Geoscience Reference
In-Depth Information
Table 1.2
Distribution of the arid zone by continent
(expressed as a percentage of the total global arid zone).
Table 1.4
Examples of arid zones with different climates
(classification as in Table 1.3).
Africa
31.9
Hot
Central Sahara; Arabia; Great Sandy Desert,
Australia
Asia
31.7
Australasia
10.8
Mild
Southern Sahara; Kalahari; Mexican; Simpson,
Australia
North America
12.0
South America
8.8
Cool
Northern Sahara; Turkish Steppes; Atacama;
Mojave
Europe
4.9
Cold
Canadian prairies; Gobi; Turkmenistan;
Chinese deserts
Source
: UNEP (1992).
Globally, arid areas embrace a range of annual temper-
ature regimes, affected by latitude, altitude and continen-
tality. Cloudsley-Thompson (1969, in Heathcote, 1983)
noted that the only common element of temperatures be-
tween different arid areas is their range. Meigs (1952)
divided arid lands into those that are hot all year round
and those with mild, cool and cold winters (Tables 1.3
and 1.4). Variations in temperature affect the seasonal
availability of moisture, by influencing evapotranspiration
rates and affecting the form of precipitation in relatively
high-latitude arid areas. For example, in the arid areas of
Canada and central Asia, winter snowfall forms an impor-
tant component of the annual precipitation budget.
1.6.1
Atmospheric stability
The major cause of aridity worldwide are the subtropical
high-pressure belts: zones of descending, stable air. Trop-
ical and subtropical deserts cover about 20 % of the global
land area (Glennie, 1987). In these areas large arid zones
are composed of central arid areas surrounded by rela-
tively small, marginal, semi-arid and dry-subhumid belts.
Precipitation is very unreliable and largely associated with
the seasonal movements of the intertropical convergence
zone.
1.6.2
Continentality
1.6
Causes of aridity
Distance from the oceans prevents the penetration of rain-
bearing winds into the centre of large continents, for exam-
ple in central Asia. Precipitation and evapotranspiration
are both usually lower than in arid areas owing their ori-
gins to atmospheric stability, while cold winters are com-
mon. In other continents, the failure of dominant easterly
trade winds to penetrate to continental interiors (Thomp-
son, 1975), such as in southern Africa, contributes to the
continentality effect. Relatively small arid areas are sur-
rounded by an extensive zone of semi-aridity.
Aridity is characterised by net surface water deficits. It
results from climatic, topographic and oceanographic fac-
tors that prevent moisture-bearing weather systems reach-
ing an area of the land surface. Four main influences can
be identified, which are not mutually exclusive.
Table 1.3
Arid land climates.
Percentage of
Mean temperature
(
◦
C)
arid lands
1.6.3
Topography
Coldest
Warmest
month
month
Arid areas can occur in the rain shadow of mountain barri-
ers. The Rockies contribute a rain shadow effect in western
North America, while in Australia the penetration of east-
erly trade winds to the interior is further inhibited by the
north-south orientation of the Great Divide. Aridity pri-
marily due to atmospheric stability or continentality can
Hot
43
10-30
>
30
Mild winter
18
10-20
10-30
Cool winter
15
0-10
10-30
Cold winter
24
<
0
10-30
