from states where we -have not met the solicitation requirements, we know of no prohibition -against accepting unsolicited donations from donors in such states who -approach us with offers to donate. - -International donations are gratefully accepted, but we cannot make -any statements concerning tax treatment of donations received from -outside the United States. U.S. laws alone swamp our small staff. - -Please check the Project Gutenberg Web pages for current donation -methods and addresses. Donations are accepted in a number of other -ways including checks, online payments and credit card donations. To -donate, please visit: www.gutenberg.org/donate - -Section 5. General Information About Project Gutenberg-tm electronic works. - -Professor Michael S. Hart was the originator of the Project -Gutenberg-tm concept of a library of electronic works that could be -freely shared with anyone. For forty years, he produced and -distributed Project Gutenberg-tm eBooks with only a loose network of -volunteer support. - -Project Gutenberg-tm eBooks are often created from several printed -editions, all of which are confirmed as not protected by copyright in -the U.S. unless a copyright notice is included. Thus, we do not -necessarily keep eBooks in compliance with any particular paper -edition. - -Most people start at our Web site which has the main PG search -facility: www.gutenberg.org - -This Web site includes information about Project Gutenberg-tm, -including how to make donations to the Project Gutenberg Literary -Archive Foundation, how to help produce our new eBooks, and how to -subscribe to our email newsletter to hear about new eBooks. - diff --git a/old/55126-0.zip b/old/55126-0.zip Binary files differdeleted file mode 100644 index b1a8467..0000000 --- a/old/55126-0.zip +++ /dev/null diff --git a/old/55126-h.zip b/old/55126-h.zip Binary files differdeleted file mode 100644 index e817c81..0000000 --- a/old/55126-h.zip +++ /dev/null diff --git a/old/55126-h/55126-h.htm b/old/55126-h/55126-h.htm deleted file mode 100644 index 2881cf4..0000000 --- a/old/55126-h/55126-h.htm +++ /dev/null @@ -1,6771 +0,0 @@ -<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" - "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> -<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> - <head> - <meta http-equiv="Content-Type" content="text/html;charset=us-ascii" /> - <meta http-equiv="Content-Style-Type" content="text/css" /> - <title> - Cloud Studies, by Arthur W. Clayden—a Project Gutenberg eBook. - </title> - <link rel="coverpage" href="images/cover.jpg" /> - <style type="text/css"> - -body { - margin-left: 10%; - margin-right: 10%; -} - - h1,h2,h3 { - text-align: center; /* all headings centered */ - clear: both; -} - - h3 {font-weight: normal;} - -p { - margin-top: .51em; - text-align: justify; - text-indent: 1em; - margin-bottom: .49em; -} - -.noindent {text-indent: 0em;} -.inset {text-indent: 0em; margin-left: 3em;} - -.hang p { - margin-left: 2em; - text-indent: -2em; -} - -.hang2 p { - margin-left: 3em; - text-indent: -1em; -} - -.ph1, .ph3 { text-align: center; text-indent: 0em; font-weight: bold; } -.ph1 { font-size: xx-large; margin: .67em auto; } -.ph3 { font-size: large; margin: .83em auto; } - -.chtitle { - font-size: 60%; - font-weight: normal; -} - -.p2 {margin-top: 2em;} -.p6 {margin-top: 6em;} - -#titlepage, #half-title, #verso { - margin-top: 6em; - text-align: center; -} - -div#titlepage p { - text-align: center; - text-indent: 0em; - line-height: 1.5em; - margin-top: 3em; -} - -#half-title {font-size: xx-large; margin-bottom: 3em;} - -#half-title p {text-align: center; text-indent: 0em;} - -#verso p {text-align: center; text-indent: 0em;} - -@media print, handheld -{ - #titlepage, #half-title, #verso - { - page-break-before: always; - page-break-after: always; - } -} - -p#printedby { - margin-top: 4em; - margin-left: 10%; - margin-right: 10%; - font-size: 70%; - border-top: solid 1px; - text-align: center; -} - -.chapter {margin-top: 6em;} - -table { - margin-left: auto; - margin-right: auto; -} - - .tdl {text-align: left;} - .tdr {text-align: right;} - .tdc {text-align: center;} - -.ml1 {margin-left: 1em;} -.ml15 {margin-left: 1.5em;} -.ml25 {margin-left: 2.5em;} -.ml35 {margin-left: 3.5em;} -.ml5 {margin-left: 5em;} - -.pr05 {padding-right: .5em;} -.pr1 {padding-right: 1em;} - -.center-block {text-align: center;} - -.block {text-align: left; display: inline-block;} - -.block p {margin-top: 0em; margin-bottom: 0em;} - -.pagenum { /* uncomment the next line for invisible page numbers */ - /* visibility: hidden; */ - position: absolute; - left: 92%; - font-size: smaller; - text-align: right; -} /* page numbers */ - -.bb {border-bottom: solid 1px;} - -.bt {border-top: solid 1px;} - -.br {border-right: solid 1px;} - -.center {text-align: center;} - -.smcap {font-variant: small-caps;} - -.gesperrt -{ - letter-spacing: 0.2em; - margin-right: -0.2em; -} - -em.gesperrt -{ - font-style: normal; -} - -.f80 {font-size: 80%;} - -.signature {text-align: right; margin-right: 2em;} - -/* Images */ -.caption p {text-align: center;} - -.capright {margin-left: 10em;} - -img {max-width: 100%; height: auto; width: auto;} - -.figcenter { - margin: 3em auto 3em auto; - text-align: center; -} - -.screenonly { display: block; } -.handonly { display: none; } - -@media handheld { - .screenonly { display: none; } - .handonly { display: block; } - -.figcenter {page-break-before: always; page-break-after: always;} -} - -/* Footnotes */ -.footnotes {border: dashed 1px;} - -.footnote {margin-left: 10%; margin-right: 10%; font-size: 0.9em;} - -.footnote .label {position: absolute; right: 84%; text-align: right;} - -.fnanchor { - vertical-align: super; - font-size: .8em; - text-decoration: - none; -} - -/* Transcriber's notes */ -.tnotes { - background-color: #eeeeee; - border: 1px solid black; - padding: 1em; -} - -@media handheld { - - .poetry - { - display: block; - margin-left: 1.5em; - } -} - </style> - </head> -<body> - - -<pre> - -The Project Gutenberg EBook of Cloud Studies, by Arthur W. Clayden - -This eBook is for the use of anyone anywhere in the United States and most -other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms of -the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - -Title: Cloud Studies - -Author: Arthur W. Clayden - -Release Date: July 16, 2017 [EBook #55126] - -Language: English - -Character set encoding: ASCII - -*** START OF THIS PROJECT GUTENBERG EBOOK CLOUD STUDIES *** - - - - -Produced by Cindy Horton, deaurider, and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - - - - -</pre> - - -<div class="tnotes screenonly"> - -<p class="ph3">Transcriber's Note</p> - -<p>Larger versions of each image can be viewed by clicking the -photographs. Your browser and device may or may not support this -feature.</p> - -</div> - -<div class="figcenter" style="width: 554px;"> -<img src="images/cover.jpg" alt="book cover" /> -</div> - -<p> - <span class="pagenum"> - <a name="Page_i" id="Page_i">[i]</a> - </span><br /> - <span class="pagenum"> - <a name="Page_ii" id="Page_ii">[ii]</a> - </span> -</p> - -<div id="half-title"> - -<p>CLOUD STUDIES</p> - -</div> - -<div class="figcenter screenonly" style="width: 260px;"> - <a href="images/i_f002b.jpg"> - <img src="images/i_f002b-tn.jpg" alt="" /> - </a> - <div class="caption"> - <p>A SUNSET SKY.</p> - <p class="capright"><i>Frontispiece.</i></p> - </div> -</div> - -<div class="figcenter handonly" style="width: 260px;"> - <img src="images/i_f002b-hh.jpg" alt="" /> - <div class="caption"> - <p>A SUNSET SKY.</p> - <p><i>Frontispiece.</i></p> - </div> -</div> - -<div class="titlepage"> - -<p><span class="pagenum"><a name="Page_iii" id="Page_iii">[iii]</a></span></p> - -<h1>CLOUD STUDIES</h1> - -<p class="ph3 p2"><span class="smcap">By</span> ARTHUR W. CLAYDEN, M.A.</p> - -<p class="center f80">PRINCIPAL OF THE<br /> -ROYAL ALBERT MEMORIAL COLLEGE, EXETER</p> - -<p class="ph3 p6">LONDON<br /> -JOHN MURRAY, ALBEMARLE STREET<br /> -1905</p> - -</div> - -<p><span class="pagenum"><a name="Page_iv" id="Page_iv">[iv]</a></span></p> - -<div id="verso"> - -<p class="f80">PRINTED BY<br /> -WILLIAM CLOWES AND SONS, LIMITED,<br /> -LONDON AND BECCLES.</p> - -</div> - -<div class="chapter"></div> - -<p><span class="pagenum"><a name="Page_v" id="Page_v">[v]</a></span></p> - -<h2>PREFACE</h2> - -<p class="noindent"><span class="smcap">To</span> the meteorologist -I hope the following pages may prove not only of some interest, but -of practical value as a small step towards that greater exactness -of language which is essential before we can attempt to explain all -the details of cloud structure, or even interchange our ideas and -observations with adequate precision. The varieties depicted and -described have been selected from many hundreds, as those which seem to -me to show such differences of form as to imply distinct differences -in the conditions to which they are due. I have not attempted to deal -with the physical causes of condensation except in a general way, -being unwilling to introduce diagrams of isothermals and adiabatics -and such purely scientific methods into a work also intended for a -wider public. For those who wish to pursue this part of the subject -I have appended a list of papers from the<span class="pagenum"><a -name="Page_vi" id="Page_vi">[vi]</a></span> <cite>Quarterly Journal of -the Royal Meteorological Society</cite> and other sources, which may -serve as references. I also hope that some more votaries of the science -may be induced to realize that meteorology does not consist solely of -the tabulation of long columns of records, but includes subjects for -investigation as much more beautiful as they are more difficult.</p> - -<p>To the artist I trust they may also be of some use, by calling -attention to the variety and exquisite beauty of the sky. Nothing is -more extraordinary in art than the general negligence of cloud-forms. -Many of them are quite as worthy of careful drawing as the leaves of a -tree, the flowers of a field, the ripples on a stream, or the texture -of a carpet, or a marble pavement. Yet it is the common rule to find -pictures, which are otherwise marvellous examples of skill and care, -disfigured by impossible skies with vague, shapeless clouds, as untrue -to nature as it would be possible to make them. Grace of outline, -delicacy of detail and texture, richness of contrast, beauty of form -and light and colour, all are present in the skies, and combine to make -a whole well worthy of the best that art can give. The illustrations -I offer are not selected for<span class="pagenum"><a name="Page_vii" -id="Page_vii">[vii]</a></span> pictorial effect; they are chosen from a -purely scientific point of view; but they are enough to indicate what -could be done if the facts of nature were treated with high artistic -skill.</p> - -<p>In addition to the meteorologist and the artist, there are a much -larger number who follow neither profession, but who love Nature in -all her moods; and to them also I hope these pages may be of interest. -Indeed, if only a few of them should be stimulated to take up a branch -of nature study which has given me many an hour of quiet enjoyment, the -labour of bringing these notes together will not have been in vain.</p> - -<p class="signature">ARTHUR W. CLAYDEN.</p> - -<p class="f80"><span class="smcap">St. John’s,<br /> -<span style="margin-left: 2em">Exeter.</span></span></p> - -<div class="chapter"></div> - -<p> - <span class="pagenum"> - <a name="Page_viii" id="Page_viii">[viii]</a> - </span><br /> - <span class="pagenum"> - <a name="Page_ix" id="Page_ix">[ix]</a> - </span> -</p> - -<h2>CONTENTS</h2> - -<div class="center"> -<table border="0" cellpadding="4" cellspacing="0" summary="Contents"> -<tr> - <td class="tdl f80" colspan="2">CHAPTER</td> - <td class="tdr f80">PAGE</td> -</tr> - -<tr> - <td class="tdr">I.</td> - <td class="tdl"><span class="smcap">Introductory</span></td> - <td class="tdr"><a href="#Page_1">1</a></td> -</tr> - -<tr> - <td class="tdr">II.</td> - <td class="tdl"><span class="smcap">Cirrus</span></td> - <td class="tdr"><a href="#Page_21">21</a></td> -</tr> - -<tr> - <td class="tdr">III.</td> - <td class="tdl"><span class="smcap">Cirro-stratus and Cirro-cumulus</span></td> - <td class="tdr"><a href="#Page_45">45</a></td> -</tr> - -<tr> - <td class="tdr">IV.</td> - <td class="tdl"><span class="smcap">Alto Clouds</span></td> - <td class="tdr"><a href="#Page_59">59</a></td> -</tr> - -<tr> - <td class="tdr">V.</td> - <td class="tdl"><span class="smcap">Lower Clouds</span></td> - <td class="tdr"><a href="#Page_71">71</a></td> -</tr> - -<tr> - <td class="tdr">VI.</td> - <td class="tdl"><span class="smcap">Cumulus</span></td> - <td class="tdr"><a href="#Page_84">84</a></td> -</tr> - -<tr> - <td class="tdr">VII.</td> - <td class="tdl"><span class="smcap">Cumulo-nimbus</span></td> - <td class="tdr"><a href="#Page_105">105</a></td> -</tr> - -<tr> - <td class="tdr">VIII.</td> - <td class="tdl"><span class="smcap">Wave Clouds</span></td> - <td class="tdr"><a href="#Page_119">119</a></td> -</tr> - -<tr> - <td class="tdr">IX.</td> - <td class="tdl"><span class="smcap">Cloud Altitudes</span></td> - <td class="tdr"><a href="#Page_137">137</a></td> -</tr> - -<tr> - <td class="tdr">X.</td> - <td class="tdl"><span class="smcap">Cloud Nomenclature</span></td> - <td class="tdr"><a href="#Page_154">154</a></td> -</tr> - -<tr> - <td class="tdr">XI.</td> - <td class="tdl"><span class="smcap">Cloud Photography</span></td> - <td class="tdr"><a href="#Page_165">165</a></td> -</tr> - -<tr> - <td class="tdr"></td> - <td class="tdl"><span class="smcap">References</span></td> - <td class="tdr"><a href="#Page_181">181</a></td> -</tr> - -<tr> - <td class="tdr"></td> - <td class="tdl"><span class="smcap">Index</span></td> - <td class="tdr"><a href="#Page_183">183</a></td> -</tr> -</table></div> - -<div class="chapter"></div> - -<p> - <span class="pagenum"> - <a name="Page_x" id="Page_x">[x]</a> - </span><br /> - <span class="pagenum"> - <a name="Page_xi" id="Page_xi">[xi]</a> - </span> -</p> - -<h2>LIST OF ILLUSTRATIONS</h2> - -<div class="center"> -<table border="0" cellpadding="4" cellspacing="0" summary="List of Illustrations"> - -<tr> - <td class="tdl f80" colspan="2">PLATE</td> - <td class="tdr f80">PAGE</td> -</tr> - -<tr> - <td class="tdr"></td> - <td class="tdl"><span class="smcap">A Sunset Sky</span></td> - <td class="tdr" style="padding-right: 2em"><a href="#Page_ii"><i>Frontispiece</i></a></td> -</tr> - -<tr> - <td class="tdr">1.</td> - <td class="tdl"><span class="smcap">Part of a Great Halo</span></td> - <td class="tdr"><a href="#Page_22">22</a></td> -</tr> - -<tr> - <td class="tdr">2.</td> - <td class="tdl"><span class="smcap">Part of a Solar Halo</span></td> - <td class="tdr"><a href="#Page_23">23</a></td> -</tr> - -<tr> - <td class="tdr">3.</td> - <td class="tdl"><span class="smcap">Cirro-nebula changing to Cirro-stratus</span></td> - <td class="tdr"><a href="#Page_24">24</a></td> -</tr> - -<tr> - <td class="tdr">4.</td> - <td class="tdl"><span class="smcap">Cirro-nebula changing to Cirro-cumulus</span></td> - <td class="tdr"><a href="#Page_27">27</a></td> -</tr> - -<tr> - <td class="tdr">5.</td> - <td class="tdl"><span class="smcap">High Cirrus.</span> (<i lang="la" xml:lang="la">Cirrus Excelsus</i>)</td> - <td class="tdr"><a href="#Page_31">31</a></td> -</tr> - -<tr> - <td class="tdr">6.</td> - <td class="tdl"><span class="smcap">Windy Cirrus.</span> (<i lang="la" xml:lang="la">Cirrus Ventosus</i>)</td> - <td class="tdr"><a href="#Page_32">32</a></td> -</tr> - -<tr> - <td class="tdr">7.</td> - <td class="tdl"><span class="smcap">Thread Cirrus.</span> (<i lang="la" xml:lang="la">Cirro-filum</i>)</td> - <td class="tdr"><a href="#Page_34">34</a></td> -</tr> - -<tr> - <td class="tdr">8.</td> - <td class="tdl"><span class="smcap">Tailed Cirrus.</span> (<i lang="la" xml:lang="la">Cirrus Caudatus</i>)</td> - <td class="tdr"><a href="#Page_35">35</a></td> -</tr> - -<tr> - <td class="tdr">9.</td> - <td class="tdl"><span class="smcap">Hazy Cirrus.</span> (<i lang="la" xml:lang="la">Cirrus Nebulosus</i>)</td> - <td class="tdr"><a href="#Page_36">36</a></td> -</tr> - -<tr> - <td class="tdr">10.</td> - <td class="tdl"><span class="smcap">Change Cirrus.</span> (<i lang="la" xml:lang="la">Cirrus Inconstans</i>)</td> - <td class="tdr"><a href="#Page_37">37</a></td> -</tr> - -<tr> - <td class="tdr">11.</td> - <td class="tdl"><span class="smcap">Common Cirrus.</span> (<i lang="la" xml:lang="la">Cirrus Communis</i>)</td> - <td class="tdr"><a href="#Page_40">40</a></td> -</tr> - -<tr> - <td class="tdr">12.</td> - <td class="tdl"><span class="smcap">Band Cirrus.</span> (<i lang="la" xml:lang="la">Cirrus Vittatus</i>)</td> - <td class="tdr"><a href="#Page_41">41</a></td> -</tr> - -<tr> - <td class="tdr">13.</td> - <td class="tdl"><span class="smcap">Band Cirrus.</span> (<i lang="la" xml:lang="la">Cirrus Vittatus</i>)</td> - <td class="tdr"><a href="#Page_42">42</a></td> -</tr> - -<tr> - <td class="tdr">14.</td> - <td class="tdl"><span class="smcap">Hazy Cirro-stratus.</span> (<i lang="la" xml:lang="la">Cirro-stratus Nebulosus</i>)</td> - <td class="tdr"><a href="#Page_46">46</a></td> -</tr> - -<tr> - <td class="tdr">15.</td> - <td class="tdl"><span class="smcap">Cirro-stratus</span></td> - <td class="tdr"><a href="#Page_47">47</a></td> -</tr> - -<tr> - <td class="tdr">16.</td> - <td class="tdl"><span class="smcap">Cirro-stratus.</span> (<i lang="la" xml:lang="la">Cirro-stratus Communis</i>)</td> - <td class="tdr"><a href="#Page_48">48</a></td> -</tr> - -<tr> - <td class="tdr">17.</td> - <td class="tdl"><span class="smcap">Flocculent Cirro-stratus.</span> (<i lang="la" xml:lang="la">Cirro-stratus Cumulosus</i>)</td> - <td class="tdr"><a href="#Page_49">49</a></td> -</tr> - -<tr> - <td class="tdr"><span class="pagenum"><a name="Page_xii" id="Page_xii">[xii]</a></span>18.</td> - <td class="tdl"><span class="smcap">Cirro-stratus and Cirro-cumulus</span></td> - <td class="tdr"><a href="#Page_50">50</a></td> -</tr> - -<tr> - <td class="tdr">19.</td> - <td class="tdl"><span class="smcap">Cirro-cumulus</span></td> - <td class="tdr"><a href="#Page_50">50</a></td> -</tr> - -<tr> - <td class="tdr">20.</td> - <td class="tdl"><span class="smcap">Hazy Cirro-cumulus.</span> (<i lang="la" xml:lang="la">Cirro-cumulus Nebulosus</i>)</td> - <td class="tdr"><a href="#Page_51">51</a></td> -</tr> - -<tr> - <td class="tdr">21.</td> - <td class="tdl"><span class="smcap">Hazy Cirro-cumulus.</span> (<i lang="la" xml:lang="la">Cirro-cumulus Nebulosus</i>)</td> - <td class="tdr"><a href="#Page_51">51</a></td> -</tr> - -<tr> - <td class="tdr">22.</td> - <td class="tdl"><span class="smcap">A Sunset Sky</span></td> - <td class="tdr"><a href="#Page_52">52</a></td> -</tr> - -<tr> - <td class="tdr">23.</td> - <td class="tdl"><span class="smcap">Speckle Cloud (Ley).</span> (<i lang="la" xml:lang="la">Cirro-macula</i>)</td> - <td class="tdr"><a href="#Page_53">53</a></td> -</tr> - -<tr> - <td class="tdr">24.</td> - <td class="tdl"><span class="smcap">Cirrus Caudatus and Cirro-macula</span></td> - <td class="tdr"><a href="#Page_55">55</a></td> -</tr> - -<tr> - <td class="tdr">25.</td> - <td class="tdl"><span class="smcap">Alto-cumulus Informis</span></td> - <td class="tdr"><a href="#Page_64">64</a></td> -</tr> - -<tr> - <td class="tdr">26.</td> - <td class="tdl"><span class="smcap">Hazy Alto-cumulus.</span> (<i lang="la" xml:lang="la">Alto-cumulus Nebulosus</i>)</td> - <td class="tdr"><a href="#Page_65">65</a></td> -</tr> - -<tr> - <td class="tdr">27.</td> - <td class="tdl"><span class="smcap">Flat Alto-cumulus.</span> (<i lang="la" xml:lang="la">Alto-cumulus Stratiformis</i>)</td> - <td class="tdr"><a href="#Page_65">65</a></td> -</tr> - -<tr> - <td class="tdr">28.</td> - <td class="tdl"><span class="smcap">High Turreted Cloud.</span> (<i lang="la" xml:lang="la">Alto-cumulus Castellatus</i>)</td> - <td class="tdr"><a href="#Page_66">66</a></td> -</tr> - -<tr> - <td class="tdr">29.</td> - <td class="tdl"><span class="smcap">High Ball Cumulus.</span> (<i lang="la" xml:lang="la">Alto-cumulus Glomeratus</i>)</td> - <td class="tdr"><a href="#Page_67">67</a></td> -</tr> - -<tr> - <td class="tdr">30.</td> - <td class="tdl"><span class="smcap">Mackerel Sky.</span> (<i lang="la" xml:lang="la">Alto-stratus Maculosus</i>)</td> - <td class="tdr"><a href="#Page_68">68</a></td> -</tr> - -<tr> - <td class="tdr">31.</td> - <td class="tdl"><span class="smcap">Mackerel Sky.</span> (<i lang="la" xml:lang="la">Alto-stratus Maculosus</i>)</td> - <td class="tdr"><a href="#Page_69">69</a></td> -</tr> - -<tr> - <td class="tdr">32.</td> - <td class="tdl"><span class="smcap">Alto-strato-cumulus</span></td> - <td class="tdr"><a href="#Page_70">70</a></td> -</tr> - -<tr> - <td class="tdr">33.</td> - <td class="tdl"><span class="smcap">Sunset.</span> (<i lang="la" xml:lang="la">Alto-cumulus Castellatus Fractus</i>)</td> - <td class="tdr"><a href="#Page_70">70</a></td> -</tr> - -<tr> - <td class="tdr">34.</td> - <td class="tdl"><span class="smcap">Three Layers of Stratiform Cloud after Rain</span></td> - <td class="tdr"><a href="#Page_73">73</a></td> -</tr> - -<tr> - <td class="tdr">35.</td> - <td class="tdl"><span class="smcap">Rain-Cloud.</span> (<i lang="la" xml:lang="la">Nimbus</i>)</td> - <td class="tdr"><a href="#Page_75">75</a></td> -</tr> - -<tr> - <td class="tdr">36.</td> - <td class="tdl"><span class="smcap">Rain-Cloud.</span> (<i lang="la" xml:lang="la">Nimbus</i>)</td> - <td class="tdr"><a href="#Page_75">75</a></td> -</tr> - -<tr> - <td class="tdr">37.</td> - <td class="tdl"><span class="smcap">Stratus Communis</span></td> - <td class="tdr"><a href="#Page_77">77</a></td> -</tr> - -<tr> - <td class="tdr">38.</td> - <td class="tdl"><span class="smcap">Strato-cumulus</span></td> - <td class="tdr"><a href="#Page_77">77</a></td> -</tr> - -<tr> - <td class="tdr">39.</td> - <td class="tdl"><span class="smcap">Strato-cumulus</span></td> - <td class="tdr"><a href="#Page_78">78</a></td> -</tr> - -<tr> - <td class="tdr">40.</td> - <td class="tdl"><span class="smcap">Stratus Maculosus</span></td> - <td class="tdr"><a href="#Page_78">78</a></td> -</tr> - -<tr> - <td class="tdr">41.</td> - <td class="tdl"><span class="smcap">Common Stratus.</span> (<i lang="la" xml:lang="la">Stratus Communis</i>)</td> - <td class="tdr"><a href="#Page_79">79</a></td> -</tr> - -<tr> - <td class="tdr">42.</td> - <td class="tdl"><span class="smcap">Roller Cloud.</span> (<i lang="la" xml:lang="la">Stratus Radius</i>)</td> - <td class="tdr"><a href="#Page_80">80</a></td> -</tr> - -<tr> - <td class="tdr">43.</td> - <td class="tdl"><span class="smcap">Small Cumulus.</span> (<i lang="la" xml:lang="la">Cumulus Minor</i>)</td> - <td class="tdr"><a href="#Page_94">94</a></td> -</tr> - -<tr> - <td class="tdr"><span class="pagenum"><a name="Page_xiii" id="Page_xiii">[xiii]</a></span>44.</td> - <td class="tdl"><span class="smcap">Cumulus</span></td> - <td class="tdr"><a href="#Page_95">95</a></td> -</tr> - -<tr> - <td class="tdr">45.</td> - <td class="tdl"><span class="smcap">Large Cumulus.</span> (<i lang="la" xml:lang="la">Cumulus Major</i>)</td> - <td class="tdr"><a href="#Page_96">96</a></td> -</tr> - -<tr> - <td class="tdr">46.</td> - <td class="tdl"><span class="smcap">Fracto-cumulus</span></td> - <td class="tdr"><a href="#Page_97">97</a></td> -</tr> - -<tr> - <td class="tdr">47.</td> - <td class="tdl"><span class="smcap">Fall Cloud.</span> (<i lang="la" xml:lang="la">Stratus Lenticularis</i>)</td> - <td class="tdr"><a href="#Page_98">98</a></td> -</tr> - -<tr> - <td class="tdr">48.</td> - <td class="tdl"><span class="smcap">Thunder-clouds forming</span></td> - <td class="tdr"><a href="#Page_109">109</a></td> -</tr> - -<tr> - <td class="tdr">49.</td> - <td class="tdl"><span class="smcap">Thunder-clouds.</span> (<i lang="la" xml:lang="la">Cumulo-nimbus</i>)</td> - <td class="tdr"><a href="#Page_110">110</a></td> -</tr> - -<tr> - <td class="tdr">50.</td> - <td class="tdl"><span class="smcap">Thunder-clouds.</span> (<i lang="la" xml:lang="la">Cumulo-nimbus</i>)</td> - <td class="tdr"><a href="#Page_111">111</a></td> -</tr> - -<tr> - <td class="tdr">51.</td> - <td class="tdl"><span class="smcap">Thunder-cloud.</span> (<i lang="la" xml:lang="la">Cumulo-nimbus</i>)</td> - <td class="tdr"><a href="#Page_111">111</a></td> -</tr> - -<tr> - <td class="tdr">52.</td> - <td class="tdl"><span class="smcap">Thunder-cloud.</span> (<i lang="la" xml:lang="la">Cumulo-nimbus</i>)</td> - <td class="tdr"><a href="#Page_111">111</a></td> -</tr> - -<tr> - <td class="tdr">53.</td> - <td class="tdl"><span class="smcap">The Flank of a Great Storm</span></td> - <td class="tdr"><a href="#Page_112">112</a></td> -</tr> - -<tr> - <td class="tdr">54.</td> - <td class="tdl"><span class="smcap">Crested Alto Waves.</span> (<i lang="la" xml:lang="la">Alto-cumulus Undatus</i>)</td> - <td class="tdr"><a href="#Page_120">120</a></td> -</tr> - -<tr> - <td class="tdr">55.</td> - <td class="tdl"><span class="smcap">Alto Waves.</span> (<i lang="la" xml:lang="la">Alto-stratus Undatus</i>)</td> - <td class="tdr"><a href="#Page_121">121</a></td> -</tr> - -<tr> - <td class="tdr">56.</td> - <td class="tdl"><span class="smcap">Cirro Ripples.</span> (<i lang="la" xml:lang="la">Cirro-cumulus Undatus</i>)</td> - <td class="tdr"><a href="#Page_122">122</a></td> -</tr> - -<tr> - <td class="tdr">57.</td> - <td class="tdl"><span class="smcap">Waved Cirro-stratus.</span> (<i lang="la" xml:lang="la">Cirro-stratus Undatus</i>)</td> - <td class="tdr"><a href="#Page_136">136</a></td> -</tr> - -<tr> - <td class="tdr">58.</td> - <td class="tdl"><span class="smcap">Camera for measuring Altitudes</span></td> - <td class="tdr"><a href="#Page_141">141</a></td> -</tr> - -<tr> - <td class="tdr">59.</td> - <td class="tdl"><span class="smcap">Print from a Negative used for measuring Altitude</span></td> - <td class="tdr"><a href="#Page_144">144</a></td> -</tr> - -<tr> - <td class="tdr">60.</td> - <td class="tdl"><span class="smcap">Pair of Prints showing the Displacement of the Cloud</span></td> - <td class="tdr"><a href="#Page_145">145</a></td> -</tr> - -<tr> - <td class="tdr">61.</td> - <td class="tdl"><span class="smcap">Cloud Camera for Studies</span></td> - <td class="tdr"><a href="#Page_171">171</a></td> -</tr> -</table></div> - -<p> - <span class="pagenum"> - <a name="Page_xiv" id="Page_xiv">[xiv]</a> - </span><br /> - <span class="pagenum"> - <a name="Page_1" id="Page_1">[1]</a> - </span> -</p> - -<div class="chapter"></div> - -<p class="ph1">CLOUD STUDIES</p> - -<h2>CHAPTER I<br /> -<span class="chtitle">INTRODUCTORY</span></h2> - -<p class="noindent"><span class="smcap">All</span> who have the -faculties proper to man must have been to some extent students of -cloud form. Go where we will, do what we will, we cannot easily escape -from the sky, or avoid noticing some of its features and coupling -them with the varying conditions of weather. We all sometimes want -to know if it is likely to rain, or whether some other change is -probable; and experience soon shows us that the clouds give the -simplest and most obvious indication of what we may expect. It is -almost impossible to avoid noticing that certain types of cloud, -or the simultaneous appearance of certain types, is the usual -accompaniment of definite kinds of weather or of particular changes. -Thus it is that most people<span class="pagenum"><a name="Page_2" -id="Page_2">[2]</a></span> acquire some small measure of weather wisdom -before their schooldays are over.</p> - -<p>Generation after generation, through all human history, the same -causes must have led to the same conclusions; and the study of clouds -must, therefore, be one of the oldest of all branches of scientific -inquiry. Yet, old as it is, it is still in its infancy, having -made very little advance indeed towards the precision of an exact -science.</p> - -<p>There are many reasons for this want of growth, and so far as -the theoretical aspects of the subject are concerned it is easy -enough to understand. Clouds are among the most inaccessible of -terrestrial objects. Except by balloon ascents, by sending up kites -bearing recording instruments, or by making observations among the -mountain-tops, we have no means of getting at them to study the -conditions under which they exist. Temperature, pressure, humidity, -have generally to be guessed at, those guesses being based on the -scanty data which have been laboriously obtained by one or another -of these cumbrous methods. Moreover, many clouds have such vast -dimensions that it is very difficult to grasp all that goes on in such -a space.</p> - -<p><span class="pagenum"><a name="Page_3" id="Page_3">[3]</a></span></p> - -<p>Besides the difficulty of attacking the problems presented by cloud -formation, it is probable that even if we could have got among the -clouds at will, we should have understood little more than we do, from -a want of sufficient certainty on many of the purely physical questions -involved. It is not many years since Mr. J. Aitken discovered the -necessity for material nuclei as a first step in the formation of cloud -particles, and not many months have elapsed since Mr. C. T. R. Wilson -showed that those particles can be formed by the action of radiation -on the air itself. There is nothing surprising, therefore, in the fact -that our theoretical knowledge of the why and wherefore of the facts -revealed by a study of clouds is limited to general principles, and -quite fails to say exactly why each special form should be assumed. The -matter for surprise is quite different.</p> - -<p>Theoretical explanations are not the first step in the working out -of a branch of science. It begins with the acquisition, by diligent and -painstaking observation, of a great mass of facts. This may go on for -centuries, the accumulation growing greater and greater, until at last -some one comes who examines the records, classifies them carefully, -and<span class="pagenum"><a name="Page_4" id="Page_4">[4]</a></span> -finally makes a summary in the form of a number of generalizations, -which are announced under the name of Laws.</p> - -<p>Two examples of such “Laws” will suffice. Astronomers -for centuries had observed the movements of the planets, always -with increasing accuracy, until Tycho Brahe made his famous series -of observations on the planet Mars. These materials fell into the -hands of Kepler, and the result of his work was the announcement of -Kepler’s Laws, which state the rules which govern the movements -of the planets in their orbits. He found that the records could not be -accounted for unless the planets moved in a certain way, but he knew -nothing of the reasons for a method and order which clearly existed.</p> - -<p>Kepler’s Laws, in fact, rest upon another set, namely, -Newton’s Laws of Gravitation, and these are themselves a second -example. They are the summary of accumulated experience, and even at -the present day we know nothing certain as to why two bodies should -attract each other, and nothing as to why that mutual attraction should -act as it was found to act by Newton.</p> - -<p>The observational part of cloud study, however,<span -class="pagenum"><a name="Page_5" id="Page_5">[5]</a></span> is still in -its infancy, in spite of the fact that it has been going on for such -countless ages. We are still in the condition of the humble observers -engaged in the comparatively humdrum task of gathering facts for -future arrangement and interpretation. Cloud observers, in all ages, -have suffered from a peculiar difficulty. They have had no common -language, no code of signs by which they could benefit from the work -of those who had gone before them, no means of transmitting their own -experience to each other, or to those who would come after them. No -progress would be possible in any study under such conditions, for -each person would begin where the previous generation began, instead -of taking up the task where others had left it. In all languages there -is an extraordinary scarcity of cloud names, and such as do exist are -frequently applied to quite different forms by different people. So -pronounced is this lack of terms, that any one who tries to describe -a sky without using any of the modern scientific names, finds himself -obliged to rely on long detailed descriptions, backed with references -to well-known objects, whose outlines or structures resemble the<span -class="pagenum"><a name="Page_6" id="Page_6">[6]</a></span> clouds more -or less vaguely; and even then he has to be a word-painter of singular -skill if his description calls up in the mind of the reader a picture -much like the original.</p> - -<p>It was to meet this want of a common tongue that Luke Howard, in -1803, proposed his scheme of cloud names. He recognized three main -types of cloud architecture, which he named Cirrus, Stratus, and -Cumulus. Cirrus included all forms which are built up of delicate -threads, like the fibres in a fragment of wool; Stratus was applied -to all clouds which lie in level sheets; and Cumulus was the lumpy -form.</p> - -<p>By combinations of these terms other clouds were described. Thus, -a quantity of cirrus arranged in a sheet was called cirro-stratus, -while high, thin clouds like cirrus, but made up of detached rounded -balls, was cirro-cumulus. Many cumulus clouds, arranged in a sheet -with little space between them, became cumulo-stratus, while the great -clouds from which our heavy rains descend partake, to some extent, -of all three types, and were therefore distinguished by a special -name—Nimbus.</p> - -<p>This system had much to recommend it. The<span class="pagenum"><a -name="Page_7" id="Page_7">[7]</a></span> three fundamental types were -obvious to all. Their names were descriptive, and were derived from a -dead language, so that no living international jealousies were raised. -It was sufficiently detailed to serve the purposes of the time, when -accurate observation was in its infancy. Hence it was universally -adopted, and will pretty certainly hold its own as the broad basis upon -which any more detailed system must necessarily rest.</p> - -<p>It has done excellent service; but although observation of clouds -in a general way is far from complete, attention is now being given to -much smaller details and much more minute differences of form, and our -vocabulary must be amplified. Precision of description is the first -essential of a satisfactory system, and the question is, what sort of -edifice can we build on Luke Howard’s foundation.</p> - -<p>The great difficulty is the infinite variety of clouds. Certain -forms may be arbitrarily selected as types, and names may be given -to them; but however well they are chosen, a very short period of -observation will show that there are all manner of intermediate -forms, which make a perfect<span class="pagenum"><a name="Page_8" -id="Page_8">[8]</a></span> gradation from one type to another. This -fact should never be forgotten. There is always a danger that the -use of any system of names based on types shall lead to the neglect -of everything not typical. A curious illustration is afforded by the -well-known fact, that in arranging collections of fossil shells, it -is frequently found that some specimens do not exactly match the type -examples to which names have been assigned. In former days it was the -custom to throw aside such “bad specimens,” as they did not -show plainly the specific characters. It is now realized that they have -a value of their own, in that they are the links in the evolutionary -chain, once supposed to be missing. Indeed, it is not unfrequent -nowadays to see carefully selected series, showing the gradual change -whereby one species passed into another, displayed in the place of -honour, while the type specimens are relegated to humbler places in the -general collection.</p> - -<p>Types there must be, no doubt, and where the series is continuous, -some one must make the selection. With clouds the series is absolutely -continuous. The task is like choosing typical links from a long chain -in which each link is almost<span class="pagenum"><a name="Page_9" -id="Page_9">[9]</a></span> exactly like its neighbours, yet no two -are alike, and the greater the distance between them the less their -likeness. Clearly any system put forward must be accompanied by -illustrations, so that all may know exactly which links have been -chosen.</p> - -<p>Many attempts have been made to meet the want; some of the systems -proposed being based on the forms assumed by the clouds, some on their -supposed mode of origin, and some on their altitudes. Those which -were not founded on Luke Howard’s types had no chance of being -accepted, while knowledge was not yet sufficiently far advanced to -make classifications based on origin of form at all possible. But the -great reason why none of the proposed schemes could come into general -use was that they were put forward without adequate illustration, -so that none but their authors knew exactly what they meant.<a -name="FNanchor_1_1" id="FNanchor_1_1"></a><a href="#Footnote_1_1" -class="fnanchor">[1]</a></p> - -<p>Matters came to a head in 1891, when an International Meteorological -Conference met at Munich. One object of this gathering was to promote -inquiries into the forms and motions of clouds, by means of concerted -observations at the various<span class="pagenum"><a name="Page_10" -id="Page_10">[10]</a></span> institutes and observatories of the globe. -Luke Howard’s system was not enough for the purpose in view, and -the addition of more detailed terms had to be settled before work could -be begun.</p> - -<p>Professor Hildebrandsson, of Upsala, and the Hon. Ralph Abercromby -jointly submitted a revised scheme, the main feature of which was the -introduction of a new class of clouds, to be distinguished by the -prefix alto-before the other name. Such alto clouds were less lofty and -denser than cirrus. This scheme was the best before the Conference, and -without waiting to discuss, and possibly improve it, it was formally -adopted, and a committee appointed to arrange and publish an atlas -showing pictures of the type-forms. This atlas did not appear until -1896, and in the mean time the Rev. W. Clement Ley had published -proposals of his own, some of which had much to recommend them. But -he was too late. The International Committee had come to a decision, -and, although it may be far from ideal, the system backed by such an -authority must be regarded as the standard until some similar gathering -has given worldwide sanction to a change, and even then it would<span -class="pagenum"><a name="Page_11" id="Page_11">[11]</a></span> be -better to modify by addition rather than by substitution.</p> - -<p>The subjects of the following pages are named in general accordance -with this International Code, but they are by no means restricted to -types. Their object is not to attempt any repetition of the work which -has already been well done by the Atlas Committee, but rather to show -the chief varieties within a type. It will, however, become abundantly -evident that the standard system is far from complete, and that any -minute and detailed study of cloud-form must take note of the precise -variety.</p> - -<p>This at once raises the question whether many of these varieties -are not sufficiently distinct to be given definite names. If a -meteorologist is told that cirrus clouds were seen on a particular -occasion, he instinctively asks—What sort of cirrus? and is -utterly unable to form any mental picture of the clouds until the -question has been answered by a detailed description. A glance at a -few of the plates further on will show the difficulty plainly, and it -occurs with other forms of cloud as well as cirrus.</p> - -<p>Is it not time that the International names were<span -class="pagenum"><a name="Page_12" id="Page_12">[12]</a></span> regarded -as those of the cloud genera, and to add specific names for those -varieties which seem to imply some difference in kind in the conditions -which have led to their formation? This has been here attempted by -translating into Latin the ordinary English term by which the variety -would naturally be described. More extended observation will probably -show that other species should be introduced, and possibly some of -those suggested in these pages may have to be subdivided. Whatever -the names may be, specific distinction of some sort is an essential -preliminary to detailed study of the why and wherefore of the -particular forms.</p> - -<p>The International system is as follows:—</p> - -<p>A. Upper clouds.</p> - -<p class="inset"> -(<i>a</i>) Cirrus.<br /> -(<i>b</i>) Cirro-stratus.</p> - -<p>B. Intermediate clouds.</p> - -<p class="inset"> -(<i>a</i>) Cirro-cumulus and alto-cumulus.<br /> -(<i>b</i>) Alto-stratus.</p> - -<p>C. Lower clouds.</p> - -<p class="inset"> -(<i>a</i>) Strato-cumulus.<br /> -(<i>b</i>) Nimbus.</p> - -<p><span class="pagenum"><a name="Page_13" id="Page_13">[13]</a></span></p> - -<p>D. Clouds of diurnal ascending currents.</p> - -<p class="inset">(<i>a</i>) Cumulus and cumulo-nimbus.</p> - -<p>E. High fogs.</p> - -<p class="inset">(<i>b</i>) Stratus.</p> - -<p>In this tabulation the forms marked (<i>a</i>) are detached and occur in -dry weather, while those marked (<i>b</i>) are widely extended. The original -scheme also gives the mean heights of the various types, but these -values have been omitted here because they are extremely variable, -and impossible to ascertain with any approach to accuracy by mere eye -estimates. They vary also with the season, and probably also with the -locality. Moreover, the altitude is no guide to the name, except that -on the whole the types occur in the order given, taking group A as the -highest and group E as the lowest. In the chapter on cloud altitudes -this subject will be further considered, and under the descriptions -of cloud-forms their average height or actual measurements for the -particular specimen figured will be given whenever possible.</p> - -<p>Before coming to the description of individual forms, it may -not be out of place to give brief consideration to the best means -of observing them in<span class="pagenum"><a name="Page_14" -id="Page_14">[14]</a></span> nature. For eye observation, of course, -no directions are needed when we are dealing with the lower and denser -varieties; but when we come to the highest groups it sometimes becomes -necessary to protect the eye from the brilliant glare which may make it -impossible to detect the real structure. Smoked glass, neutral-tinted -spectacles, or yellow glass all have something to recommend them; -but by far the most convenient means is to look, not at the clouds -themselves, but at their images formed in a black mirror. A lantern -cover glass, or a thin piece of plate-glass, blacked on the back with -some black paint, serves admirably. But all black paints are not -equally good. The best are oil paints which dry with a glossy surface, -the so-called enamels. They have the advantage that the varnish with -which they are mixed has an index of refraction not very different from -that of the glass. The consequence is that so little light is reflected -from the blackened back, compared with that which is reflected from the -front surface of the glass, that the second image can only be detected -with difficulty. If the mirror is a piece of black or deeply coloured -glass all trace of the second image is lost.