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CLOUD STUDIES

A SUNSET SKY.

Frontispiece.

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

LIST OF ILLUSTRATIONS

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.

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.

Plate 1.

PART OF A GREAT HALO.

Plate 1.

PART OF A GREAT HALO.

Plate 2.

PART OF A SOLAR HALO.

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.

Plate 3.

CIRRO-NEBULA CHANGING TO CIRRO-STRATUS.

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.

Plate 4.

CIRRO-NEBULA CHANGING TO CIRRO-CUMULUS.

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.

Plate 5.

HIGH CIRRUS.

(Cirrus Excelsus.)

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.

Plate 6.

WINDY CIRRUS.

(Cirrus Ventosus.)

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.

Plate 7.

THREAD CIRRUS.

(Cirro-filum.)

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.

Plate 8.

TAILED CIRRUS.

(Cirrus Caudatus.)

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.

Plate 9.

HAZY CIRRUS.

(Cirrus Nebulosus.)

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.

Plate 10.

CHANGE CIRRUS.

(Cirrus Inconstans.)

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.

Plate 11.

COMMON CIRRUS.

(Cirrus Communis.)

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.

Plate 12.

BAND CIRRUS.

(Cirrus Vittatus.)

Plate 12.

BAND CIRRUS.

(Cirrus Vittatus.)

Plate 13.

BAND CIRRUS.

(Cirrus Vittatus.)

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.

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.

Plate 14.

HAZY CIRRO-STRATUS.

(Cirro-stratus Nebulosus.)

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.

Plate 15.

CIRRO-STRATUS.

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.”

Plate 16.

CIRRO-STRATUS.

(Cirro-stratus Communis.)

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.

Plate 17.

FLOCCULENT CIRRO-STRATUS.

(Cirro-stratus Cumulosus.)

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.

Plate 18.

CIRRO-STRATUS AND CIRRO-CUMULUS.

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.

Plate 19.

CIRRO-CUMULUS.

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.

Plate 20.

HAZY CIRRO-CUMULUS.

(Cirro-cumulus Nebulosus.)

Plate 20.

HAZY CIRRO-CUMULUS.

(Cirro-cumulus Nebulosus.)