Geoscience Reference
In-Depth Information
Arid Landscapes
2. Rain Shadow Effect —Another environmental factor asso-
ciated with deserts is the rain shadow effect. If you recall
from Chapter 7, a rain shadow occurs on the lee side of
mountain ranges where descending air warms adiabati-
cally. This process was outlined in Figure 7.22 and is
responsible for the large zones of arid and semi-arid
climates in the western United States. It also is related to
the presence of the Atacama Desert along the west coast
of South America.
3. Distance from Large Water Bodies —The last environmen-
tal variable that is related to dry regions is the proximity
to a water body such as an ocean. Locations deep within
continents tend to receive relatively small amounts of
moisture-laden air and can therefore be quite dry. An
excellent example of this kind of situation is the Taklam-
akan and Gobi deserts in Asia (Figure 18.1), as well as the
other large areas of arid and semi-arid lands that lie deep
within that continent.
A good way to begin investigating eolian processes is by
looking more closely at the geography and character of arid
environments because these areas are often most closely
associated with the influence of flowing air. This discussion
incorporates many of the topics covered earlier in the text—
atmospheric circulation, plant geography, soils, geology, and
fluvial processes. This part of the chapter seeks to provide you
with a good understanding of desert environments and their
geography.
Figure 18.1 shows the geography of arid and semi-arid
regions on Earth. Such climates occur in both warm and cold
settings and collectively form the largest climate region on the
planet, one that covers perhaps as much as 35% of Earth's land
surface. Within the Köppen climate system (Figure 9.2), these
climate regions fall within the arid desert (BW) and semi-arid
steppe (BS) climate categories. The spatial distribution of these
dry regions is based on one of three factors:
1. Dominance of Subtropical High-Pressure Systems
In areas located between about 15° and 35° N and S
latitude, strong high-pressure systems dominate the
weather for all or significant portions of the year. These
descending air masses are components of the Hadley
cells (shown in Figures 6.16 and 6.19) that circulate
within the atmosphere in the tropical latitudes. The
subtropical high-pressure systems are very closely
associated with the largest deserts on Earth, such as
the Sahara and Kalahari in Africa, the Simpson Desert
in Australia, and the Chihuahuan Desert in Mexico
(Figure 18.1).
Desert Geomorphology
A key element that all deserts and semi-arid regions share is a
relative lack of vegetation when compared to more humid ar-
eas. As a result, descriptive terms such as sparse and barren
are frequently used to describe dry regions. This relationship of
climate and vegetation was thoroughly explored in Chapters 9
and 10 and can be seen in numerous photographs of desert
environments, such as Figures 9.5 and 10.17.
Although the barren nature of these environments may at
first seem unattractive or even foreboding, desert landscapes are
beautiful in their own way, with spectacular rock outcrops and
panoramic vistas. In the context of geomorphology, deserts are
80°
150°
120°
60°
60°
120°
150°
80°
Asia
60°
60°
North
America
Gobi
Europe
40°
40°
Sonoran
Chihuahuan
Takla-
makan
Sahara
20°
20°
Sind
Africa
Rub al-Khali
Equator
0 °
South
America
Atacama
Namib
Australia
Hot arid (desert)
climates
Semi-arid
climates
Simpson
20°
Kalahari
20°
40°
40°
60°
60°
80°
80°
Figure 18.1 Deserts of the world. Arid and semi-arid deserts collectively cover about one-third of the Earth's land
surface.
The largest deserts are associated with the belt of subtropical high-pressure systems.
 
 
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