Quantitative data are presented to illustrate the infrared spectral radiance of the sky under a variety of situations. A qualitative discussion of the dominant spectral features is given to show the importance of the emission and scattering processes, the effect of angle of elevation, angle of azimuth, sun angle, and cloud cover.
ELY E. BELL, LEONARD EISNER, JAMES YOUNG, and ROBERT A. OETJEN, "Spectral Radiance of Sky and Terrain at Wavelengths between 1 and 20 Microns. II. Sky Measurements," J. Opt. Soc. Am. 50, 1313-1317 (1960)