The index is generated for each city using that site's predicted cloud amounts and other atmospheric conditions. Rapid changes in cloud amount can alter the predicted UVI forecast. The index is valid for about a 30-mile radius around the city it is prepared for. The UV Index number is adjusted to account for the possible presence of clouds and the elevation of your location. The lower the number, the lesser the amount of UV radiation reaching the surface.
Low numbers occur when the sun is low in the sky and during overcast conditions.
A larger number occurs when the sun is high in the sky and during clear or partly cloudy conditions. The total ozone is measured by a NOAA polar orbiting satellite. This measurement is inserted into a model which also includes the latitude, day of year and time of day. The UV value derived from the model is adjusted to the elevation of the location, and is then further adjusted to account for the predicted cloud cover, which also can affect the amount of UV reaching the earth. The accuracy of the UVI forecast proved to be quite good.