Environmental Engineering Reference
In-Depth Information
Figure 3.6
Seasonal changes in the position of the ITCZ in Africa
occasion, the dust is so thick that visibility is
reduced to less than 1000 m, and, in combination
with the break-up of radio transmissions caused
by the high concentration of aerosols, this
disrupts local air traffic. The scattering of solar
radiation by the Harmattan haze also has broader
implications for such activities as agriculture,
solar energy engineering and environmental
planning (Adetunji
et al.
1979). At the personal
level, the low humidity of the dusty air
contributes to discomfort in the form of dry skin,
sore throats and cracked lips.
The ITCZ moves north again during the
northern summer, and by July and August has
reached its most northerly location at about 20°N
(see Figure 3.6). Hot, moist, maritime tropical
(mT) air flows in behind it, bringing the rainy
season. In the east, the moisture is provided by
air masses from the Indian Ocean as part of the
Asiatic monsoon system, while in the west it
arrives in the south westerly flow off the South
Atlantic. Although relatively simple to describe
governed by seasonal fluctuations in the position
of the ITCZ. Dry conditions are associated with
hot, continental tropical (cT) air, from the Sahara
in the west and the Arabian Peninsula in the east,
which moves in behind the ITCZ as it migrates
southwards during the northern hemisphere's
winter. At its most southerly extent, in January
or February, the ITCZ remains at about 8 degrees
north of the equator in West Africa, but curves
sharply southwards across the centre of the
continent to reach 15-20°S in East Africa (see
Figure 3.6). Apart from East Africa, which
receives some precipitation brought in off the
Indian Ocean by the north-east trade winds of
the winter monsoon, most of the northern part
of the continent experiences its dry season at that
time. All of West Africa, beyond a narrow strip
some 200 km wide along the coast, is under the
influence of a north-easterly airflow, from the
central Sahara. This is known locally as the
Harmattan—a hot, dry wind, which carries with
it large volumes of fine dust from the desert. On
