ULTRAVIOLET-RADIATION UNDER BROKEN CLOUD CONDITIONS AS INFERRED FROM MANY-YEAR GROUND-BASED OBSERVATIONS

The dependence of the transmittance of total ultraviolet (UV) radiatio n CQ on low-level cloud cover and on the sun duration in the warm peri od are studied from data of long-term measurements (1981-1990) at the meteorological observatory of Moscow State University. It is shown tha t the probability density of CQ for any cloud cover, except for overca st skies, can be approximated by a normal distribution. Different stat istical characteristics of CQ are studied. The probability of the grow th of UV radiation for cloudy skies as compared to clear skies is anal yzed. A technique for CQ calculation based on combined use of data on sun duration and cloud cover is proposed.