Relationship between UVB and erythemally weighted radiation

Abstract

We discuss the move from reporting damaging UV radiation in terms of UVB to the now widely accepted erythemally weighted UV radiation (UV_Ery) and the UV Index (UVI). The relationship between these quantities is given: to a good approximation, it is found that UVB_{280–315 nm} = 7.55 × UV_Ery. In terms of the UV Index, the estimated UVB_{280–315 nm} in units of W m⁻² is 18.9 times the UVI. These approximations generally hold to within ∼10% for all solar zenith angles (sza) less than 70°. For most practical purposes, this is a sufficient range, since for larger sza, the intensity of UVB is less than 10% of that for overhead sun conditions. The simple relationship above is verified using spectral measurements. However, tables are provided to enable calculation of the conversion with greater accuracy under such conditions. Similar model calculations are provided to estimate UVB_{280–320 nm}. Correction tables to convert erythemally weighted UV to other biological weightings are also presented.