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 measured clear-sky distribution and either a radiative transfer model or an empirical function showed reasonable agreement in each case. The radiance distribution measurements performed during broken cloud conditions were then investigated and compared to the clear-shy radiance. Radiances from cloudy sky locations were either higher (by up to 2.5 times) or lower (with minimum values that were 8% of the clear-shy radiances) than the clear-sky case. It has 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 greater than 60 degrees irrespective of the sight zenith angle. These results represent an advance toward the accurate description and modeling of ground UV under broken cloud conditions.