Effect of Surface Reflection on the Angular and Spectral Distribution of Skylight

Abstract

Laboratory measurements of the bidirectional reflectance of natural surfaces have been entered into the equation of radiative transfer for a plane parallel molecular atmosphere to determine the effects which are introduced into the skylight by solar radiation which is scattered downward by the atmosphere, after having been reflected one or more times from the underlying surface. Numerical results show that this surface-reflected component constitutes an important part of the total skylight, particularly at high sun elevations. Because of the normal increase of reflectance of mineral surfaces with increasing wavelength, radiation reflected from light colored sands, rocks, and desert surfaces in general can contribute as much as half of the total skylight at the longer wavelengths. The reflectance of black loam soil and the darker lavas and basalts is so low at all wavelengths that the surface-reflected component of skylight over such surfaces is only a minor part of the total field. Green vegetation in general, having a low reflectance in the visible and ultraviolet regions and a very high reflectance in the near infrared region, will have an important effect on the skylight only at the longer wavelengths. Reflection from snow, particularly new snow, will have a strong effect on the skylight at all wavelengths. The degree of polarization of skylight is generally more sensitive to surface-reflected radiation than is the skylight intensity.