The UV Index is an irradiance scale computed by multiplying the CIE erythemal irradiance integral in watts m-2 by 40. The term "erythema" refers to the reddening of the skin due to sunburn. Other irradiance integrals exist which describe other physical and biological effects of UV radiation. However, this one has become the one used most often. For a fuller description, including the calculation of CIE erythemal irradiance integral, click here.
The UV Index can be measured on the ground directly using the Brewer Spectrophotometer. They are located at 13 sites in Canada and 21 sites in the United States. (PDF or GIF). It can also be estimated indirectly from ground observations taken by pyranometers, combined with information about the ozone layer, humidity and snow. (Pyranometers measure solar radiation at all wavelengths, not just UV.) There are 42 of these sites in Canada and 43 sites in the United States. (PDF or GIF). In addition, the TOMS satellite takes readings once per day from which estimates can be made. These cover the entire Earth on a grid of approximately 100 by 100 kilometres.