UV irradiance on arbitrarily oriented surfaces: variation with atmospheric and ground properties

The dependency of erythernal weighted solar UV irradiance on tilted surfaces with different orientation is investigated with respect to solar zenith angle, variable atmospheric conditions and albedo of the location. For overcast conditions or a cloud in front of the sun, the irradiance on a horizontal surface in general is largest, with the consequence that it is reduced for surfaces with any tilted position. For cloud free conditions the irradiance on a tilted plane, in comparison to that on a horizontal flat surface, is increased for orientations towards the sun, but reduced for other orientations. The increase is strongest for low sun in combination with clear atmosphere and high ground albedo, as is typical for snow covered mountain conditions.