MIKE MOSS SAYS: Mike, Your question touches upon what is probably the single most important factor, which is cloud cover. Forecasts of UV index start with estimates of the amount of ozone aloft that will intercept and attenuate some of the incoming ultraviolet radiation. These forecasts are based in part on satellite-derived measurements of total ozone content at many locations around the globe. Once the reduced UV intensity due to ozone absorption has been estimated, then an additional reduction factor is applied based on the expected cloud cover. Finally, as you mentioned, there are a few other factors that play a role, such as the latitude of the location in question, the time of year (both of which play into the angle of incidence of incoming radiation) and the elevation above sea level of the forecast location (all else being the same, a higher elevation will receive a greater UV exposure and therefore have a higher UV index).
The Environmental Protection Agency has a very nice UV index forecast page that shows a contoured forecast (as in the image above) for the U.S. covering a four-day period. You will notice that the low-index colors on this map correspond closely to the distribution of clouds and precipitation associated with traveling weather disturbances. To see the forecasts, visit
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