The intensity of solar UV radiation at the Earth's surface is highly variable. The most important parameters for cloudless conditions are in the sequence of their significance: solar zenith angle, total ozone content of the atmosphere, amount and type of aerosols, albedo of the surrounding and altitude above sea level. Based on measurements, the effects of these parameters are discussed individually. Furthermore, clouds usually attenuate the irradiance, only in exceptional cases they can lead to an increase over short time periods. The attenuation by clouds is less strong for UV radiation compared with radiation from the whole spectral range.