1387. The cause of rain is thus accounted for by Hutton and Dalton. If two masses of air, of unequal temperatures, are, when saturated with vapour, intermixed by the ordinary currents of the winds, a precipitation ensues. If the masses are under saturation, then less precipitation takes place, or none at all, according to the degree. Also, the warmer the air, the greater is the quantity of vapour precipitated in like circumstances. Hence the reason why rains are heavier in summer than in winter, and in warm countries than in cold. 'Upon reflecting on the different degrees of rapidity with which rain falls at different times, and in different climates,' observes Hutchison, 'I am disposed to think that the capacity of the atmosphere for suspending aqueous vesicles is limited, and varies with its temperature. And from the greater density of clouds in warm climates, as well as the greater amount of rain which falls from them in a given time, it seems probable that the capacity of the air for suspending vesicles, like its capacity for holding water in invisible solution, increases with its temperature. Vesicles of a given specific gravity, upon their formation, may be supposed at a given temperature relative to that of the air, to have a tendency to descend to, and not below, a certain altitude in the atmosphere; and owing to their mutual repulsion, a given depth of atmosphere must be loaded with them, before that degree of vesicular density and compression, in which vesicular oversaturation consists, takes place.' (Meteorological Phenomena, p. 173.)
