The Work of Wind and Deserts - The Changing Earth: Exploring Geology and Evolution - page 389

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Figure 15.10 Barchan Dunes

Wind

a Barchan dunes form in areas that have a
limited amount of sand, a nearly constant wind
direction, and a generally fl at, dry surface with
little vegetation. The tips of barchan dunes point
downward.

Image not available due to copyright restrictions

AIR-PRESSURE BELTS AND
GLOBAL WIND PATTERNS
To understand the work of wind and the distribution
of deserts, we need to consider the global pattern of air-
pressure belts and winds, which are responsible for Earth's
atmospheric circulation patterns. Air pressure is the den-
sity of air exerted on its surroundings (that is, its weight).
When air is heated, it expands and rises, reducing its mass
for a given volume and causing a decrease in air pressure.
Conversely, when air is cooled, it contracts and air pressure
increases. Therefore, those areas of Earth's surface that

c A ground-level view of several barchan dunes.

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Figure 15.11 Longitudinal Dunes

Wind Wind

a Longitudinal dunes form long, parallel ridges of sand aligned
roughly parallel to the prevailing wind direction. They typically form
where sand supplies are limited.

b Longitudinal dunes, 15 m high, in the Gibson Desert, west central
Australia. The bright blue areas between the dunes are shallow pools of
rainwater, and the darkest patches are areas where the Aborigines have
set fi res to encourage the growth of spring grasses.