Methods of calculating atmospheric transmittance and radiance in the infrared

Radiation from remotely sensed objects in the earth's environment is attenuated in its passage through the atmosphere. In the infrared region of the spectrum, this is caused, for clear-sky conditions, mainly through absorption by atmospheric molecular species, i.e., the atmospheric gases. Molecular absorption is characterized by a highly variable, more-or-less random, frequency dependence which is difficult, if not impossible, to describe analytically in detail. On the other hand, most experimental work requiring an understanding of atmospheric attenuative phenomena is accomplished with instrumental spectral resolutions that are coarse enough to smooth out much of the spectral detail in molecular line structure. Therefore, methods are described for calculating atmospheric transmittance and radiance to correspond to spectral resolutions of those instruments used in remote sensing. These methods involve either numerical integration of the exact line structure over the desired spectral interval, or the use of band models derived from artificially created line structure. Limitations of these calculations are discussed in the light of factors such as the fidelity of model atmospheres used to simulate real ones and the utility of different routines regarding computer time and accuracy. A short section covers the effect of scattering.