Geoscience Reference
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Icelandic low-pressure cells (see Chapter 7C.2B). The
relative strength of these two pressure systems appears
to affect the rainfall of both northwest Africa and the
sub-Saharan zone.
cool maritime air perpetually bathes the lower western
slopes of the Andes in mist and low stratus cloud, and
Swakopmund (southwest Africa) has an average of 150
foggy days a year, little rain falls on the coastal low-
lands. Lima (Peru) has a total mean annual precipitation
of only 46 mm, although it receives frequent drizzle
during the winter months (June to September), and
Swakopmund in Namibia has a mean annual rainfall of
16 mm. Heavier rain occurs on the rare instances when
large-scale pressure changes cause a cessation of the
diurnal sea breeze or when modified air from the South
Atlantic or South Indian Ocean is able to cross the
continents at a time when the normal dynamic stability
of the trade winds is disturbed. In southwest Africa, the
inversion is most likely to break down during either
October or April, allowing convectional storms to form,
and Swakopmund recorded 51 mm of rain on a single
day in 1934. Under normal conditions, however,
the occurrence of precipitation is limited mainly to the
higher seaward mountain slopes. Further north, tropical
west coast locations in Angola and Gabon show that
cold upwelling is a more variable phenomenon in both
space and time; coastal rainfall varies strikingly with
changing sea-surface temperatures (Figure 11.55). In
South America, from Colombia to northern Peru, the
diurnal tide of cold air rolls inland for some 60 km,
rising up the seaward slopes of the western Cordillera
and overflowing into the longitudinal Andean valleys
like water over a weir (Figure 11.56). On the west-
facing slopes of the Andes of Colombia, air ascending
or banked up against the mountains may under suitable
conditions trigger off convectional instability in the
overlying trades and produce thunderstorms. In south-
west Africa, however, the 'tide' flows inland for some
130 km and rises up the 1800-m Namib Escarpment
without producing much rain because convectional
instability is not generated and the adiabatic cooling of
the air is more than offset by radiational heating from the
warm ground.
H OTHER SOURCES OF CLIMATIC
VARIATIONS IN THE TROPICS
The major systems of tropical weather and climate have
now been discussed, yet various other elements help to
create contrasts in tropical weather in both space and
time.
1 Cool ocean currents
Between the western coasts of the continents and the
eastern rims of the subtropical high-pressure cells
the ocean surface is relatively cold (see Figure 7.33).
This is the result of: the importation of water from
higher latitudes by the dominant currents; the slow
upwelling (sometimes at the rate of about 1 m in twenty-
four hours) of water from intermediate depths due to
the Ekman effect (see Chapter 7D.1); and the coastal
divergence (see Figure 7.31). This concentration of
cold water gently cools the local air to dew-point. As
a result, dry, warm air degenerates into a relatively
cool, clammy, foggy atmosphere with a comparatively
low temperature and little range along the west coast
of North America off California, off South America
between latitudes 4 and 3°S, and off southwest Africa
(8 and 32°S). Thus Callao, on the Peruvian coast, has
a mean annual temperature of 19.4°C, whereas Bahia
(at the same latitude on the Brazilian coast) has a corre-
sponding figure of 25°C.
The cooling effect of offshore cold currents is not
limited to coastal stations, as it is carried inland during
the day at all times of the year by a pronounced sea
breeze effect (see Chapter 6C.2). Along the west coasts
of South America and southwest Africa the sheltering
effect from the dynamically stable easterly trades aloft
provided by the nearby Andes and Namib Escarpment,
respectively, allows incursions of shallow tongues of
cold air to roll in from the southwest. These tongues
of air are capped by strong inversions at between 600
and 1500 m, reinforcing the regionally low trade wind
inversions (see Figure 11.6) and thereby precluding the
development of strong convective cells, except where
there is orographically forced ascent. Thus, although the
2 Topographic effects
Relief and surface configuration have a marked effect on
rainfall amounts in tropical regions, where hot, humid
airmasses are frequent. At the southwestern foot of
Mount Cameroon, Debundscha (9-m elevation) receives
11,160 mm yr -1 on average (1960 to 1980) from the
southwesterly monsoon. In the Hawaiian Islands, the
mean annual total exceeds 7600 mm on the mountains,
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