Transmittance of a cloud is wavelength-dependent in the UV-range

Spectral irradiance measured at two stations at different altitudes (the Zugspitze-summit at 2964 m a.s.l. and the Fraunhofer-Institute for Atmospheric Environmental Research at 730 m a.s.l., 8 km apart in horizontal direction) was used to examine the spectral attenuation of the UV-irradiance by a homogeneous cloud-layer at an altitude of about 1200 m. Calculating the ratio of the global spectral irradiance beneath a homogenous cloud-cover (22 October, 1995) to the global spectral irradiance on a cloudless day (24 October, 1995) both measured at the valley station, delivered the spectral transmittance of the cloud. By means of data measured on the Zugspitze on the same two days possible differences in the atmospheric conditions above 2964 m were eliminated and thus the influence of the cloud on the UV-irradiance at Garmisch-Partenkirchen was separated. The transmittance of the cloud-layer was found to be wavelength-dependent, ranging from 45% in the UVA to 60% in the UVB. Therefore it can be assumed that clouds are not generally ''grey'' (i.e. that the attenuation is dependent on wavelength).