Variability of UV-B at four stations in Europe

The variability of solar irradiance in the UV-B spectral region is studied at four stations operating well-calibrated spectroradiometers during the period 1991-96. It is confirmed that total ozone is the controlling factor in long-term changes of solar irradiance below 320 nm received at the ground. This result is supported by the similar long-term variability under clear sky and all sky conditions during the past five years. The study includes estimates of the amplitude of the annual cycle and estimates of long-term changes. An important result of this note is, that calculated changes in solar irradiance depend on the length of the observational period and that any speculations on its future changes should be treated always with caution. It is estimated that a 2.7% change per year in solar irradiance at 305 nm at Thessaloniki is a rate that could possibly continue in view of the expected continuation of the ozone decline resulting to an increase of 27% per decade, which is comparable to the amplitude of the annual cycle at that station.