Radiance Distribution on Stable Overcast Skies.

Overcast sky conditions can be classified into two groups. The one is a group of unstable and fluctuating overcast skies. The other is a group of stable overcast skies. As for the first group, it is almost impossible to derive a typical distribution pattern on a sky. As for the second group, however, it is possible to derive a typical radiance distribution pattern. The author found out that the radiance distribution pattern on a stable overcast sky can be characterized by an equation similar to but a little different from the equation for the luminance distribution on the CIE standard Overcast Sky. In a stable overcast sky, the radiance at the zenith is almost always greater than that of any other sky element.