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1. Introduction

The cloudiness and ozone are the main factors that change biologically active UVB irradiance coming to the ground. During the last years a lot of efforts were done to organize ground UVB network. But the period of continuous UVB observations is not very long and for most stations does not exceed a decade. A distinct growth of total ozone amount observed during the last years may be responsible for increase in UVB irradiance, but possible changes in cloud properties and amount may compensate or enhance the changes in UVB irradiance due to total ozone increase. Therefore it was interesting to study UVB variations due to ozone and cloudiness changes separately in order to reveal the significance of each factor and to define the geographical regions where the interannual variability of UVB irradiance is determined mainly by this or that parameter. Using historical records of cloud observations and long-term ground measurements of total ozone amount as well as satellite data we try to estimate the long-term UV irradiance variations in different geographical regions.