Page 14 - The World About Us

Monument Valley
1.1.1
2.1.2

What is the global distribution of hot deserts?

Hot deserts are found in the
tropics and sub-tropics where rainfall is Arid and semi-
below 250mm per year. These areas arid regions
are described as arid or extremely arid.
Aridity is mostly a product of the Tropic of
descending air of the Hadley and Ferrel Cancer
cells at the Sub-Tropical High Pressure
(STHP). This high pressure is created by
air moving from the tropopause Equator
towards the Earth’s surface. The air is
already dry, as most moisture will have
rained out at the ITCZ or Polar Front.
Tropic of
As there is no source of moisture in Capricorn
the upper atmosphere, the air remains
dry. This air, from the Ferrel and Hadley
cells, meets in the upper troposphere fig.33 Map of hot deserts and semi-arid regions.
and descends towards the ground
crea ng high pressure. skies allow most solar insola on to
reach the desert surface so fig.34 Rock pedestal in Egypt’s White
This falling air at the STHP begins to temperatures are high; reaching 45⁰C - Desert. They are the product of aeolian
warm as the air molecules are erosion by wind-blown sand and dust.
50⁰C in the day.
compressed together. This 'adiaba c'
warming results in any remaining The highest temperature ever
moisture evapora ng. Clouds don't recorded was in Death Valley,
form and so rainfall is low. California in 1913, where temperatures
reached a staggering 56.7°C in the
Figure 33 shows how most deserts shade. In contrast to day me highs,
are between 20⁰ and 30⁰ north and the lack of cloud cover at night can
south of the Equator. This is the global
mean temperatures fall rapidly, o en
zone dominated by the STHP. The clear
approaching freezing.
Other causes of desert aridity.

There are other causes of aridity reach, raining out nearer the coast.
which combine with the STHP to keep The world's largest desert, the
deserts so dry. Some deserts, such as Sahara, is affected by all these factors. It
the Atacama or Western Sahara, are is under the Sub-Tropical High Pressure
located in rain shadows, on the lee side zone, behind the rain shadow of the
of mountains. Air rising over the hills Atlas Mountains, and has a cold ocean
cools, clouds form and rain falls on the current running along its western shore. KEY TERMS
windward side. As the now drier air At its centre, parts of the Sahara are
descends on the lee side of the hills, it over two thousand kilometres from the Aeolian erosion: erosion caused by the
warms and any remaining moisture sea. material carried by the wind.
evaporates, leaving clear skies and li le
rainfall. Desert: a place receiving less than
250mm of rain per year.
Other deserts are located close to
the cold seas. The cold water suppresses Rain shadow: an area on the lee side of
evapora on and causes fogs to form. mountain ranges which receives less
Most water precipitates over the sea, rainfall as a result of descending and
although some may dri inland each warming air.
morning bringing welcome moisture. Sub-Tropical High Pressure (STHP): the
Many deserts are at the centre of descending limbs of the Hadley and
their con nents, where moisture Ferrel cells of atmospheric circulation
evaporated over the sea is unlikely to Fig.35 Camenchaca fog, Atacama. creating a zone of high pressure at
approximately 30 North and South.

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Ecosystems have distinct distributions and characteristics.