EFFECTS OF CLOUDINESS ON GLOBAL AND DIFFUSE UV IRRADIANCE IN A HIGH-MOUNTAIN AREA

At the high-mountain station Jungfraujoch (3576 m a.s.l., Switzerland) , measurements of the radiation fluxes were made during 16 periods of six to eight weeks by means of a Robertson-Berger sunburn meter (UVB d ata), an Eppley UVA radiometer and an Eppley pyranometer. Cloudiness, opacity and altitude of clouds were recorded at 30-minute intervals. A second set of instruments was employed for separate measurement of th e diffuse radiation fluxes using shadow bands. The global and diffuse UVA- and UVB radiation fluxes change less with cloudiness than the cor responding total radiation fluxes. When the sun is covered by clouds, the global UVA- and UVB radiation fluxes are also affected less than t he global total radiation flux. The roughly equal influence of cloudin ess on the UVA- and UVB radiation fluxes suggests that the reduction i s influenced more by scattering than by ozone. Also, the share of diff use irradiance in global irradiance is considerably higher for UVA- an d UVB irradiance than for total irradiance. At 50-degrees solar elevat ion and 0/10 cloudiness, the share is 39% for UVB irradiance, 34% for UVA irradiance and 11% for total irradiance. The increased aerosol tur bidity after the eruptions of El Chichon and Pinatubo has caused a sig nificant increase in diffuse total irradiance but has not produced any significant changes in diffuse UVA- and UVB irradiances.