Geoscience Reference
In-Depth Information
Figure 11.42
Mean number of hours
of rain per month for four West African
stations. Also shown are types of rainfall,
mean annual totals (mm) and, in
parentheses, maximum recorded daily
rainfalls (m) for Conakry (August) and
Abidjan (June). Dots show the mean
monthly rainfall intensities (mm hr
-1
).
Note the pronounced little dry season
at Abidjan. Station locations are marked
on Figure 11.41.
Source
: From Hayward and Oguntoyinbo
(1987).
3
Squall lines
. Easterly waves vary greatly in intensity.
Some give rise to little cloud and rain, whereas others
have embedded squall lines when the wave extends
down to the surface, producing updrafts, heavy
rain and thunder. Squall line formation is assisted
where surface topographic convergence of the
easterly flow occurs (e.g. the Air Mountains, the
Fouta-Jallon Plateau). These disturbance lines travel
at up to 60 km hr
-1
from east to west across southern
West Africa for distances of up to 3000 km (but
averaging 600 km) between June and September,
yielding 40 to 90 mm of rain per day. Some coastal
locations suffer about forty squall lines per year,
which account for more than 50 per cent of the
annual rainfall (see Plate 29).
Annual rainfall decreases from 2000 to 3000 mm in
the coastal belt (e.g. Conakry, Guinea) to about 1000
mm at latitude 20°N (Figure 11.42). Near the coast,
more than 300 mm per day of rain may fall during the
rainy season but further north the variability increases
due to the irregular extension and movement of the
monsoon trough. Squall lines and other disturbances
give a zone of maximum rainfall located 800 to 1000 km
south of the surface position of the monsoon trough (see
Figure 11.40B). Monsoon rains in the coastal zone of
