Solar irradiance is a highly variable and fluctuating resource. This characteristic directly and strongly affects the performance of solar energy systems, such as the electricity production from photovoltaic (PV) systems or solar thermal power plants. This entry describes databases and appropriate methods to characterize spatial and temporal variability in solar radiation. The regional distribution of solar energy depends on the meteorological situation characterized by clouds, aerosols, and water vapor content, but also on topography and ground albedo. Within this entry the focus is laid on cloud-induced fluctuations. The spatial variability caused by the topography of the terrain is not accounted for here, since it is the topic of another entry in this encyclopedia.