This was an unusually large cloud for this country, but specimens of 10, 20, or 30 cubic miles are quite common.
Now for the explanation of the series of events. To begin with, we have the production of an ordinary cumulus, but the equilibrium is unstable, the growth of the cloud, therefore, becomes more and more rapid, and the rapid condensation adds to the instability until the rising column is so much lighter than an equal column outside that a powerful updraught is created, strong enough for a time to hold up the raindrops or even hailstones. At length the condensation is complete, the upper part of the cloud consisting of snow crystals exactly like those of any other cirrus. In the mean time, the rapid ascending current necessarily involves an indraught from around, and consequent descending currents to supply it. The result is to set up a circulating system, moving inwards along the ground, upwards in the central column, and outwards in the upper disc. The downward currents are sometimes shown by a curling over of the edges of the upper disc, but the phenomenon is not often seen, as the descending movement is generally enough to dry up the cloud particles.
The rapid rising of damp air drawn from the ground brings about rapid condensation and heavy rain. The large size of thunder-drops is almost certainly due to the fact that it is only the larger drops which can fall in the teeth of the strong updraught. But when these drops begin to fall, and still more when cold hailstones begin to fall through this ascending air, it becomes chilled from top to bottom, and the column is broken or even stopped altogether. The frozen particles which make up the top subside gradually, and the chilling of the air immediately below the cloud brings the saturation level nearer the ground, and we say the cloud base descends.
The arrangement of a thunder-cloud into the upper and lower discs with a connecting uprush gives to the typical cloud a shape something like that of an anvil when seen sideways, but in the larger clouds the disc-like form is more obvious. In any ordinary thunderstorm the great majority of the discharges of lightning play between the two discs, and the larger the cloud the more frequent these are. Such discharges as pass between the cloud and the earth come exclusively from the base of the lower disc if the cloud is large, and generally follow immediately after or simultaneously with one between the two discs. The phenomena of lightning are intensely interesting, but the purpose of these pages is confined to the study of clouds and cloud forms, and it would be going beyond our scope to discuss either lightning or hail. Both are, however, so closely related to cumulo-nimbus that they can hardly be passed over in silence. One thing is certain, and that is that neither the electrical developments nor the hail has anything to do with the growth of the cloud. On the contrary, both are consequences of the cloud, the hail being due to the great altitude, and consequent low temperature of the upper part of the cloud, and also to the violent uprising currents within it; while the electrical phenomena are due to either the enormous amount of condensation, or to friction due to the rapid uprush, or more probably to the fact that considerable differences of electrical condition exist in the distant parts of the air connected by the cloud, and between which its circulating currents move. These differences are known to exist at all times, and we cannot here discuss their origin.
The formation of cumulo-nimbus and cumulus is dependent upon the presence of a large amount of water vapour. It is worth while to consider whether the atmospheric movements which bring about the condensation could exist without moisture. Wherever we find differences of temperature between neighbouring places we must get currents of hot air rising from the warmer spots, and compensating descending currents around them. But we have pointed out that if the rate of cooling as we ascend in the still air is less than the rate at which an ascending current will be dynamically cooled, such a rising current will come to rest. If, on the other hand, the rate of cooling in the ascending current be less than in the still air, the equilibrium will be unstable, and a violent uprush will result.
Now, in a climate such as our own, where the lower regions of the air contain large quantities of water vapour, any considerable rise brings about more or less condensation, and that condensation is attended by a liberation of very large quantities of heat, which retard cooling in the ascending current, and so facilitate the production of instability. But if this cause is put aside it is still possible to have a similar circulation. When discussing the causes of instability, it has been pointed out that the prime condition was an unusually rapid rate of fall of temperature in still air, such as may be produced by hot sunshine. Now, these conditions are exactly those which will give rise to the phenomenon of the mirage, and which reach their fullest development in great desert districts when the air is still.
Again, it has been pointed out that the causes which bring showers and thunderstorms to an end include the chilling of the lower parts of the ascending column by the descent of cold rain or hail from above. We may also add the shading of the underlying ground by the cloud itself, and the absorption of heat in the partial evaporation of some of the rain during the lower part of its fall. In a desert district the arising currents are so dry that even a very great ascent does not often result in visible cloud; and when it does, the cloud is produced at so great a height that the air is too rarefied to produce anything much denser than thin alto-stratus, from which no falling droplets could reach the earth. It seems, then, that there will be no such automatic check on the growth of the circulating system, and it will go on growing in volume and intensity indefinitely. As a matter of fact, this is not the case. A different check does come into operation, but not until the indraught and updraught have become so powerful as to draw up the dust and sand and generate a sandstorm, the weight and shade of which, in time, destroys the circulating currents which uplifted them.
Since, however, condensation is a considerable factor in producing instability, we should expect that such sandstorms would be rarer than thunderstorms are in an equally hot but well-watered district, which is the fact. Again, since rain and cloud are checks upon such systems, we should expect the sandstorm systems to be larger and far loftier than thunderstorms, and to consist of far more violent atmospheric movements. This also is the case, and when we know that some of these disturbances have the dimensions of a cyclonic storm, it is easy to understand how the finest dust may be raised to vast altitudes, into the great upper currents of the air, by which it may be borne hundreds of miles before returning to the ground. It is thus that the dust of the African deserts is carried across the Mediterranean to Europe, and the yellow loess from Mongolia even to the eastward of Japan.