Modelling UV irradiances on arbitrarily oriented surfaces: effects of sky obstructions

Abstract. A method is presented to calculate UV irradiances on inclined surfaces that additionally takes into account the influence of sky obstructions caused by obstacles such as mountains, houses, trees, or umbrellas. With this method it is thus possible to calculate the impact of UV radiation on biological systems, such as, for instance, the human skin or eye, in any natural or artificial environment. The method, which consists of a combination of radiation models, is explained here and the accuracy of its results is demonstrated. The effect of a natural skyline is shown for an Alpine ski area, where the UV irradiance even on a horizontal surface may increase due to reflection from snow by more than 10 percent. In contrast, in a street canyon the irradiance on a horizontal surface is reduced to 30% in shadow and to about 75% for a position in the sun.