</p> - -<p><span class="pagenum"><a name="Page_15" -id="Page_15">[15]</a></span></p> - -<p>With this simple appliance it is easy to study the details of the -thinnest clouds right up to the sun, and even the image of the sun -itself may be glanced at without serious discomfort. Nor is the general -diminution of brightness the only gain. If the glass is so held that -the light from the cloud makes an angle of about 33 degrees with the -surface, some of the blue light from the sky is suppressed altogether, -while that from the cloud is practically unaffected. The exact fraction -suppressed depends upon the part of the sky relative to the sun, and -also on the position of the mirror, but a few minutes’ trial will -show when the maximum effect has been reached.</p> - -<p>It is astonishing to see for the first time how the delicate -filaments of cirrus or the beautiful structures of cirro-cumulus stand -out shining white on the deep blue background; and the use of the black -mirror is a revelation to most. It also has one indirect advantage, -which is really more important than it seems. By gazing down into a -mirror long-continued observations can be made, and one form of cloud -may be watched changing into another, and possibly back again into its -original shape, without any danger of incurring that unpleasant result -of<span class="pagenum"><a name="Page_16" id="Page_16">[16]</a></span> -much looking upwards which is sometimes known as exhibition headache. -Such a mirror may be quite small, so that it can be carried in a -pocket-book, a point of some moment, as many of the forms of cirrus are -exceedingly transient, coming and going in a few minutes, while others -are in a state of continuous change. This is particularly often the -case with the exquisite ripple clouds, and the delicate lacework of the -higher kinds of cirrus.</p> - -<p>Still another advantage possessed by the mirror is that it makes -it easy to see the solar halos formed on the verge of a cyclone, and -to detect their iridescent colouring in a way which is quite beyond -the reach of the naked eye or any protective spectacles. Every one -is familiar with the faint halos formed round the moon, but the -corresponding solar phenomenon is comparatively little known, though it -is far commoner, much more brilliant, and often glows with colour. Its -very brightness, and that of the background on which it is projected, -hides it from the eye, except on those rare occasions when the sun is -conveniently hidden by some thicker cloud.</p> - -<p>If some permanent record is desired, much can<span -class="pagenum"><a name="Page_17" id="Page_17">[17]</a></span> be done -with a few light strokes of a pencil, but more ambitious pictures are -best secured by the use of soft pastels, aided by a liberal use of the -finger or leather stump. Ordinary paints, whether oil or water-colour, -are of little use for actual study of cloud detail, except in the hands -of a highly skilled artist who knows how to get the effect he wants in -the minimum of time.</p> - -<p>But no sketching or drawing can make records of cirrus or alto -clouds with the speed and accuracy necessary for careful study. -Photography is really the only way in which the amazing wealth of -detail can be truthfully portrayed. Yet even the camera has its -limitations. It does not record colour, and completely fails to -delineate the forms of alto-stratus, stratus, or nimbus, if they are -present in the most typical condition, that is to say, when they cover -the whole sky with a uniform tint. It is only when these forms are more -or less broken up that a photograph, or anything other than a carefully -coloured picture, will represent them at all.</p> - -<p>Cloud photography, even of the most delicate and brilliant -varieties, is easy enough when the right methods are followed; -but these are not the<span class="pagenum"><a name="Page_18" -id="Page_18">[18]</a></span> same as those which are right for -portraiture or landscape work of the usual kind. The background of blue -sky produces almost the same effect on the plate as the image of the -cloud itself, and the whole art consists in an adequate exaggeration -of the minute difference so as to reveal the details of form and -structure.</p> - -<p>A slow plate—the accompanying illustrations have -all been taken on Mawson and Swan’s photo-mechanical -plates—extremely cautious development, and sometimes -intensification of the image, are all that is necessary; but the -process becomes easier if, instead of pointing the camera to the cloud, -it is directed to the image formed in a properly constructed black -mirror. Many of the following studies have been taken by this method, -and details of the camera and processes employed will be found in a -later chapter, for the convenience of any one who may be inspired to -take up a fascinating branch of photography.</p> - -<p>It has been said that reference will be made to the average -altitudes of the different types of cloud, and to the actual -altitude of some of the varieties shown. The question will, no -doubt, have occurred<span class="pagenum"><a name="Page_19" -id="Page_19">[19]</a></span> to some as to how those altitudes have -been measured. The methods are all more or less complicated, involving -rather laborious calculation. They generally depend upon simultaneous -observations made from two stations at opposite ends of a measured -base line. Sometimes the observations are made directly by pointing -an instrument at each station to some agreed point of the cloud. It -is obvious that the two directions must converge to this point. If -the convergence is measured, the exact distance from either station -can be calculated, and if the angle between the cloud-point and the -horizon beneath it is noted, it is a simple matter to deduce the actual -altitude of the cloud. At other places the observers have relied -upon the comparison of photographs simultaneously taken from the two -stations. In this method it is necessary to know the exact direction -in which the camera is pointed, and the position of the image upon -the plate then gives the direction of the cloud as seen from that -particular station, and the subsequent calculations are the same.</p> - -<p>Measurements by one or the other of the above methods have been made -at several places, the most<span class="pagenum"><a name="Page_20" -id="Page_20">[20]</a></span> extensive series being those which -have been compiled at Upsala, and at the Blue Hill Observatory in -Massachusetts. The method employed by the writer at Exeter has -been rather different, and a description will be found later on in -the chapter on Cloud Altitudes, the fuller consideration of which -comes naturally after the different forms have been described and -compared.</p> - -<div class="chapter"></div> - -<p><span class="pagenum"><a name="Page_21" id="Page_21">[21]</a></span></p> - -<h2>CHAPTER II<br /> - -<span class="chtitle">CIRRUS</span></h2> - -<p class="noindent"><span class="smcap">A cloud</span> is sometimes -defined as any visible mass composed of small particles of ice or -water suspended in the air, and formed by condensation from the state -of vapour. As a general rule this is exact enough, but under certain -circumstances it is possible to have the particles so small, and so -thinly scattered, that it is not fully satisfied. The resulting mass -may not be actually visible. The presence of the condensed particles -may be indicated by nothing more than a slight whitening of the blue -sky, or by the formation about the sun or moon of bright circles -of light known as halos. If such a halo appears, it is generally a -phenomenon of brief duration. Sometimes the circle breaks and becomes -incomplete by the passing away of the thin patch of cloud, sometimes -the cloud increases in density until the rings are destroyed.</p> - -<p><span class="pagenum"><a name="Page_22" -id="Page_22">[22]</a></span></p> - -<p>The thinnest variety of this halo-producing structure is quite -invisible to the eye. It is so thin as to have no distinctly noticeable -effect upon the colour of the sky, but the optical results of its -presence may be very remarkable. Highly complicated systems of rings -are sometimes produced, the rings, as a rule, falling into two groups. -The commonest form has the sun (or moon) in the centre, and a circle -of pale light at a distance of about 22 degrees. Larger rings are seen -less frequently, which have an angular radius of about 46 degrees, -and as a rule have the sun situated on the ring itself. In Plate <a -href="#Plate_1">1</a> we have a part of such a great halo. The camera -was directed towards the east, and tilted upwards at an angle of about -40 degrees. The sun was behind the camera, in the south-west, and the -ring could be traced right up to it on either side.</p> - -<p>At the same time the sun was surrounded partially by a halo -of the more ordinary type, which was brightly coloured, making -an effective contrast to the dull white of the greater ring. The -phenomenon did not last more than half an hour, and the changes in its -appearance coincided with a<span class="pagenum"><a name="Page_23" -id="Page_23">[23]</a></span> growing density of cloud. When first -noticed the great ring was alone, and the sky was of a full blue, but a -silvery film came gradually up from the south-west, and the smaller and -brighter halo flashed out as the delicate curtain came near the sun. -Slowly the cloud spread to the north-east, gathering density from the -opposite point of the compass; and by the time the ordinary halo was at -its best, the great white ring had completely vanished.</p> - -<p><a id="Plate_1"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p022a.jpg"> - <img src="images/i_p022a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 1.</span></p> - <div class="caption"> - <p>PART OF A GREAT HALO.</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 400px;"> - <img src="images/i_p022a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 1.</span></p> - <div class="caption"> - <p>PART OF A GREAT HALO.</p> - </div> -</div> - -<p><a id="Plate_2"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p022d.jpg"> - <img src="images/i_p022d-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 2.</span></p> - <div class="caption"> - <p>PART OF A SOLAR HALO.</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 400px;"> - <img src="images/i_p022d-hh.jpg" alt="" /> - <p><span class="smcap">Plate 2.</span></p> - <div class="caption"> - <p>PART OF A SOLAR HALO.</p> - </div> -</div> - -<p>These circles, and the bright spots called mock-suns or mock-moons -which often accompany them, can all be explained on the assumption -that their cause is the passage of light through a veil composed of -hexagonal crystals of ice. The simple halo of 22 degrees radius is -common in most parts of the world, being very generally formed by the -film of high cloud which marks the advancing edge of a cyclonic cloud -system. A portion of one is shown in Plate <a href="#Plate_2">2</a>, -in which the rudimentary fibrous structure of the sheet of cloud is -distinctly seen. Halos of this sort are frequently coloured, often -most brilliantly so; but the tints are seldom noticed unless a black -mirror is used. They are<span class="pagenum"><a name="Page_24" -id="Page_24">[24]</a></span> sometimes quite as bright as those of an -ordinary rainbow, but instead of being projected upon a background of -dark rain-clouds, they are seen against a part of the sky which is near -the sun, and therefore exceptionally bright.</p> - -<p>The red is always on the inside of the ring, the violet outside, -thereby distinguishing them at once from the so-called coronæ, -which are formed around the sun or moon when shining through a sheet -of alto or other lower cloud made up of liquid particles. In these the -radius of the rings is much less, and the red is on the outside, the -violet actually touching the central luminary.</p> - -<p>The cloud which produces halos is called cirro-nebula. It is much -thinner, and on an average higher than cirro-stratus. Mr. Ley named it -cirro-velum (or cirro-veil), but cirro-nebula has now got to be fairly -well understood. It sometimes appears and disappears in a curious -manner, showing that it occurs in patches, which drift about or which -keep forming and melting away, only to repeat the process. If, however, -it forms part of an advancing cyclone fringe, then the sky gets whiter -and whiter, until it is covered with a sheet of undoubted<span -class="pagenum"><a name="Page_25" id="Page_25">[25]</a></span> -cirro-stratus. This process of growing density is shown in progress in -Plate 3.</p> - -<p><a id="Plate_3"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p024a.jpg"> - <img src="images/i_p024a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 3.</span></p> - <div class="caption"> - <p>CIRRO-NEBULA CHANGING TO CIRRO-STRATUS.</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p024a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 3.</span></p> - <div class="caption"> - <p>CIRRO-NEBULA CHANGING TO CIRRO-STRATUS.</p> - </div> -</div> - -<p>Cirro-nebula, as we shall call it, floats at very great altitudes in -temperate regions; but in polar latitudes, where the optical phenomena -peculiar to it are most brilliant and diversified, it seems probable -that the ice dust is much lower down, even in actual proximity to the -ground. In England its height varies greatly with the time of year, and -other circumstances, but mounts up in summer to such altitudes as nine -miles or more; the greatest height yet recorded being 9·6 miles, -or about 15,500 metres, at Exeter.</p> - -<p>The change from cirro-nebula to cirro-stratus is generally -accompanied by the formation of a distinct fibrous structure, easily -observable by the black mirror. This is not really a new feature, but -only a further development of a structure already existing, but too -minute to be easily seen. True halo-producing cirro-nebula may usually -be shown to possess more or less of a fibrous texture in an indirect -way, which is worth a brief description.</p> - -<p>In order to observe the spots on the sun and other features -of the solar surface, it is a common practice to hold a white -screen, say, about a foot<span class="pagenum"><a name="Page_26" -id="Page_26">[26]</a></span> from the eyepiece of a telescope, -while the instrument is pointed to the sun. An image, considerably -magnified, is thus projected on to the screen, and the solar -details can be studied with ease and safety. If thin clouds drift -before the sun, their images are similarly projected as they -pass across its disc, and it is possible thus to detect not only -the fibrous texture but also the movement of cirro-nebula.<a -name="FNanchor_2_2" id="FNanchor_2_2"></a><a href="#Footnote_2_2" -class="fnanchor">[2]</a></p> - -<p>The change into cirro-stratus is also attended by a marked fall in -altitude, but whether this is due to an actual descent of the cloud -particles, or to a downward spread of the conditions which give rise -to them, cannot at present be definitely settled. The balance of -probability points very strongly towards the downward spread of the -conditions. It is likely that the clouds, particularly the cyclonic -specimens, are wedge-shaped, and that as they pass overhead we see -first the thin edge, and later on the thicker parts, which project -much lower down. This is just one of those many minor problems in -cloud mechanics which we are not able to solve from the scanty -data on record.</p> <p><span class="pagenum"><a name="Page_27" -id="Page_27">[27]</a></span></p> <p>Occasionally cirro-nebula breaks -up into little detached semi-transparent cloudlets, all of them -exceedingly thin, and showing a complicated mottling, resembling, on -a minute scale, the ripple clouds of much lower altitudes. Such a sky -is depicted in Plate <a href="#Plate_4">4</a>, but no reproduction -can possibly do justice to the minute and delicate features of the -real thing. The arrangement of the faint markings was in a state of -continual flux, curiously similar to the ever-changing aspect of the -sun’s photosphere when seen under adequate power. Some parts of -the cloud stratum would at one moment break up into distinct granules -arranged in complicated patterns, other parts would assume a fibrous -texture, and yet other places would show a continuous smooth sheet. In -a minute or two all would be changed—the smooth part granulated, -the fibres vanished, and the granules fused together, and so on, no two -of a series of photographs representing the same details.</p> - -<p><a id="Plate_4"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p026b.jpg"> - <img src="images/i_p026b-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 4.</span></p> - <div class="caption"> - <p class="caption">CIRRO-NEBULA CHANGING TO CIRRO-CUMULUS.</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p026b-hh.jpg" alt="" /> - <p><span class="smcap">Plate 4.</span></p> - <div class="caption"> - <p class="caption">CIRRO-NEBULA CHANGING TO CIRRO-CUMULUS.</p> - </div> -</div> - -<p>These changes of form continued until the whole was hidden from -view by a veil of much lower stratiform cloud, one advance portion of -which is shown. Plate <a href="#Plate_4">4</a> does not represent a -type or a distinct variety of cloud. It is an intermediate form, or -a<span class="pagenum"><a name="Page_28" id="Page_28">[28]</a></span> -temporary condition, showing cirro-nebula in the act of changing into -cirro-cumulus, or possibly cirro-stratus.</p> - -<p>Cirro-nebula itself, in its simpler form, is, however, a distinct -type. It is true that it never persists over one locality for more -than an hour or two without passing into some denser form, but while -it lasts its features are so distinctive, and the optical phenomena -to which it gives rise are so striking and significant, that it is a -matter for surprise that it should in the International system have -been relegated to the position of a subordinate variety of cirrus. It -is more nearly related to cirro-stratus, but is sufficiently distinct -from that to deserve at least specific rank.</p> - -<p>True typical cirrus must have a plainly shown fibrous structure. The -fibres may cross and interlace, they may radiate in fan-like manner, -or they may curl and twist like a well-trimmed ostrich feather. The -clouds so formed must not be arranged in a continuous level sheet, or -they at once become cirro-stratus, and it is impossible to invent a -definition which will mark the exact limits of either type. Typical -cirrus consists of detached<span class="pagenum"><a name="Page_29" -id="Page_29">[29]</a></span> clouds. They cast no shadows on the -landscape, for the simple reasons that they are semi-transparent and -their component parts too narrow. If the sun is shining down obliquely -through the naked boughs of a tall tree, it will be seen that the -lowest twigs cast fairly sharp shadows on the ground, but that even -these are bordered by a fading rim; the twigs further up cast no sharp -shadows, but broader faint bands of shade; while the topmost boughs -cast no shadows which can clearly be identified. In other words, the -more distant the narrow twig is from the ground the narrower the real -shadow or umbra, and the broader the penumbra becomes, until when the -distance is sufficient the shadow is all penumbra. Cirrus filaments -throw nothing but a faint penumbra. Indeed, it is only when they lie in -the earth’s shadow, and stand against the background of a faintly -lighted sky, that they show any sign of shadow even on themselves.</p> - -<p>There is no doubt that they are composed of particles of ice. They -are formed at altitudes where the thermometer must be many degrees -below freezing-point, and not a few of the thinner examples show -fragmentary halos like those of cirro-nebula.</p> - -<p><span class="pagenum"><a name="Page_30" -id="Page_30">[30]</a></span></p> - -<p>Their actual altitudes are very variable, being greater in summer -than in winter, and reaching a maximum for any given station after a -long spell of hot weather. Exact measurements have not yet been made -in tropical latitudes or in polar regions, but there is every reason -to expect that the upper limit of cirrus for equatorial districts will -be found to be much higher than in the temperate zones where actual -observations have been made. In places nearer to the Arctic Circle it -is also almost certain that the altitudes will be less.</p> - -<p>In the New England states, as shown by the Blue Hill observations, -the maximum altitude for summer was found to be little under 15,000 -metres. At Upsala, in Sweden, it was 13,300 metres. The average -altitudes at the same observatories were, respectively, about 9900 and -8800 metres. At Exeter the writer’s own measurements give an -average for the summer months of 10,200 metres, with a minimum rather -lower than was the case in America or Sweden, and with a maximum far -above the foreign values. In winter cirrus certainly comes much lower -down, but the number of observations is fewer.</p> - -<p><span class="pagenum"><a name="Page_31" -id="Page_31">[31]</a></span></p> - -<p><a id="Plate_5"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p030b.jpg"> - <img src="images/i_p030b-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 5.</span></p> - <div class="caption"> - <p>HIGH CIRRUS.</p> - <p>(<i lang="la" xml:lang="la">Cirrus Excelsus.</i>)</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p030b-hh.jpg" alt="" /> - <p><span class="smcap">Plate 5.</span></p> - <div class="caption"> - <p>HIGH CIRRUS.</p> - <p>(<i lang="la" xml:lang="la">Cirrus Excelsus.</i>)</p> - </div> -</div> - -<p>The loftiest variety of cirrus appears in the afternoon in very hot -weather, sometimes quite late in the evening; and in autumn it is by no -means a rare event for it to suddenly form just when the sunset colours -are fading, or even after they have paled into twilight. Under such -circumstances it stands out of a shining silvery grey colour against -the background of the darkening sky. A specimen of it is shown in Plate -5, which shows its extreme slightness of form and delicacy of texture. -Sometimes it remains visible so long after the stars have begun to -show as to give the idea that it is self luminous, and the illusion -is certainly very strong. The writer has noted several instances in -which it was plainly visible, like a silvery curtain, though the sky -as a whole was so dark that stars like the five brightest points of -the Great Bear could be seen through the cloud, and much smaller -stars down to the third and fourth magnitude were plainly visible in -the clear intervals. It has sometimes been called luminous cloud, -and Mr. Ley estimated its altitude at upwards of 90,000 metres; but -if we think of it as reflecting the light of the distant colourless -twilight there is no need to<span class="pagenum"><a name="Page_32" -id="Page_32">[32]</a></span> regard it as anything fundamentally -different from other clouds, or to assume a greater altitude than we -know to have been the case. The specimen figured occurred in the early -afternoon on June 12, 1899, at Exeter, and careful measurements of its -altitude were made. This worked out as 17·02 miles, or more than -27,000 metres, a value so much greater than all other measurements of -the kind that it was only after most careful verification and reference -to duplicate records that it could be accepted. It differs in several -ways from the lower varieties, being thinner, more glistening, and in -every way more delicate. A suitable distinctive name would be high -cirrus, or cirrus excelsus.</p> - -<p><a id="Plate_6"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p032a.jpg"> - <img src="images/i_p032a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 6.</span></p> - <div class="caption"> - <p>WINDY CIRRUS.</p> - <p>(<i lang="la" xml:lang="la">Cirrus Ventosus.</i>)</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p032a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 6.</span></p> - <div class="caption"> - <p>WINDY CIRRUS.</p> - <p>(<i lang="la" xml:lang="la">Cirrus Ventosus.</i>)</p> - </div> -</div> - -<p>Lower down by thousands of metres come the feathery masses of -typical windy cirrus, such as are shown in Plate <a href="#Plate_6">6</a>. Indeed, in cold -winter weather they occur within three or four thousand metres of the -ground. In the instance figured the wind was blowing from left to -right, and the clouds were travelling swiftly. The upper filaments -appeared to be repeatedly torn away from the main masses, while the -long faint streaks which trail below and behind are evidently due to -streams of fine particles<span class="pagenum"><a name="Page_33" -id="Page_33">[33]</a></span> falling from the main centres of -condensation into a less rapidly moving stratum below. There is no -room for doubt that these clouds, like others of a similar order, are -formed by a direct passage from the vapour to the solid, or that the -fibres are made of minute snowflakes. The condensation is evidently -attended by rapid movements, which draw out the cloud, as fast as it is -formed, into long curving lines which mark lines of motion. The variety -is always, therefore, an indication of strong winds and rapid eddying -movements in the region in which it occurs. Such strong disturbances -overhead almost always accompany similar but less intense movements -at the ground-level, and when they do not accompany them they precede -them. The cloud is well named windy cirrus, which may be converted into -a specific name, cirrus ventosus.</p> - -<p>The next variety we come to (Plate <a href="#Plate_7">7</a>) is in -some ways rather similar. It is, however, thinner, more delicate, and -is entirely composed of fine threads, which are more systematically -arranged. Generally there is a bundle, or several bundles, of long -parallel fibres, which form, so to say, the quill of the feather, with -numbers of shorter threads<span class="pagenum"><a name="Page_34" -id="Page_34">[34]</a></span> branching out from them at various angles. -Cirrus ventosus was indicative of irregular movements in various -directions; this variety points also to complicated movements, but -executed in accordance with some sort of system, strangely complex -and wonderfully ordered. The specimen figured is the type of what Mr. -Ley called cirro-filum, or thread cirrus, and his name can hardly be -bettered. It is a cloud of summer, and occurs rather high up in the -cirrus zone, but no actual measurements can be quoted. It is fairly -common, but not nearly so frequent as the last.</p> - -<p><a id="Plate_7"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p034a.jpg"> - <img src="images/i_p034a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 7.</span></p> - <div class="caption"> - <p>THREAD CIRRUS.</p> - <p>(<i lang="la" xml:lang="la">Cirro-filum.</i>)</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p034a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 7.</span></p> - <div class="caption"> - <p>THREAD CIRRUS.</p> - <p>(<i lang="la" xml:lang="la">Cirro-filum.</i>)</p> - </div> -</div> - -<p>A somewhat more familiar variety is shown in Plate <a href="#Plate_8">8</a>. Little -irregular feathers of cirrus, from which long tapering streamers point -downwards in graceful curves, or else lag behind in the direction -from which the clouds have travelled. If clouds of this type are -carefully watched, it will soon be seen that each feathery head is a -centre of condensation, and that the tails or streamers are nothing -else than falling particles, which dwindle slowly away by evaporation, -and which gradually sink below the level of the heads. It is usual, -in dealing with cloud-forms like these, to speak of air-currents -of<span class="pagenum"><a name="Page_35" id="Page_35">[35]</a></span> -different velocities almost as if the winds at different levels were -as clearly separated as oil and water, or even air and water. This can -hardly be the case, for if such a thing should occur as an air-current -of one velocity flowing over another of less speed, or of a current in -one direction over another moving in a different course, the two must -inevitably mix at their junction, and in a very short time the passage -from the lower current to the upper one would be quite gradual. No -doubt we can often observe two, three, or more layers of cloud moving -in different directions; but if we were to send up a balloon, it would -be rare indeed to find its direction of horizontal movement changed in -a few metres of ascent. Different and distinct air-currents are often -invoked to explain cloud-forms quite unnecessarily. It is far more -likely that the differential movements involved in the explanation of -the features of these cirrus varieties are due to increased velocity -with greater altitude, to progressive change of direction, to irregular -eddies, or to the interaction of ascending and descending convection -currents. Indeed, it is probable that careful study of the growth -and decay of these clouds will, in time, lead to a clearer<span -class="pagenum"><a name="Page_36" id="Page_36">[36]</a></span> -understanding of atmospheric movements, and so enable us to say more -precisely why they are as we see them to be. The variety shown in Plate -8 is rare except in combination with other forms. It might well be -termed tailed cirrus or cirrus caudatus.</p> - -<p><a id="Plate_8"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p034d.jpg"> - <img src="images/i_p034d-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 8.</span></p> - <div class="caption"> - <p>TAILED CIRRUS.</p> - <p>(<i lang="la" xml:lang="la">Cirrus Caudatus.</i>)</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p034d-hh.jpg" alt="" /> - <p><span class="smcap">Plate 8.</span></p> - <div class="caption"> - <p>TAILED CIRRUS.</p> - <p>(<i lang="la" xml:lang="la">Cirrus Caudatus.</i>)</p> - </div> -</div> - -<p>The form of cirrus shown in Plate <a href="#Plate_9">9</a> is far more frequently seen -than either of those which have been described. In this the fibrous -texture is very imperfect, and the cloudlets show a tendency to arrange -themselves in a kind of ribbed structure in two directions almost at -right angles to each other. But this last is an accidental feature -of the particular example, and not in any way a specific character -of the cloud. The reason for regarding it as a distinct variety is -the total absence of sharply defined lines, not only the heads of -condensation, but even the long streamers attached to them being -uniformly hazy and ill-defined. It is a form of cirrus which comes -at all seasons, but most frequently in summer; it moves always with -great slowness, indicating a quiet atmosphere free from disturbance -of any kind. The conditions necessary for its appearance are a -nearly uniform distribution of pressure over a considerable area, -chequered by little shallow<span class="pagenum"><a name="Page_37" -id="Page_37">[37]</a></span> depressions of some trifling fraction -of an inch. In hot weather these are the conditions under which -thunder-storms develop, and this hazy cirrus, or cirrus nebulosus, may -be taken as a certain sign of such an atmospheric state.</p> - -<p><a id="Plate_9"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p036a.jpg"> - <img src="images/i_p036a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 9.</span></p> - <div class="caption"> - <p>HAZY CIRRUS.</p> - <p>(<i lang="la" xml:lang="la">Cirrus Nebulosus.</i>)</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p036a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 9.</span></p> - <div class="caption"> - <p>HAZY CIRRUS.</p> - <p>(<i lang="la" xml:lang="la">Cirrus Nebulosus.</i>)</p> - </div> -</div> - -<p>So far as permanency of form is considered, hazy cirrus is one of -the most persistent, and affords a marked contrast to the species shown -in Plate <a href="#Plate_10">10</a>, which represents the most fugitive. Five minutes before -the photograph was taken the same part of the sky was a deep, clear -blue, without any trace of cloud. Suddenly a few short curling wisps -made their appearance. These rapidly increased in number, until a -delicate filmy network extended over the greater part of the field of -view. But while the camera was being adjusted for an exposure, part of -the net had broken up into the granular structure shown in the lower -part of the photograph. The granulation rapidly spread through the -net, almost as if the fibres had been curdled, and five minutes later -the whole had been converted into a patch of cirro-cumulus which soon -fused into a uniform sheet. Meanwhile the same series of phenomena were -taking place in other parts of the sky.</p> - -<p><a id="Plate_10"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p036d.jpg"> - <img src="images/i_p036d-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 10.</span></p> - <div class="caption"> - <p>CHANGE CIRRUS.</p> - <p>(<i lang="la" xml:lang="la">Cirrus Inconstans.</i>)</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p036d-hh.jpg" alt="" /> - <p><span class="smcap">Plate 10.</span></p> - <div class="caption"> - <p>CHANGE CIRRUS.</p> - <p>(<i lang="la" xml:lang="la">Cirrus Inconstans.</i>)</p> - </div> -</div> - -<p><span class="pagenum"><a name="Page_38" id="Page_38">[38]</a></span></p> - -<p>On other occasions exactly the same set of events have been seen -to follow each other in the inverse order. Beginning with a fairly -even sheet, this broke up into granules, and they in turn seemed to be -frayed out into short hazy and wavy fibres which slowly melted away.</p> - -<p>Clearly we have here to do, not with a distinct type of cloud, but -rather with the first step towards the formation of one, or the last -stage in the life of one which is drying up. But sometimes the life of -the cloud is so short that it never passes beyond this first stage; -and it is by no means a universal rule for a growing sheet of cirrus -to pass through this stage at all. It therefore represents a peculiar -state of instability, and requires a name of its own. Sometimes patches -of it will come and go in an apparently capricious manner for an -hour or more before permanent condensation is effected or before the -sky finally clears. But this is a rare event, since the slow change -of conditions which has brought the stratum of air to the unstable -condition is generally progressive, and instead of stopping at the -critical point, goes beyond it, with the result that the condensation -grows or the cloud disappears entirely.<span class="pagenum"><a -name="Page_39" id="Page_39">[39]</a></span> Change cirrus, or cirrus -inconstans, would be an appropriate name for a kind of cloud which is -so plainly indicative of instability.</p> - -<p>The critical condition referred to is, of course, that in which a -particular stratum of air is just saturated, or is just on the point -of forming visible cloud. If any cause is brought to bear on such a -stratum which brings about even slight cooling, cloud must be produced; -and, conversely, anything which results in the slightest heating must -cause it to disappear. The shortness and haziness of the fibres, and -the fact that they gather themselves into granules, shows that the -cloud is formed in a stratum of air which is either still, or is moving -as a whole, without any of those differential movements which seem to -be necessary for the longer fibrous details.</p> - -<p>The causes which may bring about the local cooling and heating are -easy to understand when we remember how the air will be affected by the -uneven contours of the ground. As it passes over hill and valley the -up-and-down movements of the lower layers, or even the disturbances -caused by passing over a wood or clump of trees, all must be propagated -upwards. Each disturbance must slowly<span class="pagenum"><a -name="Page_40" id="Page_40">[40]</a></span> spread laterally and -diminish vertically, so that it will reach the cirrus zone as a broad -and gentle dome-like oscillation. Suppose now a series of such slight -upheavals to reach the critical level. The passage of the waves will -mean alternate expansion and compression. Expansion means cooling, and -therefore cloud-production; compression means heating, and therefore -the destruction of cloud.</p> - -<p>From the most transient form of cirrus we pass, in Plate <a href="#Plate_11">11</a>, to the -most persistent and probably the most frequent. It occurs in detached -masses which have very variable forms but are wholly fibrous, with -the details arranged in a very irregular manner. The example figured -was taken in the evening during a long spell of fine weather. If -such a cloud is watched, its permanence of detail is very striking, -and must be due to a persistence of slow eddying movements and to a -continual renewal and waste of the component particles of each wisp. -This is the kind of cirrus selected generally as the type of cirrus, -and the selection is a good one. Common cirrus, or cirrus communis, it -should be called. Settled conditions and fine weather are its usual -attendants.</p> - -<p><a id="Plate_11"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p040a.jpg"> - <img src="images/i_p040a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 11.</span></p> - <div class="caption"> - <p>COMMON CIRRUS.</p> - <p>(<i lang="la" xml:lang="la">Cirrus Communis.</i>)</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p040a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 11.</span></p> - <div class="caption"> - <p>COMMON CIRRUS.</p> - <p>(<i lang="la" xml:lang="la">Cirrus Communis.</i>)</p> - </div> -</div> - -<p><span class="pagenum"><a name="Page_41" id="Page_41">[41]</a></span></p> - -<p>We next come to a variety which is anything but a harbinger of good, -namely, the long stripes or bands of cirrus which stretch outwards -from the margin of the cloud canopy of a cyclonic storm. In some ways -these appendages to the great nimbus resemble the strips of cirriform -cloud which fringe the summit of a thunder-cloud. They look as if they -must have been formed by the blowing away, by a rapid wind, of the top -of an uprising column of vapour-charged air. Their main outline may -thus be easily accounted for, but we have only to study their detailed -structure for a few minutes to feel that they really present a problem -of a very high order. Plate <a href="#Plate_12">12</a> shows a fairly simple example, but Plate -13 represents a cloud of very great complexity. To take this last the -camera was tilted upwards at an angle of 45 degrees, so that the top of -the picture is not far from the zenith. The wonderful plume of cloud -rose from the southern horizon, and ended in a great sheaf of fibres -and films spread out like a partly opened fan whose edge was only about -50 degrees above the northern horizon. Its length as it passed overhead -lay between a point a little east of south to a little west<span -class="pagenum"><a name="Page_42" id="Page_42">[42]</a></span> of -north; and the broad band moved as a whole, without any marked internal -changes, from the south-west towards the north-east. The weather was -very unsettled. A long procession of cyclones had been sweeping along -our western shores, and the barometer was just beginning a fresh and -rapid fall. During the ensuing night a heavy gale burst over the south -of England.</p> - -<p><a id="Plate_12"></a></p> - -<div class="figcenter screenonly" style="width: 260px;"> - <a href="images/i_p040d.jpg"> - <img src="images/i_p040d-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 12.</span></p> - <div class="caption"> - <p>BAND CIRRUS.</p> - <p>(<i lang="la" xml:lang="la">Cirrus Vittatus.</i>)</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 460px;"> - <img src="images/i_p040d-hh.jpg" alt="" /> - <p><span class="smcap">Plate 12.</span></p> - <div class="caption"> - <p>BAND CIRRUS.</p> - <p>(<i lang="la" xml:lang="la">Cirrus Vittatus.</i>)</p> - </div> -</div> - -<p><a id="Plate_13"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p042a.jpg"> - <img src="images/i_p042a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 13.</span></p> - <div class="caption"> - <p>BAND CIRRUS.</p> - <p>(<i lang="la" xml:lang="la">Cirrus Vittatus.</i>)</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p042a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 13.</span></p> - <div class="caption"> - <p>BAND CIRRUS.</p> - <p>(<i lang="la" xml:lang="la">Cirrus Vittatus.</i>)</p> - </div> -</div> - -<p>The whole phenomenon was highly characteristic. These great -bands with the divergent striation might well be known as storm -bands, from their almost invariable connection with the violent -atmospheric movements to which they are most probably due. Plate <a -href="#Plate_12">12</a> shows a much less dangerous variety of the same -species, which is distinguished from it by the comparative absence of -internal detail and by the curled ends. Clouds of this character have -sometimes been called cirro-filum, but a comparison of the plates with -the typical cirro-filum of Plate <a href="#Plate_7">7</a> will show -that there is little resemblance; and the attendant weather is also in -marked contrast, both of which facts imply a fundamental difference -in the causes to which their features are due. Banded or ribboned -cirrus is the name which they<span class="pagenum"><a name="Page_43" -id="Page_43">[43]</a></span> immediately suggest, and this may be -rendered cirrus vittatus.</p> - -<p>This ends our survey of cirrus clouds. Any one who compares the -plates so far given will see that they represent forms so diverse -that it is impossible to avoid the conclusion that the conditions -under which they are produced must differ not only in degree but also -in kind. What those conditions are we have attempted here and there -to suggest, but in no case can we feel that the explanation has been -at all complete. In some cases, notably the last, we are face to -face with such complicated details that it is hopeless to attempt to -explain them in the present state of our knowledge. Fact upon fact must -be accumulated until we can give their history from their earliest -beginnings; and far more accurate and detailed knowledge of the -attendant atmospheric conditions must be acquired before we can hope to -rob such elaborate structures of their present mystery.</p> - -<p>This requires the co-operation of many eyes and many minds, and -exact specific names must be an essential preliminary. Those which have -been suggested in these pages are—</p> - -<p><span class="pagenum"><a name="Page_44" id="Page_44">[44]</a></span></p> - -<ol> - -<li>Cirro-nebula, or Cirrus haze.</li> -<li>Cirrus excelsus, or High cirrus.</li> -<li>Cirrus ventosus, or Windy cirrus.</li> -<li>Cirro-filum, or Thread cirrus.</li> -<li>Cirrus nebulosus, or Hazy cirrus.</li> -<li>Cirrus caudatus, or Tailed cirrus.</li> -<li>Cirrus vittatus, or Band cirrus.</li> -<li>Cirrus inconstans, or Change cirrus.</li> -<li>Cirrus communis, or Common cirrus.</li> - -</ol> - -<div class="chapter"></div> - -<p><span class="pagenum"><a name="Page_45" id="Page_45">[45]</a></span></p> - -<h2>CHAPTER III<br /> - -<span class="chtitle">CIRRO-STRATUS AND CIRRO-CUMULUS</span></h2> - -<p class="noindent"><span class="smcap">Several</span> of the varieties -of cirrus already discussed may gather so abundantly at some given -level in the atmosphere, that the most obvious feature comes to be -this arrangement in a sheet. The cloud then becomes cirro-stratus, -and should be so named. We have described how cirro-nebula frequently -grows in density until it fails to produce halo phenomena, and may even -reduce the sun to a hazy patch of light showing no outline. This is the -most typical of all forms of cirro-stratus. It has always a distinctly -fibrous or streaky appearance, whereby it is at once distinguished from -a similar but lower cloud which will be described later on.