442
HPa level and 8% at the 700-HPa level. In general, curved trajectories are associated with
non-polluted air coming from the North Atlantic Ocean. However, in rare cases, these air
masses could travel over some continental areas like the southwestern Iberian Peninsula or
western Africa over I or 2 days. Percentages of curved trajectories traveling over some
continental areas were lower even than those of the CLT category. The distribution of
percentages of all 850-HPa trajectories differs from that of the 700-HPa level mainly
because of the altitude of trade-wind inversion over the Canary Islands area. This altitude
oscillates around the 850-HPa surface (Huetz de Lemps, 1969). It means that in some cases
the inversion is sited between the 700 and 850 Ha surface.
The major features observed in the total 5-year study for each individual year are
shown by analyzing the percentages of a given category plotted for each individual year in
Figs 3 and 4. Despite the relatively low year-to-year variability, the differences between
1983 and 1986 must be pointed out. During 1983, trajectories from sector 1 predominated
clearly at the 850- and 700-HPa levels, whereas the percentages were less than that for all 5
years. Alternatively, for 1986, the number of cases of trajectories at the 850-HPa level
coming from sector 1 is the minimum. However, the percentage of the CT category is maximum as a result of a higher intensity Azores anticyclone. These features are similar, but not
so much as at the 700-HPa level for 1986. As during 1986, percentages of sector 1 were
low in 1985 but high percentages of curved trajectories were observed. However, percentages of local trajectories (especially CLT trajectories at 700-HPa level) and trajectories
from sector 3 increased significantly from the mean values. Air masses coming from the
North Atlantic area, i.e. air masses following trajectories sorted as sector 1 or CT,
predominated during 1987 at the 850-HPa level. Trajectories of sector 4 at both levels also
scored the highest percentages during 1987 for the whole 1983-1987 period. Percentages of
each category during 1984 are closest to the mean values for both levels.
Although not included here, percentages of trajectories of a given type were
obtained and plotted for each month over the 5-year period (Sancho et aI., 1992), so as an
analysis was made of both 850- and 700-HPa synoptic weather maps corresponding to the
months with maximum frequencies for each trajectory classification, to ascertain the cause
of these common pathways by determining the synoptic patterns responsible for them.
HPa level and 8% at the 700-HPa level. In general, curved trajectories are associated with
non-polluted air coming from the North Atlantic Ocean. However, in rare cases, these air
masses could travel over some continental areas like the southwestern Iberian Peninsula or
western Africa over I or 2 days. Percentages of curved trajectories traveling over some
continental areas were lower even than those of the CLT category. The distribution of
percentages of all 850-HPa trajectories differs from that of the 700-HPa level mainly
because of the altitude of trade-wind inversion over the Canary Islands area. This altitude
oscillates around the 850-HPa surface (Huetz de Lemps, 1969). It means that in some cases
the inversion is sited between the 700 and 850 Ha surface.
The major features observed in the total 5-year study for each individual year are
shown by analyzing the percentages of a given category plotted for each individual year in
Figs 3 and 4. Despite the relatively low year-to-year variability, the differences between
1983 and 1986 must be pointed out. During 1983, trajectories from sector 1 predominated
clearly at the 850- and 700-HPa levels, whereas the percentages were less than that for all 5
years. Alternatively, for 1986, the number of cases of trajectories at the 850-HPa level
coming from sector 1 is the minimum. However, the percentage of the CT category is maximum as a result of a higher intensity Azores anticyclone. These features are similar, but not
so much as at the 700-HPa level for 1986. As during 1986, percentages of sector 1 were
low in 1985 but high percentages of curved trajectories were observed. However, percentages of local trajectories (especially CLT trajectories at 700-HPa level) and trajectories
from sector 3 increased significantly from the mean values. Air masses coming from the
North Atlantic area, i.e. air masses following trajectories sorted as sector 1 or CT,
predominated during 1987 at the 850-HPa level. Trajectories of sector 4 at both levels also
scored the highest percentages during 1987 for the whole 1983-1987 period. Percentages of
each category during 1984 are closest to the mean values for both levels.
Although not included here, percentages of trajectories of a given type were
obtained and plotted for each month over the 5-year period (Sancho et aI., 1992), so as an
analysis was made of both 850- and 700-HPa synoptic weather maps corresponding to the
months with maximum frequencies for each trajectory classification, to ascertain the cause
of these common pathways by determining the synoptic patterns responsible for them.