Abstract. 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-sky 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-sky 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 ø and 60 ø and a sight zenith angle less than 30 ø, or with scattering angles greater than 60 ø irrespective of the sight zenith angle. These results represent an advance toward the accurate description and modeling of ground UV under broken cloud conditions. IntroductionUltraviolet (UV) radiation belongs to the part of the electromagnetic spectrum between X ray at 10 nm and the visible at 400 nm. The UV consists of three regions, the UV-C (below 280 nm), UV-B (280-320 nm), and the UV-A (320-400 nm) [Huffman, 1992]. Ozone and oxygen completely absorb the UV-C radiation in the upper and middle atmosphere. The various factors that influence ground UV-B and UV-A radiation, in order of decreasing importance, are the solar zenith angle, which determines the optical path length, the total column ozone (which mainly affects UV-B), cloud cover, the aerosol loading of the atmosphere, surface albedo, gaseous pollutants, the Sun-Earth distance, and the air pressure. Ozone depletion and clouds are two of these factors that have re-
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