</p> - -<p>A similar sheet may be formed from the fusion together of the -streaks of cirrus-nebulosus, the bands of cirrus vittatus, or the -development of cirrus<span class="pagenum"><a name="Page_46" -id="Page_46">[46]</a></span> inconstans. But the general rule is that -the cirro-stratus retains more or less of the specific characters of -the parent form.</p> - -<p>Plate <a href="#Plate_14">14</a> shows a hazy form of cirro-stratus developed from the -nebulous cirrus. Its altitude was great, being about 10,000 metres. The -processes of growth and change could be studied easily. First would -appear some faint spots and streaks; these quickly fused together -into larger patches, which again joined to their neighbours. In a few -minutes the cloud so formed would return to the mottled or streaky -appearance, and either disappear entirely or become very thin, only -to recommence the process. This went on for more than an hour, the -cloudy patches getting larger and larger, until the critical condition -was passed, and the sky was covered with a general veil of typical -cirro-stratus.</p> - -<p><a id="Plate_14"></a></p> - -<div class="figcenter screenonly" style="width: 260px;"> - <a href="images/i_p046a.jpg"> - <img src="images/i_p046a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 14.</span></p> - <div class="caption"> - <p>HAZY CIRRO-STRATUS.</p> - <p>(<i lang="la" xml:lang="la">Cirro-stratus Nebulosus.</i>)</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 455px;"> - <img src="images/i_p046a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 14.</span></p> - <div class="caption"> - <p>HAZY CIRRO-STRATUS.</p> - <p>(<i lang="la" xml:lang="la">Cirro-stratus Nebulosus.</i>)</p> - </div> -</div> - -<p>In Plate <a href="#Plate_15">15</a> we have an example of a cloud which is clearly -cirro-stratus, but the sheet is broken up into long bands, and -each of these is made up of common cirrus. In the upper part of -the picture the sprays of cirrus are forming, and as they come -into being they are arranged in rows. We have here to do with a -phenomenon of a very different<span class="pagenum"><a name="Page_47" -id="Page_47">[47]</a></span> order from the one presented by the true -banded cirrus. The arrangement into belts must here be due to some kind -of wave-movement in the air, breaking up the critical plane into long -ribs transverse to the direction of the wave-movement. The specimen -shown was moving in a direction nearly at right angles to the bands, -though the surface wind was nearly parallel to their length. The -question at once presented itself as to whether the movement of the -bands was really a drift of the cloud, or whether it was not a case -of the propagation of cloud production with the advancing wave. This -was easily answered by watching the details of a band. The advancing -side was always feathery, and careful observation showed that the -edge advanced by throwing out new threads and curls. A given thread -or curl, one moment at the edge, would in a few minutes be well in -the band. Quite opposite events were taking place in the rear of each -band. The cloud was there obviously melting away. Indeed, to sum the -matter up, the cloud bands flowed past their details just as the waves -on the sea flow past the floating foam. Evidently we have in Plate -15 the result of a plane of<span class="pagenum"><a name="Page_48" -id="Page_48">[48]</a></span> commencing cirrus formation broken into -a series of troughs and waves by an undulatory movement of the air. -But, as we have already said when speaking of cirrus inconstans, the -condition in which trifling up-and-down movements can determine whether -condensation shall, or shall not, take place seldom lasts long. It is -usually only a stage in a continuous change, and in this particular -instance the banded structure was soon replaced by a fairly continuous -sheet of typical cirro-stratus.</p> - -<p><a id="Plate_15"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p046d.jpg"> - <img src="images/i_p046d-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 15.</span></p> - <div class="caption"> - <p>CIRRO-STRATUS.</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p046d-hh.jpg" alt="" /> - <p><span class="smcap">Plate 15.</span></p> - <div class="caption"> - <p>CIRRO-STRATUS.</p> - </div> -</div> - -<p>The next plate, No. 16, shows a similar process. In the upper part -we have cirrus inconstans forming in patches out of a deep clear-blue -sky. Its hazy fibres grow closer and closer, betraying a slight -tendency to gather in narrow ripple-like bands, but the structure is -soon lost in the uniform white sheet of interlacing fibres, which -differ from common cirrus in little else than their number and -closeness. Nevertheless, the stratiform arrangement is quite obvious -enough to warrant the use of the term “cirro-stratus.”</p> - -<p><a id="Plate_16"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p048a.jpg"> - <img src="images/i_p048a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 16.</span></p> - <div class="caption"> - <p>CIRRO-STRATUS.</p> - <p>(<i lang="la" xml:lang="la">Cirro-stratus Communis.</i>)</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p048a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 16.</span></p> - <div class="caption"> - <p>CIRRO-STRATUS.</p> - <p>(<i lang="la" xml:lang="la">Cirro-stratus Communis.</i>)</p> - </div> -</div> - -<p>The change of cirro-nebula into cirro-stratus is shown in -Plate <a href="#Plate_4">4</a>, to which reference has already been made. The structures -are remarkably delicate,<span class="pagenum"><a name="Page_49" -id="Page_49">[49]</a></span> showing in the middle a distinct irregular -mottling; and rather further towards the top right-hand corner a ripple -structure appears, and in the top left-hand corner the sheet is denser -and whiter. The altitude of this cloud was evidently great, and actual -measurement showed it to be 7·6 miles. It did not last long, and -after its change into broken patches of denser cirro-stratus, still -higher clouds were revealed through the gaps.</p> - -<p>Cirro-stratus often forms almost simultaneously at more than -one level, and when that happens the full stratiform appearance is -generally reached first by the lower layer. In Plate <a href="#Plate_17">17</a> we have two -layers. The fluffy bits of cirrus nebulosus, in the lower part of -the picture, are really the higher clouds. Below them, probably by -many thousands of feet, floats the denser cloud shown in the upper -part of the picture. This is an interesting link between the fibrous -and the granular forms of cirrus, and is probably best described -as spotted cirro-stratus, or cirro-stratus maculosus. It is a form -very frequently met with, but seldom showing any persistence. It is -indicative of condensation in a calm atmosphere, and not unfrequently -marks either the small<span class="pagenum"><a name="Page_50" -id="Page_50">[50]</a></span> irregularities of pressure which form the -conditions for thunderstorms, or the beginning of the break up of an -anticyclone.</p> - -<p><a id="Plate_17"></a></p> - -<div class="figcenter screenonly" style="width: 263px;"> - <a href="images/i_p048d.jpg"> - <img src="images/i_p048d-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 17.</span></p> - <div class="caption"> - <p>FLOCCULENT CIRRO-STRATUS.</p> - <p>(<i lang="la" xml:lang="la">Cirro-stratus Cumulosus.</i>)</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 455px;"> - <img src="images/i_p048d-hh.jpg" alt="" /> - <p><span class="smcap">Plate 17.</span></p> - <div class="caption"> - <p>FLOCCULENT CIRRO-STRATUS.</p> - <p>(<i lang="la" xml:lang="la">Cirro-stratus Cumulosus.</i>)</p> - </div> -</div> - -<p>A coarser texture and greater density are shown in Plate <a href="#Plate_18">18</a>, -where we have cirro-stratus in the lower part of the picture, and -cirro-cumulus in the upper. The altitude of this cloud was only -about 4000 metres, one of the least values recorded at Exeter for -cirro-stratus of any kind. The intimate admixture of the fibrous and -granular forms is very clearly shown.</p> - -<p><a id="Plate_18"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p050a.jpg"> - <img src="images/i_p050a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 18.</span></p> - <div class="caption"> - <p>CIRRO-STRATUS AND CIRRO-CUMULUS.</p> -</div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p050a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 18.</span></p> - <div class="caption"> - <p>CIRRO-STRATUS AND CIRRO-CUMULUS.</p> -</div> -</div> - -<p>This close relation is equally obvious in Plate <a href="#Plate_19">19</a>, where the -cloudlets are arranged in loosely marshalled rows, dimly resembling -the banded structure of Plate <a href="#Plate_15">15</a>. But in this case the direction -of movement was with the long lines, and the propagation of cloud -production followed the same course. Some of the little cloudlets -have an opacity, and therefore brilliancy, quite unusual for -cirro-cumulus, but their intimate association evidently in the same -plane with undoubted cirrus shows that they must fall under that -general description. It is a cloud indicative of unsettled weather, -and the exceptional brilliancy is doubtless<span class="pagenum"><a -name="Page_51" id="Page_51">[51]</a></span> due to an unusual quantity -of vapour at the cloud plane, which must mean that the change from -the dry stratum above to the damp one below must be much more sudden -than is ordinarily the case. Clouds of this kind might well be called -cirro-stratus cumulosus.</p> - -<p><a id="Plate_19"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p050c.jpg"> - <img src="images/i_p050c-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 19.</span></p> - <div class="caption"> - <p>CIRRO-CUMULUS.</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p050c-hh.jpg" alt="" /> - <p><span class="smcap">Plate 19.</span></p> - <div class="caption"> - <p>CIRRO-CUMULUS.</p> - </div> -</div> - -<p>We now come to two companion pictures, Plates <a href="#Plate_20">20</a> and <a href="#Plate_21">21</a>, which were -taken within half a minute of each other. In the first the camera -was directed towards the west, and in the second it was facing the -north-west. The sun was nearing the horizon, and was only just outside -the field of view in each case, so that the two photographs form a -panorama of the western sky. A solar halo had disappeared about half an -hour previously, and the cirro-nebula had changed into the remarkable -forms of cloud depicted. Plate <a href="#Plate_21">21</a> shows cirrus ripples in the upper -part, and cirro-cumulus in soft, ill-defined balls in the lower part; -but they were at the same level, and are only different parts of the -same cloud plane. In Plate <a href="#Plate_22">22</a> we see similar ball-like cloudlets -ranged in long lines which run almost at right angles to the ripples -of the companion picture. Clouds like these are rare. They<span -class="pagenum"><a name="Page_52" id="Page_52">[52]</a></span> are -almost unknown during the early part of the day, and, so far as the -writer’s experience goes, they are only to be found in the -afternoon towards sunset. Some of our most gorgeous sunset skies are -due to them; for their altitude is considerable, and they do not light -up with the sunset colours until the lower clouds have become dark -shadows against the glowing background. The hottest months of the -year, the still air and great evaporation which are the contributing -causes of thunderstorms, are also the conditions under which such -skies may be seen. Indeed, while these photographs were being taken, -heavy thunderstorms were in progress within less than a hundred miles. -Cirro-cumulus nebulosus, or hazy cirro-cumulus, describes the form -correctly.</p> - -<p><a id="Plate_20"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p050f.jpg"> - <img src="images/i_p050f-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 20.</span></p> - <div class="caption"> - <p>HAZY CIRRO-CUMULUS.</p> - <p>(<i lang="la" xml:lang="la">Cirro-cumulus Nebulosus.</i>)</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p050f-hh.jpg" alt="" /> - <p><span class="smcap">Plate 20.</span></p> - <div class="caption"> - <p>HAZY CIRRO-CUMULUS.</p> - <p>(<i lang="la" xml:lang="la">Cirro-cumulus Nebulosus.</i>)</p> - </div> -</div> - -<p><a id="Plate_21"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p050h.jpg"> - <img src="images/i_p050h-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 21.</span></p> - <div class="caption"> - <p>HAZY CIRRO-CUMULUS.</p> - <p>(<i lang="la" xml:lang="la">Cirro-cumulus Nebulosus.</i>)</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p050h-hh.jpg" alt="" /> - <p><span class="smcap">Plate 21.</span></p> - <div class="caption"> - <p>HAZY CIRRO-CUMULUS.</p> - <p>(<i lang="la" xml:lang="la">Cirro-cumulus Nebulosus.</i>)</p> - </div> -</div> - -<p>The next plate, No. <a href="#Plate_22">22</a>, gives a view of an evening sky about half an -hour after sunset. The lower clouds, cirro-cumulus and cirro-stratus, -of a deep purple brown, standing out dark against a gold-coloured sheet -of higher cirro-stratus, which comes out white in the photograph, while -the purple-tinted sky comes dark. We have here three distinct layers, -all cirrus. First, the hazy cirro-cumulus,<span class="pagenum"><a -name="Page_53" id="Page_53">[53]</a></span> forming two bars across the -lower part of the picture; then long bands of cirrus or cirro-stratus, -best seen in the bottom right-hand corner; and, far above both, the -cirro-stratus which was reflecting the yellow sunlight. Such a sky -might be an indication of thunder conditions, or it might be due to an -unusual quantity of vapour in the atmosphere produced by some other -cause. The actual conditions were the gentle flow over England of -vapour-laden air from the western ocean, heralding the change from a -long spell of fine hot weather, due to a July anticyclone, to a month -of heavy rains and western gales, accompanying the passage of a long -procession of cyclones along our western shores.</p> - -<p><a id="Plate_22"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p052a.jpg"> - <img src="images/i_p052a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 22.</span></p> - <div class="caption"> - <p>A SUNSET SKY.</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p052a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 22.</span></p> - <div class="caption"> - <p>A SUNSET SKY.</p> - </div> -</div> - -<p>Again, a marked contrast is shown in Plate <a href="#Plate_23">23</a>. Here we have the -highest and thinnest form of cirro-cumulus, the one named cirro-macula -by Mr. Ley. It is rarely, if ever, seen before eleven o’clock -in the morning, and is far commoner in the afternoon. The example -shown was photographed at sunset at the close of a day which had -been almost cloudless. Cirro-macula forms here and there in a clear -sky. A hazy, whitish patch appears, which at first shows no definite -structure, but looks almost<span class="pagenum"><a name="Page_54" -id="Page_54">[54]</a></span> like a little bit of cirro-nebula. This -suddenly splits up by clear blue lanes running through it, and cutting -the patch up into irregular segments, which quickly round themselves -off into minute bits usually whiter on their edges and semi-transparent -in the centre. The process can be strikingly imitated by scattering on -water some fine powder which will float. If left without disturbance, -the particles draw together into numerous small groups, leaving lanes -of clear water between them.</p> - -<p><a id="Plate_23"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p052b.jpg"> - <img src="images/i_p052b-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 23.</span></p> - <div class="caption"> - <p>SPECKLE CLOUD (Ley).</p> - <p>(<i lang="la" xml:lang="la">Cirro-macula.</i>)</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p052b-hh.jpg" alt="" /> - <p><span class="smcap">Plate 23.</span></p> - <div class="caption"> - <p>SPECKLE CLOUD (Ley).</p> - <p>(<i lang="la" xml:lang="la">Cirro-macula.</i>)</p> - </div> -</div> - -<p>Cirro-macula frequently gives rise to the fibrous form of cirrus -we have called cirrus caudatus. The granules of the cirro-macula grow -denser, and begin to drop their frozen particles as soon as they -become large enough. Indeed, a cloudlet of cirro-macula may sometimes -be seen to turn bodily into a fine line of falling crystals, which -will be a curving line of cirrus. On the other hand, it will sometimes -remain visible for an hour or more without any trace of descending -streaks or floating fibres. Pure cirro-macula such as Plate <a href="#Plate_23">23</a> is not -often seen; it is far more frequently mixed with more solid-looking -cloudlets and descending fibres, such as are shown in Plate <a href="#Plate_24">24</a>, which -gives the same point of view as<span class="pagenum"><a name="Page_55" -id="Page_55">[55]</a></span> 23, but a quarter of an hour later, and -photographed with a longer focus lens. These two photographs, together -with 20 and 21, give excellent examples of the use of the black -mirror. In none of the four could the naked eye detect all of the -cloud structures. The whole sky was a blaze of dazzling light, but by -adjusting exposure and development the details are fully brought out -without the least difficulty.</p> - -<p><a id="Plate_24"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p054b.jpg"> - <img src="images/i_p054b-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 24.</span></p> - <div class="caption"> - <p>CIRRUS CAUDATUS AND CIRRO-MACULA.</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p054b-hh.jpg" alt="" /> - <p><span class="smcap">Plate 24.</span></p> - <div class="caption"> - <p>CIRRUS CAUDATUS AND CIRRO-MACULA.</p> - </div> -</div> - -<p>Cirro-stratus, we see from the examples which have been considered, -hardly deserves to be treated as a distinct genus of cloud. Its -formation is identical with that of many species of cirrus, or in some -cases with that of the speckle cloud, cirro-macula, or even the coarser -kinds of cirro-cumulus. The different varieties which it shows are best -rendered by reference to the specific names of the detached forms which -have similar features.</p> - -<p>Cirro-cumulus, on the other hand, does present clearly -marked varieties. Cirro-macula is so distinct that it might -well be given the name awarded to it by Mr. Ley, while the term -“cirro-cumulus” is reserved for the coarser and rounder -forms. The hazy, ripple-like structures of Plate <a href="#Plate_4">4</a> and Plate <a href="#Plate_20">20</a><span -class="pagenum"><a name="Page_56" id="Page_56">[56]</a></span> should -also have some distinctive appellation, as will be suggested later on -when dealing with wave clouds as a whole.</p> - -<p>It is difficult to find any short way of expressing the various -ideas which should be summed up in the name of a cloud. There seems -no alternative to the use of additional words, unless it be to follow -the example of chemists, and compound appalling names similar to -those which terrify the uninitiated who think they would like to read -something about, let us say, the coal-tar dyes.</p> - -<p>If a cloud belongs to the order cirrus, is in a level sheet, and -that sheet is composed of interlacing or curling fibres, like those -of common cirrus, we can hardly express the facts more briefly than -by calling it cirro-stratus communis, or common cirro-stratus. If -it consists of cirrus bands fused together, but still showing the -banded structure, it is cirro-stratus vittatus. Again, if it is finely -speckled, like cirro-macula, it may be described as cirro-stratus -maculosus, and if the structure is coarser it may be called -cirro-stratus cumulosus.</p> - -<p>As a general average, cirro-stratus lies somewhat lower in -the atmosphere than the detached forms,<span class="pagenum"><a -name="Page_57" id="Page_57">[57]</a></span> probably because the -conditions which give rise to the latter reach to greater altitudes -in patches than it is possible for them to reach in a continuous -manner. Vapour becomes rarer with increased height and with diminished -temperature, so that it must, on the whole, be less frequently present -in cloud-producing quantity as the height increases. At great altitudes -it will be seldom that the quantity is great enough to produce a -stratiform cloud, though it may well be enough for cirro-macula, or the -detached forms of cirrus, like cirrus excelsus.</p> - -<p>The production of cirro-cumulus and cirro-stratus sometimes spreads -across the sky with astonishing speed, and this rapid advance of the -edge of the cloud may lead to quite mistaken ideas as to the velocity -of the wind at that altitude. In the case of cirro-cumulus, or -cirro-macula, it is easy to fix attention on a single cloudlet. If this -has the usual ball-like form, it can only be regarded as floating in -the air and moving with it. Meanwhile new cloudlets may be forming and -growing denser, so that the cloud patch as a whole may be apparently -advancing at a much greater rate. Careless observation would then -lead to the idea that the<span class="pagenum"><a name="Page_58" -id="Page_58">[58]</a></span> cloud was moving much faster than it -really is, but if the attention is rigidly fixed on a particular -cloudlet the mistake is impossible. If the cloud is a variety of -cirro-stratus, it is not always easy, or even possible, to distinguish -between the advance of condensation and the movement of the whole, but -it can nearly always be done if the cloud shows any definite features -upon which attention can be fastened. Sometimes none sufficiently -marked can be seen, and when that happens it is still possible in most -cases, by watching the edge of the cloud-mass, to see whether new cloud -is being added to that edge. The wave-like forms present a special -case, which will be dealt with in a later chapter, after the general -principles of cloud formation have been discussed in connection with -the great clouds of the lower air, whose causes and conditions are far -better understood.</p> - -<div class="chapter"></div> - -<p><span class="pagenum"><a name="Page_59" id="Page_59">[59]</a></span></p> - -<h2>CHAPTER IV<br /> - -<span class="chtitle">ALTO CLOUDS</span></h2> - -<p class="noindent"><span class="smcap">From</span> cirro-cumulus -and cirro-stratus we pass through almost insensible gradations to -the denser forms classed together in the alto group. These clouds -are fundamentally different, in that they are always composed -of liquid particles, though there is no doubt, from their great -altitude, that their temperature must often be many degrees below the -ordinary freezing-point of water. When this is the case, they are not -unfrequently more or less mixed with streaks and filaments exactly like -those described under the name of cirrus, which have been explained -as due to slowly falling snowflakes. It is not immediately obvious -how such apparently contradictory statements can be reconciled. The -explanation is that minute droplets of water may be cooled many degrees -below freezing-point without<span class="pagenum"><a name="Page_60" -id="Page_60">[60]</a></span> changing into ice, and that such -super-cooled droplets congeal instantly if a few of them join together -to form a larger drop. Practically the same process may be watched any -day when there is a sharp frost and dense fog drifting slowly along. -The fog-particles are liquid, and produce optical effects in the -neighbourhood of any brilliant light, like an arc lamp, absolutely the -same as those which would be produced if the temperature were above -freezing-point, while there are none of the different phenomena which -might be expected if the particles were crystalline ice-dust. As these -liquid particles drift along they come in contact with branches of -trees and other obstacles, the surface stratum which surrounds them and -binds them into spheres is broken, and the drop instantly solidifies. -It is to be noted, moreover, that the drop does not freeze as such, -but merely adds some more particles to the branching crystals of hoar -frost, which grow outwards always towards the direction from which the -fog is drifting.</p> - -<p>Most liquids, when freed from contact with solid bodies, or -when surrounded by a smooth envelope of uniform character, can -be cooled below their<span class="pagenum"><a name="Page_61" -id="Page_61">[61]</a></span> normal freezing-point without -solidification taking place; but the introduction of a particle of -the solid, or sometimes of any foreign body, instantly brings about a -rapid freezing of the whole. These phenomena of surfusion, as it is -called, have long been known, and many of them are very interesting -and difficult to understand. Indeed, it is probable that we shall have -to add largely to our knowledge of the forces which bind the molecules -of a body together to form a solid, and which direct the processes of -crystallization before we shall be able to interpret with any certainty -a series of facts depending on the attributes of those very forces.</p> - -<p>Water is no exception. If finely divided, as by placing it in fine -capillary tubes, in the pores of wood, or in the narrow spaces of a -wick, it may be cooled several degrees below normal freezing-point. -In a cloud, or fog, all the conditions necessary for surfusion to -take place are undoubtedly present. The water is pure, the envelope -is uniform, the subdivision is exceedingly minute, and the drops are -free from most of the mechanical disturbances which bring about the -solidification of larger masses in the laboratory.</p> - -<p><span class="pagenum"><a name="Page_62" -id="Page_62">[62]</a></span></p> - -<p>Thus we see there is nothing at all surprising in the fact that -clouds composed of liquid particles may exist at temperatures below -the ordinary freezing-point. On the contrary, we should expect that -the solidification of the cloud particles would not take place until -the temperature was many degrees below freezing, as is certainly the -case with clouds of the cirrus order. At temperatures between this -unknown, but low value, and the normal freezing-point, the clouds will -be composed of liquid; but when the particles join together, snowflakes -will result instead of raindrops; and this will be just as true of -alto clouds as it is of the great vaporous mountains of the lower -regions of the air which bring falls of snow. The streaks often mixed -with alto-cumulus are cirrus threads, and are, no doubt, of exactly -the same nature as the tails of cirrus caudatus, or even the fibres of -cirro-filum.</p> - -<p>The simplest alto cloud is alto-stratus. When this is complete, -so as to cover the sky, it can be distinguished from cirro-stratus -by the absence of fibrous structure, and by the facts that it never -produces any halo or fragment of a halo, but instead surrounds the -sun or moon with a white blur, or, if<span class="pagenum"><a -name="Page_63" id="Page_63">[63]</a></span> it is thin enough, with -a close ring of coloured light much nearer than a halo, and with the -colours in the inverse order—that is, with the red furthest from -the centre. Some of these so-called coronæ are very beautiful -when seen in the black mirror, and some of those formed around a full -moon show quite brilliant tints to the unaided eye. Of course, these -meteorological coronæ have no relation whatever to the true solar -corona; they are simply formed by the passage of the rays of light -through the veil of small particles, and may be easily imitated. Take a -piece of glass such as a lantern-cover glass, breathe on it, and hold -it close before the eye while looking at some small source of light. If -the dew deposit is thin, bright colours are shown in a luminous ring -surrounding the light, and the thinner the deposit of dew the larger -the ring will be. Breathe heavily so as to give a thick deposit, and -the light will be seen to be the centre of a patch of white brightness -without any colour.</p> - -<p>The phenomena are due to what is known as diffraction, and if the -other conditions are unchanged the diameter of the ring is inversely -proportional to the size of the particles. Purity of colour in<span -class="pagenum"><a name="Page_64" id="Page_64">[64]</a></span> these -rings is an indication of uniformity in the size of the particles. -When the moon is shining through a sheet of alto-stratus, which thins -off to one edge, very beautiful effects may often be noticed, and the -change from the colourless blur, when a thicker part of the cloud -is interposed, to the brilliant colours of the corona formed by the -thinner edges is very striking. Similar phenomena are shown almost -equally well by any of the alto clouds, but cirrus thin enough to -produce a coloured corona will generally produce a halo.</p> - -<p>Alto-cumulus of the kind most nearly allied to cirro-cumulus -is shown in Plate <a href="#Plate_25">25</a>. The upper part of the picture shows ragged, -irregular patches, with slight indications of fibrous streaks. The -lower portion shows rounder, ball-like cloudlets, a few of the larger -of which have distinct shadows on the side away from the sun. This -plate gives alto-cumulus in a partly formed condition, but it is -not a mere passage form. Sometimes exactly such a cloud will float -overhead for hours, showing very little movement and only slow changes -of detail. It is therefore a distinct variety, and may be called -alto-cumulus informis.</p> - -<p><a id="Plate_25"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p064a.jpg"> - <img src="images/i_p064a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 25.</span></p> - <div class="caption"> - <p>ALTO-CUMULUS INFORMIS.</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p064a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 25.</span></p> - <div class="caption"> - <p>ALTO-CUMULUS INFORMIS.</p> - </div> -</div> - -<p><span class="pagenum"><a name="Page_65" id="Page_65">[65]</a></span></p> - -<p>A less definite form is shown in Plate <a href="#Plate_26">26</a>. It may also be regarded -as only partly formed, but its construction is quite different, every -part being misty and ill defined. It is a common cloud, especially in -sultry summer weather with still air. Under those circumstances, after -a hazy morning, it may be seen slowly forming during the afternoon, -growing in density as the hours go by, until it reaches a maximum about -five or six o’clock, after which it melts away, or settles down -into small patches of high stratus. Most frequent in summer, it is by -no means rare in autumn and winter, but still air is essential. From -its hazy appearance it may be called alto-cumulus nebulosus.</p> - -<p><a id="Plate_26"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p064d.jpg"> - <img src="images/i_p064d-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 26.</span></p> - <div class="caption"> - <p>HAZY ALTO-CUMULUS.</p> - <p>(<i lang="la" xml:lang="la">Alto-cumulus Nebulosus.</i>)</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p064d-hh.jpg" alt="" /> - <p><span class="smcap">Plate 26.</span></p> - <div class="caption"> - <p>HAZY ALTO-CUMULUS.</p> - <p>(<i lang="la" xml:lang="la">Alto-cumulus Nebulosus.</i>)</p> - </div> -</div> - -<p>Fixity of detail and slow movement characterize both the foregoing -forms, and in that respect our next picture (Plate <a href="#Plate_27">27</a>) shows a cloud -which is a great contrast. Its detached cloudlets are rather flatter -and thinner, and though the cloud as a whole will often persist for -hours, it is undergoing continual change, and is formed when the air -is far from still. Cloudlets form and gather into stratiform patches, -which soon break up again and disappear; and the process goes on -here and there, sometimes<span class="pagenum"><a name="Page_66" -id="Page_66">[66]</a></span> accompanied by fairly rapid movement of -the patches as a whole. This cloud may be described as alto-cumulus -stratiformis.</p> - -<p><a id="Plate_27"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p064f.jpg"> - <img src="images/i_p064f-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 27.</span></p> - <div class="caption"> - <p>FLAT ALTO-CUMULUS.</p> - <p>(<i lang="la" xml:lang="la">Alto-cumulus Stratiformis.</i>)</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p064f-hh.jpg" alt="" /> - <p><span class="smcap">Plate 27.</span></p> - <div class="caption"> - <p>FLAT ALTO-CUMULUS.</p> - <p>(<i lang="la" xml:lang="la">Alto-cumulus Stratiformis.</i>)</p> - </div> -</div> - -<p>We now come, in Plate <a href="#Plate_28">28</a>, to a cloud of singular beauty. It -forms rapidly in a clear sky, its first traces bearing a striking -resemblance to cirro-macula, but the floccules, instead of remaining -semi-transparent or dropping cirrus threads, rapidly become opaque -balls of cloud which lengthen upwards. This upward tendency causes -the formed cloudlets to have their longer axes vertical, which is -very characteristic. It might be named alto-cumulus castellatus, -or high-turreted cloud. Mr. Ley named it stratus castellatus, or -turret-cloud, but it certainly belongs to the cumulus section of -the alto group. Thunder weather is the invariable condition for -its production. If it is seen, at least in England, thunderstorms -are certain to be recorded not very far away. When this particular -photograph was being taken in South Devon, very destructive storms were -recorded in Brittany and in the English Midlands, and the anvil-shaped -tops of unmistakable thunder-clouds were visible above the horizon -while the exposure was being made.</p> - -<p><a id="Plate_28"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p066a.jpg"> - <img src="images/i_p066a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 28.</span></p> - <div class="caption"> - <p>HIGH TURRETED CLOUD.</p> - <p>(<i lang="la" xml:lang="la">Alto-cumulus Castellatus.</i>)</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p066a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 28.</span></p> - <div class="caption"> - <p>HIGH TURRETED CLOUD.</p> - <p>(<i lang="la" xml:lang="la">Alto-cumulus Castellatus.</i>)</p> - </div> -</div> - -<p><span class="pagenum"><a name="Page_67" id="Page_67">[67]</a></span></p> - -<p>Another form of almost equal beauty is shown in Plate <a href="#Plate_29">29</a>. The -rounded balls make their appearance as semi-transparent spots upon -the sky, and in their general characters might easily be mistaken for -cirro-macula. But a few minutes will be enough to decide the question. -The little spots rapidly grow denser, frequently becoming ragged at -the edges; they never drop down the slender filaments which usually -descend from cirro-macula, and their edges are never denser than their -central parts, which, it will be remembered, was a frequent feature -of the true speckle cloud. The cloudlets are obviously rounded balls -arranged in patches, which may turn gradually into alto-stratus by -their fusion, or, after an existence of minutes or hours, the whole -may disappear by a disintegration of each ball, by its breaking up -into a ragged mass and melting away. The altitude at which this cloud -forms is between 5000 and 9000 metres, according to measurements made -by the writer, the actual specimen figured being about 7000. It is -almost as characteristic of thunder weather as the last, but whereas -Plate <a href="#Plate_28">28</a> shows a variety which is most often seen before 3 p.m., since -it only occurs while the cloud<span class="pagenum"><a name="Page_68" -id="Page_68">[68]</a></span> planes are rapidly rising, the one before -us may be formed at almost any time of day, but most frequently occurs -in the afternoon. An imperfect form of it is frequently met with about -sunset, in which the rounded balls are not usually so well defined as -when the sun is high above the horizon. Alto-cumulus glomeratus would -be a suitably descriptive name.</p> - -<p><a id="Plate_29"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p066d.jpg"> - <img src="images/i_p066d-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 29.</span></p> - <div class="caption"> - <p>HIGH BALL CUMULUS.</p> - <p>(<i lang="la" xml:lang="la">Alto-cumulus Glomeratus.</i>)</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p066d-hh.jpg" alt="" /> - <p><span class="smcap">Plate 29.</span></p> - <div class="caption"> - <p>HIGH BALL CUMULUS.</p> - <p>(<i lang="la" xml:lang="la">Alto-cumulus Glomeratus.</i>)</p> - </div> -</div> - -<p>If it were possible to take a good typical example of the variety -just described and roll it flat, so that each cloudlet should be -reduced to a lenticular shape, we get a type which seems seldom to -appear during the heat of the day, and to be most frequent about -sundown. It consists, as shown in Plate <a href="#Plate_30">30</a>, of distinct cloudlets, -with considerable spaces between them, and gives the impression -of a discontinuous level sheet. But the component cloudlets are -much too definite, and preserve their individuality far too well -to suggest any idea of a broken stratus; the spotted structure is -the predominant feature, while the stratiform arrangement is almost -equally plain. Alto-stratus maculosus would be a suitable term. -It is not so high a cloud as the glomeratus type, the one shown -being at an altitude of about 5600 metres. The plate shows the -position of the setting sun,<span class="pagenum"><a name="Page_69" -id="Page_69">[69]</a></span> which is partly hidden behind some dark -patches of broken alto-stratus (fracto-alto-stratus), the hazy form -and boundaries of which form an effective contrast to the shining -cloudlets 2000 metres or so above them. Many of our most beautiful -sunsets are due to this form of cloud, particularly in the late autumn. -It is a cloud of calm weather, and often floats apparently motionless, -and undergoing little change, like flakes of glowing fire against the -background of a fading sky long after the sun has disappeared. It is -not indicative of thunder conditions, and it may occur on the margins -of an anticyclone.</p> - -<p><a id="Plate_30"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p068a.jpg"> - <img src="images/i_p068a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 30.</span></p> - <div class="caption"> - <p>MACKEREL SKY.</p> - <p>(<i lang="la" xml:lang="la">Alto-stratus Maculosus.</i>)</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p068a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 30.</span></p> - <div class="caption"> - <p>MACKEREL SKY.</p> - <p>(<i lang="la" xml:lang="la">Alto-stratus Maculosus.</i>)</p> - </div> -</div> - -<p>A lower and coarser form of the spotted alto-stratus is shown in -Plate <a href="#Plate_31">31</a>, where it is seen through the gaps in a thin sheet of broken -stratus. In this case also the sun was getting low in the sky, being -hidden by the denser bit of stratus in the bottom left-hand corner.</p> - -<p><a id="Plate_31"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p068d.jpg"> - <img src="images/i_p068d-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 31.</span></p> - <div class="caption"> - <p>MACKEREL SKY.</p> - <p>(<i lang="la" xml:lang="la">Alto-stratus Maculosus.</i>)</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p068d-hh.jpg" alt="" /> - <p><span class="smcap">Plate 31.</span></p> - <div class="caption"> - <p>MACKEREL SKY.</p> - <p>(<i lang="la" xml:lang="la">Alto-stratus Maculosus.</i>)</p> - </div> -</div> - -<p>Alto-stratus does not often, if ever, grow from the fusion of the -cloudlets of the maculosus type. But it does come from alto-cumulus -glomeratus, and also from a form shown in Plate <a href="#Plate_32">32</a>. Here we have -alto-stratus in process of growth. Small irregular lumps of cloud -forming on the right-hand<span class="pagenum"><a name="Page_70" -id="Page_70">[70]</a></span> side of the picture grow larger and more -irregular, begin to fuse together towards the centre, and on the -left-hand side the fusion is almost complete. Still, although the sky -is covered with cloud, the lumpy form is plainly visible. The term -“alto-strato-cumulus” is suitable, as it differs from the -more frequent and much lower cloud, which will be described further on -as strato-cumulus, in little else than altitude and general massiveness -of texture. This high strato-cumulus is common enough, too common, -indeed, in England, as it produces many a dull grey sky both in summer -and in winter. In the latter season it is not unfrequent with the cold -east winds of February and March. It is probably the lowest of the alto -clouds; the lowest measurement made by the writer being 1828 metres at -Exeter, but lower altitudes seem to have been recorded elsewhere.</p> - -<p><a id="Plate_32"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p070a.jpg"> - <img src="images/i_p070a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 32.</span></p> - <div class="caption"> - <p>ALTO-STRATO-CUMULUS.</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p070a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 32.</span></p> - <div class="caption"> - <p>ALTO-STRATO-CUMULUS.</p> - </div> -</div> - -<p>Alto-cumulus castellatus, which is breaking up and disappearing, is -shown in Plate <a href="#Plate_33">33</a>. It was photographed with a long-focus lens, so that -the scale of representation is about eight times as great as that of -Plate <a href="#Plate_27">27</a>. This view was taken at Exeter while a thunderstorm was in -progress at Bristol.</p> - -<p><a id="Plate_33"></a></p> - -<div class="figcenter screenonly" style="width: 266px;"> - <a href="images/i_p070c.jpg"> - <img src="images/i_p070c-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 33.</span></p> - <div class="caption"> - <p>SUNSET.</p> - <p>(<i lang="la" xml:lang="la">Alto-cumulus Castellatus Fractus.</i>)</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 460px;"> - <img src="images/i_p070c-hh.jpg" alt="" /> - <p><span class="smcap">Plate 33.</span></p> - <div class="caption"> - <p>SUNSET.</p> - <p>(<i lang="la" xml:lang="la">Alto-cumulus Castellatus Fractus.</i>)</p> - </div> -</div> - -<div class="chapter"></div> - -<p><span class="pagenum"><a name="Page_71" id="Page_71">[71]</a></span></p> - -<h2>CHAPTER V<br /> - -<span class="chtitle">LOWER CLOUDS</span></h2> - -<p class="noindent"><span class="smcap">The</span> clouds of the lower -portions of the atmosphere are formed in regions where water vapour -is abundant, and frequently in easy reach of the strong ascending -and descending air currents produced by the varying temperatures and -irregular surface of the ground. It is sufficient to recite these -conditions to show that these lower clouds will be denser, larger, -coarser in texture, and characterized by greater definiteness of form -than those we have, so far, considered.</p> - -<p>In the International system they are classified thus—</p> - -<p>Group C. Lower clouds.</p> - -<p class="inset">(<i>a</i>) Strato-cumulus.<br /> -(<i>b</i>) Nimbus.</p> - -<p>Group D. Clouds of diurnal ascending currents.</p> - -<p class="inset">Cumulus and cumulo-nimbus.</p> - -<p><span class="pagenum"><a name="Page_72" id="Page_72">[72]</a></span></p> - -<p>Group E. High fogs.</p> - -<p class="inset">Stratus.