Among the objectives of the Plan de Actuacion de Lucha Antigranizo, (P
ALA, Hail Suppression Activity Plan), is the on-going analysis of the
mechanisms operating during the formation of storms. There is a relati
onship between the processes at work in the interior of cloud masses a
nd the concentration of natural ice nuclei (IN) at ground level. Throu
gh the rigorous analysis of 954 measurements made with an isothermal c
loud chamber, it has been possible to establish that a dependence exis
ts 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 originat
ing from the Sahara desert. Furthermore, from the analysis of 4-day ba
ck trajectories, it has been observed that slow-moving maritime air ma
sses gradually become altered during their passage over the peninsula,
which causes an increase in the number of active IN. (C) 1998 Elsevie
r Science B.V. All rights reserved.