TRANSMITTANCE OF A CLOUD IS WAVELENGTH-DEPENDENT IN THE UV-RANGE

Spectral irradiance measured at two stations at different altitudes (t he 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 hori zontal direction) was used to examine the spectral attenuation of the UV-irradiance by a homogeneous cloud-layer at an altitude of about 120 0 m. Calculating the ratio of the global spectral irradiance beneath a homogenous cloud-cover (22 October, 1995) to the global spectral irra diance on a cloudless day (24 October, 1995) both measured at the vall ey station, delivered the spectral transmittance of the cloud. By mean s of data measured on the Zugspitze on the same two days possible diff erences in the atmospheric conditions above 2964 m were eliminated and thus the influence of the cloud on the UV-irradiance at Garmisch-Part enkirchen was separated. The transmittance of the cloud-layer was foun d to be wavelength-dependent, ranging from 45% in the UVA to 60% in th e UVB. Therefore it can be assumed that clouds are not generally ''gre y'' (i.e. that the attenuation is dependent on wavelength).