Measurements of the diffuse UV sky radiance during broken cloud conditions

Sky radiance distributions pertaining to surface UV have been investigated under clear-sky and broken cloud conditions. Comparison between the measure d clear-sky distribution and either a radiative transfer model or an empiri cal function showed reasonable agreement in each case. The radiance distrib ution measurements performed during broken cloud conditions were then inves tigated and compared to the clear-shy radiance. Radiances from cloudy sky l ocations were either higher (by up to 2.5 times) or lower (with minimum val ues that were 8% of the clear-shy radiances) than the clear-sky case. It ha s been shown that clouds have an enhancing effect on the ground UV radiance at locations with scattering angles between 30 degrees and 60 degrees and a sight zenith angle less than 30 degrees, or with scattering angles greate r than 60 degrees irrespective of the sight zenith angle. These results rep resent an advance toward the accurate description and modeling of ground UV under broken cloud conditions.