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The solar spectrum outside the Earth's atmosphere contains radiation in a wide range of wavelengths. Large parts of it are absorbed in the atmosphere. The atmosphere is only transparent for visible light, and parts of ultraviolet (UV) and infrared (IR) radiation. In general, the damaging effect of radiation increases with decreasing wavelength. But nearly all radiation with wavelengths shorter than 280 nm (UV-C) is absorbed by oxygen. The radiation between 280 and 325 nm (UV-B) is absorbed by atmospheric ozone, while the UV-A part (325-400 nm) is much less attenuated. The human skin is particularly sensitive to radiation in the UV-B region. Too much exposure to UV-B may cause sunburn on short term, but also some types of skin cancer on the long term.
The intensity of radiation in the UV-B region is governed by
- the elevation of the sun: the higher the elevation of the sun, the higher the radiation, which depends on the time of day, the time of the year and the location where you are;
- the cloudiness: clouds absorb (decrease), but can also reflect (increase) UV-B radiation;
- the total ozone content of the atmosphere (mainly stratospheric ozone): more ozone absorbs more radiation;
- the altitude at which you are: at high altitude the UV-B intensity is higher;
- the albedo of the surroundings: higher reflecting surroundings (snow, white sand) increase the intensity of the UV-B radiation;
- other atmospheric constituents (aerosols, trace gases): when present they decrease the radiation. Aerosols play an important role in climate forcing: they can reduce the UV-B at the ground with about 5 to 35% (see pdf: 344 kb).
Since there are signals of a decreasing thickness of the ozone layer, and the changing behaviour of the population (outdoor activities, travelling) there is concern about the risk of the UV-B radiation for the health of the population.
In order to inform the public about the expected level of UV radiation the use of the so-called UV-index has been recommended by the WHO (World health Organisation) and WMO (World Meteorological Organisation).
Methods to forecast UV radiation were studied in the frame of the COST 713 action on UV forecast of the European Union.
To inform the public about the risks related to the UV radiation, the UV index is forecasted and disseminated together with the weather forecast. An image with both measured and forecasted UV-indices for Uccle is updated several times per day.