</p> - -<p>This is certainly the least satisfactory part of the whole -scheme, and it is not at all easy to see upon what grounds it was -adopted by the International Committee. Group D—cumulus and -cumulo-nimbus—do show important differences from the other -groups, though it is often difficult to say whether the sky should be -described as covered with strato-cumulus or as covered with numerous -small cumulus. It is the separation of stratus—placing it in a -group by itself, and making that the lowest—which is the worst -point. As a matter of fact, stratus may exist at any altitude from -sea-level up to such heights that we should not hesitate to call it -alto-stratus. Indeed, there is no essential character of alto-stratus -which distinguishes it from some of the lower forms. Whatever its -altitude, its thickness and the size of its particles may vary in a -precisely similar manner. We may have the particles exceedingly small, -when the fog will be dry, and such a stratus may be so thin as hardly -to dim the sun; or it may be so thick as to completely hide it. On -the other hand, the fog may<span class="pagenum"><a name="Page_73" -id="Page_73">[73]</a></span> consist of particles easily visible to the -naked eye, forming the so-called Scotch mist, or the “dry” -fog of Dartmoor, which will wet things as rapidly and more thoroughly -than a smart shower. When such a fog accumulates to a sufficient -depth, the particles in their fall pick up others, and the result is -a distinct fine rain. This may occur not only near the ground, but at -almost any level below that at which the cloud would pass into the -region of cirrus.</p> - -<p>Plate <a href="#Plate_34">34</a> shows three layers of stratus, in each case much broken up. -The highest layer is a good example of alto-stratus maculosus. Lower -down, by half a mile or more, come parts of a grey sheet considerably -denser and thicker. It is a matter of taste whether this should be -called high stratus or low alto-stratus. There is no test by which -the one can be distinguished from the other. Lower again come the -detached darker clouds, which are fragments of a sheet of stratus -which is breaking up and disappearing. The photograph was taken in the -afternoon, after a wet morning, and all three layers were probably -relics of the great rain-cloud system, or nimbus, which produced the -rainfall.</p> - -<p><a id="Plate_34"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p072b.jpg"> - <img src="images/i_p072b-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 34.</span></p> - <div class="caption"> -<p>THREE LAYERS OF STRATIFORM CLOUD AFTER RAIN.</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p072b-hh.jpg" alt="" /> - <p><span class="smcap">Plate 34.</span></p> - <div class="caption"> -<p>THREE LAYERS OF STRATIFORM CLOUD AFTER RAIN.</p> - </div> -</div> - -<p>We have referred to the production of fine rain<span -class="pagenum"><a name="Page_74" id="Page_74">[74]</a></span> from a -thick fog. If now such a thick layer of coarse-grained fog—if -we may use such a phrase—is suspended overhead at a moderate -altitude, the result is a drizzling rain underneath, and the -cloud at once becomes a nimbus. When Luke Howard adopted the term -“nimbus,” he proposed to employ it, apparently, for a -vast mass of cloud such as that which forms the rainy region of a -cyclone; a huge pile of clouds containing representatives of all his -other types in some unknown but close relationship. It was, in fact, -a comprehensive term, and as such there was a good use for it. At -present it is applied to any cloud from which rain is falling, except -when the cloud can be identified as a variety of cumulus which is -called cumulo-nimbus. But we have already said that a stratus may be -a rain-cloud, and so may other varieties. Moreover, whenever a nimbus -breaks sufficiently for us to be able to see its upper surface, we -invariably find that, if it were viewed from above, we should, without -a moment’s hesitation, place it in one of the other groups. It is -only when we are underneath it we can see its rain-producing character, -and give it the orthodox name. The real fact is that nimbus should -be<span class="pagenum"><a name="Page_75" id="Page_75">[75]</a></span> -an adjective, meaning rain-producing, and not a substantive.</p> - -<p>However, it has its allotted place in the International system, and -it is better to adhere as far as possible to a defective but widely -recognized system until it can be authoritatively amended, rather than -to make an individual attempt to ignore it. The facts are sufficiently -obvious, and the days of nimbus as a type are numbered. The two plates, -Nos. 35 and 36, are fair typical representations of the clouds usually -known as nimbus; but they are both of them only the under-surfaces of -other clouds, Plate <a href="#Plate_36">36</a> showing the under-surfaces of a group of heavy -cumulo-nimbus all joined together so as to cover the sky, while Plate -35 shows a mass of dense strato-cumulus. The rain-cloud is always -a form of either stratus or cumulus, or a combination of the two, -sometimes in further combination with clouds of the alto class, or even -extending upwards to cirro-stratus and cirro-nebula. Where it consists -of a single layer, that layer differs from its rainless representative -only in greater thickness from base to summit, or in greater density; -and when there are several distinct layers<span class="pagenum"><a -name="Page_76" id="Page_76">[76]</a></span> of cloud, so that the -lowest is shaded by the higher, rain may fall, even though they differ -in no visible way from clouds which would be rainless if alone. Plate -33 is an example.</p> - -<p><a id="Plate_35"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p074b.jpg"> - <img src="images/i_p074b-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 35.</span></p> - <div class="caption"> -<p>RAIN-CLOUD.</p> -<p>(<i lang="la" xml:lang="la">Nimbus.</i>)</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p074b-hh.jpg" alt="" /> - <p><span class="smcap">Plate 35.</span></p> - <div class="caption"> -<p>RAIN-CLOUD.</p> -<p>(<i lang="la" xml:lang="la">Nimbus.</i>)</p> - </div> -</div> - -<p><a id="Plate_36"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p074d.jpg"> - <img src="images/i_p074d-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 36.</span></p> - <div class="caption"> -<p>RAIN-CLOUD.</p> -<p>(<i lang="la" xml:lang="la">Nimbus.</i>)</p> - </div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p074d-hh.jpg" alt="" /> - <p><span class="smcap">Plate 36.</span></p> - <div class="caption"> -<p>RAIN-CLOUD.</p> -<p>(<i lang="la" xml:lang="la">Nimbus.</i>)</p> - </div> -</div> - -<p>Nimbus, indeed, is not a type-form, but is merely a typical -condition, and when used as a substantive is only a convenient way -of expressing our ignorance as to the real form of the cloud we so -describe.</p> - -<p>The altitude of the base of a rain-cloud may vary considerably. -It may be anything from sea-level up to heights which vary with the -geographical conditions and with the conditions of temperature and -pressure, but probably in this country never greater than 7000 or 8000 -metres.</p> - -<p>Rain, or snow, often falls from clouds at greater altitudes than -these, but unless in its descent it passes through other lower clouds, -the drops, as a rule, will dry up and disappear. The author has often -seen quite heavy rain descending from a cloud, and disappearing -completely within a thousand feet or so of the cloud-base. On rarer -occasions a still more remarkable thing may be seen—namely, a -shower falling from an upper cloud into a lower, and none between this -lower cloud and the ground.<span class="pagenum"><a name="Page_77" -id="Page_77">[77]</a></span> This curious phenomenon can only be -explained by supposing that the convection currents which make the -lower cloud are strong enough to support the small raindrops.</p> - -<p>Pure stratus is a level sheet of cloud with little variation of -thickness, not ascending every here and there into rounded lumps. Its -most typical form covers the whole sky with a uniform grey pall, which -may or may not completely hide the sun. Such a cloud does not lend -itself to pictorial representation. A frequent form, in which the sheet -is more or less broken, is shown in Plate <a href="#Plate_37">37</a>. This is a variety which -is frequent in the summer mornings, and generally breaks up and clears -away before eleven o’clock. If, however, it appears in autumn and -winter with layers of alto cloud above, it may grow denser, and turn -into a stratiform nimbus, or it may go on drifting overhead for several -days without sign of change.</p> - -<p><a id="Plate_37"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p076b.jpg"> - <img src="images/i_p076b-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 37.</span></p> - <div class="caption"> -<p>STRATUS COMMUNIS.</p></div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p076b-hh.jpg" alt="" /> - <p><span class="smcap">Plate 37.</span></p> - <div class="caption"> -<p>STRATUS COMMUNIS.</p></div> -</div> - -<p>Break up such a sheet of cloud by numerous meandering cracks, -and round off the detached pieces so as to give them a more or less -rounded or pyramidal section, and the cloud becomes strato-cumulus, -typical representations of which are shown<span class="pagenum"><a -name="Page_78" id="Page_78">[78]</a></span> in Plates <a href="#Plate_38">38</a> and <a href="#Plate_39">39</a>, -which depict different parts of the same sky. In Plate <a href="#Plate_38">38</a> the camera -was pointed due west, and in Plate <a href="#Plate_39">39</a> it was turned round to the -north-west, so that the two views do not quite meet.</p> - -<p><a id="Plate_38"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p076d.jpg"> - <img src="images/i_p076d-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 38.</span></p> - <div class="caption"> -<p>STRATO-CUMULUS.</p></div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p076d-hh.jpg" alt="" /> - <p><span class="smcap">Plate 38.</span></p> - <div class="caption"> -<p>STRATO-CUMULUS.</p></div> -</div> - -<p><a id="Plate_39"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p078a.jpg"> - <img src="images/i_p078a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 39.</span></p> - <div class="caption"> -<p>STRATO-CUMULUS.</p></div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p078a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 39.</span></p> - <div class="caption"> -<p>STRATO-CUMULUS.</p></div> -</div> - -<p>Plate <a href="#Plate_40">40</a> is a different variety. It is stratiform, each component -cloudlet being rather ragged at the edges. In some ways it resembles -cirro-macula and the speckled varieties of alto cloud, but it -is coarser in texture and obviously at no great altitude. The -International system would call it strato-cumulus, but Mr. Ley gives -a representation from another negative taken at the same time as the -type of what he calls stratus maculosus, a name which seems far more -suitable, since the cloud bears a much closer relation to stratus than -to cumulus. In the particular instance figured, the broken structure -did not last long; the spaces gradually closed in, and a complete -stratus was the result.</p> - -<p><a id="Plate_40"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p078c.jpg"> - <img src="images/i_p078c-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 40.</span></p> - <div class="caption"> -<p>STRATUS MACULOSUS.</p></div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p078c-hh.jpg" alt="" /> - <p><span class="smcap">Plate 40.</span></p> - <div class="caption"> -<p>STRATUS MACULOSUS.</p></div> -</div> - -<p>Strato-cumulus often lasts for hours, with little or no perceptible -change, but stratus maculosus rarely persists for more than half -an hour. The first is a cloud of fairly stable conditions, the -latter is dependent for its existence upon the near approach<span -class="pagenum"><a name="Page_79" id="Page_79">[79]</a></span> to -critical conditions at one particular level, and, as we have said -in other cases, such a critical state is almost always soon passed, -with the result that the cloud either masses into a denser form, or -else breaks up and disappears. If the up and down currents are strong -enough to persist, the result will be strato-cumulus and not stratus -maculosus.</p> - -<p>A kind of stratus which is frequently seen in the daytime is shown -in Plate <a href="#Plate_41">41</a>. This is literally a lifted fog, having been formed -about midday, after ground fog in the early morning. It would be -called common stratus, or stratus communis. When it appears it is a -fairly persistent form, sometimes breaking up or swelling up into -strato-cumulus, but more often splitting into long rolls of cloud, with -margins like those of cumulus. This phenomenon is shown in Plate <a href="#Plate_42">42</a>, -which was taken in December at 11 a.m., on a day which opened with a -thick ground fog. A precisely similar cloud is frequent in the early -hours of a summer morning, as a stage in the dispersal of a radiation -ground fog. The fog first lifts from the ground, until it reaches a -height of a few hundred metres, when it splits into the long rolls -whose axes are at right angles<span class="pagenum"><a name="Page_80" -id="Page_80">[80]</a></span> to the direction of drift. The consequence -is very strange if you stand on a hilltop close under the drifting -mass, and look towards the horizon in the direction of drift. The -changing shadows give the impression that the clouds are actually -rolling along, though of course no such thing is really taking place. -As time goes on the rolls grow larger and the interspaces wider; then -transverse fissures appear, and gradually the rolls break up into small -detached cumulus. Cumulus radius, from the Latin for a rolling-pin, -might be a suitably descriptive name, but it should not be forgotten -that it is only an intermediate link between stratus and cumulus, -and, indeed, is more nearly related to the former, since it is never -produced except on the break up of stratus, while it may dry up and -disappear without reaching the cumulus stage at all. Stratus radius -would therefore be a better name.</p> - -<p><a id="Plate_41"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p078f.jpg"> - <img src="images/i_p078f-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 41.</span></p> - <div class="caption"> -<p>COMMON STRATUS.</p> -<p>(<i lang="la" xml:lang="la">Stratus Communis.</i>)</p></div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p078f-hh.jpg" alt="" /> - <p><span class="smcap">Plate 41.</span></p> - <div class="caption"> -<p>COMMON STRATUS.</p> -<p>(<i lang="la" xml:lang="la">Stratus Communis.</i>)</p></div> -</div> - -<p><a id="Plate_42"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p080a.jpg"> - <img src="images/i_p080a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 42.</span></p> - <div class="caption"> -<p>ROLLER CLOUD.</p> -<p>(<i lang="la" xml:lang="la">Stratus Radius.</i>)</p></div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p080a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 42.</span></p> - <div class="caption"> -<p>ROLLER CLOUD.</p> -<p>(<i lang="la" xml:lang="la">Stratus Radius.</i>)</p></div> -</div> - -<p>Cumulus is closely related to another form of stratus, which Mr. Ley -has named stratus lenticularis, but this appears to be so frequently -the last stage in a disappearing cumulus that its history will come -better later on. It is mentioned here as it is, after all, one of the -commonest of all forms of<span class="pagenum"><a name="Page_81" -id="Page_81">[81]</a></span> stratus, the form which appears at, or -after, sunset, and is one of the few clouds which have an English -popular name—Fall cloud. Plate <a href="#Plate_47">47</a> gives a representation of it, -standing out dark against an evening sky, with a sheet of alto-stratus -far above it in the upper part of the photograph.</p> - -<p>To sum up, then, we have among the lower clouds of more or less -stratiform pattern—<br /> - -<span class="inset">Stratus communis, or Common stratus.</span><br /> - -<span class="inset">Stratus lenticularis, the Fall cloud.</span><br /> - -<span class="inset">Stratus radius, or Roll cloud.</span><br /> - -<span class="inset">Stratus maculosus, or Mottled stratus.</span><br /> - -<span class="inset">Strato-cumulus, or Sheet cumulus.</span><br /> - -The last leads naturally to the consideration of cumulus and -cumulo-nimbus, while the term “nimbus” does not belong to -any one type-form, but sometimes to one, sometimes to another, and -generally to a mixture of two or more.</p> - -<p>A good many years ago the writer made a series of measurements -of the thickness of detached clouds of the stratus and cumulus -types, such as those which may produce a shower. The conclusions -reached in consequence of those determinations have since been amply -confirmed by subsequent<span class="pagenum"><a name="Page_82" -id="Page_82">[82]</a></span> observations. In winter no rain will fall -from a cloud unless it reaches a minimum thickness of at least 100 -metres, while in summer it must have rather greater thickness. There -is one exception, and that is in winter, when the temperature is so -low that the drop starts on its downward journey as a flake of snow. -When this is the case, rain may fall from a layer of thin lifted fog, -not quite thick enough to hide the blue colour of the sky. But under -ordinary conditions of temperature, if the cloud has a thickness less -than 2000 feet, or 616 metres, rain is unlikely, but if it does come, -the drops will be small and the fall of rain quite trifling.</p> - -<p>Above this thickness the heaviness of the rain and size of the drops -increases, so that if the distance from base to summit be between -2000 and 3000 feet, or 600 to 1000 metres, the fall will be gentle. A -thickness of 4000 to 6000 feet, or 1200 to 1800 metres, gives large -drops and a fairly heavy shower, while, in summer time at least, -cold heavy rain and hail come from clouds measuring 6000 to 10,000 -feet, or in round numbers 1800 to 3000 metres or more. In winter the -necessary dimensions seem to<span class="pagenum"><a name="Page_83" -id="Page_83">[83]</a></span> be less, but the rule still holds equally -good, that the rain-cloud does not necessarily differ in any way from -the rainless one, except in thickness, and that when the requisite -thickness is present rain is not always the result.</p> - -<div class="chapter"></div> - -<p><span class="pagenum"><a name="Page_84" id="Page_84">[84]</a></span></p> - -<h2>CHAPTER VI<br /> - -<span class="chtitle">CUMULUS</span></h2> - -<p class="noindent"><span class="smcap">Under</span> the general term -cumulus there are grouped the most common, the best known, and the -grandest forms of cloud. Indeed, beautiful as the cirrus and alto -clouds may be, there is a solid grandeur about the greater forms -of cumulus which gives them a beauty of their own quite comparable -with the charm afforded by the delicate tracery of their more lofty -rivals.</p> - -<p>Cumulus can be divided into several types, which are best considered -in the order of growth. They are all formed in the lower part of -the atmosphere, their under-surfaces varying in altitude from about -600 metres, or even less, up to 3000 metres, or slightly more. The -writer’s own measurements vary from a minimum of 584 metres to -a maximum of 2286 metres, with an average of a little more than 1000 -metres.</p> - -<p><span class="pagenum"><a name="Page_85" -id="Page_85">[85]</a></span></p> - -<p>They are described in the International system as “clouds -in a rising current,” and there is no doubt the description is -correct. Each cumulus must be looked upon as simply the visible top of -an ascending pillar of damp air. The vapour which makes its appearance -in the cloud is present in the transparent air beneath, and the base of -the cloud is simply the level at which that vapour begins to condense -into visible liquid particles. Since cumulus clouds are caused by -ascending currents, these currents must be brought about either by -the general disturbance of the air due to a cyclonic movement, or by -the local irregularities of temperature on the ground produced by the -sun’s heat. As a matter of fact, we do get cumulus produced in -great abundance in the rear of every cyclone, and we get them also -under the conditions of still air and hot sun, which specially favour -evaporation and the development of differences of temperature. The -cyclone cumulus may come at any hour of the day or night, though -comparatively rare between midnight and the morning. Heat cumulus is -generally formed during the afternoon, and it is only under relatively -uncommon conditions that it<span class="pagenum"><a name="Page_86" -id="Page_86">[86]</a></span> persists during the night. If the cloud -has not grown to very great size it usually begins to break up and -disappear about sunset, but if it has grown to the enormous dimensions -of a summer thunder-cloud it may go on growing, piling mass on to -mass, until it generates a thunderstorm, even in the hours of early -morning.</p> - -<p>In the case of some of the higher kinds of cloud, we are not able to -give any certain account of the mechanics of their production from a -study of those clouds themselves. We have already referred incidentally -to some of the speculations as to their origin and some of the facts -definitely known, but considerable light can be thrown on the genesis -of all the varieties of cirro-cumulus and alto-cumulus by a careful -study of their larger and more accessible representatives of lower -regions.</p> - -<p>The cyclone cumulus does not differ in any essential from the clouds -of calm weather. The only difference is that the uprising currents are -perhaps partly eddies, and the rate of fall of temperature with ascent -is often more rapid.</p> - -<p>Given any mass of air at a particular temperature, it can take -up and hold in the form of invisible<span class="pagenum"><a -name="Page_87" id="Page_87">[87]</a></span> vapour a fixed quantity of -water, and no more. When it holds the maximum possible it is said to be -saturated. If it is nearly saturated it would be called damp; if far -from saturated, dry. Now, the warmer the air the larger the quantity of -vapour necessary to saturate it, so that if a quantity is saturated at -a high temperature, and is then cooled, it will no longer be able to -retain all its moisture in the invisible form, but the surplus quantity -will make its appearance as liquid particles, that is to say, as mist -or cloud.</p> - -<p>Similarly, if a quantity of air is not fully saturated at its -particular temperature, and is then cooled, it will approach nearer and -nearer to saturation, and if the process is continued long enough the -result will be cloud formation.</p> - -<p>All clouds, without exception, are produced by exactly such cooling -of air containing water vapour, first to the temperature at which the -quantity it contains is the maximum possible, and then beyond that -point. Now, if we start with very warm air, and cool it 1 degree, -we decrease its vapour-holding power, and the decrease per degree -grows less and less as the temperature<span class="pagenum"><a -name="Page_88" id="Page_88">[88]</a></span> falls. Suppose, for -instance, we have air saturated at 61 degrees and cool it to 60 -degrees, the quantity of vapour condensed will be equal to the -difference of holding power. Suppose, again, we have air saturated -at 31 degrees and we cool it to 30 degrees, the quantity of vapour -condensed will again be equal to the difference of holding power; -but this quantity will be very greatly less than in the former case. -Cooling air saturated at 61 degrees to 60 degrees might produce a dense -cloud; but applying a similar reduction of 1 degree to air saturated at -31 degrees, if we take the same volume of air, will only produce a very -much thinner result. Here we see one good reason why the highest clouds -are the thinnest and the alto clouds of intermediate density.</p> - -<p>The necessary cooling may be brought about in several ways. Firstly, -the air is capable of radiating its heat into space, and therefore -of cooling. But we know little of the laws which govern atmospheric -radiation, and presumably, if cloud could be produced by such -means, it ought to make its appearance most frequently in the small -hours of the morning before sunrise. We are, however, unaware<span -class="pagenum"><a name="Page_89" id="Page_89">[89]</a></span> of -any variety of cloud which answers those conditions, unless it be -the ground fogs which so often form during the night; and these, we -know, are certainly due to the chilling of the air by contact with -the ground, which has been cooled by radiating away its heat. On -the contrary, it is well known to astronomers that the sky is, on -the whole, clearer and freer from clouds after midnight than in the -earlier hours of the night—a circumstance which is particularly -unfortunate for the amateur star-gazer, who has to be up and about at -the same time as the rest of the working world. Cooling by radiation we -may then dismiss as a cause of cloud formation of no great efficacy, -and certainly one which has little to do with the production of -cumulus.</p> - -<p>Cooling by contact with a cold body is another and more potent -cause. We often see it in a mountain district, where a frost-bound -peak stands facing the wind with glittering snow-slopes on which the -sun is shining, while a long tongue of cloud hangs like a banner on -its leeward side. In such a case it is easy to understand how the air -sweeping by the icy mass is chilled below its saturation point; but -as it passes on, the chilled portions become<span class="pagenum"><a -name="Page_90" id="Page_90">[90]</a></span> mixed with the rest, and -the cloud evaporates again. It is not quite so easy to see how far this -cause is responsible for the clouds which are formed when the warm damp -air of the ocean drifts over a comparatively cold land. It is probable -that the contact chilling is in this case only part of the explanation, -and that other causes co-operate.</p> - -<p>The mixing of warm damp air with cold has often been adduced as a -cause of clouds. No doubt it might be, and some of the stratiform types -may possibly be formed at the junction between a warm damp stratum of -air and a cold one, but no example is certainly known. It may also be a -contributing cause in producing the sharply defined upper surfaces of -some cumulus or strato-cumulus clouds, but these are in the main most -certainly due to the chief cause of cloud production—namely, what -is known as dynamic cooling.</p> - -<p>If a quantity of air exists under a certain pressure and at a -certain temperature, on reducing the pressure it will expand, and -in the act of expanding it will become cooler. This may easily -be illustrated with an air-pump. Let a damp sponge or a piece of -wet blotting-paper stand under a glass<span class="pagenum"><a -name="Page_91" id="Page_91">[91]</a></span> receiver over an air-pump -until the air has become damp. If the apparatus is in a darkened -room, and a powerful beam of light from a lantern is sent through the -receiver, the damp air will be seen to be quite clear; but a stroke or -two of the pump removes some of the air, the remainder is chilled by -its own expansion, and a dense cloud is precipitated. If this cloud be -viewed closely, it will be seen to be composed of minute particles, -which, on looking towards the light, glow with the colours of a corona. -In a few minutes the cloud will disappear, but it can be recalled -again and again by successive strokes of the pump, getting thinner and -thinner as the air gets more and more rarefied; an illustration of a -second reason why the high clouds are thinner than the lower.</p> - -<p>Some years ago Mr. John Aitken showed that if the damp air used in -this experiment were carefully filtered, so as to remove all foreign -particles, no cloud was produced, and the introduction of a puff of -unfiltered air was attended by immediate condensation. The deduction -was that vapour, even below its saturation temperature, cannot produce -cloud unless nuclei of some sort are already<span class="pagenum"><a -name="Page_92" id="Page_92">[92]</a></span> present, presumably dust -particles. Later on it was shown by Mr. Shelford Bidwell and others -that gaseous particles, such as those produced by the burning of -sulphur, would serve the purpose, and that the brush discharge from an -electrified point was in some mysterious way particularly effective. -It has recently been shown by Mr. C. T. R. Wilson that causes such as -the radiations of radium, or the impact of ultra-violet rays, acting on -the air itself, splits up some of its particles into the smaller bodies -known as ions, and that these are efficient nuclei. These experiments -open up many most interesting questions, but, unless it is to explain -the extreme density and darkness of a thunder-cloud, they do not seem -to play any important part in determining the forms to be assumed. -Nuclei in sufficient abundance are probably always present at any -height which can be reached by enough vapour to form a cloud.</p> - -<p>Now, if we have a quantity of air, say at sea-level, damp but not -saturated, and it is caused to ascend, either because it is warmer and -therefore lighter than the surrounding air, or for some other reason, -as it moves upwards the pressure upon it<span class="pagenum"><a -name="Page_93" id="Page_93">[93]</a></span> will decrease, it will -expand, and in the act it will be steadily cooled. This cooling may -after a time bring it down to the same temperature as the rest of the -air at its particular level. If so, it will no longer be lighter, and -the ascent will come to an end. But before this state of affairs is -attained it may have reached its saturation point, and cloud production -will begin.</p> - -<p>It is true that the rarefaction of the air tends to enable it to -retain more vapour than it could if it were cooled without change -of density. The temperature of the air being fixed, its holding -power increases with decrease of pressure. But this increase is much -less than the diminution due to cooling, and the result in nature -must be similar to what we can see happen under the receiver of the -air-pump.</p> - -<p>The condensation of water introduces another factor of great -importance. It has just been said that the ascending air may be -cooled so rapidly as to be reduced to the same temperature as the -rest of the air at that level, and if so the ascent will end. -Clearly the cessation or persistence of the upward motion depends -upon whether the diminution<span class="pagenum"><a name="Page_94" -id="Page_94">[94]</a></span> of temperature per 100 metres of ascent is -most rapid in the rising column or in the air outside it. As long as -the ascending air is warmer than that outside, but at its own level, so -long will ascent continue. Now, as long as no condensation was taking -place, the rate of cooling would follow a simple law which produces -a cooling of 1 degree for about 100 metres of ascent; but as soon as -water vapour begins to pass into the liquid form, a large quantity -of heat is set free, and the rate of cooling is consequently greatly -lessened. Cloud production tends, therefore, to accelerate ascent, and -the greater the amount of condensation, the more important will this -consideration become; though, on the other hand, when once the cloud -is formed, it tends to stop the rising current by shading the air and -ground beneath it.</p> - -<p>On an ordinary day the rate of decrease of temperature as we ascend -is rather less than the value given above, and uprising currents are -soon checked. If they do extend far enough to reach cloud production, -the clouds will be small, forming the smallest variety of cumulus. This -is shown in Plate <a href="#Plate_43">43</a>. Small irregular uprising currents have<span -class="pagenum"><a name="Page_95" id="Page_95">[95]</a></span> just -been able to reach far enough up to have their summits tipped with -cloud.</p> - -<p><a id="Plate_43"></a></p> - -<div class="figcenter screenonly" style="width: 265px;"> - <a href="images/i_p094a.jpg"> - <img src="images/i_p094a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 43.</span></p> - <div class="caption"> -<p>SMALL CUMULUS.</p> -<p>(<i lang="la" xml:lang="la">Cumulus Minor.</i>)</p></div> -</div> - -<div class="figcenter handonly" style="width: 463px;"> - <img src="images/i_p094a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 43.</span></p> - <div class="caption"> -<p>SMALL CUMULUS.</p> -<p>(<i lang="la" xml:lang="la">Cumulus Minor.</i>)</p></div> -</div> - -<p>After the foregoing explanation, it is easy to see why at a given -time the floating cloudlets should have a common base level. This is -the height to which the air must attain before reaching its saturation -temperature. Each cloudlet marks an uprising current, and the intervals -show the position of the counterbalancing descending streams.</p> - -<p>A larger variety is shown in Plate <a href="#Plate_44">44</a>. In this the level base and -generally pyramidal shape is shown, and also the hard, rounded upper -surface. The thickness of this cloud was about 500 metres. When clouds -like these are visible, they may be the beginning of larger ones, and -the only way to judge whether they are likely to develop into rain- or -shower-clouds is to watch them. If they are seen to be growing larger, -and particularly if detached fragments are developing into clouds, -further growth is almost certain, and rain is probable.</p> - -<p><a id="Plate_44"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p094d.jpg"> - <img src="images/i_p094d-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 44.</span></p> - <div class="caption"> -<p>CUMULUS.</p></div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p094d-hh.jpg" alt="" /> - <p><span class="smcap">Plate 44.</span></p> - <div class="caption"> -<p>CUMULUS.</p></div> -</div> - -<p>If great towering masses are making their appearance with little -dark fragments between them, as shown in Plate <a href="#Plate_45">45</a>, then smart showers -may be confidently expected. The cloud figured<span class="pagenum"><a -name="Page_96" id="Page_96">[96]</a></span> was a shower-cloud, and -the distance is seen through the veil of falling rain. The height -and thickness of this particular cloud were measured just after its -photograph had been taken. Its base was 1200 metres above the ground, -and its summit was 1500 metres further. Its thickness from summit to -base was, therefore, not much short of a mile, and the total contents -of the cloud were probably between one and a half and two cubic miles. -The upper contour is hard and rounded, as in the smaller cloud of Plate -44, but the whole cloud is much larger.</p> - -<p><a id="Plate_45"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p096a.jpg"> - <img src="images/i_p096a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 45.</span></p> - <div class="caption"> -<p>LARGE CUMULUS.</p> -<p>(<i lang="la" xml:lang="la">Cumulus Major.</i>)</p></div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p096a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 45.</span></p> - <div class="caption"> -<p>LARGE CUMULUS.</p> -<p>(<i lang="la" xml:lang="la">Cumulus Major.</i>)</p></div> -</div> - -<p>We have already explained that there seems to be a definite -connection between the thickness of such clouds and the amount of -precipitation from them. Small cumulus, less than 120 metres thick, -rarely produces rain, and nothing like a heavy shower is likely -unless the thickness exceeds 400 metres. In winter, especially in -hard frost, snow crystals may fall from the smallest cloud, even from -little fragments only a few metres thick, but the quantity of water so -precipitated will, of course, be small.</p> - -<p>As long as the top of the cumulus is rounded<span -class="pagenum"><a name="Page_97" id="Page_97">[97]</a></span> and -clearly defined, the conditions of aërial equilibrium are stable, -and the growth of the cloud has been brought to an end by a stoppage of -the ascending current. In Plate <a href="#Plate_45">45</a> the ascent has been hindered both -by the mechanical action of the falling raindrops and by the cooling -of the lower parts of the ascending column by the descent into it of -the cool drops from its colder upper part. This is probably one of the -chief reasons why a shower-cloud never maintains its activity as a rain -producer for more than a very limited period. As the cloud drifts over -the landscape, it seldom maintains its showery character for more than -ten or twenty miles, often for much less.</p> - -<p>Cumulus, like any of these three, is a cloud of the daytime. It -generally begins about ten or eleven o’clock in the morning, -grows larger until about four o’clock, and then begins to -break up and disappear. After the ascending currents have ceased, -the component cloud particles slowly settle down into the warmer air -beneath, until the mass has lost its proper pyramidal form, and has -become an irregular cloud, such as is shown in Plate 46. This is known -as degraded or fracto-cumulus.</p> - -<p><a id="Plate_46"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p096d.jpg"> - <img src="images/i_p096d-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 46.</span></p> - <div class="caption"> -<p>FRACTO-CUMULUS.</p></div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p096d-hh.jpg" alt="" /> - <p><span class="smcap">Plate 46.</span></p> - <div class="caption"> -<p>FRACTO-CUMULUS.</p></div> -</div> - -<p><span class="pagenum"><a name="Page_98" id="Page_98">[98]</a></span></p> - -<p>One consequence of the arrest of the uprising currents is the -formation of lenticular patches of stratus, called by Mr. Ley stratus -lenticularis. This is often formed about sunset, and has been named -fall cloud, from its appearance at the fall of night. The name is -appropriate in another way. The ascending currents having ceased, -the cloud particles slowly subside until they dry up in some warmer -stratum. The water vapour does not continue its descent, but slowly -diffuses in all directions, and if the fall of cloud particles is -sufficient, this stratum, which is approximately coincident with the -base of the original cumulus, soon becomes saturated, and further -particles which fall into it remain visible. This saturated zone will -slowly sink lower and lower with the descent of the particles, until it -reaches regions in which the temperature is high enough for the whole -to be evaporated without reaching saturation point. Evening stratus in -calm weather always goes through this sequence of changes. It usually -forms at, or soon after, sundown, and begins to break up and disappear -as the stars are becoming visible in the darkening sky. Plate 47 shows -a specimen of this evening stratus.</p> - -<p><a id="Plate_47"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p098a.jpg"> - <img src="images/i_p098a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 47.</span></p> - <div class="caption"> -<p>FALL CLOUD.</p> -<p>(<i lang="la" xml:lang="la">Stratus Lenticularis.</i>)</p></div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p098a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 47.</span></p> - <div class="caption"> -<p>FALL CLOUD.</p> -<p>(<i lang="la" xml:lang="la">Stratus Lenticularis.</i>)</p></div> -</div> - -<p><span class="pagenum"><a name="Page_99" id="Page_99">[99]</a></span></p> - -<p>A curious feature is sometimes shown on the underside of a thick -cloud, which is probably due to the upper part of the ascending column -having been carried beyond its position of equilibrium by its own -inertia, and then falling back again in the teeth of the still rising -lower part. The result is to give the base of the cloud an appearance -of a number of rounded masses hanging downwards below the cloud, very -suggestive of the idea that the cloud is upside down. Such an event -will not often occur, and when it does the conditions are quite wanting -in stability, and the consequent features will be very transient. When -the base of a cumulus or cumulo-nimbus is so affected, the cloud is -known as festooned cumulus, or cumulus mammatus. A precisely similar -structure may be seen under strato-cumulus, or even thick stratus. In -some countries it seems to be frequently observed, but in England it is -so uncommon that the writer has only noted it about a dozen times in -twenty years, and on no one of these did it last long enough to allow -of its portrait being taken. It is an indication of very disturbed -conditions, and is usually followed by heavy rain.</p> - -<p><span class="pagenum"><a name="Page_100" -id="Page_100">[100]</a></span></p> - -<p>When cumulus clouds are formed in air which is steadily moving as -a whole, that is to say, when there is a steady breeze, they have -a very decided tendency to follow each other in long lines. It may -often be noticed that in a particular place with a certain direction -of wind these long processions follow definite tracks in relation to -the geographical features. The phenomenon does not seem to have been -recorded except in hilly country, but has frequently been observed by -the writer. It is not the same thing as the formation of stationary -belts of cloud transverse to the wind. These cumulus float along with -the movement of the air, and the question to be answered is, why should -they follow each other so persistently, and why should the intervening -belts of sky be so continuously free from cloud.</p> - -<p>If we consider that the warm damp air which supplies them is drawn -from the ground, it seems that any cause which tends to direct this -warm stratum into definite channels, as it is carried on by the wind, -will be a competent cause of the whole phenomenon. This we find in -the presence of lofty hills which stand in the way of the warm<span -class="pagenum"><a name="Page_101" id="Page_101">[101]</a></span> -surface winds, causing them to follow more or less the general trend -of the valleys, and so delivering the rising convection currents of -cloud-producing air at the same spot.</p> - -<p>It is easy to conceive that other causes, such as a difference in -temperature or dampness of neighbouring tracts, resulting from whether -they are bare or wooded, marshland or sandy plain, might equally -suffice; or might, at least, powerfully co-operate with, or counteract, -the effect of hill and vale. But in any case it is plain that the -geographical conditions to the windward of the place of observation not -only may affect the occurrence and distribution of cloud, but if the -wind is steady it is difficult to see how they could avoid affecting -it.</p> - -<p>Another puzzling phenomenon, sometimes presented by cloud and fog, -is that our instruments for detecting humidity show that the air within -them is not always fully saturated. It seems probable that this is due -to such cloud or fog having begun the process of drying up, or that -in some way not fully understood the presence of the cloud particles -after they have first come into existence may cause the withdrawal of -some of the moisture from the<span class="pagenum"><a name="Page_102" -id="Page_102">[102]</a></span> intervening damp air. The surface of -each minute droplet exerts a pressure on its interior similar to the -pressure exerted by the film of a soap-bubble on the air within it, -and it is conceivable that some of the uncondensed vapour from outside -may diffuse through this enclosing surface film, and be retained -there in consequence of the pressure. If this is so, and subsequent -investigation can alone decide the matter, it will follow that when -once cloud production has begun it will be continued until the air -between the cloud particles is reduced so far below its saturation -point that the tendency of the drops to evaporate, that is to say, for -the imprisoned water to escape through the confining film, balances the -retaining pressure.