Sometimes the stratum is cooled by being wafted over ranges of mountains, sometimes by encountering and becoming mingled with cooler currents of air; and sometimes, again, by being driven in winds toward a higher, and, consequently, cooler latitude.

If, on the other hand, air moves from cold mountains toward warm and sunny plains, or from higher latitudes to lower, or if, among the various currents into which it falls, it becomes mixed with air warmer than itself, its capacity for containing vapor in solution is increased, and, consequently, instead of releasing its hold upon the waters which it has already in possession, it becomes thirsty for more.

It moves over a country, under these circumstances, as a warm and drying wind.

Under a reverse of circumstances it would have formed drifting mists, or, perhaps, even copious showers of rain.
It will be evident, from these considerations, that the frequency of the showers, and the quantity of the rain which will fall, in the various regions respectively which the surface of the earth presents, must depend on the combined influence of many causes, such as the warmth of the climate, the proximity and the direction of mountains and of seas, the character of the prevailing winds, and the reflecting qualities of the soil.

These and other similar causes, it is found, do, in fact, produce a vast difference in the quantity of rain which falls in different regions.