Geoscience Reference
In-Depth Information
2 Warm airmasses
generates a steep lapse rate, but despite its instability
the low moisture content prevents the development of
cloud and precipitation. In the southern hemisphere, cT
air is rather more prevalent in winter over the subtropical
continents, except for South America. In summer, much
of southern Africa and northern Australia is affected by
mT air, while there is a small source of cT air over
Argentina (see Figure 9.4B).
The characteristics of the primary airmasses are
illustrated in Figures 9.3 and 9.5. In some cases, move-
ment away from the source region has considerably
affected their properties, and this question is discussed
below (see p. 181).
Source regions can also be defined from analysis of
airstreams. Streamlines of the mean resultant winds (see
Note 1) in individual months may be used to analyse
areas of divergence representing airmass source regions,
downstream airflow and the confluence zones between
different airstreams. Figure 9.6A shows airmass domi-
nance in the northern hemisphere in terms of annual
duration. Four sources are indicated: the subtropical
North Pacific and North Atlantic anticyclones, and
their southern hemisphere counterparts. For the entire
year, air from these sources covers at least 25 per
cent of the northern hemisphere; for six months of
each year they affect almost three-fifths of the hemi-
sphere. In the ocean-dominated southern hemisphere,
These have their origins in the subtropical high-pressure
cells and, during the summer season, in the bodies of
warm surface air that characterize the heart of large land
areas.
The tropical (T) sources are: (1) maritime (mT),
originating in the oceanic subtropical high-pressure
cells; (2) continental (cT), either originating from the
continental parts of these subtropical cells (e.g. as does
the North African
Harmattan
); or (3) associated with
regions of generally light variable winds, assisted by
upper tropospheric subsidence, over the major conti-
nents in summer (e.g. Central Asia). In the southern
hemisphere, the source area of mT air covers about half
of the hemisphere. There is no significant temperature
gradient between the equator and the oceanic sub-
tropical convergence at about 40°S.
The mT type is characterized by high temperatures
(accentuated by the warming due to subsidence), high
humidity of the lower layers over the oceans, and stable
stratification. Since the air is warm and moist near the
surface, stratiform cloud commonly develops as the air
moves poleward from its source. The continental type
in winter is restricted mainly to North Africa (see Figure
9.2), where it is a warm, dry and stable airmass. In
summer, warming of the lower layers by the heated land
Figure 9.5
The average vertical temperature
structure for selected airmasses affecting
North America in summer.
Sources
: After Godson (1950), Showalter (1939),
and Willett.
