Atmospheric Back Radiation

long-wave, or infrared, atmospheric radiation directed toward the surface of the earth. Under a cloudless sky, the amount of atmospheric back radiation depends primarily on the temperature and absolute humidity of the air. It also varies over the course of a day, with maximum values observed at about noon. However, during the winter over a snow-covered surface, the amount of radiation remains practically unchanged for 24 hours. Under a clear sky, the amount usually ranges from 200 to 400 watts/m². The figure increases significantly in cloudy weather, and under a completely overcast sky it will be from 20 to 25 percent higher. Atmospheric back radiation is measured with pyrgeometers and balance meters.

The distribution of energy in the back radiation outside of the spectral region adjoining the transparency window of the atmosphere, at wavelengths from 8 to 12 microns, differs little from the distribution in the spectrum of a blackbody at the temperature of air near the surface of the earth.