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westerly winds intervening between the two
Trade Wind belts ( Figures 7.12 and 7.14 ). This
westerly system is well marked over Africa and
South Asia in the Northern Hemisphere summer,
when thermal heating over the continents assists
the northward displacement of the Equatorial
Trough (see Figure 11.1 ). Over Africa, the
westerlies reach 2-3km and over the Indian Ocean
to 5-6km. In Asia, these winds are known as
the 'Summer Monsoon', but this is now
recognized to be a complex phenomenon, the
cause of which is partly global and partly regional
in origin (see Chapter 11C). The equatorial
westerlies are not simply trades of the opposite
hemisphere that recurve (due to the changed
direction of the Coriolis deflection) on crossing
the equator. There is on average a westerly
component in the Indian Ocean at 2-3
3 The mid-latitude (Ferrel)
westerlies
These are the winds of the mid-latitudes emanating
from the poleward sides of the subtropical high
pressure cell (see Figure 7.12 ). They are far more
variable than the trades in both direction and
intensity, since in these regions the path of air
movement is frequently affected by transient cells
of low and high pressure, which, although guided
by the meandering long Rossby Waves discussed
earlier, travel generally eastward. In addition, in the
Northern Hemisphere the preponderance of land
areas with their irregular relief and changing
seasonal pressure patterns tend to obscure the
generally westerly airflow. The Isles of Scilly, off
southwest England, lying in the southwesterlies,
record 46 percent of winds from between south-
west and northwest, but fully 29 percent from the
opposite sector, between northeast and southeast.
The westerlies of the Southern Hemisphere are
stronger and more constant in direction than
those of the Northern Hemisphere because the
S in June
and July and at 2-3°N in December and January.
Over the Pacific and Atlantic Oceans, the
ITCZ does not shift sufficiently far from the
equator to permit the development of this westerly
wind belt.
°
60°N
60°N
30°N
30°N
0
30°S
30°S
January
July
60°S
60°S
Figure 7.14 Distribution of the equatorial westerlies in any layer below 3km (about 10,000ft) for January and July.
Source: After Flohn in Indian Meteorological Department (1960).
 
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