</p> - -<p>This consideration, however, is quite incompetent to affect the -general explanation of cloud formation which has been given. Its -result would be to carry condensation a little further than the exact -saturation point, and to retard equally slightly the subsequent -evaporation of the cloud particles.</p> - -<p>We have spoken of the typical cumulus as having a roughly -pyramidal shape, and if the horizontal movement of the air is -small, the loftiest<span class="pagenum"><a name="Page_103" -id="Page_103">[103]</a></span> point of the cloud will be situated -approximately above the centre of its base. But if the horizontal -movement increases in velocity, so that the top is in a more rapidly -moving stratum than the base, it will lean forward in the direction of -movement. This is a very common phenomenon, being generally shown by -cumulus on a windy day.</p> - -<p>On much rarer occasions the converse occurs, and the top of the -cloud lags behind the base, the explanation being a lessening of the -velocity of the wind as the height above ground increases. But such -conditions rarely occur, and when they do they are due to local eddies -and affect only a limited area. Hence such clouds are isolated, and -indicate a disturbed state of the air and uncertainty of weather. The -clouds which lean forward are formed under conditions which are spread -over wide districts, such as the rear of a large cyclone, and cumulus -of that kind may follow one another across the sky for hours or even -days as long as the wind persists.</p> - -<p>So far we have considered only the round-topped types of -cumulus—those which mark the tops of ascending currents -whose ascent has been<span class="pagenum"><a name="Page_104" -id="Page_104">[104]</a></span> stopped at a comparatively early stage, -or those whose ascent is still in that early stage, though the upward -movement has not yet come to an end. The full story of the growth of -a cumulus is identical with that of the youth of a cumulo-nimbus, the -later stages of which we will consider in another chapter.</p> - -<div class="chapter"></div> - -<p><span class="pagenum"><a name="Page_105" id="Page_105">[105]</a></span></p> - -<h2>CHAPTER VII<br /> - -<span class="chtitle">CUMULO-NIMBUS</span></h2> - -<p class="noindent"><span class="smcap">Grandest</span> of all clouds -are the huge mountains of vapour which are the parents of summer -thunder-storms. They are at once distinguished from ordinary cumulus -by their upper parts, which sometimes reach beyond the region of -the alto clouds high into the realm of cirrus, and extend outwards -as a broad disc, which is occasionally indistinguishable from the -cirro-nebula and cirro-stratus which form the van of a cyclone cloud -canopy. Indeed, there seems to be no essential dividing line between a -large cumulo-nimbus and the cloud pile of a small cyclone, and no real -difference between them except their size.</p> - -<p>As a matter of fact, the term cumulo-nimbus would only be given to -the cloud when a large fraction of the whole can be seen at once.</p> - -<p><span class="pagenum"><a name="Page_106" -id="Page_106">[106]</a></span></p> - -<p>In dealing with common cumulus, it has been pointed out that the -cessation of the uprising convection currents which determines the -maximum height to which the cloud will grow is due to the rate of -cooling within the ascending column being greater than the rate of -cooling outside it. It follows that when the ascending current has -reached a certain height it will, as a whole, be just as heavy as an -equal column outside. Ascent must then cease. The equilibrium of the -air in such a case is said to be stable, and the condition of such -stability is simply that the general rate of fall of temperature per -100 metres of ascent is less than the rate of cooling dynamically -produced in an ascending current.</p> - -<p>If, however, the general rate of fall of temperature is greater than -that produced dynamically, the consequence will be that the upward -tendency of the rising air will increase as it moves upward, and the -taller the column becomes the greater will be the difference of weight -between the inside and outside columns. In such a case the equilibrium -is said to be unstable, and the result will be the production of -cumulo-nimbus.</p> - -<p><span class="pagenum"><a name="Page_107" -id="Page_107">[107]</a></span></p> - -<p>Just as cumulus may be divided into heat cumulus and the clouds of -the rear of a cyclone, so cumulo-nimbus may be divided into the same -two groups. In the case of the heat thunder-clouds the instability -of the air is effected by the rapid heating of its lower layers in -contact with the ground, those lower layers being so quickly warmed -that there is not time for them to become mixed with the overlying -air in which the rate of decrease is normal. If there is much wind we -rarely get cumulo-nimbus, because the heated air is mixed mechanically -with the overlying parts, and the rate of decrease is approximately -normal throughout. Calm air and hot sun are then one set of necessary -conditions for the production of instability.</p> - -<p>But it is well known that thunder-showers and lesser examples of -cumulo-nimbus are by no means infrequent in the rear of a cyclone, and -such storm clouds are usually attended by considerable wind. They are, -as a rule, much smaller than those produced by heat, but they have -the same form, and are evidently due to instability in the lower part -of the air, and the question is how can that condition be produced. -In order to find the answer<span class="pagenum"><a name="Page_108" -id="Page_108">[108]</a></span> it is necessary to refer to the -temperature phenomena of a cyclonic area. If a cyclone be divided -into four quadrants by two lines drawn through its centre, one in the -direction in which the system is travelling and the other at right -angles to it, then the front right-hand quadrant is the warmest and -the rear right-hand quadrant much colder. The cumulo-nimbus clouds of -a cyclone are limited to the first part of this cold quadrant, that is -to say, to the portion of the storm in which a great volume of cold -air is flowing over a district which has just been warmed and wetted -by the preceding part. The result is that, the air being warmed by -contact with damp ground at a temperature many degrees above that of -the air itself, we have produced exactly the same unstable state at a -low temperature as we have at a high temperature in the case of heat -storms. The lower temperature of the whole is enough to account for -the smaller volume of the cloud, and that in turn explains why cyclone -thunderstorms are, generally speaking, on a much smaller scale than -heat storms.</p> - -<p>The life history of a cumulo-nimbus is easily studied on a suitable -day. The rapid heating of<span class="pagenum"><a name="Page_109" -id="Page_109">[109]</a></span> the lower layers of air causes -them to expand bodily, and as they do so they lift the overlying -air, frequently in broad domes or waves. The first result is the -expansion of these upper zones, which are lightened by the flowing -away of still higher layers. Expansion means chilling, and sooner or -later its effects become visible in the formation of cirro-stratus, -cirro-cumulus, or alto-cumulus. Simultaneously the heated air near -the ground begins to rise up in tall columns, while the cooler air -from a little higher descends to take its place. Soon patches of lower -cloud appear, at first hazy and indistinct, but gradually shaping -themselves into cumulus with hazy base and rounded summits. These -rapidly assume the typical pyramidal shape, with level base and sharply -contoured top, and so far there is little to distinguish them from -an ordinary cumulus (see Plate <a href="#Plate_48">48</a>). But watch them carefully. Here -and there some will be growing taller than their fellows, and as they -grow their rate of growth increases until the top begins to show signs -of spreading outwards. Rapidly the bulging summit throws out long -fingers of cloud, radiating from the central column almost as if<span -class="pagenum"><a name="Page_110" id="Page_110">[110]</a></span> -propelled by some repulsive force. At first, these fingers are merely -projecting lumps of cloud with rounded ends, but in a few minutes they -undergo a sudden and striking change. The whole summit becomes frayed -out, drawn out into long radiating lines, which thin off against the -blue sky exactly like the edges of a sheet of cirro-stratus. False -cirrus is the name commonly given to this, but there seems no valid -reason why it should be regarded as “false.” The top of -the cloud rapidly spreads horizontally, forming a disc of cirriform -cloud, which sometimes spreads several miles ahead of the rest of the -storm. Meanwhile, the original cumulus column loses all its deep folds -and convolutions, and other round-topped cumulus arise around it until -the completed system consists of a more or less disc-shaped mass of -cumulus, with a common base, rising higher and higher towards some -central point, where these are connected, by an uprushing column of -vapour, to an upper disc with cirriform margins.</p> - -<p><a id="Plate_48"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p108b.jpg"> - <img src="images/i_p108b-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 48.</span></p> - <div class="caption"> -<p>THUNDER-CLOUDS FORMING.</p></div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p108b-hh.jpg" alt="" /> - <p><span class="smcap">Plate 48.</span></p> - <div class="caption"> -<p>THUNDER-CLOUDS FORMING.</p></div> -</div> - -<p>In Plate 49 we have on the left hand a specimen in which -the outspreading is just beginning, and the same cloud is -shown half an hour<span class="pagenum"><a name="Page_111" -id="Page_111">[111]</a></span> later on the left of plate <a href="#Plate_50">50</a>. A -complete cumulo-nimbus in full work is shown on the right of Plate 49, -and the same appears on the right in Plate 50.</p> - -<p><a id="Plate_49"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p110a.jpg"> - <img src="images/i_p110a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 49.</span></p> - <div class="caption"> -<p>THUNDER-CLOUDS.</p> -<p>(<i lang="la" xml:lang="la">Cumulo-nimbus.</i>)</p></div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p110a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 49.</span></p> - <div class="caption"> -<p>THUNDER-CLOUDS.</p> -<p>(<i lang="la" xml:lang="la">Cumulo-nimbus.</i>)</p></div> -</div> - -<p><a id="Plate_50"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p110d.jpg"> - <img src="images/i_p110d-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 50.</span></p> - <div class="caption"> -<p>THUNDER-CLOUDS.</p> -<p>(<i lang="la" xml:lang="la">Cumulo-nimbus.</i>)</p></div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p110d-hh.jpg" alt="" /> - <p><span class="smcap">Plate 50.</span></p> - <div class="caption"> -<p>THUNDER-CLOUDS.</p> -<p>(<i lang="la" xml:lang="la">Cumulo-nimbus.</i>)</p></div> -</div> - -<p>These clouds were thunder-clouds, the larger one being a smart -thunderstorm with heavy hail. They were photographed in the evening, -and in the second picture the sun was just below the horizon.</p> - -<p>But, to continue the story of a thunder-cloud, we always find that -after a time the cirriform top flattens out and gradually subsides, -and this is usually accompanied by a descent of the cloud base to a -lower level. Meanwhile, it frequently happens that the whole series of -phenomena is repeated in one of the attendant cumulus. Plates <a href="#Plate_51">51</a> and <a href="#Plate_52">52</a> -are also two views of the same cloud at different times. In Plate 51 we -have the main part of the storm on the right, while on the extreme left -a lower part of the cloud is rising rapidly into a tall dome. In Plate -52 the central top has lost its cirriform margin and has distinctly -flattened, while the left-hand dome has risen much higher and is -beginning to throw out the projecting bits.</p> - -<p><a id="Plate_51"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p110f.jpg"> - <img src="images/i_p110f-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 51.</span></p> - <div class="caption"> -<p>THUNDER-CLOUD.</p> -<p>(<i lang="la" xml:lang="la">Cumulo-nimbus.</i>)</p></div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p110f-hh.jpg" alt="" /> - <p><span class="smcap">Plate 51.</span></p> - <div class="caption"> -<p>THUNDER-CLOUD.</p> -<p>(<i lang="la" xml:lang="la">Cumulo-nimbus.</i>)</p></div> -</div> - -<p><a id="Plate_52"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p110h.jpg"> - <img src="images/i_p110h-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 52.</span></p> - <div class="caption"> -<p>THUNDER-CLOUD.</p> -<p>(<i lang="la" xml:lang="la">Cumulo-nimbus.</i>)</p></div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p110h-hh.jpg" alt="" /> - <p><span class="smcap">Plate 52.</span></p> - <div class="caption"> -<p>THUNDER-CLOUD.</p> -<p>(<i lang="la" xml:lang="la">Cumulo-nimbus.</i>)</p></div> -</div> - -<p><span class="pagenum"><a name="Page_112" id="Page_112">[112]</a></span></p> - -<p>The hard-topped cumulus which fringe the lower disc, and the -vast pile of cirriform and hazy cloud which forms the centre of a -cumulo-nimbus, are shown in Plate <a href="#Plate_53">53</a>, which represents part of the side -of a great thunder-cloud. In this case the diameter of the lower disc -was about 15 miles, and the upper disc was rather larger. The uprising -column in the middle was about 7 miles across, and the height from base -to summit about 3 miles. The whole system contained between 100 and 150 -cubic miles of cloud. When photographed it was over the northern part -of Salisbury Plain. Lightning played repeatedly between the back of the -white cumulus and the hazy mass behind it, and the rumble of thunder -was all but continuous for nearly half an hour as the great cloud -passed by.</p> - -<p><a id="Plate_53"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p112a.jpg"> - <img src="images/i_p112a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 53.</span></p> - <div class="caption"> -<p>THE FLANK OF A GREAT STORM.</p></div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p112a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 53.</span></p> - <div class="caption"> -<p>THE FLANK OF A GREAT STORM.</p></div> -</div> - -<p>This was an unusually large cloud for this country, but specimens of -10, 20, or 30 cubic miles are quite common.</p> - -<p>Now for the explanation of the series of events. To begin with, we -have the production of an ordinary cumulus, but the equilibrium is -unstable, the growth of the cloud, therefore, becomes more and more -rapid, and the rapid condensation adds to<span class="pagenum"><a -name="Page_113" id="Page_113">[113]</a></span> the instability until -the rising column is so much lighter than an equal column outside -that a powerful updraught is created, strong enough for a time to -hold up the raindrops or even hailstones. At length the condensation -is complete, the upper part of the cloud consisting of snow crystals -exactly like those of any other cirrus. In the mean time, the rapid -ascending current necessarily involves an indraught from around, and -consequent descending currents to supply it. The result is to set up -a circulating system, moving inwards along the ground, upwards in the -central column, and outwards in the upper disc. The downward currents -are sometimes shown by a curling over of the edges of the upper disc, -but the phenomenon is not often seen, as the descending movement is -generally enough to dry up the cloud particles.</p> - -<p>The rapid rising of damp air drawn from the ground brings about -rapid condensation and heavy rain. The large size of thunder-drops is -almost certainly due to the fact that it is only the larger drops which -can fall in the teeth of the strong updraught. But when these drops -begin to fall, and still more when cold hailstones begin to fall<span -class="pagenum"><a name="Page_114" id="Page_114">[114]</a></span> -through this ascending air, it becomes chilled from top to bottom, and -the column is broken or even stopped altogether. The frozen particles -which make up the top subside gradually, and the chilling of the air -immediately below the cloud brings the saturation level nearer the -ground, and we say the cloud base descends.</p> - -<p>The arrangement of a thunder-cloud into the upper and lower discs -with a connecting uprush gives to the typical cloud a shape something -like that of an anvil when seen sideways, but in the larger clouds -the disc-like form is more obvious. In any ordinary thunderstorm the -great majority of the discharges of lightning play between the two -discs, and the larger the cloud the more frequent these are. Such -discharges as pass between the cloud and the earth come exclusively -from the base of the lower disc if the cloud is large, and generally -follow immediately after or simultaneously with one between the two -discs. The phenomena of lightning are intensely interesting, but the -purpose of these pages is confined to the study of clouds and cloud -forms, and it would be going beyond our scope to discuss either -lightning or hail. Both are,<span class="pagenum"><a name="Page_115" -id="Page_115">[115]</a></span> however, so closely related to -cumulo-nimbus that they can hardly be passed over in silence. One -thing is certain, and that is that neither the electrical developments -nor the hail has anything to do with the growth of the cloud. On the -contrary, both are consequences of the cloud, the hail being due to -the great altitude, and consequent low temperature of the upper part -of the cloud, and also to the violent uprising currents within it; -while the electrical phenomena are due to either the enormous amount of -condensation, or to friction due to the rapid uprush, or more probably -to the fact that considerable differences of electrical condition exist -in the distant parts of the air connected by the cloud, and between -which its circulating currents move. These differences are known to -exist at all times, and we cannot here discuss their origin.</p> - -<p>The formation of cumulo-nimbus and cumulus is dependent upon the -presence of a large amount of water vapour. It is worth while to -consider whether the atmospheric movements which bring about the -condensation could exist without moisture. Wherever we find differences -of temperature between neighbouring places we must get<span -class="pagenum"><a name="Page_116" id="Page_116">[116]</a></span> -currents of hot air rising from the warmer spots, and compensating -descending currents around them. But we have pointed out that if the -rate of cooling as we ascend in the still air is less than the rate at -which an ascending current will be dynamically cooled, such a rising -current will come to rest. If, on the other hand, the rate of cooling -in the ascending current be less than in the still air, the equilibrium -will be unstable, and a violent uprush will result.</p> - -<p>Now, in a climate such as our own, where the lower regions of the -air contain large quantities of water vapour, any considerable rise -brings about more or less condensation, and that condensation is -attended by a liberation of very large quantities of heat, which retard -cooling in the ascending current, and so facilitate the production of -instability. But if this cause is put aside it is still possible to -have a similar circulation. When discussing the causes of instability, -it has been pointed out that the prime condition was an unusually rapid -rate of fall of temperature in still air, such as may be produced by -hot sunshine. Now, these conditions are exactly those which will give -rise to the phenomenon of<span class="pagenum"><a name="Page_117" -id="Page_117">[117]</a></span> the mirage, and which reach their -fullest development in great desert districts when the air is still.</p> - -<p>Again, it has been pointed out that the causes which bring showers -and thunderstorms to an end include the chilling of the lower parts of -the ascending column by the descent of cold rain or hail from above. -We may also add the shading of the underlying ground by the cloud -itself, and the absorption of heat in the partial evaporation of some -of the rain during the lower part of its fall. In a desert district -the arising currents are so dry that even a very great ascent does -not often result in visible cloud; and when it does, the cloud is -produced at so great a height that the air is too rarefied to produce -anything much denser than thin alto-stratus, from which no falling -droplets could reach the earth. It seems, then, that there will be -no such automatic check on the growth of the circulating system, -and it will go on growing in volume and intensity indefinitely. As -a matter of fact, this is not the case. A different check does come -into operation, but not until the indraught and updraught have become -so powerful as to draw up the dust and sand and generate a sandstorm, -the weight and shade of<span class="pagenum"><a name="Page_118" -id="Page_118">[118]</a></span> which, in time, destroys the circulating -currents which uplifted them.</p> - -<p>Since, however, condensation is a considerable factor in producing -instability, we should expect that such sandstorms would be rarer than -thunderstorms are in an equally hot but well-watered district, which -is the fact. Again, since rain and cloud are checks upon such systems, -we should expect the sandstorm systems to be larger and far loftier -than thunderstorms, and to consist of far more violent atmospheric -movements. This also is the case, and when we know that some of these -disturbances have the dimensions of a cyclonic storm, it is easy to -understand how the finest dust may be raised to vast altitudes, into -the great upper currents of the air, by which it may be borne hundreds -of miles before returning to the ground. It is thus that the dust of -the African deserts is carried across the Mediterranean to Europe, and -the yellow loess from Mongolia even to the eastward of Japan.</p> - -<div class="chapter"></div> - -<p><span class="pagenum"><a name="Page_119" id="Page_119">[119]</a></span></p> - -<h2>CHAPTER VIII<br /> - -<span class="chtitle">WAVE CLOUDS</span></h2> - -<p class="noindent"><span class="smcap">Reference</span> has already -been made on more than one occasion to the remarkable rippled or wavy -structure sometimes assumed by clouds. The waves may be of almost any -dimensions, from the broad bands into which a sheet of cirro-stratus -or of alto-stratus is sometimes divided, down to the most minute -ripples. Sometimes they are ranged in long straight lines, sometimes -they are bent into sharp angles, and sometimes curved in very elaborate -patterns; but whether they be large or small, straight or curved, no -one can see them and fail to conclude that they must be due to an -action more or less analogous to the causes which produce waves on the -sea or ripple marks upon the sand.</p> - -<p>Wave clouds occur at all heights where clouds are formed. The -break-up of a lifting fog into<span class="pagenum"><a name="Page_120" -id="Page_120">[120]</a></span> roller clouds is probably the lowest -example, but it may more frequently be seen in higher clouds of the -alto or cirrus kinds.</p> - -<p>A low example is given in Plate <a href="#Plate_40">40</a>, which represents stratus -maculosus, and which has already been described. A higher type is shown -in Plate <a href="#Plate_54">54</a>, which is a wave-like arrangement of alto-cumulus. Rather -higher come the long zig-zag bands of Plate <a href="#Plate_55">55</a>, in which the stratiform -arrangement is more obvious, and which would be best described as a -wave-form of alto-stratus. These two plates form striking contrasts. -The clouds shown in the first are distinctly of the cumulus order, -and a prominent feature is the way in which the right-hand side of -each wave has a clear-cut rounded contour like that of the upper edge -of a small cumulus, while the left-hand edge of each band is frayed -out into a ragged fringe. The whole cloud was moving slowly in a -direction nearly, but not quite, at right angles to the waves, and -the fringed edge formed the rear. It is evident that this peculiar -structure must be due to a series of narrow waves intersecting a plane -in which the air is just on the point of producing alto-cumulus. If -there were no such waves, the<span class="pagenum"><a name="Page_121" -id="Page_121">[121]</a></span> little uprising currents, with their -intervening down currents, would be irregularly distributed, and -all the wave disturbances have had to do is to arrange them. The -consequence is that as the waves pass along the stratum the air is -alternately raised and lowered. Where it is rising condensation takes -place, where it is falling evaporation results.</p> - -<p><a id="Plate_54"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p120a.jpg"> - <img src="images/i_p120a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 54.</span></p> - <div class="caption"> -<p>CRESTED ALTO WAVES.</p> -<p>(<i lang="la" xml:lang="la">Alto-cumulus Undatus.</i>)</p></div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p120a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 54.</span></p> - <div class="caption"> -<p>CRESTED ALTO WAVES.</p> -<p>(<i lang="la" xml:lang="la">Alto-cumulus Undatus.</i>)</p></div> -</div> - -<p><a id="Plate_55"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p120d.jpg"> - <img src="images/i_p120d-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 55.</span></p> - <div class="caption"> -<p>ALTO WAVES.</p> -<p>(<i lang="la" xml:lang="la">Alto-stratus Undatus.</i>)</p></div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p120d-hh.jpg" alt="" /> - <p><span class="smcap">Plate 55.</span></p> - <div class="caption"> -<p>ALTO WAVES.</p> -<p>(<i lang="la" xml:lang="la">Alto-stratus Undatus.</i>)</p></div> -</div> - -<p>The cloud, like most other wave clouds, did not retain its -features for any length of time, but the gaps closed slowly in as -the cloud-bands increased in size, until a sheet of alto-stratus was -produced. Since the time of day was the morning, it is almost certain -that the plane of saturation was rising in accordance with the general -law, which is that the planes of condensation rise steadily, until -about two or three o’clock in the afternoon, and then slowly -descend.</p> - -<p>In Plate <a href="#Plate_55">55</a> each band is much flatter and less dense. They are -just as evidently formed by wave movements intersecting the plane of -condensation; but this was formed in the evening when the sun was -nearing the horizon, and at a time when the cloud planes are as a rule -rapidly descending.</p> - -<p>Among the alto clouds wave-forms sometimes<span class="pagenum"><a -name="Page_122" id="Page_122">[122]</a></span> persist for a fairly -long time, and in this case the bands moved steadily onward in a -direction equally inclined to their length and breadth, that is to -say, from the bottom left-hand corner of the photograph to the top -right-hand corner. As they passed across the sky new bands kept on -making their appearance at about the same spot, each band persisting -with little change until it had passed out of sight.</p> - -<p>Going much higher up into the region of cirrus, we meet with the -most minute and delicate ripple clouds. Some of these have already -been referred to. They are connected with either cirro-macula, -cirro-cumulus, or cirro-stratus, just as the coarser textured waves we -have been considering are connected with alto-cumulus or alto-stratus. -In Plate <a href="#Plate_56">56</a> we have an example in which we can see the stages in -the process. Nearest to the zenith we have cirro-cumulus, which is -here and there irregularly distributed, but is generally arranged in -delicate ripples, which are variously curved. Nearer the horizon the -troughs of the waves are filled in, and sheets of cirro-stratus are the -result. Here, again, the wave-form is evidently not typical. It<span -class="pagenum"><a name="Page_123" id="Page_123">[123]</a></span> is -an arrangement of either cirro-cumulus or cirro-stratus, produced by -the intersection of the plane of condensation by a series of wave -movements.</p> - -<p><a id="Plate_56"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p122a.jpg"> - <img src="images/i_p122a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 56.</span></p> - <div class="caption"> -<p>CIRRO RIPPLES.</p> -<p>(<i lang="la" xml:lang="la">Cirro-cumulus Undatus.</i>)</p></div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p122a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 56.</span></p> - <div class="caption"> -<p>CIRRO RIPPLES.</p> -<p>(<i lang="la" xml:lang="la">Cirro-cumulus Undatus.</i>)</p></div> -</div> - -<p>The arrangement is, however, so striking a feature when it is well -shown that any description of the cloud which contains no reference to -the waves is manifestly incomplete, and this would be best effected -by adding the word undatus or waved to the name of the cloud. Plate -54 will then be alto-cumulus undatus, Plate <a href="#Plate_55">55</a> alto-stratus undatus, -and Plate <a href="#Plate_56">56</a> would be described as cirro-cumulus undatus, passing into -cirro-stratus undatus and cirro-stratus. In popular language Plate 55 -might be called alto waves, Plate <a href="#Plate_54">54</a> crested alto waves, and Plate 56 -cirro ripples.</p> - -<p>If we are satisfied that the wave clouds are due to a wave movement -intersecting a plane of incipient cloud formation, the whole question -of their mode of production resolves itself into two parts—how is -that plane of incipient condensation produced? and how can we account -for the intersecting waves?</p> - -<p>The first question has by far the greater importance, since -it amounts to asking for a general explanation of the production -of high clouds,<span class="pagenum"><a name="Page_124" -id="Page_124">[124]</a></span> especially the forms of cirro-cumulus, -cirro-stratus, cirro-macula, and the corresponding alto varieties. -There are, again, two divisions also to this question. How does the -water vapour reach the stratum in sufficient quantity to saturate it? -and when condensation takes place, why does it so frequently assume -the characteristic mottled and granular forms like crowds of little -cumulus clouds arranged in one level? This last sentence gives the -clue. They are, in truth, little cumulus clouds, and must be formed -in exactly the same way as their vastly larger prototypes of lower -regions. It has been explained that low cumulus is the result of large -upward moving air columns or convection currents, each one being -initially caused by the heating of the vapour-laden air near the -ground, and each uprising column being supplied by cooler descending -air which flows down in the intervening spaces. It has also been -explained that these movements result in changes of temperature, which -tend to check those movements and restore the original equilibrium. -Suppose this to occur, as it constantly does, without any column -reaching sufficiently high to produce a cloud. There will be no visible -effect, but, nevertheless, an<span class="pagenum"><a name="Page_125" -id="Page_125">[125]</a></span> important change has taken place. Every -ascending current has lifted some water vapour with it to a higher -level, and the descending drier air has come down in contact with -the ground or damper air to become equally charged with moisture in -its turn. The process will be repeated again and again, and at one -level after another, so that the water vapour travels ever higher and -higher.</p> - -<p>This process of interchange between ascending and descending -air has been called by Mr. Ley inversion, but the term does not -seem very suitable, and interconvection would be better. The two -opposite currents pass through each other, as if the ascending air -gathered itself into definite channels, and passed through holes -in the descending mass like the passage of water upwards through a -descending plate of perforated metal. Moreover, just as the holes in -such a descending plate might have any size, so that the ascending -streams might vary in breadth from the finest hair to a column of huge -diameter, in exactly the same way the ascending columns of air may vary -from the smallest imaginable size to the great cumulo-nimbus currents. -It is the little currents which account for<span class="pagenum"><a -name="Page_126" id="Page_126">[126]</a></span> the constant quiver of -the margins of any object which is viewed through a large telescope -by day, and for the haze, so characteristic of a hot day, which makes -distant objects seem ill-defined and in a state of continual tremble. -The rays of light in passing through the intersecting streams are bent -a little, now this way, now that, as the air currents sway to and -fro.</p> - -<p>The near neighbourhood of the ground is not essential. As long -as the temperature of the air at any level is rising, so long -interconvection must occur. The process will be independent of the -presence or absence of wind. All that wind can do is to mix up the air -at different levels, breaking the system of currents and reducing it -to, so to say, a finer texture, or producing eddies, if strong enough, -which direct the currents and gather them into definite channels. The -final result in any case is that, with rising temperature, water vapour -is steadily borne upwards from the ground.</p> - -<p>As it ascends the air becomes cooler, and yet retains its water -vapour. When the rising currents are large they mix little with the -descending dry air, and on reaching a certain level condensation<span -class="pagenum"><a name="Page_127" id="Page_127">[127]</a></span> -takes place, and we have the beginning of a cumulus. If they are of a -more moderate size they will ascend less rapidly, the admixture with -descending air will bear a larger proportion to the whole, and the -plane at which condensation will begin will be higher, and then each -small column will be tipped with a ball of alto-cumulus. Make the -interconvection currents smaller still, and the cloud plane will be -lifted yet higher, and we shall have cirro-cumulus or cirro-macula.</p> - -<p>Now, the more even the distribution of temperature on the ground -the less the probability of coarse interconvection, and the same is -true of any higher stratum of air, provided it is free from disturbing -influences from outside. If, therefore, we have large currents near the -ground, ending, as they must, in cumulus, it has already been explained -that these clouds stop the action, and the general system of large -currents will be restricted to the region in which they occur. At some -distance above the lower clouds the only difference will be that water -vapour has been brought up to their level in great abundance. Smaller -systems of interconvection can then exist, and so we may have<span -class="pagenum"><a name="Page_128" id="Page_128">[128]</a></span> the -spectacle of several layers of cloud—cumulus capping the great -currents of lower regions, alto-cumulus forming the summits of the -smaller currents of intermediate regions, and cirro-cumulus floating -far above both.</p> - -<p>Frequently it happens that before the ascent of vapour has gone -quite far enough to produce a cloud, other causes co-operate, and -the cloud makes its appearance suddenly over considerable patches of -sky. The most potent of these is a fall of the barometric pressure, -which is brought about by some of the air far above the region of -even the highest clouds flowing away to some other district. The -air at all lower levels being thus relieved of the superincumbent -pressure, immediately expands, and is thereby cooled throughout. -Consequently, if at any level it was near its point of saturation, -it will be carried beyond that point, and cloud will rapidly make -its appearance over a large part of the sky, possibly at more -than one level. Stratiform arrangements will be the rule; but if -interconvection is going on at the time, its presence will be betrayed -by a granular or cumuloid structure. Interconvection clouds should -then be most frequent, and<span class="pagenum"><a name="Page_129" -id="Page_129">[129]</a></span> best formed when the air as a whole -is still or moving slowly (so as not to create great eddies), when -the temperature is rising rapidly, and when the barometer is making a -sudden fall. All these conditions are met in thunder weather, and at -the time when a summer anticyclone is giving way. It will be remembered -that many of the most beautiful forms have been described as forming -under one or the other of these very conditions.</p> - -<p>A second contributing cause, and one which tends to make the -condensation in patches or long broad bands ranged roughly at right -angles to the direction in which the air is moving, has been referred -to earlier. It is the passage of the air over an undulating country; -the up-and-down movements of the lower air being transmitted upwards -to great altitudes, as ever broadening and flattening waves. If the -upper air is flowing more rapidly than the lower, these broad waves may -be far ahead of their real cause, which will, therefore, quite escape -recognition, but the phenomenon is constantly to be detected in the -arrangement of the lower clouds. Two instances in the writer’s -experience will suffice. It was desired one morning to<span -class="pagenum"><a name="Page_130" id="Page_130">[130]</a></span> -measure the altitude of some small clouds which were passing from -the north-west at a height of probably between 2000 and 4000 metres, -over a hill only about 150 metres higher than the valley in which the -apparatus was fixed. In order to make the measurement, it was necessary -for the cloud to cross the valley and appear in the same field of view -as the sun, according to the method that will be described further -on. But in order to cross the valley the air had to descend, and so, -of course, had the cloud stratum, though to a less extent. But small -as the descent was, it was enough to dry up the clouds entirely, and -for more than a couple of hours the clouds came sailing over the -hill, disappearing entirely, and then reforming so far beyond that -no measurement was possible, since not one single fragment came near -enough to the position of the sun, which remained shining brightly -through a broad clear gap between two patches of cloud-strewn sky.</p> - -<p>On another occasion considerable preparations had been made for -some photographic observations during an eclipse of the sun. The -observatory stands on the eastern side of the valley of the Exe,<span -class="pagenum"><a name="Page_131" id="Page_131">[131]</a></span> which -is flanked on its western side by a long ridge of hills going up to 800 -feet above the sea. Beyond these hills lies the deep, narrow valley of -the Teign, and beyond that the granite ramparts of Dartmoor, 1000 feet -above the sea. The wind was blowing gently across the two valleys, and -shortly before the eclipse began a broad strip of thin cloud formed -above and rather towards the eastern side of the Exe valley, just where -the sun was, while at the same time the sky was practically clear half -a mile further east, and bright sunlight was streaming down on the -ridge between the two rivers a few miles towards the west. The cloud -was never thick enough to quite hide the sun, so that the eclipse was -easy to watch with the naked eye; but in spite of fairly rapid movement -of the cloud masses as they drifted before the sun, they kept on -forming in just the same place, and completely prevented the carrying -out of the programme planned. It is almost certain that the phenomenon -was brought about by an upward moving wave marking the place where the -level of approaching saturation was upheaved by the disturbance caused -by crossing the two valleys and intervening ridge.</p> - -<p><span class="pagenum"><a name="Page_132" -id="Page_132">[132]</a></span></p> - -<p>These two instances are not quoted as examples of a rare occurrence, -but as definite simple instances of a phenomenon which may be -constantly observed, and as proof that the conformation of the ground -does exercise an influence upon the distribution of cloud.</p> - -<p>But no irregularities of the ground will suffice to explain the -minute waves and ripples which have been described at the beginning -of this chapter. These must be due to wave disturbances in the air -itself. They have been explained as due to two different currents of -air, either a warm damp current flowing over a cold one, or <i lang="la" xml:lang="la">vice -versâ</i>. Now, such an occurrence as a warm damp current flowing -over a cold one must be very rare, though it is impossible to deny that -it might occur. The immediate contact of a cold current above a warm -damp one is equally unlikely, unless the general atmospheric condition -were greatly disturbed, which is the same thing as saying that wave -clouds would not occur. They are most frequent at just those times -when interconvection has freest play, and this is amply sufficient -to account for a plane of saturation without any necessity for a -hypothesis of two layers of air at different temperatures all<span -class="pagenum"><a name="Page_133" id="Page_133">[133]</a></span> but -producing cloud at their junction. No convincing evidence of cloud -production by such means has yet been adduced, and it is better to rely -upon causes which we know do operate than to call in theories as to -what might possibly happen. This is one of those points in the study of -clouds which need investigation, and until proof is forthcoming it is -better to say that the admixture of two strata of air might conceivably -produce cloud, but most forms can be accounted for by other causes of -which we have more positive evidence.</p> - -<p>Still, the wave clouds are due to waves, and there seems no other -way of accounting for them than the supposition of gentle differential -currents. But if such currents occur the ripples and waves will not -be limited to a definite surface, so to say, of contact, but will be -propagated upwards and downwards for considerable distances from the -level of greatest disturbance. Whether, therefore, the level at which -the natural operation of interconvection has produced saturation is -high or low in this region, the result will be the marshalling of the -ascending and descending elements of the convection system in the -characteristic waves.</p> - -<p><span class="pagenum"><a name="Page_134" -id="Page_134">[134]</a></span></p> - -<p>The differential currents, then, which cause the waves must not be -conceived as producing those waves at a surface of contact, nor must -the currents be thought of as separated by any definite surface, but -rather by a region of variable but usually considerable depth, in which -the air is disturbed by a series of small slow eddies and oscillatory -movements. When the waves are parallel straight lines the air currents -may be really portions of a whole, having the upper part more rapid -than the lower. In such a case the direction of movement should be -at right angles to the cloud lines. If the upper current differs in -direction as well as velocity, the direction of movement of the clouds -will be intermediate, and will resemble that of the upper or lower -current, according to their relative distances from the plane at which -the clouds are formed.