Precipitation Chemistry and Air Pollution
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a
10/ 4/90
c
d
-'.
1?/ 3/89
12/ 4/91
Fig. 14.2. Examples of computed back trajectories at three levels (L 1000 hPa; M 850 hPa;
U 700 hPa) for episodes of a oceanic, b African, c continental, and d mixed continental and African origins
work referred to above, back trajectories were calculated for 23 episodes of
several precipitation days chosen to minimize local influences (Carratahi
1993; Carratalci et al. 1996). Three main routes of transport were found:
(1) oceanic, (2) continental, and (3) African. Continental rains produced the
greatest amount of precipitation (62%), followed by oceanic (20%) and African rains (18%).
Oceanic trajectories were associated with fronts crossing the Iberian Peninsula from W to E (coming from the Atlantic Ocean), and were characterized by rains of pH close to neutral and low ionic content (Fig. 14.2a). Continental fronts were characterized by high pressures over south Scandinavia or
eastern Europe and low pressures around the strait of Gibraltar, with continental cold air from eastern Europe being channelled over the Mediterra-