Theory And Parameterization Of Thermal Infrared Radiative Transfer

Abstract We shall confine our discussion to the condition of local thermodynamic equilibrium (LTE) under which Kirchhoff’s law is applicable. In the localized portion, the atmosphere absorbs radiation of a particular wave number (or wavelength), and at the same time also emits radiation of the same wavenumber. Because there is equilibrium, the amount of radiation absorbed must be equal to that emitted. Otherwise, equilibrium would not be possible as to define a uniform temperature, and this would violate the second law of thermodynamics. A blackbody absorbs the maximum possible radiation, and it has to emit that same amount of radiation. The rate at which emission takes place is a function of temperature and wave number in accord with Eq. (2.0.1). A gray body is characterized by incomplete absorption and emission, and is said to have emissivity less than unity. It is in the context of LTE that we begin our discussion on the thermal infrared (also referred to as ir or Jong-wave) radiative transfer.