UV Index: How is It Computed?

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The NOAA/EPA UV Index is computed using forecasted ozone data, a radiative transfer model, forecasted cloud amounts, climatological aerosol loading, variable snow and constant bare earth albedo, and elevation.

Forecasted total ozone amounts for the entire globe are obtained via the NCEP/Global Forecast System model. The operational forecast model assimilates NOAA SBUV/2 total and profile ozone observations plus additional ozone observations from the GOME-2 instrument on MetOP and from the OMI and MLS instruments on the NASA Aura research satellite. A radiative transfer model (TUV) is used to determine the UV irradiances from 290 to 400 nm, using the time of day, 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 an 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), surface albedo (an increase over bare ground is given 2%, while the increase over snow is variable from 30% to 100%), and tropospheric aerosol loading (Aerosol Optical Depth AOD and Single Scattering Albedos – SSA) decreases the UV dose rate by 0 to 20%. This results in a final clear sky UV dose rate forecast (milliWatts/sq meter).

The NCEP/GFS is also used to provide the forecast due to the presence of clouds. Instead of using the cloud amount and then using some relationship between observed cloud amount and UV transmission. The ratio of down- welling UV band radiation at the surface with clouds vs without clouds is used. This ratio gives the cloud transmittion in the UV. The transmission is nearly linear and nearly 1.0 until cloud amounts of 60% occur after which the transmission curves toward 0% at cloud amounts of 100%. However, due to the variability of the opacity of the clouds there is quite a large range in the transmission amout as cloud amounts increase. This is a much better estimate since the model has included all the various effects of multiple scattering by the clouds in these values. This transmission ratio is then applied to the clear sky forecast to provide the ‘cloudy sky’ UV dose rate.

The UV Index value is determined by dividing the dose rate (milliWatts per square meter) 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.

Currently, the tropospheric aerosol loading is derived from a semi-annual climatology. When daily observed and forcasted aerosol loading values becomes available they will be included in the forecast computation.