Let us consider,
for example, the day of September 12. 1999 mentioned above. First of all,
the synoptic situation at 6 hours UTC shows an area of slightly low pressure
centered on Sicily, qualified as "shallow depression", propitious to local
meteorological phenomena. Moreover, according to our observations, strong
ascending currents are forming over the continental coast. One can assume
that they will produce a sea breeze whose influence may extend beyond the
Lipari archipelago since the wind there blows towards the continental coast
(west wind). Insolation is such that clouds develop and form cumulo-nimbus.
In the evening, as the breeze effect disappears and the prevailing south-east
wind rises again, these thunderclouds are driven to the islands were they
cause the famous squall. It is interesting that the satellite analysis of
September 12. 1999 confirmed the presence of these local thunderstorms.
analysis, 12TH september 1999.
From a temporal point of view the observed chronology confirms this hypothesis
: ascending currents in the afternoon, clouds progressively extending into
cumulo-nimbus, east wind rising again in the late afternoon when the breeze
drops, squall breaking at about 8 p.m. for a short time, since clouds are
just passing over the archiopelago. From a spatial point of view, on the
contrary, the situation observed verifies this hypothesis but reveals a
strong phenomenon, as already suggested by the progressive formation of
cumulo-nimbus. Moreover, the Lipari Islands are located some 30 or 40 miles
off the continental coast and the west wind observed in the islands suggests
a thermal breeze extent that is typical of tropical zones. This could be
due to the large vertical extent of the ascending currents indicated by
the formation of cumulo-nimbus clouds. In addition, according to the direction
of the smokes issuing from the volcano that culminates at 1000 meters the
width of this breeze is still important at this distance. A so wide limit
layer is however quite possible.