Analytic formula for the clear-sky UV index

An approximate formula for the UV Index (UVI) under cloud-free, unpolluted, low surface albedo conditions is: UVI approximately 12.5mu(o)(2.42)(Omega/300)(-1.23) where mu(o) is the cosine of the solar zenith angle and Omega is the total vertical ozone column (in Dobson Units, DU). The dependence on mu(o) and Omega is based on a simple physical model of biologically weighted atmospheric transmission in the UV-B and UV-A spectral bands, with coefficients tuned to a detailed radiative transfer model, and is accurate to 10% or better over 0-60 degrees and 200-400 DU. Other factors (clouds, haze, ground, etc.) mostly conserve this dependence and scale simply.