Classification

An initial categorisation of air masses was made by Tor Bergeron in 1928. Primarily they are classed according to the latitude of the source area (i.e. its temperature) and secondarily according to whether the source area was continental or maritime. For instance, mP refers to an air mass of marine polar origin, coming from the southern ocean. There is sometimes a third subdivision in terms of the stability of the air mass. The last is indicated by the direction of movement: an air mass travelling to higher latitudes, for instance, is warmer than the surface and therefore labelled 'w'; such an air mass becomes increasingly stable as its base becomes

Table 13. 1 Kinds of air mass in the southern hemisphere, using two-letter labels: 'c' refers to continental and 'm' to maritime air masses, from the equator 'E', temperate region 'T', polar region 'P' (i.e. high latitude) or Antarctica 'A'. Characteristic surface daily mean temperature (T°C), mixing ratio (r) and stability are listed for winter/summer

Approx latitude or source

Label

(C)

r (gAgl

cE

25/25

18/18

mE

25/25

19/19

cT

15/25

4/6

mT

18/22

14/17

cP*

0/10

3/5

mPt

4/8

5/8

cA

-40/-20

Equatorial (0-10°S) Subtropical (30°S) Polar (50°S) Antarctic moist neutral moist neutral stable/dry neutral stable stable/dry neutral moist neutral stable

* Found only in Patagonia.

t Called the maritime Southern in the southern oceans.

cooled. Conversely, the label 'k' (for 'kalt', German for 'cold') means an increasingly unstable air mass.

Dominant air masses in the southern hemisphere are labelled mP and mT, arising from subtropical highs over the oceans (Figure 12.1). The two kinds differ appreciably in temperature (Table 13.1). Areas south of about 35°S obtain most of their rain from northward excursions of mP air, while those north of 35°S receive it from mT air, except that in monsoonal areas, such as Darwin (Section 12.1), it comes from mE or cE air. A wintertime cP/ mP air mass in South America occasionally penetrates north of 20°S in the lee of the Andes (Section 13.3).

Air masses change character according to the surface they traverse. They are affected by surface heating or cooling, by wet or dry surfaces, by mixing with other air masses and by radiative cooling. In fact, the susceptibility to change is a weakness of the notion of definable air masses, and accounts for the reduced interest in the concept since the 1960s. Another reason is the fact that the atmosphere does not move en bloc, because the upper air travels faster, so that the stability of an air mass can change even in the absence of any changes at the surface. Also, there is the difficulty of categorising air whose properties do not match those in Table 13.1. Nevertheless, the concept remains helpful in describing climates (Chapter 15) and in explaining atmospheric 'fronts', for instance.

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