Geoscience Reference
In-Depth Information
it is possible to classify air masses on the basis of their
source area. Four main types are recognized: Arctic (and
Antarctic), Polar, Tropical and Equatorial, and these are
further subdivided into continental (for those forming
over large land masses) and maritime (for those
developing over the oceans) ( Table 7.1 , Figure 7.1 ).
As the air mass moves away from its source area its
character changes, owing to the influence of the new
underlying surface. Air moving towards the poles
generally comes into contact with cooler surfaces. This
causes it to be cooled from below, so that it may become
saturated, with the result that low clouds are formed,
LONGITUDE
150
120
90
60
30
0
30
60
90
120
150
180
Ac
80
80
(a) January
60
Pc
Pc
40
40
Tc
20
20
Tm
Tm
Tc
Tm
Tm
0
0
Tm
Tm
20
20
Tc
Tm
Tc
Tc
Tm
40
40
Maritime Polar
Maritime Equatorial
60
60
Ac
90
60
30
0
30
60
90
150
120
120
150
180
LONGITUDE
150
120
90
60
30
0
30
60
90
120
150
180
Ac
80
80
(b) July
60
40
40
Tc
Tm
Tm
Tc
Tm
20
20
0
0
Tm
Tc
Tm
Tc
Tm
Tm
20
20
Tc
40
40
Maritime Polar
Maritime equatorial
SW monsoon air
60
60
Ac
150
120
90
60
30
0
30
60
90
120
150
180
Figure 7.1 Air mass source regions, (a) January, (b) July. Ac Continental Arctic; Pc Continental Polar; Tc Continental Tropical;
TmMaritime Tropical.
 
 
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