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The UltraViolet Index (UVI) Bulletin contains forecasts of the amount of skin damaging UV radiation expected to reach the earth's surface at the time when the sun is highest in the sky (solar noon) of the next day in selected US cities. The UVI forecast products also include hourly forecasts in gridded maps. The forecasts of the amount of UV radiation reaching the surface is primarily related to the elevation of the sun in the sky, the amount of ozone in the stratosphere, and the amount of clouds and aerosols present. The index is calculated using GFS ozone forecasts and a radiative transfer model to calculate the flux of UV radiation at ground level for a range of wavelengths.These fluxes are then adjusted using an action spectrum that weights the response of human skin to UV radiation across the wavelengths. This dose rate forecast is then scaled and rounded to the nearest whole number to obtain the UVI value in the unit of 25 milli-Watts/m2.
The UVI has been in operation at NWS since 1994 and is maintained by the Climate Prediction Center (CPC). Currently, the UVI products are issued daily and consist of the following: 1) a text bulletin of the next day noontime UVI forecast for 58 cities, 2) gridded output (0.5 degrees) of the daily noontime forecast out to 4 days, and 3) gridded output (0.5 degrees) of the hourly forecast at leads out to 120 hours.