A model-derived global climatology of UV irradiation at the earth's surface

We present calculations of the geographical distribution of the dose rate at the surface of UVB (280-320 nm), UVA (320-400 nm) and, using biological action spectra, the effective radiation for erythema, cataracts; and keratitis, A multistream radiative transfer model is used in conjunction with a multiyear climatology of ozone, cloud, surface pressure, surface albedo, temperature and a rudimentary representation of aerosols to calculate the clear-sky and all-sky irradiances. Model outputs are evaluated using daily UV measurements and found to be accurate to about +/-10% for clear skies and +/-20% for all-sky conditions. The effects of UV- weighted sur face albedo, surface altitude, sun-earth separation and the vertical distribution of ozone and temperature are included. The results show that the sun's position is the most important factor in determining the geographical pattern of global daily UV rather than column ozone, cloud, surface pressure, daylength or surface albedo. Over elevated regions, the effect of the differences in surface pressure on daily doses was found to be more significant than the effect of the differences in column ozone. Clouds reduce the clear-sky UV dose from a few percent over arid and semiarid regions to 45% in regions with frequent mid-latitude depressions.