CHAPTER 3 ABSORPTION, EMISSION, REFLECTION, AND …

CHAPTER 3 absorption , EMISSION, REFLECTION, AND SCATTERING absorption and Emission As noted earlier, blackbody radiation represents the upper limit to the amount of radiation that a real substance may emit at a given temperature. At any given wavelength , emissivity is defined as the ratio of the actual emitted radiance, R , to that from an ideal blackbody, B , = R / B . Emissivity is a measure of how strongly a body radiates at a given wavelength; it ranges between zero and one for all real substances. A gray body is defined as a substance whose emissivity is independent of wavelength. In the atmosphere, clouds and gases have emissivities that vary rapidly with wavelength. The ocean surface has near unit emissivity in the visible regions.

The factor of one-fourth arises because the cross sectional area of the earth disc to solar radiation, πr2, is one-fourth the earth radiating surface, 4 πr2. Recalling that S = 1380 Wm-2, if the earth albedo is 30 percent, then E = 241 Wm-2. 3.4 Selective Absorption and Emission

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