CHAPTER XX

Summary

The annual supply of heat which the earth receives from the sun is always the same, and it is annually radiated into space, so that it neither accumulates in the earth nor in the atmosphere. Its distribution is very unequal, but certain it is that an excess of heat in one part of the globe is compensated by a deficiency in another; an unusually warm summer is balanced by a cold one elsewhere. Diurnal variations of heat are perceptible only at a small distance below the surface, because the earth is a bad conductor, the annual heating influence of the sun penetrates much deeper. The heat which enters the earth in summer, returns during winter; and before passing into space, tempers the cold in the higher latitudes. At the equator, where the heat is the greatest, it descends deeper than elsewhere, with a diminishing intensity; but there, and everywhere throughout the globe, there is a stratum, at the depth of from 40 to 100 feet below the surface of the ground, where the temperature never varies, and is nearly the same with the mean heat of the surface.

At least one-third of the sun's heat is absorbed by the air before reaching the earth, but the atmosphere is chiefly warmed by the radiation of the sun's heat from the earth in its return to space, which takes place most abundantly when the sky is clear and blue.