Abstract

For the environmental factors there appears a domain where the impact is most suitable for living organisms. The present paper discusses briefly the measurement methods of UV radiation, the available UV dose and its variations at a typical Estonian rural site, at the Tartu-Tõravere Meteorological Station. The summer half-year between the vernal equinox and the autumnal equinox accounts for about 80% of annual total solar radiation and nearly 90% of annual erythemally weighted radiation. The interannual variations of erythemal radiation are lower than the variations of global solar radiation. Four years out of five deviations from the average remain between  5.5% and 8%. The extreme highest half-yearly doses have reached 11% and 14.5% above average. At the geographic latitudes above 50° there is a period in each winter when the level of short-wave ultraviolet radiation (UVR) remains below the threshold needed for vitamin D synthesis in the human skin. The “vitamin D winter” at the Tartu-Tõravere Meteorological Station site lasts from about Nov 6 to Feb 19 in cloudless weather and in climatic total ozone conditions. During the 10 days closest to this conventional “vitamin D winter”, in the most unfavourable conditions, there can be no availability of vitamin D synthesizing irradiance above its threshold level.