Satellite-derived maps of surface UV irradiance are currently limited by their poorly assessed accuracy. Here we use an extensive data set of ground-based spectral measurements from two Alpine sites to examine the level of uncertainty expected in model results due to the time-limited nature of some satellite data. When the ozone column, cloud optical depth, and aerosol optical depth supplied to a radiative transfer model are fixed to a single noontime value, the root-mean-square difference between calculations and measurements of the erythemal daily dose is about 20%. The corresponding uncertainty in the monthly dose is less than 5%. The modeled results also show a systematic error that depends on cloud optical depth. The results suggest that satellite-derived maps of UV irradiance cannot be expected to produce accurate values of the daily dose if they rely on a single estimate of the cloud conditions but may be able to provide reasonable estimates of the monthly dose. ¿ 2000 American Geophysical Union