Description
The UV Index is a linear scale, with higher values representing a greater risk level of sunburn (which is correlated with other health risks) due to UV exposure. An index of 0 corresponds to zero UV irradiation, as is essentially the case at night. While an index of 10 corresponds roughly to midday, summer sun and a clear sky, indices greater than 11 are quite common in summer at low latitudes, or in areas with above average ozone layer depletion. The numbers are related to the amount of UV radiation reaching the surface of the earth, measured in W/m2, but the relationship is not simple (see below for definition) because UV power is spread unevenly over a spectrum of wavelengths; shorter wavelengths are much more damaging. By weighting their effect the index aims to create a number that cannot be expressed in physical units, but is a good indication of likely skin damage.
While the UV Index can be calculated from a direct measurement of the UV spectral power at a given place, the value given in weather forecasts is usually a prediction based on a computer model. While this may be in error, as with all predictions, it is usually the same index as that which would be measured.
The UV Index announced in weather forecasts is a prediction of how strong the actual UV intensity will be at the sun's highest point in the day, called solar noon. This generally occurs between 1130 and 1230, or 1230 and 1330 in areas where daylight saving time is being observed. The prediction is made by a computer model that accounts for the effects of sun altitude, ozone concentration, altitude, the surface albedo, and cloud cover, which increase or decrease the amount of UV radiation that will reach the surface. The calculations are weighted in favor of the UV wavelengths to which human skin is most sensitive according to the McKinlay-Diffey Erythema action spectrum. Providing the public with an easy-to-understand daily and hourly forecast of UV intensity is the main purpose of the UV index.
Some inexpensive portable devices also measure actual UV Index.
Read more about this topic: Ultraviolet Index
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