</p> - -<p>The behaviour of the clouds will depend upon the relative shares -in their production borne by interconvection pure and simple and by -the wave oscillations. If the stratum is one in which cloud would -actually be formed independently of the up-and-down movements, all -this will be able to do<span class="pagenum"><a name="Page_135" -id="Page_135">[135]</a></span> will be to arrange the cloudlets at -their birth, and these will then continue to exist, drifting with the -general horizontal movement of the air like any other cloud of the same -order.</p> - -<p>On the other hand, if the production of cloud is dependent upon the -vertical oscillations, the cloudlets or lines of cloud will move with -the air waves, and their rate of motion and direction of motion will -be determined by the rate and direction of the waves, which may be -quite different from that of the air at that stratum as a whole. The -ascending waves will be marked by lines of cloud generally rounder and -better defined on their advancing sides, while the descending troughs -will be marked by clear intervals.</p> - -<p>Wave movements of the necessary kind are frequently very -complicated, and it is not by any means a rare occurrence to see the -wave lines in one part of the sky at all sorts of angles with similar -lines in other parts, or even to see two or more sets of waves at -different altitudes crossing one another. Either phenomenon is always -accompanied by rapid changes in the cloud, and the rippled structure is -short-lived. This was the case<span class="pagenum"><a name="Page_136" -id="Page_136">[136]</a></span> with the clouds shown in Plate <a href="#Plate_54">54</a>. -Plate <a href="#Plate_53">53</a>, on the contrary, shows great uniformity in the wave lines, -and although the vertical oscillation is probably the main cause of -condensation, the form was unusually persistent.</p> - -<p>Irregular patches of wave disturbance, affecting a plane occupied by -cirro-stratus vittatus, are shown in Plate <a href="#Plate_57">57</a>. In this case the wave -systems only touch the cloud plane here and there, and the places of -contact varied rapidly. It is pretty clear from this photograph that -the idea of the waves being formed at a surface of contact between two -diverse currents will not suffice. The bands of the cirro-stratus are -for the most part unbroken and unaffected; it is only here and there -that the wave region touches them.</p> - -<p><a id="Plate_57"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p136a.jpg"> - <img src="images/i_p136a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 57.</span></p> - <div class="caption"> -<p>WAVED CIRRO-STRATUS.</p> -<p>(<i lang="la" xml:lang="la">Cirro-stratus Undatus.</i>)</p></div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p136a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 57.</span></p> - <div class="caption"> -<p>WAVED CIRRO-STRATUS.</p> -<p>(<i lang="la" xml:lang="la">Cirro-stratus Undatus.</i>)</p> - </div> -</div> - -<p>The conclusions at which we have arrived are simple, and there is -little room for doubt as to their main correctness, but there are -numerous minute features presented by these beautiful cloud patterns -which await interpretation, and they reveal complicated oscillatory -movements in the air which are difficult to account for, whether we -seek their originating causes or the mechanics of their motions.</p> - -<div class="chapter"></div> - -<p><span class="pagenum"><a name="Page_137" id="Page_137">[137]</a></span></p> - -<h2>CHAPTER IX<br /> - -<span class="chtitle">CLOUD ALTITUDES</span></h2> - -<p class="noindent"><span class="smcap">During</span> an extended -experience of cloud photography, it was found that it was quite -possible to get pictures which showed the cloud detail even when -the sun was in the field of view. Sometimes the solar image was -reversed, but if the exposure was very short this was not the case. -In such photographs the structure of the cloud was exceedingly clear -and sharply defined quite close to the sun. Indeed, the intense -illumination seemed to reveal minute details of internal arrangement -which could not be detected in similar clouds some distance away.</p> - -<p>The methods which had been employed for the measurement of cloud -altitudes elsewhere have already been briefly referred to. Some of -them required two observers, who were equally responsible, each of -them having to direct his apparatus or camera to the same point of -the cloud, and to record<span class="pagenum"><a name="Page_138" -id="Page_138">[138]</a></span> the exact direction in which the -instrument was pointed. The instruments, if accurate, were costly, -and there were many opportunities for error in reading the graduated -circles which gave the directions. Moreover, in most of these methods -the two observers were connected by telephone, and had to agree on -the exact point towards which their instruments should be directed; -either the exact point of the cloud, or the precise direction as shown -by the mounting of the camera or other instrument. At Kew some of -these sources of error were avoided by fixing the two cameras with the -axes of the lenses and centres of the plates in a vertical position -and exposing the two plates simultaneously. The Kew observations -were not long continued, and for some years the only measurements in -progress were those carried out abroad, particularly at the Blue Hill -Observatory and at Upsala.</p> - -<p>The experience gained in photographing clouds in order to record -their forms suggested a way in which many of the sources of error in -previous measurements of altitude could be avoided, especially by -simplifying and reducing the operations at the moment of making the -observation.</p> - -<p><span class="pagenum"><a name="Page_139" -id="Page_139">[139]</a></span></p> - -<p>If two cameras are placed at the opposite ends of a measured base -line, whose direction is known, and if they are both pointed towards -the sun, on making the exposures by electrical means at the same -moment, the position of the image of the sun upon the plate gives the -direction in which the cameras are pointed. It will be in the same -direction as seen from both ends of the line.</p> - -<p>Now, if we note the time at which the exposure is made, this with -the date gives all that is required for ascertaining the sun’s -position in the sky, and is, therefore, the only exact observation -which need be made at the time of taking the photographs. Mistakes -are almost impossible, as each plate contains its own record of the -sun’s position, and even if some of the plates should get mixed -the images of the clouds will generally suffice to pair them properly. -For general measurements there is one grave defect in the method, and -that is that it can only be used when the sun and cloud can be got into -the same field of view. But with the higher varieties of cloud this -is generally possible, and it was just these higher sorts about which -knowledge was least certain, and which it was proposed to study.</p> - -<p><span class="pagenum"><a name="Page_140" -id="Page_140">[140]</a></span></p> - -<p>An initial difficulty was the finding of a level site, flat land -being very uncommon in Devonshire, but fortunately a suitable place was -found in some artificially levelled ground close to Exeter, belonging -to the London and South Western Railway Company. It was a stretch of -ground intended to be covered with sidings, but had not been finished, -and had become overgrown with grass, stunted sallows, and other wild -plants. Being railway ground, it was, comparatively, though by no means -entirely, free from mischievous and inquisitive people. The next point -was a suitable camera. It must have fairly long focus in order to give -a large image, and therefore large displacement; it must be capable -of being pointed in any direction and clamped there; and it must be -capable of standing considerable extremes of temperature and variations -of dampness, as it was intended that they should be kept on the spot in -wooden structures, which served for stands as well as to contain the -apparatus.</p> - -<p>The pattern finally decided upon is represented in Plate 58, which -shows one of the cameras pointed up to the sky and standing on one of -the stands. These cameras were to take plates of whole plate<span -class="pagenum"><a name="Page_141" id="Page_141">[141]</a></span> -size, two double dark slides of the ordinary pattern being attached to -each.</p> - -<p><a id="Plate_58"></a></p> - -<div class="figcenter screenonly" style="width: 274px;"> - <a href="images/i_p140b.jpg"> - <img src="images/i_p140b-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 58.</span></p> - <div class="caption"> -<p>CAMERA FOR MEASURING ALTITUDES.</p></div> -</div> - -<div class="figcenter handonly" style="width: 473px;"> - <img src="images/i_p140b-hh.jpg" alt="" /> - <p><span class="smcap">Plate 58.</span></p> - <div class="caption"> -<p>CAMERA FOR MEASURING ALTITUDES.</p></div> -</div> - -<p>The camera looks rather complicated, but it is really simple. Its -body consists of front and back, each attached to a central part by a -short bellows and sliding on a base board, to which it can be clamped -by screws of the usual pattern. The central part carries trunnions, -such as are used for looking-glasses, which swing in sockets carried -by two upright supports, so as to give the whole free motion in a -vertical plane. In order to be able to fix it firmly at any angle, the -base board of the camera body carries on its underside a thin board -projecting beneath it and forming a segment of a circle whose centre -would be the horizontal axis through the trunnions. The board passes -between the jaws of a small wooden clamping vice in front, which is -carried by the square base to which the uprights are fixed. The whole -is firmly made of well-seasoned pine, and has stood well the hard usage -of half a dozen years.</p> - -<p>There is no focusing screen. Focusing was done with great care -once for all, and then a coat of hard varnish was put over all -the adjusting screws.<span class="pagenum"><a name="Page_142" -id="Page_142">[142]</a></span> A small view-finder is attached to one -side, and it was by this that the camera was pointed in the desired -direction.</p> - -<p>In order to lessen risk of mistake, it was so arranged that the two -slides belonging to one camera would not fit the other. The lenses, -of 18 inches focus, and giving sharp detail all over the plate, were -carefully matched, and the focus adjusted until the images given by -them when placed side by side appeared to coincide exactly. They were -provided with iris diaphragms, which were shut down to an aperture of -a quarter of an inch, and with shutters which could be released at the -same moment by an electric current, acting through the electro-magnet -shown under the lens on the front of the camera.</p> - -<p>The shutters were of the kind known as the “Chronolux,” -which will give any exposure from the sixty-fourth of a second up to -three seconds. But it was found in practice that the highest speed was -sufficient and gave satisfactory results. Of course, there was no idea -of adjusting matters on each occasion so as to get the best possible -negatives capable of yielding good prints. Measurement was the<span -class="pagenum"><a name="Page_143" id="Page_143">[143]</a></span> -object, and if the negative showed the sun and sufficient cloud detail -for the identification of cloud points, that was all that was wanted. -The shutters gave a good deal of trouble at first. Their sliding parts -were made of ebonite, and when the cameras were left in their stands -with an August sun shining down upon them, everything inside got -very hot and the ebonite warped; but the difficulty was got over by -substituting aluminium.</p> - -<p>The two camera stands were placed 200 yards apart, and were -connected by a line of telegraph wire carried on short poles. At each -end of the wire an insulated connecting piece was brought down to the -camera stand, and to the batteries and other apparatus. The current -which was sent through this wire by pressing a contact at one end of -the line did not directly make the exposures; but two similar relays -were brought into action, and each of these sent the current from a -local battery of Leclanché cells through the electro-magnet on -the camera and made the exposure.</p> - -<p>After development the two negatives showed the image of the -sun, not far from the centre of the field of view, and the cloud -whose altitude was<span class="pagenum"><a name="Page_144" -id="Page_144">[144]</a></span> required. Since this was taken from -two different points of view, the negatives were not alike, but the -distances between the centre of the sun’s disc and any special -point of the cloud were different. For instance, if the cloud were east -of the sun, with its edge just apparently touching the solar image as -photographed from the eastern station, then the negative taken from the -western end of the base would show an interval of clear sky between the -two, which would be greater as the cloud was lower.</p> - -<p>It often happened that after developing the plates the image -of the sun was lost in a black blur, but it was easy to reduce -this part of the image by local application of a reducing agent<a -name="FNanchor_3_3" id="FNanchor_3_3"></a><a href="#Footnote_3_3" -class="fnanchor">[3]</a> by means of a paint-brush, until the disc -became clear enough. Two lines were then drawn on the negative, -one vertical and the other horizontal, intersecting each other at -the centre of the sun’s image. These lines served as the -starting-points for exactly measuring the distance from their point of -intersection to any selected point of the cloud.</p> - -<p>The distances could generally be determined to<span -class="pagenum"><a name="Page_145" id="Page_145">[145]</a></span> a -fiftieth or a hundredth part of an inch, and their difference was, -of course, dependent upon the direction of the sun relative to the -base line and the altitude of the cloud, but for low level clouds the -difference was sometimes so great that no pair of corresponding points -could be detected, while it was often as much as an inch. With higher -clouds the differences were smaller, but unless the sun was very low in -the sky, either east or west, the displacements of the cloud image were -great enough to give reliable measures. Specimen prints from pairs of -negatives are shown in Plates 59 and 60.</p> - -<p><a id="Plate_59"></a></p> - -<div class="figcenter screenonly" style="width: 258px;"> - <a href="images/i_p144a.jpg"> - <img src="images/i_p144a-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 59.</span></p> - <div class="caption"> -<p>PRINT FROM A NEGATIVE USED FOR MEASURING ALTITUDE.</p></div> -</div> - -<div class="figcenter handonly" style="width: 450px;"> - <img src="images/i_p144a-hh.jpg" alt="" /> - <p><span class="smcap">Plate 59.</span></p> - <div class="caption"> -<p>PRINT FROM A NEGATIVE USED FOR MEASURING ALTITUDE.</p></div> -</div> - -<p><a id="Plate_60"></a></p> - -<div class="figcenter screenonly" style="width: 400px;"> - <a href="images/i_p144d.jpg"> - <img src="images/i_p144d-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 60.</span></p> - <div class="caption"> -<p>PAIR OF PRINTS SHOWING THE DISPLACEMENT OF THE CLOUD.</p></div> -</div> - -<div class="figcenter handonly" style="width: 700px;"> - <img src="images/i_p144d-hh.jpg" alt="" /> - <p><span class="smcap">Plate 60.</span></p> - <div class="caption"> -<p>PAIR OF PRINTS SHOWING THE DISPLACEMENT OF THE CLOUD.</p></div> -</div> - -<p>The processes by which the measurements are worked out are -laborious,<a name="FNanchor_4_4" id="FNanchor_4_4"></a><a -href="#Footnote_4_4" class="fnanchor">[4]</a> and consist -of two parts,<span class="pagenum"><a name="Page_146" -id="Page_146">[146]</a></span> the first being the determination of -the exact position of the sun from the date, hour, and latitude and -longitude of the place, and the second, the determination of the -position of the cloud. Two points which represent the same part of the -cloud are selected, and their respective distances from the two lines -drawn through the sun are measured as accurately as possible. Now, a -certain distance on the negative corresponds with a definite angular -displacement, and a scale can be constructed showing how much should be -added to or subtracted from the sun’s position to get the exact -position of the cloud. This being done, it is then a simple piece of -trigonometry to deduce the actual height of<span class="pagenum"><a -name="Page_147" id="Page_147">[147]</a></span> the cloud above the -place of observation. The work of computation, however, was greatly -lightened by the fact that many of the pairs of negatives showed more -than one layer of cloud; thus Plate <a href="#Plate_59">59</a>, which is a fair specimen, shows -three layers, and, consequently, one determination of the sun’s -position sufficed for three distinct results.</p> - -<p>For the highest clouds the displacements were, of course, small, -and could only be made with certainty of a correct result within about -three hours of noon. Earlier than 9 a.m., or later than 3 p.m., the -sun was too nearly in a line with the two stations, or too low in the -sky, to give a sufficient displacement of image. A base line of 400 -yards instead of 200 would have been better for the high clouds. But, -on the other hand, when low level clouds are viewed from two different -spots their outlines may seem so changed that it may be impossible to -identify a pair of corresponding points, and the same difficulty may -also arise when high clouds are seen through a gap in a lower stratum. -The longer the base line the more frequent and more obtrusive would -this perspective difficulty become, so the distance of 200 yards<span -class="pagenum"><a name="Page_148" id="Page_148">[148]</a></span> -between the stations was adopted as a convenient mean.</p> - -<p>The method of making the observations was simple. Each observer was -provided with some signal flags, by which the necessary communications -were made in accordance with a simple code. Call the two observers A -and B, and suppose A directed the operations. He watched the sky until -a favourable opportunity seemed to be approaching. He then signalled -to B, and both cameras were turned to the sun, the dark slides were -inserted, the shutters set, and everything made ready. Signals were -then interchanged, to signify that preparations were complete, and when -A saw that the edge of the cloud had reached a suitable position to be -in the same field of view with the sun, the contact key was pressed and -the plates simultaneously exposed. At the moment when this was done the -time was noted. Several observations were thus made in a short time.</p> - -<p>Measurements were carried out as opportunity allowed over -four consecutive seasons, from the beginning of April until -the end of October. During the last of the four years, the -site had become less<span class="pagenum"><a name="Page_149" -id="Page_149">[149]</a></span> convenient owing to an extension of the -railway work, and early in November the series was brought to an abrupt -conclusion by a heavy gale, which snapped off all the poles carrying -the connecting wire. But by that time 423 measurements had been -obtained, the great majority of which referred to clouds of the cirrus -and alto groups.</p> - -<p>The general results may be tabulated thus, giving heights in -metres:—</p> - -<div class="center"> -<table border="0" cellpadding="4" cellspacing="0" summary="Altitude of Various Clouds"> -<tr> - <td class="tdc bt br"></td> - <td class="tdc bt br">Number of<br /> observations.</td> - <td class="tdc bt br">Maximum <br />altitude.</td> - <td class="tdc bt br">Minimum<br /> altitude.</td> - <td class="tdc bt">Mean<br /> altitude.</td> -</tr> - -<tr> - <td class="tdl br">Cirrus</td> - <td class="tdc bt br">58</td> - <td class="tdr pr1 bt br">27,413</td> - <td class="tdr pr1 bt br">4,114</td> - <td class="tdr pr05 bt">10,230</td> -</tr> - -<tr> - <td class="tdl br">Cirro-stratus</td> - <td class="tdc br">64</td> - <td class="tdr pr1 br">15,503</td> - <td class="tdr pr1 br">3,840</td> - <td class="tdr pr05">9,540</td> -</tr> - -<tr> - <td class="tdl br"><span class="ml1">„</span> <span class="ml1">cumulus</span></td> - <td class="tdc br">63</td> - <td class="tdr pr1 br">11,679</td> - <td class="tdr pr1 br">3,657</td> - <td class="tdr pr05">8,624</td> -</tr> - -<tr> - <td class="tdl br">Alto-cumulus</td> - <td class="tdc br">83</td> - <td class="tdr pr1 br">9,390</td> - <td class="tdr pr1 br">1,828</td> - <td class="tdr pr05">5,348</td> -</tr> - -<tr> - <td class="tdl br">Cumulus top</td> - <td class="tdc br">42</td> - <td class="tdr pr1 br">4,582</td> - <td class="tdc br">—</td> - <td class="tdr pr05">3,006</td> -</tr> - -<tr> - <td class="tdl br"><span class="ml15">„</span> <span class="ml15">base</span></td> - <td class="tdc br">48</td> - <td class="tdr pr1 br">1,959</td> - <td class="tdr pr1 br">584</td> - <td class="tdr pr05">1,290</td> -</tr> - -<tr> - <td class="tdl br">Strato-cumulus</td> - <td class="tdc br">27</td> - <td class="tdr pr1 br">6,926</td> - <td class="tdr pr1 br">823</td> - <td class="tdr pr05">2,248</td> -</tr> - -<tr> - <td class="tdl br">Cumulo-nimbus top</td> - <td class="tdc br">15</td> - <td class="tdr pr1 br">6,409</td> - <td class="tdr pr1 br">2,004</td> - <td class="tdr pr05">8,002</td> -</tr> - -<tr> - <td class="tdl bb br"><span class="ml15">„</span> <span class="ml25">„</span> <span class="ml15">base</span></td> - <td class="tdc bb br">15</td> - <td class="tdr pr1 bb br">2,286</td> - <td class="tdr pr1 bb br">766</td> - <td class="tdr pr05 bb">1,045</td> -</tr> -</table></div> - -<p>These values are not very different, on the whole, from those -which have been arrived at elsewhere, and in making a comparison it -must be borne in mind that there is always a little want of precision -in cloud nomenclature. As a whole, the Exeter maxima are greater -than the foreign ones,<span class="pagenum"><a name="Page_150" -id="Page_150">[150]</a></span> and this is very markedly so in the -case of cirrus, for which the American highest record is 14,930 -metres, the Swedish record is 13,376, while the Exeter value is 27,413 -metres, or about 17 miles. But this extreme measurement, and several -others unusually large, were made in one morning, a day of very hot -damp weather, when cloud formed at seven different levels: cumulus -at a height of 1·9 miles, alto-cumulus at 3·9 miles, -cirro-cumulus at 4·7 miles, cirro-stratus (No. 1) at 8 miles, -cirro-stratus (No. 2) at 9·6 miles, cirrus at 11·5 -miles, and cirrus excelsus at 17 miles. By about half-past one in the -afternoon the sky was completely overcast with dull grey clouds, which -cleared off at half-past four, and at half-past five in the evening -the cirrus had fallen to 7·9 miles, and the cirro-cumulus -to 4·3 miles. If this one day’s observations had been -omitted, the Exeter maximum would only have been little more than 1000 -metres above the record from across the Atlantic, but 1000 metres is a -height worth noting.</p> - -<p>While the Exeter maxima are all rather greater, we find the minima -for cirrus, cirro-stratus, and cirro-cumulus are rather less than at -the foreign stations; that is to say, that clouds are formed over<span -class="pagenum"><a name="Page_151" id="Page_151">[151]</a></span> -Devonshire both at lower and at higher levels than seems to be the case -in Massachusetts or Sweden. It seems probable that this is due to a -greater humidity on our western coasts, such as we should suppose would -be the case from their position and the prevailing winds and ocean -currents. If so, we should expect the great convection clouds to be -larger. Thus, at Exeter, out of only fifteen examples of cumulo-nimbus, -the top varied from 2004 metres to 6409, with an average base level -of 1045. At Upsala the maximum was 5970 and the minimum 1400, with an -average base level of 1400. The mean thickness of the Swedish clouds -was only 1400 metres, while that of the Devonshire specimens was more -than 2000 metres.</p> - -<p>Again and again, during the progress of these measurements, it was -found that the greatest altitudes and the richest development of the -higher varieties occurred towards the end of a spell of fine calm -weather, when convection had had free play day after day. A slight -fall of the barometer, only the hundredth part of an inch, would -usually, under those circumstances, bring about abundant formation of -high clouds, frequently of the undatus kind.<span class="pagenum"><a -name="Page_152" id="Page_152">[152]</a></span> All the cumulus -clouds, by which we mean to include alto-cumulus and cirro-cumulus, -are most frequent when the levels of condensation are rising, while -the stratiform clouds are an indication of no vertical movement or -of active descent. Pure cirrus is indicative rather of movement in a -horizontal direction, and may occur when the condensation levels are -stationary, or when they are rapidly changing either way.</p> - -<p>In broken weather the natural movements of the atmosphere and of -its vapour are masked and disturbed by the strong eddies brought by -the cyclonic systems. It not unfrequently happens that the region of -disturbance does not reach up to the level of the highest cirrus, or, -what is more probable, the cyclonic system leans so far forward that we -may have in its rear the upper clouds floating quietly far above the -comparatively shallow region of disturbance, while in front the upper -part of the storm system projects above undisturbed air.</p> - -<p>The frequent appearance of cloud almost at the same time at more -than one level is at first rather difficult to understand, but it will -be noticed that when this occurs the barometer almost invariably<span -class="pagenum"><a name="Page_153" id="Page_153">[153]</a></span> -falls. Now, if we suppose that the air is nearly saturated at more than -one level, and that the whole is then bodily relieved of some of the -superincumbent mass, so that the barometer falls, the mass of air will -at once swell up, being cooled from top to bottom simultaneously, and -wherever it is damp enough cloud will be formed.</p> - -<p>The converse is equally true. If we have cloud at several levels, -and the whole is compressed by the addition of more air above, which is -the case when the barometer rises, that compression will be accompanied -by the generation of heat and the consequent disintegration and -disappearance of the clouds.</p> - -<div class="chapter"></div> - -<p><span class="pagenum"><a name="Page_154" id="Page_154">[154]</a></span></p> - -<h2>CHAPTER X<br /> - -<span class="chtitle">CLOUD NOMENCLATURE</span></h2> - -<p class="noindent"><span class="smcap">Since</span> a considerable -number of new terms have been suggested in the foregoing pages, it may -be convenient to collect them and tabulate them, so as to show their -relation to those already recognized by the International system.</p> - -<p>In the atlas put forward by the committee, sixteen varieties are -recognized by distinct names, and these are drawn up in tabular form -with appropriate abbreviations for use in making records.</p> - -<p>The names are—</p> - -<div class="center-block"> - -<div class="block"> - -<p>Cirrus. Ci.</p> -<p>Cirro-stratus. Ci. S.</p> -<p>Cirro-cumulus. Ci. Cu.</p> -<p>Alto-cumulus. A. Cu.</p> -<p>Alto-stratus. A. S.</p> -<p>Strato-cumulus. S. Cu.</p> -<p>Nimbus. N.</p> -<p>Cumulus. Cu.</p> -<p>Cumulo-nimbus. Cu. N.</p> -<p>Stratus. S.</p> -<p>Fracto-cumulus. Fr. Cu.</p> -<p>Fracto-nimbus. Fr. N.</p> -<p>Fracto-stratus. Fr. S.</p> -<p>Stratus-cumuliformis. S. Cf.</p> -<p>Nimbus-cumuliformis. N. Cf.</p> -<p>Mammato-cumulus. M. Cu.</p> - -</div></div> - -<p><span class="pagenum"><a name="Page_155" id="Page_155">[155]</a></span></p> - -<p>During our survey of these groups we have found that some of them -include clouds of many shapes, which must be due to very diverse -conditions. It follows that if observations are to be made on the -occurrence of these special kinds, with a view to arriving at a -thorough understanding of the circumstances to which they owe their -forms, it becomes necessary to devise a code of names and symbols -whereby an interchange of ideas and records may be rendered possible. -Specific names have been proposed as each form was considered, and -it only remains to sum them up concisely. Subsequent observation, -particularly in other climates, may show that further additions should -be made; but if the principle of specific names be once admitted, it -will be easy to fill any omission.</p> - -<h3><span class="smcap">Group Cirrus.</span></h3> - -<p>Under the general head of cirrus we have found nine distinct -forms—</p> - -<p>1. <i lang="la" xml:lang="la">Cirro-nebula</i> (Ley) (Plates <a href="#Plate_2">2</a> and <a href="#Plate_3">3</a>). Cirrus veil.</p> - -<p>Characterized by comparative absence of structure and by the -formation of halo. Ci. Na.</p> - -<p><span class="pagenum"><a name="Page_156" -id="Page_156">[156]</a></span></p> - -<p>2. <i lang="la" xml:lang="la">Cirro-filum</i> (Ley) (Plate <a href="#Plate_7">7</a>). Thread cirrus.</p> - -<p>Built up of fine long threads, straight, curved, or crossing, but -free from hazy curling or flocculent structures. Ci. F.</p> - -<p>3. <i lang="la" xml:lang="la">Cirrus excelsus</i> (Plate <a href="#Plate_5">5</a>). High cirrus.</p> - -<p>Characterized by great altitude, thinness, irregular branching -structure. Ci. Ex.</p> - -<p>4. <i lang="la" xml:lang="la">Cirrus ventosus</i> (Plate <a href="#Plate_6">6</a>). Windy cirrus.</p> - -<p>Characterized by curving branches leaning forward in the direction -of movement, and other long curving streamers lagging behind and below. -Fluffy parts are usually present, and mark the origins of the long -curling fibres. Ci. V.</p> - -<p>5. <i lang="la" xml:lang="la">Cirrus nebulosus</i> (Plate <a href="#Plate_9">9</a>). Hazy cirrus.</p> - -<p>Characterized by the absence of sharply defined lines, fibres, or -streamers; all parts of the cloud being hazy, and suggestive of other -varieties of cirrus out of focus. Ci. Neb.</p> - -<p>6. <i lang="la" xml:lang="la">Cirrus caudatus</i> (Plate <a href="#Plate_8">8</a>). Tailed cirrus.</p> - -<p>Characterized by small hazy or fluffy heads behind or below which -hang long streamers, which taper away more or less to a point. The -tails are sharply defined, and so are the edges of the heads. Ci. -Ca.</p> - -<p><span class="pagenum"><a name="Page_157" -id="Page_157">[157]</a></span></p> - -<p>7. <i lang="la" xml:lang="la">Cirrus vittatus</i> (Plates <a href="#Plate_12">12</a> and <a href="#Plate_13">13</a>). Ribbon cirrus.</p> - -<p>Characterized by formation in long bands of cloud, sometimes made of -parallel long fibres with cirrus haze linking them together, sometimes -consisting of a long bundle of fibres, from which others diverge at an -angle as shown in the plate. Ci. Vt.</p> - -<p>8. <i lang="la" xml:lang="la">Cirrus inconstans</i> (Plate <a href="#Plate_10">10</a>). Change cirrus.</p> - -<p>Characterized by a peculiar ragged, wavy appearance. It is -generally only the beginning or the end of a mass of cirro-stratus -or cirro-cumulus, but occasionally it vanishes shortly after its -appearance, without reaching the further stage. Ci. In.</p> - -<p>9. <i lang="la" xml:lang="la">Cirrus communis</i> (Plate <a href="#Plate_11">11</a>). Type cirrus or common cirrus.</p> - -<p>Characterized by short irregularly curling fibres collected together -in considerable patches. No definite arrangement into any of the forms -already described. Ci. Com.</p> - -<h3><span class="smcap">Group Cirro-stratus.</span></h3> - -<p>Under this group the cloud usually shows some structure, being -apparently built up from a massing together of detached forms -at a common level.<span class="pagenum"><a name="Page_158" -id="Page_158">[158]</a></span> When this is so it should be described -by adding the specific name of the detached form most nearly -related.</p> - -<p>1. <i lang="la" xml:lang="la">Cirro-stratus nebulosus</i> (Plates <a href="#Plate_3">3</a>, <a href="#Plate_4">4</a>, and <a href="#Plate_14">14</a>). Hazy -cirro-stratus.</p> - -<p>Characterized by absence of visible structure. Ci. S. Neb.</p> - -<p>2. <i lang="la" xml:lang="la">Cirro-stratus communis</i> (Plate <a href="#Plate_16">16</a>). Common cirro-stratus.</p> - -<p>Characterized by the presence of short curling fibres matted -together. Ci. S. Com.</p> - -<p>3. <i lang="la" xml:lang="la">Cirro-stratus vittatus</i> (Plate <a href="#Plate_57">57</a>). Ribboned cirro-stratus.</p> - -<p>Characterized by being made up of long stripes or bands of cloud. -Ci. S. Vt.</p> - -<p>4. <i lang="la" xml:lang="la">Cirro-stratus cumulosus</i> (Plate <a href="#Plate_17">17</a>). Flocculent -cirro-stratus.</p> - -<p>Characterized by an obscurely granular structure. Ci. S. Cu.</p> - -<p>Many forms of cirro-stratus are arranged in waves or ripples. -This is indicated by attaching the word undatus, or waved, after the -ordinary specific name, or the letter U after the abbreviation.</p> - -<p><span class="pagenum"><a name="Page_159" id="Page_159">[159]</a></span></p> - -<h3><span class="smcap">Group Cirro-cumulus.</span> Divisible into -three species.</h3> - -<p>1. <i lang="la" xml:lang="la">Cirro-macula</i> (Ley) (Plate <a href="#Plate_23">23</a>). Speckle cloud.</p> - -<p>Characterized by semi-transparency, by the fact that the particles -are frequently whiter and more opaque on their edges. A patch of -cirro-macula always looks like a thin sheet which has curdled. Ci. -Ma.</p> - -<p>2. <i lang="la" xml:lang="la">Cirro-cumulus nebulosus</i> (Plates <a href="#Plate_20">20</a> and <a href="#Plate_21">21</a>). Hazy -cirro-cumulus.</p> - -<p>Characterized as rounded balls of semi-transparent cloud, but -ill-defined and hazy. No shadows. Ci. Cu. Neb.</p> - -<p>3. <i lang="la" xml:lang="la">Cirro-cumulus</i> (Plates <a href="#Plate_18">18</a> and <a href="#Plate_19">19</a>).</p> - -<p>Characterized as opaque rounded balls clearly defined, but showing -no shadows on their under sides. Ci. Cu. Com.</p> - -<p>Wave forms again are indicated by the addition of the word -undatus.</p> - -<h3><span class="smcap">Group Alto Clouds.</span> Divisible into nine -species.</h3> - -<p>1. <i lang="la" xml:lang="la">Alto-stratus.</i> High stratus.</p> - -<p>A uniform veil of cloud showing no details of<span -class="pagenum"><a name="Page_160" id="Page_160">[160]</a></span> -structure except local variation in density in patches. Rarely dense -enough to completely hide the sun, or even the full moon. A. S.</p> - -<p>2. <i lang="la" xml:lang="la">Alto-stratus maculosus</i> (Plate <a href="#Plate_30">30</a>). Mackerel sky.</p> - -<p>Characterized as numerous nearly equal and small lenticular patches -ranged on a level and about equi-distant from each other. A. S. Mac.</p> - -<p>3. <i lang="la" xml:lang="la">Alto-stratus fractus</i> (Plate <a href="#Plate_34">34</a>).</p> - -<p>Patches and bits of cloud of irregular shape, but resembling broken -bits of a level sheet. A. S. Fr.</p> - -<p>4. <i lang="la" xml:lang="la">Alto-strato-cumulus</i> (Plate <a href="#Plate_32">32</a>).</p> - -<p>Intermediate between alto-stratus and alto-cumulus. A. S. Cu.</p> - -<p>5. <i lang="la" xml:lang="la">Alto-cumulus informis</i> (Plate <a href="#Plate_25">25</a>).</p> - -<p>Characterized as more or less rounded cloudlets interspersed with -ragged bits of cloud and occasionally with streaks of cirrus, the -cloudlets showing no clear-cut outlines, but having distinct shadows. -A. Cu. In.</p> - -<p>6. <i lang="la" xml:lang="la">Alto-cumulus nebulosus</i> (Plate <a href="#Plate_26">26</a>).</p> - -<p>Hazy alto-cumulus. A. Cu. Neb.</p> - -<p>7. <i lang="la" xml:lang="la">Alto-cumulus castellatus</i> (Plate <a href="#Plate_28">28</a>). Turret cloud.</p> - -<p><span class="pagenum"><a name="Page_161" -id="Page_161">[161]</a></span></p> - -<p>A high cloud resembling a number of tall narrow cumulus clouds on a -very diminutive scale. The cloudlets show distinct shadows, are very -opaque, and their upper margins are sharply defined. Vertical axes -longer than the horizontal ones. A. Cu. Ca.</p> - -<p>8. <i lang="la" xml:lang="la">Alto-cumulus glomeratus</i> (Plate <a href="#Plate_29">29</a>).</p> - -<p>Characterized by the roundness and regularity of the cloudlets, -which have sharp margins, cast distinct shadows, and have their axes -about equal in all directions. A. Cu. Gl.</p> - -<p>9. <i lang="la" xml:lang="la">Alto-cumulus communis.</i></p> - -<p>Small high cumulus of the ordinary pyramidal pattern. A. Cu. Com.</p> - -<p>10. <i lang="la" xml:lang="la">Alto-cumulus stratiformis</i> (Plate <a href="#Plate_27">27</a>).</p> - -<p>Flattened cloudlets gathering into small detached sheets. A. Cu. -S.</p> - -<h3>Lower clouds. <span class="smcap">Group Stratus.</span></h3> - -<p>1. <i lang="la" xml:lang="la">Stratus communis</i> (Plates <a href="#Plate_37">37</a> and <a href="#Plate_41">41</a>).</p> - -<p>In its most typical state, stratus consists of a sheet of cloud -of approximately uniform thickness. The most common form, however, -does vary considerably, though usually dense enough to hide the<span -class="pagenum"><a name="Page_162" id="Page_162">[162]</a></span> sun. -Portions of such a sheet would take the same specific name, unless the -portions are very small and ragged, which would be expressed by adding -the word fractus. S. Com.</p> - -<p>2. <i lang="la" xml:lang="la">Stratus maculosus</i> (Plate <a href="#Plate_40">40</a>).</p> - -<p>Formed either by the appearance of cloud in lumps, which are always -lenticular in shape, and ultimately join together to form a stratus, or -by the break up of the typical stratus. S. Mac.</p> - -<p>3. <i lang="la" xml:lang="la">Stratus radius</i> (Plate <a href="#Plate_42">42</a>). Roll cloud.</p> - -<p>Formed during the break up of a low stratus, which separates up into -a number of parallel lines of cloud. S. R.</p> - -<p>4. <i lang="la" xml:lang="la">Stratus lenticularis</i> (Plate <a href="#Plate_47">47</a>). Fall cloud.</p> - -<p>Formed by the collapse of cumulus or strato-cumulus. A cloud of -evening, easily recognized as lenticular patches. S. L.</p> - -<p>5. <i lang="la" xml:lang="la">Strato-cumulus</i> (Plates <a href="#Plate_38">38</a> and <a href="#Plate_39">39</a>).</p> - -<p>A term applied to either a stratus which has thickened every here -and there into cumulus, or a number of cumulus which have joined -together so as to show a nearly continuous common base. S. Cu.</p> - -<p><span class="pagenum"><a name="Page_163" id="Page_163">[163]</a></span></p> - -<h3><span class="smcap">Group Cumulus.</span></h3> - -<p>1. <i lang="la" xml:lang="la">Cumulus minor</i> (Plate <a href="#Plate_43">43</a>). Small cumulus.</p> - -<p>Cumulus clouds so small as to present the appearance of rounded -lumps, no definite pyramidal form or flattened base. Cu. Mi.</p> - -<p>2. <i lang="la" xml:lang="la">Cumulus major</i> (Plates <a href="#Plate_44">44</a> and <a href="#Plate_45">45</a>). Large cumulus.</p> - -<p>Characterized by a flattened base and rounded clear-cut upper -surfaces. Cu. Ma.</p> - -<p>3. <i lang="la" xml:lang="la">Cumulo-nimbus</i> (Plates <a href="#Plate_49">49</a> to <a href="#Plate_52">52</a>). Storm cloud.</p> - -<p>Characterized by the expanded, anvil-shaped, or disc-shaped top, -cirrifying at its edges.</p> - -<h3><span class="smcap">General Terms</span></h3> - -<p><i lang="la" xml:lang="la">Nimbus</i>, a term applied to a cloud from which rain is falling. When -the form of the cloud is visible, the term should be attached to that -belonging to the cloud. It may, however, be used as a substantive alone -when there is nothing to show from what sort of cloud, or combination -of clouds, the rain is falling (Plates <a href="#Plate_35">35</a> and <a href="#Plate_36">36</a>).</p> - -<p>Nimbus is either heavy stratus, massive strato-cumulus,<span -class="pagenum"><a name="Page_164" id="Page_164">[164]</a></span> or a -combination of these with stratiform clouds above, and possibly ragged -masses of fracto-cumulus below. N. either alone or after the sign of -the cloud.</p> - -<p><i lang="la" xml:lang="la">Fracto-</i> is a term placed as a prefix before the name of a cloud to -indicate that the cloud has ragged irregular margins, as if it had been -more or less torn to pieces. It is sometimes less awkward to append the -word fractus after the name of the cloud.</p> - -<p>A convenient abbreviation would be to write F. after the name of the -cloud.</p> - -<p><i lang="la" xml:lang="la">Undatus</i>, or waved, should always be added to the name of any cloud -which shows the arrangement so described.</p> - -<div class="chapter"></div> - -<p><span class="pagenum"><a name="Page_165" -id="Page_165">[165]</a></span></p> - -<h2>CHAPTER XI<br /> - -<span class="chtitle">CLOUD PHOTOGRAPHY</span></h2> - -<p class="noindent"><span class="smcap">Reference</span> has been -made in the first chapter to the fact that those who wish to make a -photographic study of clouds must follow a special course of procedure. -For every photographic purpose there is some particular process or some -special kind of apparatus which is better fitted for the end in view -than any other, and half the difficulty in attaining success is to find -out the best tools and the best methods.</p> - -<p>There is no difficulty whatever in securing excellent photographs -of heavy grey clouds, or of clouds which stand out dark against -a twilight sky. Any camera and any plate can be used, and in an -experienced hand will ensure success after a few trials, but except -under these special conditions, cirrus, in all its varieties, the -alto clouds, and even<span class="pagenum"><a name="Page_166" -id="Page_166">[166]</a></span> many of the lower ones, present a real -difficulty due to two causes. In the first place, they and their -surroundings are so brilliant that a very short exposure is sufficient, -far shorter than would be needed for a sunlit landscape; and in the -second place, the actinic value of the light they reflect is very -little greater than that received from the background of blue sky. When -so minute a difference comes to be represented in the monochrome of the -ordinary photograph, the eye fails to appreciate it, and all the finer -details are lost.</p> - -<p>Now, if proper care is taken in the development of a negative, -satisfactory results may be attained even if the exposure is twice as -great, or only half as great, as it should have been to get the best -result. But if the exposure is four or more times the best duration, -the negative will generally yield but poor contrasts, if any result -at all can be coaxed out. Again, if the exposure is only a quarter or -less of the ideal time, little or no image will come out. Suppose, -now, we have a brilliant object, and the correct exposure for the -plate and aperture of lens employed should be one-fiftieth of a -second; if we make an error either in judging or in effecting<span -class="pagenum"><a name="Page_167" id="Page_167">[167]</a></span> the -exposure, which amounts to one twenty-fifth of a second too much, we -get the negative exposed three times as much as it should be. Suppose, -again, the object is less brilliant, and the correct exposure should -be one-fifth of a second, an equal error of one twenty-fifth will -make little difference. But in photographing cirrus and such clouds, -if we used the same plates and the same lens apertures as we employ -for ordinary landscape work, we should want exposures of the order of -those given by a focal plane shutter, and a mistake either in judging -or in making the exposure, of even the hundredth part of a second, -would be fatal to good results, and would probably completely spoil the -plate. Evidently one of our first steps must be to lengthen the correct -exposure.