Page:Popular Science Monthly Volume 54.djvu/100

92 rays of the sun, which they almost wholly absorb, these clouds are warmed up again in the interior, and budding protuberances are seen, which are especially developed on the upper parts of the cloud. These protuberances are formed and grow so rapidly as to almost suggest the presence of a steam generator within every cloud. The external parts of the cloud, however, cool very soon by radiation, evaporation, or dissolution, but especially by their contact with the cold air, into which they continue going. Hence, when the vapors emitted by the cloud reach its periphery, they are cooled at once as if in a condenser; they then take on a rapid movement of descent, which is easily distinguished, and suffer condensation in their lower parts. As the surface of the cloud in contact with the cold allaround it is considerable in proportion to that which receives the influence of the solar rays, the warm ascending currents slacken speed and are extinguished, because the cloudy mass, drawn on by the higher currents, removes from the place where it is formed, or because it stops the rays of the sun and prevents their reaching the ground. There results a more and more complete condensation, and the watery vapor is at last transformed into drops of rain. The condensation into rain is accelerated and augmented when the mass of cloud rises with great rapidity, especially when it enters abruptly into very cold atmospheric strata. A sudden mixture of the cloud with the air around it takes place then, and sudden and abundant rains result like those which are produced at the instant of thunderstorms.

The formation and mixture of masses of air of different temperatures are effected by ascending currents in zones of restricted extent, but sometimes very numerous. Local showers and thunderstorms are produced in this way. The phenomenon becomes much more important and at the same time extends over vast regions, when it is brought about by the aid of the wind and the larger movements of the atmosphere, and general rains result.

Babinet, in his Studies on the Sciences of Observation, explains the formation of rain by supposing that when the wind meets an obstacle, it ascends; the moving air cools in rarefying, and deposits its excess of vapor over saturation. This fact, when it occurs, should indeed contribute to the condensation of the vapor contained in the air; but it does not afford an adequate explanation of all rains; for, first, how can it rain on the vast oceans which present no obstacles to cause the air to ascend? It is necessary to suppose that internal movements of the atmosphere intervene in the production of rain.

Monk, Mason, de Saussure, and many others fix the prime condition for the formation of rain in the superposition of two beds of cloud. This assertion, although it is still repeated in a number of treatises on physics, is inexact. A single stratum of cloud—yes, a