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. For a body in local thermodynamic equilibrium the amount of thermal energy emitted must be equal to the energy absorbed; otherwise the body would heat up or cool down in time, contrary to the assumption of equilibrium.

3-3 Since a L > a S, the irradiance and hence the radiative equilibrium temperature at the earth surface is increased by the presence of the atmosphere. With a L = .8 and a S = .1 and E = 241 Wm-2, Stefans Law yields a blackbody temperature …

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