</p> - -<p>There are four ways in which this can be done—by using a -slow-acting plate, by lessening the aperture of the lens, by putting -some transparent screen in front of the lens to shut off some of the -light, and, finally, by pointing the camera, not at the cloud itself, -but at its image in a black mirror.</p> - -<p>Of these, of course the slow plate and small aperture are the -simplest to adopt, and all the cloud<span class="pagenum"><a -name="Page_168" id="Page_168">[168]</a></span> studies shown in the -illustrations to these pages have been taken on plates prepared -for photo-mechanical purposes or for transparencies. There seems -to be nothing to choose between these two brands. Orthochromatic, -isochromatic, double-coated, and many other special types of plate -had previously been tried, both with coloured filters in front of the -lens and without them, without showing any marked superiority over an -ordinary plate of low rapidity. At last the photo-mechanical plates -were tried, and the efforts made to get satisfactory cloud portraits, -which had previously been marked only now and then with satisfactory -results, became uniformly and continuously successful.</p> - -<p>If the slow plates are exposed in the camera without either a -screen or the black mirror, the diaphragm should be reduced to a small -size and the exposure suitably adjusted. The length of exposure may -generally be judged by looking at the image on the focusing screen, and -reducing the aperture until the picture shows its detail easily. Then, -regarding the picture as that of a sunlit sea or distant landscape, -judge the necessary exposure by the brightness of the image.</p> - -<p><span class="pagenum"><a name="Page_169" -id="Page_169">[169]</a></span></p> - -<p>No definite rule can be given. The light varies enormously from day -to day, and hour to hour, and especially with the position occupied by -the cloud relative to the sun. Thus, working with a lens of six inches -focus and an aperture of a quarter of an inch, the exposure may vary -from the quickest snap of a Thornton-Pickard roller blind to as much as -a quarter of a second, or even more. Again, using a lens of eighteen -inches focus and an exposure of a fiftieth of a second, the necessary -aperture might vary from an eighth of an inch up to an inch and a -half. But if we suppose that we are dealing with an ordinary bright -summer sky between 9 a.m. and 5 p.m., and that the clouds are cirrus -or cirro-cumulus, an aperture of about one thirty-second of the focal -length will probably give some sort of image with a snap-shot exposure. -At first the failures will be many, but a little practice will soon -enable very respectable pictures to be taken by varying either the -diaphragm or the speed of shutter. Heavier clouds of the alto types -will need rather longer exposure or larger aperture.</p> - -<p>The lens may be of any kind, as long as it gives a well-defined -image, but there are many advantages<span class="pagenum"><a -name="Page_170" id="Page_170">[170]</a></span> in using one of the -rectilinear type provided with an iris diaphragm. A rapid lens is not -needed; indeed, it has been pointed out that slowness is a very great -desideratum, and if the camera is provided with a rapid lens it must -be ruthlessly stopped down. For general cloud purposes the best kind -of lens is a wide-angle rectilinear, but many occasions will present -themselves on which a lens of longer focus will be wanted in order to -give more insight into the details of some specially delicate clouds. -If the lenses are good, and the focusing is accurate, enlargements will -go a long way towards revealing the minuter structures, but the results -can never be quite so well defined as a direct photograph in a long -camera.</p> - -<p>A shutter will be essential, and it should be one which opens in -the middle, or which travels across the lens. The shutters which are -ingeniously contrived to give more exposure to the lower part of the -picture than to its upper part are useless for the purpose in view. It -should have some latitude of exposure, from about one-sixtieth of a -second up to a full second or more.</p> - -<p>Then as to the camera. Any light-tight camera<span -class="pagenum"><a name="Page_171" id="Page_171">[171]</a></span> will -do, and, as the objects will all be at a great distance, it may very -well be a fixed-focus one, or may be kept set up and fixed in focus for -a distant object. If not, on setting it up it should be focused on the -horizon or most distant object possible, and not on the cloud itself. -As, however, the clouds present themselves at all heights above the -horizon, even in the zenith, it becomes necessary to have some means -of pointing the camera in such directions. To a certain extent the -ordinary stand does allow of tilting, but a special support which will -allow the camera to be fixed firmly in any position is of the greatest -convenience.</p> - -<p>If the study is meant to be at all prolonged, the best plan is -to make a suitable camera, once for all, which can be left in fixed -focus, so as to be always ready, and which can be directed with equal -ease to any part of the sky, from the horizon to the zenith. If it is -intended to use a black mirror, then a special mount becomes almost -essential.</p> - -<p>Many of the most delicate of the photographs reproduced here have -been taken with a camera of peculiar pattern, the structure of which -is shown in Plate <a href="#Plate_61">61</a>. The lens is an ordinary rapid rectilinear,<span -class="pagenum"><a name="Page_172" id="Page_172">[172]</a></span> and -the stop used was generally one-sixteenth of the focal length. The -shutter is a light slip of aluminium, which can be drawn across from -side to side at any desired pace. The body of the camera is mahogany, -with a bellows part for getting correct focus, but when once this was -obtained the back was clamped to the tail-board and a little varnish -brushed over the clamping screws.</p> - -<p><a id="Plate_61"></a></p> - -<div class="figcenter screenonly" style="width: 329px;"> - <a href="images/i_p170b.jpg"> - <img src="images/i_p170b-tn.jpg" alt="" /> - </a> - <p><span class="smcap">Plate 61.</span></p> - <div class="caption"> -<p>CLOUD CAMERA FOR STUDIES.</p></div> -</div> - -<div class="figcenter handonly" style="width: 568px;"> - <img src="images/i_p170b-hh.jpg" alt="" /> - <p><span class="smcap">Plate 61.</span></p> - <div class="caption"> -<p>CLOUD CAMERA FOR STUDIES.</p></div> -</div> - -<p>The camera swings on a couple of screws, which act as trunnions. -These pass through two upright arms, which spring on either side from -the base board, which is attached to the ordinary camera stand. This -base board can be rotated into any horizontal position desired, and the -camera can be tilted through any vertical angle by swinging it between -the uprights, and can be clamped by tightening the two trunnion screws. -These screws are so placed on the front of the camera that the lens and -its attachments on the one side nearly balance the back part of the -camera on the other side, and so lessen the danger of slipping.</p> - -<p>Supported in front of the lens by light brass-work is the -black mirror, made of a very dark glass optically worked on the -front face. It is a<span class="pagenum"><a name="Page_173" -id="Page_173">[173]</a></span> curious fact that, although bits of -plate-glass blackened on the back seem to the naked eye to give a -single image of sufficient truth, if such a mirror is placed in front -of the camera the second faint image formed by reflection from the -blackened surface is almost always to be detected. Moreover, the lens -with its large aperture at once detects irregularities in the surface -of the glass, which are quite imperceptible through the narrow limits -of the pupil. Black glass, with a truly worked surface, is essential -then, but the surface need not be of the high order of excellence -required for mirrors used for telescopic work, since the first image is -not, as a rule, intended to be highly magnified.</p> - -<p>The mirror is held so that its surface makes an angle of about 33 -degrees with the axis of the lens, and the block carrying shutter and -mirror can be turned round into any position by slipping it round the -lens mount as an axis. The mirror thus always retains the correct -angle.</p> - -<p>The action of the mirror is to a large extent due to mere diminution -of brightness, but it also partly extinguishes the blue light of the -sky without exerting any such influence on the white light from<span -class="pagenum"><a name="Page_174" id="Page_174">[174]</a></span> a -cloud. This is due to the fact that the blue light of the sky is partly -polarized, while that reflected from the cloud is not. Now, polarized -light which falls upon a black mirror held in a particular position is -not reflected by it. This position depends upon various circumstances, -but one condition is that the reflected ray must make an angle of about -33 degrees with the surface of the glass. The amount of the polarized -component of the blue light varies greatly, but is at a maximum at all -points 90 degrees away from the sun. This, then, is the best possible -position for photographing a cloud, as the whole of this polarized -component may be suppressed by adjusting the mirror to the proper -position, and then the most delicate cirrus fibres stand out brilliant -on an almost black background.</p> - -<p>The black mirror could with some advantage be replaced by a -Nicol’s prism mounted between the components of the lens, so -that it could be turned in any position; but Nicol’s prisms -are expensive, and such an arrangement would cost many times the sum -sufficient for an excellent mirror, and then would narrow down the -field of view in a very inconvenient way.</p> - -<p><span class="pagenum"><a name="Page_175" -id="Page_175">[175]</a></span></p> - -<p>With this apparatus exposures of a tenth to a fifth of a second -were usually required for high clouds in bright daylight, while longer -times, up to a second, might be required under less actively actinic -conditions.</p> - -<p>The exposure having been made, the next step is development.</p> - -<p>Now, every practical photographer has his own pet formula, his own -particular favourite among the numerous developing compounds now on the -market. It is, therefore, rather a thankless task to offer advice as -to which should be selected. In all probability as good results may be -got by other methods and other formulæ, and the description which -follows must be understood rather as an account of the process actually -adopted, than advice as to that which should be chosen.</p> - -<p>The developer used has been always pyro and ammonia, made up in -accordance with the formula—</p> - -<div class="center"> -<table border="0" cellpadding="4" cellspacing="0" summary=""> -<tr> - <td class="tdl">Pyro</td> - <td class="tdc">30</td> - <td class="tdl">grains</td> -</tr> - -<tr> - <td class="tdl">Potassium metabisulphite</td> - <td class="tdc">30</td> - <td class="tdc">„</td> -</tr> - -<tr> - <td class="tdl">Ammonium bromide</td> - <td class="tdc">30</td> - <td class="tdc">„</td> -</tr> - -<tr> - <td class="tdl">Water</td> - <td class="tdc">10</td> - <td class="tdc">ozs.</td> -</tr> -</table></div> - -<p class="noindent">But if much work was anticipated the solution<span -class="pagenum"><a name="Page_176" id="Page_176">[176]</a></span> was -made up in a more concentrated form, and diluted to this strength of 3 -grains of pyro per ounce for actual use.</p> - -<p>The ammonia solution is prepared by mixing 3 drams ammonia fortiss. -with 20 ozs. of water.</p> - -<p>In developing it is necessary to remember that our object is to -make the most of a very small difference in effect. The plate is first -flowed over with a mixture of sufficient developer, with not more than -a quarter of its bulk of the ammonia. If the cloud should flash out in -a few seconds add more of the pyro solution, but unless the exposure -has been much overdone this will not happen. If the image begins to -appear after from thirty to forty seconds it is probable that the best -result will be reached by leaving it alone, but if there is any hanging -back of the detail another quarter bulk of ammonia should be put into -the glass, the developer mixed with it, and the whole returned to the -developing dish.</p> - -<p>If no image appears after about forty seconds, add more ammonia as -above described, and leave for another forty seconds, and so on, until -by this method of trial the right quantity of alkali for the<span -class="pagenum"><a name="Page_177" id="Page_177">[177]</a></span> -particular exposure has been ascertained. The development must never be -hurried, or the background of sky will blacken too soon, and in some -cases it may take a quarter of an hour or more to get enough density on -the cloud. But as a general rule the image is fully out in about two -minutes, and the plate is then washed and fixed in the usual way.</p> - -<p>If a black mirror is used there will seldom be any necessity for -intensification, but if not, it may frequently be required, especially -for the more delicate kinds of cirrus. Indeed, the image may sometimes -be so thin that the common process of intensification by mercury and -ammonia does not give density enough. If that seems at all likely to be -the case, it is wiser to use the formula known as Monckhoven’s, -since that simply adds silver to silver instead of replacing the silver -image by some other body, and the process can consequently be repeated -more than once, if sufficient density is not secured by the first -application. The formula does not seem to be very often used, so it may -be best to quote it.</p> - -<p><span class="pagenum"><a name="Page_178" id="Page_178">[178]</a></span></p> - -<div class="center"> -<table border="0" cellpadding="4" cellspacing="0" summary=""> - -<tr> - <td class="tdl">A.</td> - <td class="tdl">Potassium bromide</td> - <td class="tdr">100</td> - <td class="tdl">grains</td> -</tr> - -<tr> - <td class="tdl"></td> - <td class="tdl">Mercuric chloride</td> - <td class="tdr">100</td> - <td class="tdc">„</td> -</tr> - -<tr> - <td class="tdl"></td> - <td class="tdl">Water</td> - <td class="tdr">10</td> - <td class="tdl">ozs.</td> -</tr> - -<tr> - <td class="tdl">B.</td> - <td class="tdl">Potassium cyanide (pure)</td> - <td class="tdr">100</td> - <td class="tdl">grains</td> -</tr> - -<tr> - <td class="tdl"></td> - <td class="tdl">Silver nitrate</td> - <td class="tdr">100</td> - <td class="tdc">„</td> -</tr> - -<tr> - <td class="tdl"></td> - <td class="tdl">Water</td> - <td class="tdr">10</td> - <td class="tdl">ozs.</td> -</tr> -</table></div> - -<p>Place the washed negative in A until it has gone white, then rinse -it well and transfer to B, in which the image turns to a velvety black. -After washing, the process can be repeated.</p> - -<p>Intensification is, however, only a way of saving photographs which -cannot be secured again. If the first photograph of a particular -variety of cloud is not satisfactory, it ought at least to tell the -operator where he had gone wrong, and a second attempt should produce -a better result than any image built up by chemical action on an -imperfect base.</p> - -<p>There is nothing novel in any of these methods, and there is -no doubt that other formulæ would be as good; but the one -thing essential is to have a developer whose action can be held -under control, and to apply that developer in such a way that very -considerable over-exposure will not result in the ruin of the plate. If -a number of photographs have<span class="pagenum"><a name="Page_179" -id="Page_179">[179]</a></span> been taken in about the same part of -the sky, and within a short time of each other, then the correct -proportions of developer and alkali will be nearly the same for all, -but the first of such a batch will always have to be attacked in the -cautious step-by-step method. Patience and perseverance, backed by a -steady refusal to be discouraged by the failures which are at first -inevitable, are as certain to be crowned by success as they are in -other studies.</p> - -<p>The workers are few, and there is much to be done; for it is mainly -to those who will photograph the higher clouds, and so trace the stages -of their growth and decay, that we must look for the data which will -enable us to solve the problems they present, and so enlarge the narrow -boundaries of our knowledge of some of the most beautiful things in -Nature.</p> - -<div class="chapter"></div> - -<p> - <span class="pagenum"> - <a name="Page_180" id="Page_180">[180]</a> - </span><br /> - <span class="pagenum"> - <a name="Page_181" id="Page_181">[181]</a> - </span> -</p> - -<h2>REFERENCES</h2> - -<div class="hang"> - -<p>1. “International Atlas of Clouds” (Atlas International -des Nuages). Hildebrandsson, Riggenbach, and Teisserenc de Bort. Paris. -1896.</p></div> - -<p>This is the atlas referred to in the text. The letter-press is -short, and is repeated in English, French, and German.</p> - -<div class="hang"> - -<p>2. “Annals of the Astronomical Observatory of Harvard -College.” Vol. XXX. Observations made at the Blue Hill -Meteorological Observatory. Part III. Measurement of Cloud Heights and -Velocities. By H. H. Clayton and S. P. Fergusson. Part IV. Discussion -of the Cloud Observations. By H. H. Clayton.</p></div> - -<p>This last gives a very concise account of all the different -proposals which have been made for the systematic naming of clouds.</p> - -<div class="hang"> - -<p>3. “Études International des Nuages.” 1896-1897. Observations -et Mesures de la Suède. I., II. Publication de -l’Observatoire Météorologique de l’Université Roy. d’Upsala. -H. H. Hildebrandsson.</p></div> - -<p>An account of the Upsala observations referred to in the text.</p> - -<div class="hang"> - -<p>4. <cite>Quarterly Journal of the Royal Meteorological Society</cite>.</p> - -</div> - -<div class="hang2"> - -<p>Helm Wind. Marriott. 1886 and 1889.</p> - -<p>The Thickness of Shower Clouds. Clayden. 1886.</p> - -<p><span class="pagenum"><a name="Page_182" -id="Page_182">[182]</a></span></p> - -<p>Methods of Cloud Measurement. Ekholm. 1888.</p> - -<p>Cirrus Formation. Clayton. 1890.</p> - -<p>Nomenclature of Clouds. Hildebrandsson. 1887.</p> - -<p><span class="ml35">„</span> <span class="ml5">„</span> -<span class="ml15">Abercromby.</span> 1887.</p> - -<p><span class="ml35">„</span> <span class="ml5">„</span> -<span class="ml15">Wilson-Barker.</span> 1890.</p> - -<p><span class="ml35">„</span> <span class="ml5">„</span> -<span class="ml15">Gaster.</span> 1893.</p> - -<p><span class="ml35">„</span> <span class="ml5">„</span> -<span class="ml15">Scott.</span> 1895.</p> - -<p>A New Instrument for Cloud Measurement. Ekholm. 1893.</p> - -<p>Calculation of Photographic Cloud Measurements. Olsson. 1894.</p> - -<p>The Motion of Clouds. Shaw. 1895.</p> - -</div> - -<div class="hang"> - -<p>5. Reports of the British Association. Reports of the Committee -on Meteorological Photography. Clayden. 1891 to 1900.</p> - -</div> - -<p>The reports for 1896 and 1900 refer mainly to the measurements -described in the text.</p> - -<div class="hang"> - -<p>6. “Cloudland.” Clement Ley.</p></div> - -<p>The work in which Mr. Ley set forth his proposed scheme.</p> - -<div class="hang"> - -<p>7. “A Popular Treatise on the Winds.” Ferrel.</p></div> - -<p>Not a “popular” work in the usual sense, but contains lucid -descriptions of the mechanics of the atmosphere.</p> - -<div class="hang"> - -<p>8. There are many excellent text-books on meteorology, all of -which deal more or less with the movements of the atmosphere -and the formation of clouds.</p></div> - -<div class="chapter"></div> - -<p><span class="pagenum"><a name="Page_183" id="Page_183">[183]</a></span></p> - -<h2>INDEX</h2> - -<div> -<span class="smcap">Abercromby</span>, <a href="#Page_10">10</a><br /> -Aitken, <a href="#Page_3">3</a>, <a href="#Page_91">91</a><br /> -Altitude of clouds, <a href="#Page_19">19</a>, <a href="#Page_149">149</a><br /> -—— of rain-clouds, <a href="#Page_76">76</a><br /> -Alto clouds, <a href="#Page_59">59</a>, <a href="#Page_159">159</a><br /> -Alto-cumulus communis, <a href="#Page_161">161</a><br /> -—— castellatus, <a href="#Page_66">66</a>, <a href="#Page_160">160</a><br /> -—— glomeratus, <a href="#Page_67">67</a>, <a href="#Page_161">161</a><br /> -—— informis, <a href="#Page_64">64</a>, <a href="#Page_160">160</a><br /> -—— nebulosus, <a href="#Page_65">65</a>, <a href="#Page_160">160</a><br /> -—— stratiformis, <a href="#Page_65">65</a>, <a href="#Page_161">161</a><br /> -Alto-strato-cumulus, <a href="#Page_70">70</a>, <a href="#Page_160">160</a><br /> -Alto-stratus maculosus, <a href="#Page_68">68</a>, <a href="#Page_160">160</a><br /> -Ascent of vapour, <a href="#Page_124">124</a><br /> -Atlas, International, <a href="#Page_11">11</a><br /> -<br /> -<span class="smcap">Band</span> cirrus, <a href="#Page_41">41</a><br /> -Bidwell, Shelford, <a href="#Page_92">92</a><br /> -Black mirror, <a href="#Page_14">14</a>, <a href="#Page_173">173</a><br /> -Blue Hill, <a href="#Page_20">20</a>, <a href="#Page_138">138</a><br /> -<br /> -<span class="smcap">Cameras</span>, <a href="#Page_140">140</a>, <a href="#Page_171">171</a><br /> -Change cirrus, <a href="#Page_37">37</a><br /> -—— of velocity, <a href="#Page_35">35</a><br /> -Cirriform top of thunder-cloud, <a href="#Page_110">110</a><br /> -Cirro-cumulus, <a href="#Page_45">45</a>, <a href="#Page_159">159</a><br /> -—— nebulosus, <a href="#Page_52">52</a>, <a href="#Page_159">159</a><br /> -Cirro-filum, <a href="#Page_33">33</a>, <a href="#Page_156">156</a><br /> -Cirro-macula, <a href="#Page_53">53</a>, <a href="#Page_159">159</a><br /> -Cirro-nebula, <a href="#Page_26">26</a>, <a href="#Page_27">27</a>, <a href="#Page_155">155</a><br /> -Cirro-stratus, <a href="#Page_45">45</a>, <a href="#Page_157">157</a><br /> -—— communis, <a href="#Page_47">47</a>, <a href="#Page_158">158</a><br /> -Cirro-stratus cumulosus, <a href="#Page_48">48</a>, <a href="#Page_158">158</a><br /> -—— nebulosus, <a href="#Page_45">45</a>, <a href="#Page_158">158</a><br /> -—— vittatus, <a href="#Page_158">158</a><br /> -Cirro-velum, <a href="#Page_24">24</a><br /> -Cirrus, <a href="#Page_21">21</a>, <a href="#Page_155">155</a><br /> -—— altitudes, <a href="#Page_30">30</a>, <a href="#Page_149">149</a><br /> -—— communis, <a href="#Page_40">40</a>, <a href="#Page_157">157</a><br /> -—— caudatus, <a href="#Page_34">34</a>, <a href="#Page_156">156</a><br /> -—— excelsus, <a href="#Page_31">31</a>, <a href="#Page_156">156</a><br /> -—— inconstans, <a href="#Page_37">37</a>, <a href="#Page_157">157</a><br /> -—— nebulosus, <a href="#Page_36">36</a>, <a href="#Page_156">156</a><br /> -—— ripples, <a href="#Page_51">51</a><br /> -—— ventosus, <a href="#Page_32">32</a>, <a href="#Page_156">156</a><br /> -—— vittatus, <a href="#Page_41">41</a>, <a href="#Page_156">156</a><br /> -Cloud altitudes, <a href="#Page_149">149</a><br /> -—— nuclei, <a href="#Page_3">3</a><br /> -—— photography, <a href="#Page_17">17</a>, <a href="#Page_165">165</a><br /> -Condensation, <a href="#Page_93">93</a><br /> -Cooling by contact, <a href="#Page_89">89</a><br /> -—— by expansion, <a href="#Page_92">92</a><br /> -—— by mixture, <a href="#Page_90">90</a><br /> -—— by radiation, <a href="#Page_88">88</a><br /> -<br /> -<span class="smcap">Development</span>, <a href="#Page_175">175</a><br /> -Differential currents, <a href="#Page_133">133</a><br /> -Diffraction, <a href="#Page_63">63</a><br /> -Dimensions of clouds, <a href="#Page_112">112</a><br /> -Dry fog, <a href="#Page_73">73</a><br /> -<br /> -<span class="smcap">Equilibrium</span>, stable and unstable, <a href="#Page_106">106</a><br /> -Exeter measurements, <a href="#Page_137">137</a><br /> -<span class="pagenum"><a name="Page_184" id="Page_184">[184]</a></span>Exposure, <a href="#Page_167">167</a><br /> -<br /> -<span class="smcap">Fall cloud</span>, <a href="#Page_81">81</a>, <a href="#Page_88">88</a><br /> -False cirrus, <a href="#Page_110">110</a><br /> -Fog particles, <a href="#Page_60">60</a><br /> -Fracto, <a href="#Page_164">164</a><br /> -<br /> -<span class="smcap">Gravitation</span>, <a href="#Page_4">4</a><br /> -Great waves, <a href="#Page_129">129</a><br /> -<br /> -<span class="smcap">Halos</span>, <a href="#Page_21">21</a>, <a href="#Page_22">22</a>, <a href="#Page_23">23</a><br /> -Heat cumulus, <a href="#Page_85">85</a><br /> -—— thunderstorms, <a href="#Page_107">107</a><br /> -Hildebrandsson, <a href="#Page_10">10</a><br /> -Hoar frost, <a href="#Page_60">60</a><br /> -Howard, Luke, <a href="#Page_9">9</a>, <a href="#Page_10">10</a><br /> -<br /> -<span class="smcap">Intensification</span>, <a href="#Page_177">177</a><br /> -Interconvection, <a href="#Page_125">125</a><br /> -International Code, <a href="#Page_11">11</a><br /> -—— Committee, <a href="#Page_10">10</a><br /> -—— System, <a href="#Page_12">12</a>, <a href="#Page_71">71</a><br /> -Ions as nuclei, <a href="#Page_92">92</a><br /> -Irregularities of ground, <a href="#Page_39">39</a><br /> -<br /> -<span class="smcap">Kepler</span>, <a href="#Page_4">4</a><br /> -Kew, <a href="#Page_138">138</a><br /> -<br /> -<span class="smcap">Ley, Clement</span>, <a href="#Page_24">24</a>, <a href="#Page_34">34</a>, <a href="#Page_55">55</a><br /> -Lightning, <a href="#Page_114">114</a><br /> -Lower clouds, <a href="#Page_71">71</a><br /> -<br /> -<span class="smcap">Mammato-cumulus</span>, <a href="#Page_99">99</a><br /> -Meteorological Conference, <a href="#Page_9">9</a><br /> -Methods of computing altitudes, <a href="#Page_143">143</a><br /> -Munich, <a href="#Page_9">9</a><br /> -<br /> -<span class="smcap">Newton</span>, <a href="#Page_4">4</a><br /> -Nimbus, <a href="#Page_74">74</a>, <a href="#Page_163">163</a><br /> -Nuclei, <a href="#Page_92">92</a><br /> -<br /> -<span class="smcap">Photographic</span> methods, <a href="#Page_17">17</a>, <a href="#Page_165">165</a><br /> -<br /> -<span class="smcap">Rain-clouds</span>, altitude, <a href="#Page_76">76</a><br /> -——, thickness, <a href="#Page_81">81</a><br /> -Rate of fall of temperature, <a href="#Page_94">94</a><br /> -<br /> -<span class="smcap">Sandstorms</span>, <a href="#Page_117">117</a><br /> -Saturation, <a href="#Page_87">87</a><br /> -Scotch mist, <a href="#Page_73">73</a><br /> -Slow plates, <a href="#Page_167">167</a><br /> -Spread of condensation, <a href="#Page_57">57</a><br /> -Stable equilibrium, <a href="#Page_106">106</a><br /> -Strato-cumulus, <a href="#Page_77">77</a><br /> -Stratus, <a href="#Page_72">72</a>, <a href="#Page_161">161</a><br /> -—— communis, <a href="#Page_77">77</a>, <a href="#Page_161">161</a><br /> -—— lenticularis, <a href="#Page_80">80</a>, <a href="#Page_98">98</a>, <a href="#Page_162">162</a><br /> -—— maculosus, <a href="#Page_78">78</a>, <a href="#Page_162">162</a><br /> -—— radius, <a href="#Page_79">79</a>, <a href="#Page_162">162</a><br /> -Subsidence of cloud top, <a href="#Page_111">111</a><br /> -Surfusion, <a href="#Page_61">61</a><br /> -Swing stand, <a href="#Page_171">171</a><br /> -<br /> -<span class="smcap">Turreted</span> cloud, <a href="#Page_66">66</a><br /> -Tycho Brahe, <a href="#Page_4">4</a><br /> -Types, <a href="#Page_8">8</a><br /> -<br /> -<span class="smcap">Umbra</span> and penumbra, <a href="#Page_29">29</a><br /> -Undatus, <a href="#Page_164">164</a><br /> -Unsaturated cloud, <a href="#Page_101">101</a><br /> -Upsala, <a href="#Page_10">10</a>, <a href="#Page_138">138</a><br /> -<br /> -<span class="smcap">Wave</span> clouds, <a href="#Page_47">47</a>, <a href="#Page_119">119</a>, <a href="#Page_133">133</a><br /> -Wilson, C. T. R., <a href="#Page_92">92</a><br /> -</div> - -<p class="center p2 f80">THE END</p> - -<p id="printedby">PRINTED BY WILLIAM CLOWES AND SONS, LIMITED, LONDON -AND BECCLES.</p> - -<div class="chapter"></div> - -<div class="footnotes"> - -<p class="ph3">FOOTNOTES:</p> - -<div class="footnote"> - -<p><a name="Footnote_1_1" id="Footnote_1_1"></a><a -href="#FNanchor_1_1"><span class="label">[1]</span></a> See reference -No. 2 on p. <a href="#Page_181">181</a>.</p></div> - -<div class="footnote"> - -<p><a name="Footnote_2_2" id="Footnote_2_2"></a><a -href="#FNanchor_2_2"><span class="label">[2]</span></a> The telescope -with which these observations have been made is a 6·8-inch -refractor equatorially mounted.</p></div> - -<div class="footnote"> - -<p><a name="Footnote_3_3" id="Footnote_3_3"></a><a -href="#FNanchor_3_3"><span class="label">[3]</span></a> Ferricyanide of -potassium and hyposulphite of soda.</p></div> - -<div class="footnote"> - -<p><a name="Footnote_4_4" id="Footnote_4_4"></a><a -href="#FNanchor_4_4"><span class="label">[4]</span></a> From the -declination of the sun corrected for variation and from the known -latitude, the meridian zenith distance is calculated.</p> - -<p>From the Greenwich time, the longitude, and the equation of time, -the hour angle is obtained.</p> - -<p>Now, if H be the hour angle, D the reduced declination, and M the -meridian zenith distance, the sun’s altitude may be calculated by -the formula—</p> - -<p class="center">log versin H + L cos lat. + L cos D-20 = log <i>n</i>,</p> - -<p class="noindent">where <i>n</i> is a natural number, and</p> - -<p class="center"><i>n</i> + vers M = covers alt.</p> - -<p>Again, to find the azimuth—</p> - -<p class="center">vers sup. (lat. + alt.)-vers polar dist. = <i>m</i>,</p> - -<p class="noindent">where <i>m</i> is another natural number, and</p> - -<p class="center">log <i>m</i> + L sec. lat. + L sec. alt.-20 = log vers azim.,</p> - -<p class="noindent">reckoned from the south.</p> - -<p>Hence the position of the sun is ascertained for both negatives.</p> - -<p>By actual measurements on the plates and reference to a previously -constructed scale the position of the cloud as seen from each camera -is next determined, and the angle subtended by the base line at a -point X vertically beneath the cloud is calculated. If A and B are -the stations, and <i>a</i> and <i>b</i> the angles from them respectively, the -distance AX is given thus—</p> - -<p class="center">log AX = L sin <i>b</i>-L sin AXB + log AB,</p> - -<p class="noindent">and the height <i>h</i> of the cloud above X is given -by—</p> - -<p class="center">log <i>h</i> = log AX + L tan alt.-10.</p> - -</div> - -</div> - -<div class="chapter"></div> - -<div class="tnotes"> - -<p class="ph3">Transcriber’s Note</p> - -<p>Variations in hyphenation (i.e. thunderstorm and thunder-storm) -have been retained. The following apparent typographical errors were -corrected:</p> - -<p>Page <a href="#Page_171">171</a>, “focussed” changed to -“focused.” (it should be focused on the horizon)</p> - -<p>Page <a href="#Page_173">173</a>, “aperature” changed to -“aperture.” (the lens with its large aperture)</p> - -</div> - - - - - - - - -<pre> - - - - - -End of the Project Gutenberg EBook of Cloud Studies, by Arthur W. Clayden - -*** END OF THIS PROJECT GUTENBERG EBOOK CLOUD STUDIES *** - -***** This file should be named 55126-h.htm or 55126-h.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/5/5/1/2/55126/ - -Produced by Cindy Horton, deaurider, and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. Special rules, set forth in the General Terms of Use part -of this license, apply to copying and distributing Project -Gutenberg-tm electronic works to protect the PROJECT GUTENBERG-tm -concept and trademark. Project Gutenberg is a registered trademark, -and may not be used if you charge for the eBooks, unless you receive -specific permission. If you do not charge anything for copies of this -eBook, complying with the rules is very easy. You may use this eBook -for nearly any purpose such as creation of derivative works, reports, -performances and research. They may be modified and printed and given -away--you may do practically ANYTHING in the United States with eBooks -not protected by U.S. copyright law. Redistribution is subject to the -trademark license, especially commercial redistribution. - -START: FULL LICENSE - -THE FULL PROJECT GUTENBERG LICENSE -PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK - -To protect the Project Gutenberg-tm mission of promoting the free -distribution of electronic works, by using or distributing this work -(or any other work associated in any way with the phrase "Project -Gutenberg"), you agree to comply with all the terms of the Full -Project Gutenberg-tm License available with this file or online at -www.gutenberg.org/license. - -Section 1. General Terms of Use and Redistributing Project -Gutenberg-tm electronic works - -1.A. By reading or using any part of this Project Gutenberg-tm -electronic work, you indicate that you have read, understand, agree to -and accept all the terms of this license and intellectual property -(trademark/copyright) agreement. If you do not agree to abide by all -the terms of this agreement, you must cease using and return or -destroy all copies of Project Gutenberg-tm electronic works in your -possession. If you paid a fee for obtaining a copy of or access to a -Project Gutenberg-tm electronic work and you do not agree to be bound -by the terms of this agreement, you may obtain a refund from the -person or entity to whom you paid the fee as set forth in paragraph -1.E.8. - -1.B. "Project Gutenberg" is a registered trademark. It may only be -used on or associated in any way with an electronic work by people who -agree to be bound by the terms of this agreement. There are a few -things that you can do with most Project Gutenberg-tm electronic works -even without complying with the full terms of this agreement. See -paragraph 1.C below. There are a lot of things you can do with Project -Gutenberg-tm electronic works if you follow the terms of this -agreement and help preserve free future access to Project Gutenberg-tm -electronic works. See paragraph 1.E below. - -1.C. The Project Gutenberg Literary Archive Foundation ("the -Foundation" or PGLAF), owns a compilation copyright in the collection -of Project Gutenberg-tm electronic works. Nearly all the individual -works in the collection are in the public domain in the United -States. If an individual work is unprotected by copyright law in the -United States and you are located in the United States, we do not -claim a right to prevent you from copying, distributing, performing, -displaying or creating derivative works based on the work as long as -all references to Project Gutenberg are removed. Of course, we hope -that you will support the Project Gutenberg-tm mission of promoting -free access to electronic works by freely sharing Project Gutenberg-tm -works in compliance with the terms of this agreement for keeping the -Project Gutenberg-tm name associated with the work. You can easily -comply with the terms of this agreement by keeping this work in the -same format with its attached full Project Gutenberg-tm License when -you share it without charge with others. - -1.D. The copyright laws of the place where you are located also govern -what you can do with this work. Copyright laws in most countries are -in a constant state of change. If you are outside the United States, -check the laws of your country in addition to the terms of this -agreement before downloading, copying, displaying, performing, -distributing or creating derivative works based on this work or any -other Project Gutenberg-tm work. The Foundation makes no -representations concerning the copyright status of any work in any -country outside the United States. - -1.E. Unless you have removed all references to Project Gutenberg: - -1.E.1. The following sentence, with active links to, or other -immediate access to, the full Project Gutenberg-tm License must appear -prominently whenever any copy of a Project Gutenberg-tm work (any work -on which the phrase "Project Gutenberg" appears, or with which the -phrase "Project Gutenberg" is associated) is accessed, displayed, -performed, viewed, copied or distributed: - - This eBook is for the use of anyone anywhere in the United States and - most other parts of the world at no cost and with almost no - restrictions whatsoever. You may copy it, give it away or re-use it - under the terms of the Project Gutenberg License included with this - eBook or online at www.gutenberg.org. If you are not located in the - United States, you'll have to check the laws of the country where you - are located before using this ebook. - -1.E.2. If an individual Project Gutenberg-tm electronic work is -derived from texts not protected by U.S. copyright law (does not -contain a notice indicating that it is posted with permission of the -copyright holder), the work can be copied and distributed to anyone in -the United States without paying any fees or charges. If you are -redistributing or providing access to a work with the phrase "Project -Gutenberg" associated with or appearing on the work, you must comply -either with the requirements of paragraphs 1.E.1 through 1.E.7 or -obtain permission for the use of the work and the Project Gutenberg-tm -trademark as set forth in paragraphs 1.E.8 or 1.E.9. - -1.E.3. If an individual Project Gutenberg-tm electronic work is posted -with the permission of the copyright holder, your use and distribution -must comply with both paragraphs 1.E.1 through 1.E.7 and any -additional terms imposed by the copyright holder. Additional terms -will be linked to the Project Gutenberg-tm License for all works -posted with the permission of the copyright holder found at the -beginning of this work. - -1.E.4. Do not unlink or detach or remove the full Project Gutenberg-tm -License terms from this work, or any files containing a part of this -work or any other work associated with Project Gutenberg-tm. - -1.E.5. Do not copy, display, perform, distribute or redistribute this -electronic work, or any part of this electronic work, without -prominently displaying the sentence set forth in paragraph 1.E.1 with -active links or immediate access to the full terms of the Project -Gutenberg-tm License. - -1.E.6. You may convert to and distribute this work in any binary, -compressed, marked up, nonproprietary or proprietary form, including -any word processing or hypertext form. However, if you provide access -to or distribute copies of a Project Gutenberg-tm work in a format -other than "Plain Vanilla ASCII" or other format used in the official -version posted on the official Project Gutenberg-tm web site -(www.gutenberg.org), you must, at no additional cost, fee or expense -to the user, provide a copy, a means of exporting a copy, or a means -of obtaining a copy upon request, of the work in its original "Plain -Vanilla ASCII" or other form. Any alternate format must include the -full Project Gutenberg-tm License as specified in paragraph 1.E.1. - -1.E.7. Do not charge a fee for access to, viewing, displaying, -performing, copying or distributing any Project Gutenberg-tm works -unless you comply with paragraph 1.E.8 or 1.E.9. - -1.E.8. You may charge a reasonable fee for copies of or providing -access to or distributing Project Gutenberg-tm electronic works -provided that - -* You pay a royalty fee of 20% of the gross profits you derive from - the use of Project Gutenberg-tm works calculated using the method - you already use to calculate your applicable taxes. The fee is owed - to the owner of the Project Gutenberg-tm trademark, but he has - agreed to donate royalties under this paragraph to the Project - Gutenberg Literary Archive Foundation. Royalty payments must be paid - within 60 days following each date on which you prepare (or are - legally required to prepare) your periodic tax returns. Royalty - payments should be clearly marked as such and sent to the Project - Gutenberg Literary Archive Foundation at the address specified in - Section 4, "Information about donations to the Project Gutenberg - Literary Archive Foundation." - -* You provide a full refund of any money paid by a user who notifies - you in writing (or by e-mail) within 30 days of receipt that s/he - does not agree to the terms of the full Project Gutenberg-tm - License. You must require such a user to return or destroy all - copies of the works possessed in a physical medium and discontinue - all use of and all access to other copies of Project Gutenberg-tm - works. - -* You provide, in accordance with paragraph 1.F.3, a full refund of - any money paid for a work or a replacement copy, if a defect in the - electronic work is discovered and reported to you within 90 days of - receipt of the work. - -* You comply with all other terms of this agreement for free - distribution of Project Gutenberg-tm works. - -1.E.9. If you wish to charge a fee or distribute a Project -Gutenberg-tm electronic work or group of works on different terms than -are set forth in this agreement, you must obtain permission in writing -from both the Project Gutenberg Literary Archive Foundation and The -Project Gutenberg Trademark LLC, the owner of the Project Gutenberg-tm -trademark. Contact the Foundation as set forth in Section 3 below. - -1.F. - -1.F.1. Project Gutenberg volunteers and employees expend considerable -effort to identify, do copyright research on, transcribe and proofread -works not protected by U.S. copyright law in creating the Project -Gutenberg-tm collection. Despite these efforts, Project Gutenberg-tm -electronic works, and the medium on which they may be stored, may -contain "Defects," such as, but not limited to, incomplete, inaccurate -or corrupt data, transcription errors, a copyright or other -intellectual property infringement, a defective or damaged disk or -other medium, a computer virus, or computer codes that damage or -cannot be read by your equipment. - -1.F.2. LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the "Right -of Replacement or Refund" described in paragraph 1.F.3, the Project -Gutenberg Literary Archive Foundation, the owner of the Project -Gutenberg-tm trademark, and any other party distributing a Project -Gutenberg-tm electronic work under this agreement, disclaim all -liability to you for damages, costs and expenses, including legal -fees. YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT -LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE -PROVIDED IN PARAGRAPH 1.F.3. YOU AGREE THAT THE FOUNDATION, THE -TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE -LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR -INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH -DAMAGE. - -1.F.3. LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a -defect in this electronic work within 90 days of receiving it, you can -receive a refund of the money (if any) you paid for it by sending a -written explanation to the person you received the work from. If you -received the work on a physical medium, you must return the medium -with your written explanation. The person or entity that provided you -with the