Atmospheric transmission and emission are very important in the infrared region of the spectrum. In the visible region the atmosphere is essentially nonabsorbing, so the lack of transmission is due almost entirely to scattering. In the infrared there are regions in which there is almost total absorption and others in which the absorption is quite low. Scattering is minimal, since the wavelengths are so much longer and Rayleigh scattering depends on the inverse fourth power of the wavelength.
Atmospheric emissivity is essentially the ones complement of the atmospheric transmissivity. The transmission, or absorption, is a function of the composition of the atmosphere and the amount of absorption of the various important species.
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