Geoscience Reference
In-Depth Information
(A)
(B)
0°
180°
mT
cT
mT
mT
mP
mP
trans
cT
cT
trans
cP
cA
(mA)
90°W
90°E
90°W
90°E
mT
cP
or
mT
mP
trans
cT
mP
mP
mT
cT
cT
MONSOON
AIR
cT
mT
MONSOON
AIR
E Q U
0°
180°
Figure 9.4
Air masses in summer. A: Northern Hemisphere; B: Southern Hemisphere.
Sources: A: After Petterssen (1950) and Crowe (1965). B: After Taljaard et al. (1969) and Newton (1972).
equator and the oceanic subtropical 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 air
mass. In summer, warming of the lower layers by
the heated land generates a steep lapse rate, but
despite its instability the low moisture content
prevents the development of cloud and precipita-
tion. In the Southern Hemisphere, cT air is rather
more prevalent in winter over the subtropical
continents, except in 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 air masses are
illustrated in
Figures 9.3
and
9.5
. In some cases,
movement away from the source region has
considerably affected their properties, and this
question is discussed below (see p. 231).
Source regions can also be defined from
analysis of airstreams. Streamlines of the mean
resultant winds (see Note 1) in individual
months can be used to analyse areas of divergence
representing air-mass source regions, downstream
airflow, and the confluence zones between
different airstreams.
Figure 9.6A
shows air-mass
dominance 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 percent of the Northern
Hemisphere; for six months of each year they
affect almost three-fifths of the hemisphere. In
the ocean-dominated Southern Hemisphere, the
airstream climatology is much simpler (
Figure
9.6B
). Source areas are the oceanic subtropical
anticyclones. Antarctica is the major continental
source, with another mainly in winter over
Australia.
C AIR-MASS MODIFICATION
As an air mass moves away from its source region
it is affected by different heat and moisture
