Properties of Diffuse Solar Radiation under Overcast Skies with Stratified Cloud

I. Angular Distribution

Abstract

The intensity distribution of diffuse solar radiation with the zenith angle under overcast sky with stratified clouds (hereafter we call merely angular distribution) were measured at wavelengths of 400, 500, 600, 700 and 750nm. The detailed analysis of their features was also carried out with the aid of numerical computations. The main results obtained are as follows. The angular distribution in the case of overcast sky has a maximum at the zenith and decreases from the zenith to the horizon in contrast with the case of clear sky in which the angular distribution increases from the zenith to the horizon except for the surroundings of the sun. And this feature of the angular distribution is almost constant beyond a critical thickness of cloud (about 150m thick) and does not almost depend on the cloud thickness, the cloud species, the wavelength, the solar zenith angle, the cloud base height and so on but depends much on the albedo of ground surface. This suggests the possibility of obtaining the surface albedo by measurement of the angular distribution under overcast sky. From the measurements and the numerical analysis of the angular distribution in this study, it becomes clear that the conventional empirical equation expresses the angular distribution well.