A device for the convenient quantitative measurement of the thermal radiation from the atmosphere has been developed. In the instrument the emission spectrum of the earth’s atmosphere as observed at ground level is compared automatically with a spectrum approximating that of a blackbody at the boiling point of liquid nitrogen at atmospheric pressure. The most prominent features of the atmospheric spectrum between 4 µ and 15.5 µ, observed during daylight and darkness when the sky is clear, are due to emission by carbon dioxide, ozone, and water vapor; the intensity of the water vapor emission shows pronounced variations with atmospheric temperature and humidity. The spectrum of an overcast sky resembles that of a blackbody. By comparing the recorder traces of the atmospheric spectra with similar traces obtained with a blackbody source, it is possible to estimate the effective radiation temperature of various portions of the sky for various atmospheric conditions.
RAYMOND SLOAN, JOHN H. SHAW, and DUDLEY WILLIAMS, "Infrared Emission Spectrum of the Atmosphere," J. Opt. Soc. Am. 45, 455-457 (1955)