This contribution shows some results obtained analysing a long time series of meteorological data taken at two of the most important astronomical sites in the world: the site of Observatorio del Roque de Los Muchachos (ORM, Canary Islands) and the site of Paranal Observatory (Chile). The aim of this analysis is to derive correlations between synoptic parameters and the physical properties of the atmosphere in both short and long time baseline in order to have the best scientific result from the astronomical observations.