The climatology of hail in the plain of Friuli Venezia Giulia

Hail is quite common on the plain of Friuli Venezia Giulia (FVG), for this reason a network of polystyrene hailpads was established in 1988. This network remains active today monitoring hail frequency and aspect. We use the data of this network to define the hail climatology for Friuli Venezia Giulia, in particular, its spatial and temporal frequency, and to give a physical interpretation of the observed behavior. The main features that emerge from the analysis are that, even if hail is a local phenomenon, its climatology can be interpreted only by accounting for the mesoscale mechanisms triggered by the interplay between synoptic perturbations and the complex orography of Friuli Venezia Giulia. The analysis also reveals the importance of the lower level (say below 1500 m) water vapor amount for the occurrence of hail. The yearly frequency of hail days in Friuli was also compared with that of other regions and Countries, as well as with the time series of the North Atlantic Oscillation index. The results reveal some significant, even if weak, connections. (C) 2003 Elsevier B.V. All rights reserved.