If, however, the general rate of fall of temperature is greater than that produced dynamically, the consequence will be that the upward tendency of the rising air will increase as it moves upward, and the taller the column becomes the greater will be the difference of weight between the inside and outside columns. In such a case the equilibrium is said to be unstable, and the result will be the production of cumulo-nimbus.
Just as cumulus may be divided into heat cumulus and the clouds of the rear of a cyclone, so cumulo-nimbus may be divided into the same two groups. In the case of the heat thunder-clouds the instability of the air is effected by the rapid heating of its lower layers in contact with the ground, those lower layers being so quickly warmed that there is not time for them to become mixed with the overlying air in which the rate of decrease is normal. If there is much wind we rarely get cumulo-nimbus, because the heated air is mixed mechanically with the overlying parts, and the rate of decrease is approximately normal throughout. Calm air and hot sun are then one set of necessary conditions for the production of instability.
But it is well known that thunder-showers and lesser examples of cumulo-nimbus are by no means infrequent in the rear of a cyclone, and such storm clouds are usually attended by considerable wind. They are, as a rule, much smaller than those produced by heat, but they have the same form, and are evidently due to instability in the lower part of the air, and the question is how can that condition be produced. In order to find the answer it is necessary to refer to the temperature phenomena of a cyclonic area. If a cyclone be divided into four quadrants by two lines drawn through its centre, one in the direction in which the system is travelling and the other at right angles to it, then the front right-hand quadrant is the warmest and the rear right-hand quadrant much colder. The cumulo-nimbus clouds of a cyclone are limited to the first part of this cold quadrant, that is to say, to the portion of the storm in which a great volume of cold air is flowing over a district which has just been warmed and wetted by the preceding part. The result is that, the air being warmed by contact with damp ground at a temperature many degrees above that of the air itself, we have produced exactly the same unstable state at a low temperature as we have at a high temperature in the case of heat storms. The lower temperature of the whole is enough to account for the smaller volume of the cloud, and that in turn explains why cyclone thunderstorms are, generally speaking, on a much smaller scale than heat storms.
The life history of a cumulo-nimbus is easily studied on a suitable day. The rapid heating of the lower layers of air causes them to expand bodily, and as they do so they lift the overlying air, frequently in broad domes or waves. The first result is the expansion of these upper zones, which are lightened by the flowing away of still higher layers. Expansion means chilling, and sooner or later its effects become visible in the formation of cirro-stratus, cirro-cumulus, or alto-cumulus. Simultaneously the heated air near the ground begins to rise up in tall columns, while the cooler air from a little higher descends to take its place. Soon patches of lower cloud appear, at first hazy and indistinct, but gradually shaping themselves into cumulus with hazy base and rounded summits. These rapidly assume the typical pyramidal shape, with level base and sharply contoured top, and so far there is little to distinguish them from an ordinary cumulus (see Plate [48]). But watch them carefully. Here and there some will be growing taller than their fellows, and as they grow their rate of growth increases until the top begins to show signs of spreading outwards. Rapidly the bulging summit throws out long fingers of cloud, radiating from the central column almost as if propelled by some repulsive force. At first, these fingers are merely projecting lumps of cloud with rounded ends, but in a few minutes they undergo a sudden and striking change. The whole summit becomes frayed out, drawn out into long radiating lines, which thin off against the blue sky exactly like the edges of a sheet of cirro-stratus. False cirrus is the name commonly given to this, but there seems no valid reason why it should be regarded as “false.” The top of the cloud rapidly spreads horizontally, forming a disc of cirriform cloud, which sometimes spreads several miles ahead of the rest of the storm. Meanwhile, the original cumulus column loses all its deep folds and convolutions, and other round-topped cumulus arise around it until the completed system consists of a more or less disc-shaped mass of cumulus, with a common base, rising higher and higher towards some central point, where these are connected, by an uprushing column of vapour, to an upper disc with cirriform margins.
Plate 48.
THUNDER-CLOUDS FORMING.
