Variability of UV irradiance in Europe.

The diurnal and annual variability of solar UV radiation in Europe is described for different latitudes, seasons and different biologic weighting functions. For the description of this variability under cloudless skies the widely used one-dimensional version of the radiative transfer model UVSPEC is used. We reconfirm that the major factor influencing the diurnal and annual variability of UV irradiance is solar elevation. While ozone is a strong absorber of UV radiation its effect is relatively constant when compared with the temporal variability of clouds. We show the significant role that clouds play in modifying the UV climate by analyzing erythemal irradiance measurements from 28 stations in Europe in summer. On average, the daily erythemal dose under cloudless skies varies between 2.2 kJ m(-2) at 70 degrees N and 5.2 kJ m(-2) at 35 degrees N, whereas these values are reduced to 1.5-4.5 kJ m(-2) if clouds are included. Thus clouds significantly reduce the monthly UV irradiation, with the smallest reductions, on average, at lower latitudes, which corresponds to the fact that it is often cloudless in the Mediterranean area in summer.

Europe's darker atmosphere in the UV-B.

Irradiation in the ultraviolet wavelength range is found to be up to 50% lower in the European summer compared to sites with comparable latitudes in New Zealand. We have developed a method to quantitatively attribute the causes for such differences between sites by analysis of spectra. We conclude that these large differences are caused mainly by differences in total ozone, cloudiness, aerosol loading and Sun-Earth separation. The relative contribution of clouds varies from year to year and it is site dependent. Averaged over several years we find a strong latitudinal gradient of the cloud impact within Europe, with much less cloud attenuation in southern Europe. Due to the differences in total ozone and aerosol loading, the UV-B levels are generally lower in Europe compared to New Zealand. It is likely that inter-hemispheric differences will change in coming decades due to a combination of changes in ozone concentrations, air pollution and cloudiness as a result of climate change. However, since the future evolution of these major parameters is highly uncertain, the magnitude and even the sign of such changes are not known yet.