Environmental Engineering Reference
In-Depth Information
Figure 7.1
Air mass source regions: (a) January, (b) July.
Source: After Crowe (1971).
Table 7.1 Average thermal properties of air masses
Air mass
Symbol
Properties
Mean temperature
(°C)
Specific humidity (g
kg
-1
)
Continental Arctic
Ac
Very cold, dry
-20
0·1
Continental Polar
Pc
Cold, dry
(winter)
-10
1·4
Maritime Polar
Pm
Cool, moist
5
4·4
Continental
Tropical
Tc
Warm, dry
25
11
Maritime Tropical
Tm
Warm, moist
20
17
Maritime
Equatorial
Em
Warm, very
moist
27
19
area. Four main types are recognized: Arctic (or 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, but it is a relatively slow
process. In addition, the air is made more stable, so rainfall is less likely (Figure 7.2).
Conversely, air moving towards the equator becomes warmer as it meets warmer
surfaces. As we saw in Chapter 4, warming of the lower layers of the air steepens the
lapse rate, making the air less stable and convectional showers more likely. With
instability, the process of transformation is more rapid.
Changes in air masses by these means are particularly marked in the mid-latitudes.
Here, depressions draw in air from several sources; the air is modified by the new
surfaces it encounters and is gradually mixed as it rises around the cyclone centre. Its
precise thermal properties will depend upon the origin of the air, its track and the speed
of its movement from the source area.
