This is an example of TOMS global image of daily erythemal UV exposure. Ultraviolet radiation is at shorter wavelengths than the visible spectrum (400 to 700 nm) and is divided into three components: UV-A (315 to 400 nm), UV-B (280 to 315 nm) and UV-C (less than 280 nm). The shorter wavelengths that comprise UV-B are the most dangerous portion of UV radiation that can reach ground level. Atmospheric ozone shields life at the surface from most of the UV-B and almost all of the UV-C. UV-A and UV-B are reduced by a small amount from Rayleigh scattering in the atmosphere. All forms of UV radiation are reduced by cloud cover. Persistent lack of cloud cover in some regions (e.g. Australia and South Africa) increases the danger from UV radiation compared to similar latitudes in the Northern Hemisphere. Chemical processes in the atmosphere can affect the amount of ozone, and therefore, the level of protection provided by the ozone in the statosphere and troposphere. This thinning of the atmospheric ozone leads to elevated levels of UV-B at the earth's surface and increases the risks of DNA damage and other cellular damage in living organisms.
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