Our planet is continuously bathed in solar radiation. Although we who
are confined to a fixed location on the globe experience day and night,
the earth does not. It is always day in the sense that the sun is shining on
half of the globe. Much of the incoming solar radiation, about 30%, is
scattered back to space by clouds, atmospheric gases and particles, and
objects on the earth. The remaining 70% is, therefore, absorbed mostly
at the earth's surface. This absorbed radiation gives up its energy to
whatever absorbed it, thereby causing its temperature to increase. Because solar radiation is absorbed continuously by the earth, it might be
supposed that its temperature should continue to increase. It does not, of
course, because the earth also emits radiation, the spectral distribution
of which is quite different from that of the incoming solar radiation. The
higher the earth's temperature, the more infrared radiation it emits. At a
sufficiently high temperature, the total rate of emission of infrared
radiation equals the rate of absorption of solar radiation. Radiative
equilibrium has been achieved, although it is a dynamic equilibrium:
absorption and emission go on continuously at equal rates. The temperature
at which this occurs is called the radiative equilibrium temperature
of the earth. This is an average temperature, not the
temperature at any one location or at any one time. It is merely the
temperature that the earth, as a blackbody, must have in order to emit as
much radiant energy as the earth absorbs solar energy.
