Continuous measurements of ultraviolet irradiance in the 300--380 nm spectral range (UV380) carried out at the Meteorological Observatory of the Moscow State University are analyzed for the 1968--1997 warm periods. Variability in UV irradiance is examined together with variations of ozone content, aerosol optical thickness, cloud amount, and cloud optical thickness. We estimated the maximum contribution of each parameter to interannual variability of UV380 and erythemally weighted (EW) irradiance. The most important parameters are cloud amount, whose variation contributed to changes of up to 12% in the level of UV irraidance, and total ozone content, which contributed to changes of up to 8% in the level of EW irradiance during this period. According to the developed algorithm the variability in EW irradiance was calculated for the entire 30 year period. The analysis of variability in UV380 and EW irradiance shows a slight increase in UV values since the middle of the 1980s, though no statistically significant UV trend over the 30 year period is observed. Comparison between measurements of UV380 in a cloudless atmosphere and the results of radiative transfer modeling, using a two stream delta-Eddington approximation, reveals a good agreement. Since the end of the 1980s there is an increase of UV380 and especially EW irradiance under cloudless conditions. Extremely high values of EW irradiance and corresponding atmospheric conditions are of particular concern. It is shown that approximately in 50% of the cases, these extremely high values are detected in the presence of clouds. ¿ 2000 American Geophysical Union