Global fields of UV Index forecasts at hourly intervals out to 120 hours are available via the NCEP Product Access in the uv.YYYYMMDD directories (where YYYY is the year, MM is the month, and DD is the day). The file names are in the form: uv.t12z.grbfXX, where XX is the forecast hour (01 to 120). These 0.5x0.5 degree gridded fields are in GRIB2 format. The fields are generated from output fields of the 12 UTC run of the GFS. The data (erythemal dose rates) have the units of Watts per sq meter. To compute the UV Index value multiply the gridded data value by 40.0.

The erythemally weighted downwelling UV fluxes are determined using the TUV radiative transfer code. The fluxes is altered by the effects of:

• sun-earth distance
• solar zenith angle
• total ozone amount
• tropospheric aerosol optical depth
• elevation
• snow/ice reflectivity
• cloud transmission

https://uvb.nrel.colostate.edu/UVB/ Initial comparisons with USDA/Colorado State Univ UV-B Monitoring and Research Program and NOAA SURFRAD observations are very good. Feedback is encouraged for improvement of this product. Send constructive comments to the CPC web master.