Ultraviolet light, UV for short, is an often ignored part of the light spectrum, mostly because it is invisible to the naked eye. The short-term dangers of UV exposure are usually fairly minor and merely annoying. However, the long-term dangers of UV light pose a serious risk that all too often ends in severe disabilities, and even death. Some people falsely believe that one can train the body to handle UV rays; this is not true.

UV light has extremely short wavelengths, which result in very high levels of radiating energy. When you stand outside and absorb too many UV rays, the skin weakens, then blood is sent to the "burn" area, hence the red coloration of a sunburn. Not everyone gets sunburned equally, or at all. Darker-skinned people have more melanin, which safely absorbs UV energy.

The UV index is computed on a longitude-latitude grid of 0.5 by 0.5 degrees (at the equator this grid measures about 50 kilometers) and subsequently stored at this resolution. The value of the UV index reported for a certain location is thus an average value, valid for the whole grid cell within which the given location lies.

Erythemal UV index from SCIAMACHY