In recent years the availability and quality of UV radiation data have improved appreciably. However, high quality measurements from intercalibrated ground-based spectrometers are available at only a few sites worldwide. These have shown large differences in UV between mid latitudes in the northern and southern hemisphere. Satellite derived UV irradiances offer the advantages of global coverage and avoid the problems of instrument intercalibrations when investigating geographic differences in UV. However, with the satellite sensors in common use, the range of sampled altitudes does not generally extend all the way down to the Earth’s surface. Consequently these retrievals can be subject to errors due to variability in the distribution of ozone and aerosols in the lower atmosphere. Previous studies have shown good agreement between satellite-derived UV and ground-based measurements at pristine locations, but with increasing overestimations of the surface UV at more polluted sites, which are characterised by larger concentrations of ozone and aerosols in the troposphere. Consequently, the contrast in UV between pristine locations and more polluted locations may be underestimated by the satellite retrievals. Here we investigate the relative importance of differences in ozone, differences in vertical profiles of ozone and temperature, differences in aerosols extinction, and seasonal differences in Sun-Earth separation in modulating UV. We also investigate how these impact on the accuracy of satellite-derived UV at the surface.