Stratosphere around at midlatitudes, and at about at the poles. Temperatures range from an average of near the tropopause to an average of near the mesosphere. Stratospheric temperatures also vary within the stratosphere as the seasons change, reaching particularly low temperatures in the polar night (winter). Winds in the stratosphere can far exceed those in the troposphere, reaching near in the Southern polar vortex. The mechanism describing the formation of the ozone layer was described by British mathematician Sydney Chapman in 1930. Molecular oxygen absorbs high energy sunlight in the UV-C region, at wavelengths shorter than about 240 nm. Radicals produced
Sun tanning tanning and UV radiation exposure, either from the sun or from indoor tanning. Production of vitamin D is essential for human health. Moderate exposure (avoiding sunburn) to UV radiation provides benefits such as increased vitamin D, as well as other possible benefits that are still being studied. Several tanning activators have used different forms of psoralen, which are known to be photocarcinogenic. Health authorities have banned psoralen since July 1996. Tanning has gone in and out of fashion. In the United States and Western Europe before about the 1920s, tanned skin was associated with the lower classes, because they worked