Coastal regions with fiords and mountains with seasonally varying snowcover form complex environments for ultraviolet radiative transfer. The presence of snow causes enhanced levels of UV radiation, the quantitative interpretation of which is only possible with detailed three-dimensional radiative transfer simulations. Such calculations of the irradiance at 340 nm have been performed for the location of Troms¿, Norway, under cloudless and overcast conditions, moving the snowline from 0 to 1000 m.a.s.l. and finally for snow free conditions. The evolution of the radiation enhancement as the snowline moves up is compared with measurements made by a narrow bandwidth multichannel filter instrument. For a cloudless sky the simulations and measurements exhibit a similar radiation enhancement of 23--27%. For the investigated overcast situation the radiation enhancement is about 40--60%. ¿ 2001 American Geophysical Union