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Among the objectives of the Plan de Actuacion de Lucha Antigranizo, (PALA, Hail Suppression Activity Plan), is the on-going analysis of the mechanisms operating during the formation of storms. There is a relationship between the processes at work in the interior of cloud masses and the concentration of natural ice nuclei (IN) at ground level. Through the rigorous analysis of 954 measurements made with an isothermal cloud chamber, it has been possible to establish that a dependence exists between the background concentration of IN and the type of air mass. The work shows that the concentration of IN in the samples analyzed in Leon was, on average, higher for continental than for maritime air masses, with a substantial presence of continental air masses originating from the Sahara desert. Furthermore, from the analysis of 4-day back trajectories, it has been observed that slow-moving maritime air masses gradually become altered during their passage over the peninsula, which causes an increase in the number of active IN. (C) 1998 Elsevier Science B.V. All rights reserved.