Cloud Studies

CHAPTER I. Introductory

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.

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