Ultraviolet spectral irradiance in the French Alps: Results of two campaigns

The ultraviolet spectral irradiance has been measured in the Alps in Brianc on (altitude 1300 m asl) in July 1996 and March 1997, with the spectroradio meter of the University of Lille. The erythemal dose rate has been measured simultaneously with a broadband instrument. The aerosol optical depth, nec essary as input for modeling, was measured with a Sun photometer. On cloudy days, irradiance is highly variable, with values exceeding those of clear days when the direct solar beam is not obstructed by clouds. For clear days , measured values are compared with the results of a radiative transfer mod el, and the influence of altitude and ground surface reflectance is analyze d. For the same solar zenith angle, irradiance in UVA is 8-10% higher in Br iancon than in Brussels, half of the difference being due to the higher alt itude of Briancon. Comparison of winter and summer values shows an amplific ation of similar to 10-15% in winter owing to the snow reflectance; it can be explained by an effective reflectance of 0.3-0.4.