Influence of snow and clouds on erythemal UV radiation: Analysis of Swiss measurements and comparison with models

Global, direct and diffuse measurements of erythemal UV radiation with UV-B iometer and radiative transfer calculations are used to estimate the influe nces of (1) a snow-covered surface, (2) an overcast sky, and (3) a joined s now-covered surface and overcast sky at Davos (1610 m asl, Swiss Alps). The influence of total ozone, zenith angle, and aerosol are first investigated . The influence of surface albedo and clouds are then estimated as a functi on of the zenith angle after normalization of the data to a fixed ozone amo unt. If the ground is covered with snow on clear-sky days at Davos, erythem al irradiance increases by 15 to 25% due to multiple reflections between th e surface and the atmosphere. This relative increase may reach 80% on overc ast days.-The highly significant dependence of the albedo on solar elevatio n is most probably due to inhomogeneities in the surrounding terrain leadin g to a strong non-Lambertian behavior. The impact of clouds on radiation is highly variable: on overcast days with a snow free surface, erythemal UV r adiation at Davos is reduced to a level ranging between 8% (very thick clou d cover) and 70% (thin cloud layer) relative to the radiation under clear-s ky conditions.