Overview

Radiation is an important component of the climate system because it is the dominant form ofenergy exchange between the earth and its surroundings. The sun is the primary source of radiantenergy for the earth and solar energy has its maximum intensity at wavelengths that arecharacteristic of visible light, although the full spectrum does extend from ultravioletwavelengths to the much longer infrared wavelengths. The earth intercepts solar radiation as a nearparallel beam so that the overall intensity is strongest at the surface when the sun appears to bedirectly overhead. Also, at any point in time, half of the earth s surface is in darkness. Theintensity of solar radiation decreases as the angle between the local vertical and the apparentdirection of the sun increases. As a consequence, the intensity of solar radiation is greatest overthe tropics.

The intensity of the solar radiation varies with the square of the distance from the sun. Ifthe earth s orbit is perfectly circular then the total amount of solar radiation intercepted eachday (the global insolation) would not vary through the year. However the orbit is slightlyelliptical and the global insolation when the earth is closest to the sun is about 5 percent morethan when it is at its farthest point. Overall, the tropics receive more solar insolation than thepolar regions but the amount is highly variable with season and location.

Radiation emitted from the earth has its maximum intensity at longer infrared wavelengths.The earth is continually emitting terrestrial radiation to space in all directions and theintensity is...