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The meteorological features of the Canary Islands are dominated by the strength and
position of the subtropical anticyclone, which is normally situated over the Azores Islands
to the northwest of the station. A subsidence inversion layer is usually present below 2000
m above sea level, resulting in two distinct air masses. The lower air mass is cooler and
more moist, while the upper one is warmer and drier. This occurs about 90% of the time in
summer and 70% of the time in winter. The inversion layer prevents the arrival at the
Observatory of polluted air masses from the lower layer because the height of the inversion
top is normally below the station altitude (Hernandez et al., 1988).
The northeastward movement of the anticyclone, sometimes as far as western
Europe, gives rise to the arrival of polar air masses to the Canary Islands. These air masses
pass over the European continent and/or northwestern Africa en route to Izafia. When this
situation occurs in winter, the weather at Izaiia station is dry, sunny and chilly. During
summer, some short-duration thermal low-pressure systems appear over the Sahara Desert.
This circumstance added to the movement of the so-called Intertropical Convergence Zone
northwards, causes the arrival of hot and dry airflows, loaded with dust, at Izaiia (Dubief,
1979). Sometimes, the influence of a low-pressure system produces airflows of maritime
polar or subtropical origin, depending on the strength and relative position of the cyclone.
Heavy rain and decreased atmospheric stability usually result.
To establish a climatology of trajectories, the directions of the trajectories were
classified according to the meteorological features of the Canary Island region and to the
geographic locations of pollutant source areas. A 2000-km radius circle was drawn with its
center at Izaiia Observatory in order to distinguish between long- and short-range
transports. Trajectories coming from beyond 2000 km represent the strongest flow and have
a minimum average wind speed of 5 mls along the 5-day trajectory. The classification
criterion is shown in Fig. 1 and is described as follows:
• Sector one (270°-10°) Air masses from North Atlantic excluding Europe .
• Sector two (10°-60°) Air masses (potentially polluted) arriving from Europe.
• Sector three (60°-150°) Saharan air masses sometimes carrying high dust concentrations
(Carlsson and Prospero, 1972; Prospero et al., 1979; Schutz, 1979).
• Sector four (150°-270°) Air masses arriving from the equatorial Atlantic Ocean. These air
masses, as well as those in sector 1, usually reflect background measurements at the Izafia
The meteorological features of the Canary Islands are dominated by the strength and
position of the subtropical anticyclone, which is normally situated over the Azores Islands
to the northwest of the station. A subsidence inversion layer is usually present below 2000
m above sea level, resulting in two distinct air masses. The lower air mass is cooler and
more moist, while the upper one is warmer and drier. This occurs about 90% of the time in
summer and 70% of the time in winter. The inversion layer prevents the arrival at the
Observatory of polluted air masses from the lower layer because the height of the inversion
top is normally below the station altitude (Hernandez et al., 1988).
The northeastward movement of the anticyclone, sometimes as far as western
Europe, gives rise to the arrival of polar air masses to the Canary Islands. These air masses
pass over the European continent and/or northwestern Africa en route to Izafia. When this
situation occurs in winter, the weather at Izaiia station is dry, sunny and chilly. During
summer, some short-duration thermal low-pressure systems appear over the Sahara Desert.
This circumstance added to the movement of the so-called Intertropical Convergence Zone
northwards, causes the arrival of hot and dry airflows, loaded with dust, at Izaiia (Dubief,
1979). Sometimes, the influence of a low-pressure system produces airflows of maritime
polar or subtropical origin, depending on the strength and relative position of the cyclone.
Heavy rain and decreased atmospheric stability usually result.
To establish a climatology of trajectories, the directions of the trajectories were
classified according to the meteorological features of the Canary Island region and to the
geographic locations of pollutant source areas. A 2000-km radius circle was drawn with its
center at Izaiia Observatory in order to distinguish between long- and short-range
transports. Trajectories coming from beyond 2000 km represent the strongest flow and have
a minimum average wind speed of 5 mls along the 5-day trajectory. The classification
criterion is shown in Fig. 1 and is described as follows:
• Sector one (270°-10°) Air masses from North Atlantic excluding Europe .
• Sector two (10°-60°) Air masses (potentially polluted) arriving from Europe.
• Sector three (60°-150°) Saharan air masses sometimes carrying high dust concentrations
(Carlsson and Prospero, 1972; Prospero et al., 1979; Schutz, 1979).
• Sector four (150°-270°) Air masses arriving from the equatorial Atlantic Ocean. These air
masses, as well as those in sector 1, usually reflect background measurements at the Izafia