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UV Index: How is It Computed?
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:
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