Cloud Studies

by Arthur William Clayden

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.