UV Index: Information

Unlike some countries, the NOAA/EPA UV Index is not based upon surface observations. Rather, it is computed using the forecasted ozone data, a radiative transfer model, forecasted cloud amounts and the elevation of the forecast cities.

Total ozone amounts for the entire globe are obtained via the TOVS (TIROS Operational Vertical Sounder) or the SBUV/2 (Solar Backscatter UltraViolet/2) instruments on board NOAA polar orbiting satellites. The observed data is then used to produce a forecast of the ozone data for tomorrow. This is done using the thermal-dynamical relationship between total ozone and heights at 100 and 500 hPa and temperatures at 50 hPa. NCEP provides the necessary analyzes and forecasts to used to determine the forecasted ozone data. This forecasted ozone data has been shown to be much more accurate than just using persistence. A radiative transfer model is used to determine the UV irradiances from 290 to 400 nm, using the time of day (solar noon), day of year, and latitude. The irradiances are weighted by the McKinlay-Diffey Erythema action spectrum (weighting function) so as to reflect the human skins response to each wavelength. These weighted irradiances are integrated over the 290 to 400 nm range resulting in the erythema dose rate. So from the total ozone amounts, erythema dose rates are determined. The erythema dose rates are adjusted for the effects of elevation( an increase of about 6% per kilometer).

The NCEP forecast models also provide forecasts of the cloud conditions at the time the UV Index would verify at. We have determined from earlier data that:

• Clear skies allow 100% transmission of UV radiation to the surface.
• Scattered cloud conditions allow 89% transmission of UV radiation to the surface.
• Broken cloud conditions allow 73% transmission of UV radiation to the surface.
• And overcast cloud conditions allow 32% transmission of UV radiation to the surface.

Given these transmission amounts, the forecast cloud conditions can be converted into a forecast transmission. This percentage is then applied to the elevation adjusted erythema dose rate at each of the forecast cities. This value is then scaled (divided) by the standard of 25 milliWatts per square meter. 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.

Currently, the computation of the UV Index does not include the effects of variable surface reflection, atmospheric pollutants or haze.

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