Source: Adapted from Kiehl and Trenberth, 1997.
When averaged over the globe, the earth's surface absorbs about 168 W m-2 of solar radiation every year and effectively radiates 66 W m-2 of longwave energy to the atmosphere. The difference, +102 W m-2, is the net radiation gain of the earth's surface. Likewise, the net radiation balance of the atmosphere comes to -102 W m-2 per year. Thus, the atmosphere loses as much radiative energy in a year as the earth's surface gains. To keep the thermal balance in equilibrium, energy is transferred from the earth's surface to the atmosphere. This vertical heat exchange occurs mainly through the evaporation of water from the surface of the earth (heat loss), through condensation in the atmosphere (heat gain), and by the conduction of sensible heat from the surface and transfer to the atmosphere through convection.
Solar radiation is the energy source that sustains organic life on earth. Crop production is in fact an exploitation of solar radiation. The three broad spectra of solar energy described in this section are significant to plant life.
The shorter-than-visible wavelength radiation segment in the solar spectrum is chemically very active. When plants are exposed to excessive amounts of this radiation, the effects are detrimental. However, the atmosphere acts as a regulator in this type of solar radiation, and none of the cosmic, gamma, and X rays reach the earth (Evans, 1973). The ultraviolet radiation of this segment reaching the earth's surface is very low and is normally tolerated by plants.
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