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. [Illustration: Plate 48. THUNDER-CLOUDS FORMING.] In Plate 49 we have on the left hand a specimen in which the outspreading is just beginning, and the same cloud is shown half an hour later on the left of plate 50. A complete cumulo-nimbus in full work is shown on the right of Plate 49, and the same appears on the right in Plate 50. [Illustration: Plate 49. THUNDER-CLOUDS. (_Cumulo-nimbus._)] [Illustration: Plate 50. THUNDER-CLOUDS. (_Cumulo-nimbus._)] These clouds were thunder-clouds, the larger one being a smart thunderstorm with heavy hail. They were photographed in the evening, and in the second picture the sun was just below the horizon.