Visual surveillance in outdoor environments requires the monitoring of both objects and events. The analysis is generally driven by the target application which, in turn, determines the set of relevant events and objects to be analyzed. In this paper we concentrate on the analysis of outdoor scenes, in particular for vehicle traffic control. In this scenario, the analysis of weather conditions is considered to signal particular and potentially dangerous situations like the presence of snow, fog, or heavy rain. The developed system uses a statistical framework based on the mixture of Gaussians to identify changes both in the spatial and temporal frequencies which characterize specific meteorological events. Several experiments performed on standard databases and real scenes demonstrate the applicability of the proposed approach.