Erythemal UV index from SCIAMACHY
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 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.
There are three types of UV rays:
Ultraviolet A rays (UVA)
The atmosphere does little to shield these rays—most UVA radiation reaches Earth’s surface. UVA rays cause skin aging and eye damage, and can lower your body’s ability to fight off illness. UVA rays also contribute to the risk of skin cancer.
Ultraviolet B rays (UVB)
The Earth’s atmosphere shields us from most UVB rays—the amount of UVB rays that reach the Earth’s surface depends on latitude, altitude, time of year and other factors. UVB rays cause sunburns, skin cancer, skin aging, and snow blindness (a sunburn to your cornea that causes a temporary loss of vision) and can lower your body’s ability to fight illness.
Ultraviolet C rays (UVC)
UVC rays do not reach the Earth’s surface because they are completely absorbed the atmosphere. Harmful effects from UVC rays are minimal.
The UV radiation that reaches the Earth’s surface is mostly UVA and some UVB. Almost half the daytime total of the more harmful UVB radiation is received between the hours of 10 a.m. and 4 p.m. Even on a cloudy day, you can be sunburned UVB radiation.