Near the earth's surface the effects of water vapor are dominant. Consequently, most of the downward atmospheric radiation comes from the moist lower layers below heights of one or two kilometers.
Dotted curve shows blackbody spectrum.
The existence of the atmospheric window causes the intensity of radiation emitted by air at a particular temperature to be less than the intensity of blackbody radiation at the same temperature. This allows objects on the ground to radiate energy through the atmosphere into space. This process is most noticable in radiative cooling under a clear sky at night. The measurement of atmospheric radiation is difficult because the instrument used is at a temperature close to that of the radiation itself. All radiometers measure the exchange of energy between the receiving surface and its immediate surroundings.