LARGE CUMULUS.
(Cumulus Major.)
We have already explained that there seems to be a definite connection between the thickness of such clouds and the amount of precipitation from them. Small cumulus, less than 120 metres thick, rarely produces rain, and nothing like a heavy shower is likely unless the thickness exceeds 400 metres. In winter, especially in hard frost, snow crystals may fall from the smallest cloud, even from little fragments only a few metres thick, but the quantity of water so precipitated will, of course, be small.
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.
Plate 46.
FRACTO-CUMULUS.
