How is the UV Index Calculated?

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Unlike some countries' indices, the US UV Index is not based upon surface observations. Rather, it is computed using forecasted ozone levels, a computer model that relates ozone levels to UV incidence on the ground, forecasted cloud amounts, and the elevation of the forecast cities.

The calculation starts with measurements of current total ozone amounts for the entire globe, obtained via two satellites operated by the National Oceanic and Atmospheric Administration (NOAA). These data are then used to produce a forecast of ozone levels for the next day at various points around the country. A radiative transfer model is then used to determine the amount of UV radiation reaching the ground from 290 to 400 nm in wavelength, using the time of day (solar noon), day of year, and latitude. As an example, assume the following UV levels for each wavelength are predicted for a given location (these are totally made up numbers, and not even the ratios represent reality):

Wavelength        Incidence
290nm                 10
350nm                 20
400nm                 50

This information is then weighted according to how human skin responds to each wavelength; it is more important to protect people from wavelengths that harm skin than from wavelengths that do not damage people's skin. The weighting function is called the McKinlay-Diffey Erythema action spectrum. For illustration purposes only (these numbers are not correct), assume 290nm radiation causes three times as much damage as 350nm radiation and five times as much damage as 400 nm radiation. Then, if in some unit 290nm UV radiation did 15 units of damage, 350nm radiation would do 5 units and 400 nm radiation would do 3 units. At each wavelength, multiply the actual incoming radiation level by the weighting:

Wavelength       Incidence       Weight      Result
290nm                10            15         150
350nm                20             5         100
400nm                50             3         150

These weighted irradiances are summed up, or integrated, over the 290 to 400 nm range resulting in a value representing the total effect a given day's UV radiation will have on skin. For our example, the total is 400.

These estimates are then adjusted for the effects of elevation and clouds. UV at the surface increases about 6% per kilometer above sea level. Clear skies allow 100% of the incoming UV radiation from the sun to reach the surface, whereas scattered clouds transmit 89%, broken clouds transmit 73%, and overcast conditions transmit 31%. If we assume that the example location is at 1 kilometer in elevation, and that there will be broken clouds, then the calculation is:

400 x 1.06 x 0.73 = 309.5

Once adjusted for elevation and clouds, this value is then scaled (divided) by a conversion factor of 25 and rounded to the nearest whole number. This results in a number that usually ranges from 0 (where there is no sun light) to the mid teens. This value is the UV Index. Thus, the UV Index for the example city would be:

309.5 / 25 = 12.4, rounded to 12

Currently, the computation of the UV Index does not include the effects of variable surface reflection (e.g., sand, water, or snow), atmospheric pollutants or haze.

To use the UV Index, read the UV Index and your health.

ANd for an extreme level of detail - try the Climate Prediction Center's pages.

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