The UV Index (or UVI) was originally created as a predictive tool for the public to get a sense as to how intense the UV rays from sunlight will be. The US National Weather Service model takes into account the predicted filtering from the ozone layer, cloud cover, and compensates for latitude and elevation.

The UV index is a number linearly related to the intensity of sunburn-producing UV radiation at a given point on the Earth’s surface. To accomplish this the index provides an erythemally weighted factor for various wavelengths of UV light (from 280 – 400 nm). It cannot be simply related to the irradiance because the shorter wavelengths are much more damaging to skin, hence the weighting factor.  The combined elements of this predictive calculation are combined and normalized to a scale of 0 to the mid-teens.

For additional information regarding the UV Index please visit these sites:
https://www.epa.gov/sunsafety/calculating-uv-index-0
http://www.cpc.ncep.noaa.gov/products/stratosphere/uv_index/uv_compute.shtml
https://en.wikipedia.org/wiki/Ultraviolet_index

It is interesting to note that other countries use slightly different models to determine their UV Index values.

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