Diffuse sky radiation

From Glossary of Meteorology

diffuse sky radiation

Solar radiation that is scattered at least once before it reaches the surface.

As a percentage of the global radiation, diffuse radiation is a minimum, less than 10% of the total, under clear sky conditions and overhead sun. The percentage rises with increasing solar zenith angle and reaches 100% for twilight, overcast, or highly turbid conditions. It is measured by a shadow band pyranometer.