defective work may elect to provide a replacement copy in -lieu of a refund. If you received the work electronically, the person -or entity providing it to you may choose to give you a second -opportunity to receive the work electronically in lieu of a refund. If -the second copy is also defective, you may demand a refund in writing -without further opportunities to fix the problem. - -1.F.4. Except for the limited right of replacement or refund set forth -in paragraph 1.F.3, this work is provided to you 'AS-IS', WITH NO -OTHER WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT -LIMITED TO WARRANTIES OF MERCHANTABILITY OR FITNESS FOR ANY PURPOSE. - -1.F.5. Some states do not allow disclaimers of certain implied -warranties or the exclusion or limitation of certain types of -damages. If any disclaimer or limitation set forth in this agreement -violates the law of the state applicable to this agreement, the -agreement shall be interpreted to make the maximum disclaimer or -limitation permitted by the applicable state law. The invalidity or -unenforceability of any provision of this agreement shall not void the -remaining provisions. - -1.F.6. INDEMNITY - You agree to indemnify and hold the Foundation, the -trademark owner, any agent or employee of the Foundation, anyone -providing copies of Project Gutenberg-tm electronic works in -accordance with this agreement, and any volunteers associated with the -production, promotion and distribution of Project Gutenberg-tm -electronic works, harmless from all liability, costs and expenses, -including legal fees, that arise directly or indirectly from any of -the following which you do or cause to occur: (a) distribution of this -or any Project Gutenberg-tm work, (b) alteration, modification, or -additions or deletions to any Project Gutenberg-tm work, and (c) any -Defect you cause. - -Section 2. Information about the Mission of Project Gutenberg-tm - -Project Gutenberg-tm is synonymous with the free distribution of -electronic works in formats readable by the widest variety of -computers including obsolete, old, middle-aged and new computers. It -exists because of the efforts of hundreds of volunteers and donations -from people in all walks of life. - -Volunteers and financial support to provide volunteers with the -assistance they need are critical to reaching Project Gutenberg-tm's -goals and ensuring that the Project Gutenberg-tm collection will -remain freely available for generations to come. In 2001, the Project -Gutenberg Literary Archive Foundation was created to provide a secure -and permanent future for Project Gutenberg-tm and future -generations. To learn more about the Project Gutenberg Literary -Archive Foundation and how your efforts and donations can help, see -Sections 3 and 4 and the Foundation information page at -www.gutenberg.org - - - -Section 3. Information about the Project Gutenberg Literary Archive Foundation - -The Project Gutenberg Literary Archive Foundation is a non profit -501(c)(3) educational corporation organized under the laws of the -state of Mississippi and granted tax exempt status by the Internal -Revenue Service. The Foundation's EIN or federal tax identification -number is 64-6221541. Contributions to the Project Gutenberg Literary -Archive Foundation are tax deductible to the full extent permitted by -U.S. federal laws and your state's laws. - -The Foundation's principal office is in Fairbanks, Alaska, with the -mailing address: PO Box 750175, Fairbanks, AK 99775, but its -volunteers and employees are scattered throughout numerous -locations. Its business office is located at 809 North 1500 West, Salt -Lake City, UT 84116, (801) 596-1887. Email contact links and up to -date contact information can be found at the Foundation's web site and -official page at www.gutenberg.org/contact - -For additional contact information: - - Dr. Gregory B. Newby - Chief Executive and Director - gbnewby@pglaf.org - -Section 4. Information about Donations to the Project Gutenberg -Literary Archive Foundation - -Project Gutenberg-tm depends upon and cannot survive without wide -spread public support and donations to carry out its mission of -increasing the number of public domain and licensed works that can be -freely distributed in machine readable form accessible by the widest -array of equipment including outdated equipment. Many small donations -($1 to $5,000) are particularly important to maintaining tax exempt -status with the IRS. - -The Foundation is committed to complying with the laws regulating -charities and charitable donations in all 50 states of the United -States. Compliance requirements are not uniform and it takes a -considerable effort, much paperwork and many fees to meet and keep up -with these requirements. We do not solicit donations in locations -where we have not received written confirmation of compliance. To SEND -DONATIONS or determine the status of compliance for any particular -state visit www.gutenberg.org/donate - -While we cannot and do not solicit contributions from states where we -have not met the solicitation requirements, we know of no prohibition -against accepting unsolicited donations from donors in such states who -approach us with offers to donate. - -International donations are gratefully accepted, but we cannot make -any statements concerning tax treatment of donations received from -outside the United States. U.S. laws alone swamp our small staff. - -Please check the Project Gutenberg Web pages for current donation -methods and addresses. Donations are accepted in a number of other -ways including checks, online payments and credit card donations. To -donate, please visit: www.gutenberg.org/donate - -Section 5. General Information About Project Gutenberg-tm electronic works. - -Professor Michael S. Hart was the originator of the Project -Gutenberg-tm concept of a library of electronic works that could be -freely shared with anyone. For forty years, he produced and -distributed Project Gutenberg-tm eBooks with only a loose network of -volunteer support. - -Project Gutenberg-tm eBooks are often created from several printed -editions, all of which are confirmed as not protected by copyright in -the U.S. unless a copyright notice is included. Thus, we do not -necessarily keep eBooks in compliance with any particular paper -edition. - -Most people start at our Web site which has the main PG search -facility: www.gutenberg.org - -This Web site includes information about Project Gutenberg-tm, -including how to make donations to the Project Gutenberg Literary -Archive Foundation, how to help produce our new eBooks, and how to -subscribe to our email newsletter to hear about new eBooks. - - - -</pre> - -</body> -</html> diff --git a/old/55126-h/images/cover.jpg b/old/55126-h/images/cover.jpg Binary files differdeleted file mode 100644 index 80bd130..0000000 --- a/old/55126-h/images/cover.jpg +++ /dev/null diff --git a/old/55126-h/images/i_f002b-hh.jpg b/old/55126-h/images/i_f002b-hh.jpg Binary files differdeleted file mode 100644 index ea3597d..0000000 --- a/old/55126-h/images/i_f002b-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_f002b-tn.jpg b/old/55126-h/images/i_f002b-tn.jpg Binary files differdeleted file mode 100644 index 8350c02..0000000 --- a/old/55126-h/images/i_f002b-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_f002b.jpg b/old/55126-h/images/i_f002b.jpg Binary files differdeleted file mode 100644 index d4114bb..0000000 --- a/old/55126-h/images/i_f002b.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p022a-hh.jpg b/old/55126-h/images/i_p022a-hh.jpg Binary files differdeleted file mode 100644 index dab7d79..0000000 --- a/old/55126-h/images/i_p022a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p022a-tn.jpg b/old/55126-h/images/i_p022a-tn.jpg Binary files differdeleted file mode 100644 index 9ac46f1..0000000 --- a/old/55126-h/images/i_p022a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p022a.jpg b/old/55126-h/images/i_p022a.jpg Binary files differdeleted file mode 100644 index dd7b55c..0000000 --- a/old/55126-h/images/i_p022a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p022d-hh.jpg b/old/55126-h/images/i_p022d-hh.jpg Binary files differdeleted file mode 100644 index 20203dd..0000000 --- a/old/55126-h/images/i_p022d-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p022d-tn.jpg b/old/55126-h/images/i_p022d-tn.jpg Binary files differdeleted file mode 100644 index 198e7a7..0000000 --- a/old/55126-h/images/i_p022d-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p022d.jpg b/old/55126-h/images/i_p022d.jpg Binary files differdeleted file mode 100644 index 22d19c1..0000000 --- a/old/55126-h/images/i_p022d.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p024a-hh.jpg b/old/55126-h/images/i_p024a-hh.jpg Binary files differdeleted file mode 100644 index 3d9b4a1..0000000 --- a/old/55126-h/images/i_p024a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p024a-tn.jpg b/old/55126-h/images/i_p024a-tn.jpg Binary files differdeleted file mode 100644 index dd6f7b8..0000000 --- a/old/55126-h/images/i_p024a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p024a.jpg b/old/55126-h/images/i_p024a.jpg Binary files differdeleted file mode 100644 index ce9a2bf..0000000 --- a/old/55126-h/images/i_p024a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p026b-hh.jpg b/old/55126-h/images/i_p026b-hh.jpg Binary files differdeleted file mode 100644 index af1f285..0000000 --- a/old/55126-h/images/i_p026b-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p026b-tn.jpg b/old/55126-h/images/i_p026b-tn.jpg Binary files differdeleted file mode 100644 index 53c8b2c..0000000 --- a/old/55126-h/images/i_p026b-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p026b.jpg b/old/55126-h/images/i_p026b.jpg Binary files differdeleted file mode 100644 index bf0d6bb..0000000 --- a/old/55126-h/images/i_p026b.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p030b-hh.jpg b/old/55126-h/images/i_p030b-hh.jpg Binary files differdeleted file mode 100644 index 8ceaf79..0000000 --- a/old/55126-h/images/i_p030b-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p030b-tn.jpg b/old/55126-h/images/i_p030b-tn.jpg Binary files differdeleted file mode 100644 index eda893d..0000000 --- a/old/55126-h/images/i_p030b-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p030b.jpg b/old/55126-h/images/i_p030b.jpg Binary files differdeleted file mode 100644 index 80bd1c3..0000000 --- a/old/55126-h/images/i_p030b.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p032a-hh.jpg b/old/55126-h/images/i_p032a-hh.jpg Binary files differdeleted file mode 100644 index 0f35a87..0000000 --- a/old/55126-h/images/i_p032a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p032a-tn.jpg b/old/55126-h/images/i_p032a-tn.jpg Binary files differdeleted file mode 100644 index ea7398a..0000000 --- a/old/55126-h/images/i_p032a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p032a.jpg b/old/55126-h/images/i_p032a.jpg Binary files differdeleted file mode 100644 index 8a608dc..0000000 --- a/old/55126-h/images/i_p032a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p034a-hh.jpg b/old/55126-h/images/i_p034a-hh.jpg Binary files differdeleted file mode 100644 index af1184c..0000000 --- a/old/55126-h/images/i_p034a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p034a-tn.jpg b/old/55126-h/images/i_p034a-tn.jpg Binary files differdeleted file mode 100644 index 05d9b1b..0000000 --- a/old/55126-h/images/i_p034a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p034a.jpg b/old/55126-h/images/i_p034a.jpg Binary files differdeleted file mode 100644 index eb930f1..0000000 --- a/old/55126-h/images/i_p034a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p034d-hh.jpg b/old/55126-h/images/i_p034d-hh.jpg Binary files differdeleted file mode 100644 index d42bbb9..0000000 --- a/old/55126-h/images/i_p034d-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p034d-tn.jpg b/old/55126-h/images/i_p034d-tn.jpg Binary files differdeleted file mode 100644 index 161b309..0000000 --- a/old/55126-h/images/i_p034d-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p034d.jpg b/old/55126-h/images/i_p034d.jpg Binary files differdeleted file mode 100644 index a177840..0000000 --- a/old/55126-h/images/i_p034d.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p036a-hh.jpg b/old/55126-h/images/i_p036a-hh.jpg Binary files differdeleted file mode 100644 index b51b29a..0000000 --- a/old/55126-h/images/i_p036a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p036a-tn.jpg b/old/55126-h/images/i_p036a-tn.jpg Binary files differdeleted file mode 100644 index 2609d91..0000000 --- a/old/55126-h/images/i_p036a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p036a.jpg b/old/55126-h/images/i_p036a.jpg Binary files differdeleted file mode 100644 index 2b87f7c..0000000 --- a/old/55126-h/images/i_p036a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p036d-hh.jpg b/old/55126-h/images/i_p036d-hh.jpg Binary files differdeleted file mode 100644 index 51bb87c..0000000 --- a/old/55126-h/images/i_p036d-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p036d-tn.jpg b/old/55126-h/images/i_p036d-tn.jpg Binary files differdeleted file mode 100644 index fb0513f..0000000 --- a/old/55126-h/images/i_p036d-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p036d.jpg b/old/55126-h/images/i_p036d.jpg Binary files differdeleted file mode 100644 index a6ab38e..0000000 --- a/old/55126-h/images/i_p036d.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p040a-hh.jpg b/old/55126-h/images/i_p040a-hh.jpg Binary files differdeleted file mode 100644 index 16129ea..0000000 --- a/old/55126-h/images/i_p040a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p040a-tn.jpg b/old/55126-h/images/i_p040a-tn.jpg Binary files differdeleted file mode 100644 index 23f1d68..0000000 --- a/old/55126-h/images/i_p040a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p040a.jpg b/old/55126-h/images/i_p040a.jpg Binary files differdeleted file mode 100644 index c1762d8..0000000 --- a/old/55126-h/images/i_p040a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p040d-hh.jpg b/old/55126-h/images/i_p040d-hh.jpg Binary files differdeleted file mode 100644 index 82391c7..0000000 --- a/old/55126-h/images/i_p040d-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p040d-tn.jpg b/old/55126-h/images/i_p040d-tn.jpg Binary files differdeleted file mode 100644 index d80b43a..0000000 --- a/old/55126-h/images/i_p040d-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p040d.jpg b/old/55126-h/images/i_p040d.jpg Binary files differdeleted file mode 100644 index 7b06aa9..0000000 --- a/old/55126-h/images/i_p040d.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p042a-hh.jpg b/old/55126-h/images/i_p042a-hh.jpg Binary files differdeleted file mode 100644 index b48b5be..0000000 --- a/old/55126-h/images/i_p042a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p042a-tn.jpg b/old/55126-h/images/i_p042a-tn.jpg Binary files differdeleted file mode 100644 index c1d0379..0000000 --- a/old/55126-h/images/i_p042a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p042a.jpg b/old/55126-h/images/i_p042a.jpg Binary files differdeleted file mode 100644 index ef0bdd8..0000000 --- a/old/55126-h/images/i_p042a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p046a-hh.jpg b/old/55126-h/images/i_p046a-hh.jpg Binary files differdeleted file mode 100644 index 2ab608f..0000000 --- a/old/55126-h/images/i_p046a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p046a-tn.jpg b/old/55126-h/images/i_p046a-tn.jpg Binary files differdeleted file mode 100644 index 56c5100..0000000 --- a/old/55126-h/images/i_p046a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p046a.jpg b/old/55126-h/images/i_p046a.jpg Binary files differdeleted file mode 100644 index 8b6abb4..0000000 --- a/old/55126-h/images/i_p046a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p046d-hh.jpg b/old/55126-h/images/i_p046d-hh.jpg Binary files differdeleted file mode 100644 index 7609984..0000000 --- a/old/55126-h/images/i_p046d-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p046d-tn.jpg b/old/55126-h/images/i_p046d-tn.jpg Binary files differdeleted file mode 100644 index 2b83ca3..0000000 --- a/old/55126-h/images/i_p046d-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p046d.jpg b/old/55126-h/images/i_p046d.jpg Binary files differdeleted file mode 100644 index 7b4b8d2..0000000 --- a/old/55126-h/images/i_p046d.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p048a-hh.jpg b/old/55126-h/images/i_p048a-hh.jpg Binary files differdeleted file mode 100644 index 4385c6f..0000000 --- a/old/55126-h/images/i_p048a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p048a-tn.jpg b/old/55126-h/images/i_p048a-tn.jpg Binary files differdeleted file mode 100644 index 7bd4da8..0000000 --- a/old/55126-h/images/i_p048a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p048a.jpg b/old/55126-h/images/i_p048a.jpg Binary files differdeleted file mode 100644 index 9d50117..0000000 --- a/old/55126-h/images/i_p048a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p048d-hh.jpg b/old/55126-h/images/i_p048d-hh.jpg Binary files differdeleted file mode 100644 index 86f8e9a..0000000 --- a/old/55126-h/images/i_p048d-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p048d-tn.jpg b/old/55126-h/images/i_p048d-tn.jpg Binary files differdeleted file mode 100644 index fb5094a..0000000 --- a/old/55126-h/images/i_p048d-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p048d.jpg b/old/55126-h/images/i_p048d.jpg Binary files differdeleted file mode 100644 index 093f5ae..0000000 --- a/old/55126-h/images/i_p048d.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p050a-hh.jpg b/old/55126-h/images/i_p050a-hh.jpg Binary files differdeleted file mode 100644 index e866aad..0000000 --- a/old/55126-h/images/i_p050a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p050a-tn.jpg b/old/55126-h/images/i_p050a-tn.jpg Binary files differdeleted file mode 100644 index c3f417a..0000000 --- a/old/55126-h/images/i_p050a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p050a.jpg b/old/55126-h/images/i_p050a.jpg Binary files differdeleted file mode 100644 index a2ac5e1..0000000 --- a/old/55126-h/images/i_p050a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p050c-hh.jpg b/old/55126-h/images/i_p050c-hh.jpg Binary files differdeleted file mode 100644 index 29adc99..0000000 --- a/old/55126-h/images/i_p050c-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p050c-tn.jpg b/old/55126-h/images/i_p050c-tn.jpg Binary files differdeleted file mode 100644 index 7c39fb1..0000000 --- a/old/55126-h/images/i_p050c-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p050c.jpg b/old/55126-h/images/i_p050c.jpg Binary files differdeleted file mode 100644 index 35b6870..0000000 --- a/old/55126-h/images/i_p050c.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p050f-hh.jpg b/old/55126-h/images/i_p050f-hh.jpg Binary files differdeleted file mode 100644 index 04bc7ac..0000000 --- a/old/55126-h/images/i_p050f-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p050f-tn.jpg b/old/55126-h/images/i_p050f-tn.jpg Binary files differdeleted file mode 100644 index a90c5a0..0000000 --- a/old/55126-h/images/i_p050f-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p050f.jpg b/old/55126-h/images/i_p050f.jpg Binary files differdeleted file mode 100644 index 1fec99c..0000000 --- a/old/55126-h/images/i_p050f.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p050h-hh.jpg b/old/55126-h/images/i_p050h-hh.jpg Binary files differdeleted file mode 100644 index f9f9cc3..0000000 --- a/old/55126-h/images/i_p050h-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p050h-tn.jpg b/old/55126-h/images/i_p050h-tn.jpg Binary files differdeleted file mode 100644 index c2f8e13..0000000 --- a/old/55126-h/images/i_p050h-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p050h.jpg b/old/55126-h/images/i_p050h.jpg Binary files differdeleted file mode 100644 index 7322f20..0000000 --- a/old/55126-h/images/i_p050h.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p052a-hh.jpg b/old/55126-h/images/i_p052a-hh.jpg Binary files differdeleted file mode 100644 index aa3fcd6..0000000 --- a/old/55126-h/images/i_p052a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p052a-tn.jpg b/old/55126-h/images/i_p052a-tn.jpg Binary files differdeleted file mode 100644 index 5f6a243..0000000 --- a/old/55126-h/images/i_p052a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p052a.jpg b/old/55126-h/images/i_p052a.jpg Binary files differdeleted file mode 100644 index 1db92c7..0000000 --- a/old/55126-h/images/i_p052a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p052b-hh.jpg b/old/55126-h/images/i_p052b-hh.jpg Binary files differdeleted file mode 100644 index f46d4b4..0000000 --- a/old/55126-h/images/i_p052b-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p052b-tn.jpg b/old/55126-h/images/i_p052b-tn.jpg Binary files differdeleted file mode 100644 index 65fcefe..0000000 --- a/old/55126-h/images/i_p052b-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p052b.jpg b/old/55126-h/images/i_p052b.jpg Binary files differdeleted file mode 100644 index 0b07a82..0000000 --- a/old/55126-h/images/i_p052b.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p054b-hh.jpg b/old/55126-h/images/i_p054b-hh.jpg Binary files differdeleted file mode 100644 index 834deed..0000000 --- a/old/55126-h/images/i_p054b-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p054b-tn.jpg b/old/55126-h/images/i_p054b-tn.jpg Binary files differdeleted file mode 100644 index 7a8d9f0..0000000 --- a/old/55126-h/images/i_p054b-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p054b.jpg b/old/55126-h/images/i_p054b.jpg Binary files differdeleted file mode 100644 index 6db04bf..0000000 --- a/old/55126-h/images/i_p054b.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p064a-hh.jpg b/old/55126-h/images/i_p064a-hh.jpg Binary files differdeleted file mode 100644 index bd84e0a..0000000 --- a/old/55126-h/images/i_p064a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p064a-tn.jpg b/old/55126-h/images/i_p064a-tn.jpg Binary files differdeleted file mode 100644 index 493964b..0000000 --- a/old/55126-h/images/i_p064a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p064a.jpg b/old/55126-h/images/i_p064a.jpg Binary files differdeleted file mode 100644 index f3b27dc..0000000 --- a/old/55126-h/images/i_p064a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p064d-hh.jpg b/old/55126-h/images/i_p064d-hh.jpg Binary files differdeleted file mode 100644 index 63b9719..0000000 --- a/old/55126-h/images/i_p064d-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p064d-tn.jpg b/old/55126-h/images/i_p064d-tn.jpg Binary files differdeleted file mode 100644 index a3b9665..0000000 --- a/old/55126-h/images/i_p064d-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p064d.jpg b/old/55126-h/images/i_p064d.jpg Binary files differdeleted file mode 100644 index f1a3893..0000000 --- a/old/55126-h/images/i_p064d.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p064f-hh.jpg b/old/55126-h/images/i_p064f-hh.jpg Binary files differdeleted file mode 100644 index d653967..0000000 --- a/old/55126-h/images/i_p064f-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p064f-tn.jpg b/old/55126-h/images/i_p064f-tn.jpg Binary files differdeleted file mode 100644 index cca63c1..0000000 --- a/old/55126-h/images/i_p064f-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p064f.jpg b/old/55126-h/images/i_p064f.jpg Binary files differdeleted file mode 100644 index 28d5143..0000000 --- a/old/55126-h/images/i_p064f.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p066a-hh.jpg b/old/55126-h/images/i_p066a-hh.jpg Binary files differdeleted file mode 100644 index e0048c9..0000000 --- a/old/55126-h/images/i_p066a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p066a-tn.jpg b/old/55126-h/images/i_p066a-tn.jpg Binary files differdeleted file mode 100644 index f86b94e..0000000 --- a/old/55126-h/images/i_p066a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p066a.jpg b/old/55126-h/images/i_p066a.jpg Binary files differdeleted file mode 100644 index fbc3611..0000000 --- a/old/55126-h/images/i_p066a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p066d-hh.jpg b/old/55126-h/images/i_p066d-hh.jpg Binary files differdeleted file mode 100644 index c806f16..0000000 --- a/old/55126-h/images/i_p066d-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p066d-tn.jpg b/old/55126-h/images/i_p066d-tn.jpg Binary files differdeleted file mode 100644 index f279a31..0000000 --- a/old/55126-h/images/i_p066d-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p066d.jpg b/old/55126-h/images/i_p066d.jpg Binary files differdeleted file mode 100644 index 20bc1af..0000000 --- a/old/55126-h/images/i_p066d.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p068a-hh.jpg b/old/55126-h/images/i_p068a-hh.jpg Binary files differdeleted file mode 100644 index c6c734f..0000000 --- a/old/55126-h/images/i_p068a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p068a-tn.jpg b/old/55126-h/images/i_p068a-tn.jpg Binary files differdeleted file mode 100644 index b02d440..0000000 --- a/old/55126-h/images/i_p068a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p068a.jpg b/old/55126-h/images/i_p068a.jpg Binary files differdeleted file mode 100644 index 5f77496..0000000 --- a/old/55126-h/images/i_p068a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p068d-hh.jpg b/old/55126-h/images/i_p068d-hh.jpg Binary files differdeleted file mode 100644 index a562938..0000000 --- a/old/55126-h/images/i_p068d-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p068d-tn.jpg b/old/55126-h/images/i_p068d-tn.jpg Binary files differdeleted file mode 100644 index c12d7b9..0000000 --- a/old/55126-h/images/i_p068d-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p068d.jpg b/old/55126-h/images/i_p068d.jpg Binary files differdeleted file mode 100644 index c111722..0000000 --- a/old/55126-h/images/i_p068d.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p070a-hh.jpg b/old/55126-h/images/i_p070a-hh.jpg Binary files differdeleted file mode 100644 index b8b7a6e..0000000 --- a/old/55126-h/images/i_p070a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p070a-tn.jpg b/old/55126-h/images/i_p070a-tn.jpg Binary files differdeleted file mode 100644 index fe3a0f6..0000000 --- a/old/55126-h/images/i_p070a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p070a.jpg b/old/55126-h/images/i_p070a.jpg Binary files differdeleted file mode 100644 index b182153..0000000 --- a/old/55126-h/images/i_p070a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p070c-hh.jpg b/old/55126-h/images/i_p070c-hh.jpg Binary files differdeleted file mode 100644 index 32a67bf..0000000 --- a/old/55126-h/images/i_p070c-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p070c-tn.jpg b/old/55126-h/images/i_p070c-tn.jpg Binary files differdeleted file mode 100644 index c6e400c..0000000 --- a/old/55126-h/images/i_p070c-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p070c.jpg b/old/55126-h/images/i_p070c.jpg Binary files differdeleted file mode 100644 index 8a4be4e..0000000 --- a/old/55126-h/images/i_p070c.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p072b-hh.jpg b/old/55126-h/images/i_p072b-hh.jpg Binary files differdeleted file mode 100644 index 954169f..0000000 --- a/old/55126-h/images/i_p072b-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p072b-tn.jpg b/old/55126-h/images/i_p072b-tn.jpg Binary files differdeleted file mode 100644 index 3376790..0000000 --- a/old/55126-h/images/i_p072b-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p072b.jpg b/old/55126-h/images/i_p072b.jpg Binary files differdeleted file mode 100644 index e5e043b..0000000 --- a/old/55126-h/images/i_p072b.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p074b-hh.jpg b/old/55126-h/images/i_p074b-hh.jpg Binary files differdeleted file mode 100644 index ef01201..0000000 --- a/old/55126-h/images/i_p074b-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p074b-tn.jpg b/old/55126-h/images/i_p074b-tn.jpg Binary files differdeleted file mode 100644 index 0da91ee..0000000 --- a/old/55126-h/images/i_p074b-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p074b.jpg b/old/55126-h/images/i_p074b.jpg Binary files differdeleted file mode 100644 index 01f4a50..0000000 --- a/old/55126-h/images/i_p074b.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p074d-hh.jpg b/old/55126-h/images/i_p074d-hh.jpg Binary files differdeleted file mode 100644 index 889ddbe..0000000 --- a/old/55126-h/images/i_p074d-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p074d-tn.jpg b/old/55126-h/images/i_p074d-tn.jpg Binary files differdeleted file mode 100644 index 0868227..0000000 --- a/old/55126-h/images/i_p074d-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p074d.jpg b/old/55126-h/images/i_p074d.jpg Binary files differdeleted file mode 100644 index ea2f09d..0000000 --- a/old/55126-h/images/i_p074d.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p076b-hh.jpg b/old/55126-h/images/i_p076b-hh.jpg Binary files differdeleted file mode 100644 index a75ed2d..0000000 --- a/old/55126-h/images/i_p076b-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p076b-tn.jpg b/old/55126-h/images/i_p076b-tn.jpg Binary files differdeleted file mode 100644 index 8e5ca7c..0000000 --- a/old/55126-h/images/i_p076b-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p076b.jpg b/old/55126-h/images/i_p076b.jpg Binary files differdeleted file mode 100644 index d13424d..0000000 --- a/old/55126-h/images/i_p076b.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p076d-hh.jpg b/old/55126-h/images/i_p076d-hh.jpg Binary files differdeleted file mode 100644 index c61b40c..0000000 --- a/old/55126-h/images/i_p076d-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p076d-tn.jpg b/old/55126-h/images/i_p076d-tn.jpg Binary files differdeleted file mode 100644 index 006a183..0000000 --- a/old/55126-h/images/i_p076d-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p076d.jpg b/old/55126-h/images/i_p076d.jpg Binary files differdeleted file mode 100644 index 76ba65b..0000000 --- a/old/55126-h/images/i_p076d.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p078a-hh.jpg b/old/55126-h/images/i_p078a-hh.jpg Binary files differdeleted file mode 100644 index 92fa19d..0000000 --- a/old/55126-h/images/i_p078a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p078a-tn.jpg b/old/55126-h/images/i_p078a-tn.jpg Binary files differdeleted file mode 100644 index 7f78074..0000000 --- a/old/55126-h/images/i_p078a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p078a.jpg b/old/55126-h/images/i_p078a.jpg Binary files differdeleted file mode 100644 index 462fedf..0000000 --- a/old/55126-h/images/i_p078a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p078c-hh.jpg b/old/55126-h/images/i_p078c-hh.jpg Binary files differdeleted file mode 100644 index 4edd0ef..0000000 --- a/old/55126-h/images/i_p078c-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p078c-tn.jpg b/old/55126-h/images/i_p078c-tn.jpg Binary files differdeleted file mode 100644 index 58f0255..0000000 --- a/old/55126-h/images/i_p078c-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p078c.jpg b/old/55126-h/images/i_p078c.jpg Binary files differdeleted file mode 100644 index 6be709f..0000000 --- a/old/55126-h/images/i_p078c.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p078f-hh.jpg b/old/55126-h/images/i_p078f-hh.jpg Binary files differdeleted file mode 100644 index 7f9b8a0..0000000 --- a/old/55126-h/images/i_p078f-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p078f-tn.jpg b/old/55126-h/images/i_p078f-tn.jpg Binary files differdeleted file mode 100644 index 19a179a..0000000 --- a/old/55126-h/images/i_p078f-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p078f.jpg b/old/55126-h/images/i_p078f.jpg Binary files differdeleted file mode 100644 index a7ae8a9..0000000 --- a/old/55126-h/images/i_p078f.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p080a-hh.jpg b/old/55126-h/images/i_p080a-hh.jpg Binary files differdeleted file mode 100644 index 44c042b..0000000 --- a/old/55126-h/images/i_p080a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p080a-tn.jpg b/old/55126-h/images/i_p080a-tn.jpg Binary files differdeleted file mode 100644 index c2a08ea..0000000 --- a/old/55126-h/images/i_p080a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p080a.jpg b/old/55126-h/images/i_p080a.jpg Binary files differdeleted file mode 100644 index 2ccb6a6..0000000 --- a/old/55126-h/images/i_p080a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p094a-hh.jpg b/old/55126-h/images/i_p094a-hh.jpg Binary files differdeleted file mode 100644 index 9a9f66f..0000000 --- a/old/55126-h/images/i_p094a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p094a-tn.jpg b/old/55126-h/images/i_p094a-tn.jpg Binary files differdeleted file mode 100644 index 1d301c6..0000000 --- a/old/55126-h/images/i_p094a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p094a.jpg b/old/55126-h/images/i_p094a.jpg Binary files differdeleted file mode 100644 index 2e72593..0000000 --- a/old/55126-h/images/i_p094a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p094d-hh.jpg b/old/55126-h/images/i_p094d-hh.jpg Binary files differdeleted file mode 100644 index 41f3d3d..0000000 --- a/old/55126-h/images/i_p094d-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p094d-tn.jpg b/old/55126-h/images/i_p094d-tn.jpg Binary files differdeleted file mode 100644 index db9662b..0000000 --- a/old/55126-h/images/i_p094d-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p094d.jpg b/old/55126-h/images/i_p094d.jpg Binary files differdeleted file mode 100644 index 3f56a14..0000000 --- a/old/55126-h/images/i_p094d.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p096a-hh.jpg b/old/55126-h/images/i_p096a-hh.jpg Binary files differdeleted file mode 100644 index 323fa97..0000000 --- a/old/55126-h/images/i_p096a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p096a-tn.jpg b/old/55126-h/images/i_p096a-tn.jpg Binary files differdeleted file mode 100644 index 6aade89..0000000 --- a/old/55126-h/images/i_p096a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p096a.jpg b/old/55126-h/images/i_p096a.jpg Binary files differdeleted file mode 100644 index 29c74de..0000000 --- a/old/55126-h/images/i_p096a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p096d-hh.jpg b/old/55126-h/images/i_p096d-hh.jpg Binary files differdeleted file mode 100644 index ee752b8..0000000 --- a/old/55126-h/images/i_p096d-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p096d-tn.jpg b/old/55126-h/images/i_p096d-tn.jpg Binary files differdeleted file mode 100644 index 0915fa6..0000000 --- a/old/55126-h/images/i_p096d-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p096d.jpg b/old/55126-h/images/i_p096d.jpg Binary files differdeleted file mode 100644 index 2bd13fe..0000000 --- a/old/55126-h/images/i_p096d.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p098a-hh.jpg b/old/55126-h/images/i_p098a-hh.jpg Binary files differdeleted file mode 100644 index e03473d..0000000 --- a/old/55126-h/images/i_p098a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p098a-tn.jpg b/old/55126-h/images/i_p098a-tn.jpg Binary files differdeleted file mode 100644 index f39379a..0000000 --- a/old/55126-h/images/i_p098a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p098a.jpg b/old/55126-h/images/i_p098a.jpg Binary files differdeleted file mode 100644 index 60626d6..0000000 --- a/old/55126-h/images/i_p098a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p108b-hh.jpg b/old/55126-h/images/i_p108b-hh.jpg Binary files differdeleted file mode 100644 index ee2a920..0000000 --- a/old/55126-h/images/i_p108b-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p108b-tn.jpg b/old/55126-h/images/i_p108b-tn.jpg Binary files differdeleted file mode 100644 index a306277..0000000 --- a/old/55126-h/images/i_p108b-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p108b.jpg b/old/55126-h/images/i_p108b.jpg Binary files differdeleted file mode 100644 index ff783fa..0000000 --- a/old/55126-h/images/i_p108b.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p110a-hh.jpg b/old/55126-h/images/i_p110a-hh.jpg Binary files differdeleted file mode 100644 index 8ad0c90..0000000 --- a/old/55126-h/images/i_p110a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p110a-tn.jpg b/old/55126-h/images/i_p110a-tn.jpg Binary files differdeleted file mode 100644 index 3ab399d..0000000 --- a/old/55126-h/images/i_p110a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p110a.jpg b/old/55126-h/images/i_p110a.jpg Binary files differdeleted file mode 100644 index c8fb66d..0000000 --- a/old/55126-h/images/i_p110a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p110d-hh.jpg b/old/55126-h/images/i_p110d-hh.jpg Binary files differdeleted file mode 100644 index e88b8b9..0000000 --- a/old/55126-h/images/i_p110d-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p110d-tn.jpg b/old/55126-h/images/i_p110d-tn.jpg Binary files differdeleted file mode 100644 index d618b16..0000000 --- a/old/55126-h/images/i_p110d-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p110d.jpg b/old/55126-h/images/i_p110d.jpg Binary files differdeleted file mode 100644 index 969d2ff..0000000 --- a/old/55126-h/images/i_p110d.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p110f-hh.jpg b/old/55126-h/images/i_p110f-hh.jpg Binary files differdeleted file mode 100644 index 52284d9..0000000 --- a/old/55126-h/images/i_p110f-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p110f-tn.jpg b/old/55126-h/images/i_p110f-tn.jpg Binary files differdeleted file mode 100644 index 6d4febe..0000000 --- a/old/55126-h/images/i_p110f-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p110f.jpg b/old/55126-h/images/i_p110f.jpg Binary files differdeleted file mode 100644 index 15be280..0000000 --- a/old/55126-h/images/i_p110f.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p110h-hh.jpg b/old/55126-h/images/i_p110h-hh.jpg Binary files differdeleted file mode 100644 index 56e5435..0000000 --- a/old/55126-h/images/i_p110h-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p110h-tn.jpg b/old/55126-h/images/i_p110h-tn.jpg Binary files differdeleted file mode 100644 index 4d7e09f..0000000 --- a/old/55126-h/images/i_p110h-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p110h.jpg b/old/55126-h/images/i_p110h.jpg Binary files differdeleted file mode 100644 index 0ebbe0c..0000000 --- a/old/55126-h/images/i_p110h.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p112a-hh.jpg b/old/55126-h/images/i_p112a-hh.jpg Binary files differdeleted file mode 100644 index b6f4f23..0000000 --- a/old/55126-h/images/i_p112a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p112a-tn.jpg b/old/55126-h/images/i_p112a-tn.jpg Binary files differdeleted file mode 100644 index 9a894e8..0000000 --- a/old/55126-h/images/i_p112a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p112a.jpg b/old/55126-h/images/i_p112a.jpg Binary files differdeleted file mode 100644 index 1cbebf0..0000000 --- a/old/55126-h/images/i_p112a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p120a-hh.jpg b/old/55126-h/images/i_p120a-hh.jpg Binary files differdeleted file mode 100644 index 493558b..0000000 --- a/old/55126-h/images/i_p120a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p120a-tn.jpg b/old/55126-h/images/i_p120a-tn.jpg Binary files differdeleted file mode 100644 index d238e5a..0000000 --- a/old/55126-h/images/i_p120a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p120a.jpg b/old/55126-h/images/i_p120a.jpg Binary files differdeleted file mode 100644 index b025904..0000000 --- a/old/55126-h/images/i_p120a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p120d-hh.jpg b/old/55126-h/images/i_p120d-hh.jpg Binary files differdeleted file mode 100644 index 0b724e0..0000000 --- a/old/55126-h/images/i_p120d-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p120d-tn.jpg b/old/55126-h/images/i_p120d-tn.jpg Binary files differdeleted file mode 100644 index b4b13a4..0000000 --- a/old/55126-h/images/i_p120d-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p120d.jpg b/old/55126-h/images/i_p120d.jpg Binary files differdeleted file mode 100644 index 5440254..0000000 --- a/old/55126-h/images/i_p120d.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p122a-hh.jpg b/old/55126-h/images/i_p122a-hh.jpg Binary files differdeleted file mode 100644 index 5ae42e6..0000000 --- a/old/55126-h/images/i_p122a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p122a-tn.jpg b/old/55126-h/images/i_p122a-tn.jpg Binary files differdeleted file mode 100644 index 5065da1..0000000 --- a/old/55126-h/images/i_p122a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p122a.jpg b/old/55126-h/images/i_p122a.jpg Binary files differdeleted file mode 100644 index 87e109f..0000000 --- a/old/55126-h/images/i_p122a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p136a-hh.jpg b/old/55126-h/images/i_p136a-hh.jpg Binary files differdeleted file mode 100644 index 5311dca..0000000 --- a/old/55126-h/images/i_p136a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p136a-tn.jpg b/old/55126-h/images/i_p136a-tn.jpg Binary files differdeleted file mode 100644 index b569a81..0000000 --- a/old/55126-h/images/i_p136a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p136a.jpg b/old/55126-h/images/i_p136a.jpg Binary files differdeleted file mode 100644 index 1259a89..0000000 --- a/old/55126-h/images/i_p136a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p140b-hh.jpg b/old/55126-h/images/i_p140b-hh.jpg Binary files differdeleted file mode 100644 index 70c297f..0000000 --- a/old/55126-h/images/i_p140b-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p140b-tn.jpg b/old/55126-h/images/i_p140b-tn.jpg Binary files differdeleted file mode 100644 index 6511009..0000000 --- a/old/55126-h/images/i_p140b-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p140b.jpg b/old/55126-h/images/i_p140b.jpg Binary files differdeleted file mode 100644 index faa7d19..0000000 --- a/old/55126-h/images/i_p140b.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p144a-hh.jpg b/old/55126-h/images/i_p144a-hh.jpg Binary files differdeleted file mode 100644 index ad48ef3..0000000 --- a/old/55126-h/images/i_p144a-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p144a-tn.jpg b/old/55126-h/images/i_p144a-tn.jpg Binary files differdeleted file mode 100644 index 968d60e..0000000 --- a/old/55126-h/images/i_p144a-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p144a.jpg b/old/55126-h/images/i_p144a.jpg Binary files differdeleted file mode 100644 index 3b30509..0000000 --- a/old/55126-h/images/i_p144a.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p144d-hh.jpg b/old/55126-h/images/i_p144d-hh.jpg Binary files differdeleted file mode 100644 index 37c892b..0000000 --- a/old/55126-h/images/i_p144d-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p144d-tn.jpg b/old/55126-h/images/i_p144d-tn.jpg Binary files differdeleted file mode 100644 index e094942..0000000 --- a/old/55126-h/images/i_p144d-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p144d.jpg b/old/55126-h/images/i_p144d.jpg Binary files differdeleted file mode 100644 index c653e38..0000000 --- a/old/55126-h/images/i_p144d.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p170b-hh.jpg b/old/55126-h/images/i_p170b-hh.jpg Binary files differdeleted file mode 100644 index a73bc0e..0000000 --- a/old/55126-h/images/i_p170b-hh.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p170b-tn.jpg b/old/55126-h/images/i_p170b-tn.jpg Binary files differdeleted file mode 100644 index 9b187c7..0000000 --- a/old/55126-h/images/i_p170b-tn.jpg +++ /dev/null diff --git a/old/55126-h/images/i_p170b.jpg b/old/55126-h/images/i_p170b.jpg Binary files differdeleted file mode 100644 index dac4006..0000000 --- a/old/55126-h/images/i_p170b.jpg +++ /dev/null |
