In 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 human skin. Vitamin D is produced with maximum efficiency at wavelengths around 297 nm. We proceed from the assumption that spectral irradiance at 306 nm is closely related to that at the wavelength 297 nm. The daily totals of the continuously recorded 306 nm spectral irradiance above the threshold value for vitamin D production have been studied during spring and autumnal transition periods as well as in summer. The "vitamin D winter" at the Tartu-Toravere Meteorological Station site (58.16'N, 26°.28'E, 70 m a.s.l.) lasted for 100-105 days from about Nov 6 to Feb 19 in cloudless weather and climatic total ozone conditions. During the closest 10 days to the conventional "vitamin D winter" in the most unfavorable conditions there could be no availability of the vitamin D synthesizing irradiance above its threshold level. In the most favorable conditions in separate cases small doses above the threshold could be available even about two weeks before the conventional end of the "vitamin D winter".
The objectives of the COST action 726 are to establish long-term changes of UV-radiation in the past, which can only be derived by modelling with good and available proxy data. To find the best available models and input data, 16 models have been tested by modelling daily doses for two years of data measured at four stations distributed over Europe. The modelled data have been compared with the measured data, using different statistical methods. Models that use Cloud Modification Factors for the UV spectral range, derived from co-located measured global irradiance, give the best results.
Use has been made of a technique to determine the distribution in atmosphere of the so-called optically active components (aerosol, ozone, water vapor, etc.) by the brightness profiles of atmosphere on a horizon, measured from space. Developed are the PHASE eight-channel aerospace teleradiometer and laboratory equipment for its study and graduation (including the complex for an absolute graduation and collimator for reading the characteristics of the teleradiometer field of vision). The PHASE teleradiometer has been installed on board of orbital complex `Salyut-7' - `Cosmos-1686' - `Sojuz-T14' and more than 10,000 brightness profiles were obtained over various regions of the Earth.