Ultraviolet rays have the most pronounced effect on living organisms. This part of the solar spectrum is biologically active. It accounts for about 5% of the solar radiation reaching the Earth's surface. Length ultraviolet rays in the range of 100 to 400 nm. Distinguish 3 sub-band: A (UVA) — 315-400 nm, B (UVB) — 280-315 nm, C (UVC) — 100-280 nm. Fortunately, the hard ultraviolet (UVC whole and 90% UVB) is absorbed by ozone, oxygen, water vapor and carbon dioxide in the atmosphere. That is, the surface of the Earth reaches the remaining 10% UVB and all UVA. In 1994, the World Meteorological Organization (WMO), the World Health Organization (WHO) for the numerical evaluation of UV suggested a UV-index, which is the maximum daily dietary exposure to radiation or exposure. For clear sky UV-index corresponds to the value of irradiance at noon and true to the highest possible risk of UV exposure during the day. Scale to determine the index has a range from 1 to 10. In the middle latitudes in summer UVI values depend mainly on the total ozone in the atmosphere, clouds and surface albedo. The higher the index value, the greater the possibility for damage to the skin and eye. The level of UV rays during the day changes, so changes and UV-index value. It reaches a maximum value for the four-hour period around noon (± 2 hours from noon). In many countries, during the summer months on the UVI value reported in weather forecasts on the site of the national meteorological services and the media.