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.

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.

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.

As long as the top of the cumulus is rounded 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 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.

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.

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.