Desert -- The Formation of
Before looking at the creation of deserts, let' s have a look at the rainfall cycle:
Evaporation lifts pure water up from the sea while the salt remains in the sea. Onshore
winds from the oceans carry air that is rich in moisture over the land. Air rises and
cools under the influence of upcurrents created by the warmer land surface. Water vapour
condense and falls as rain. It provides vital sustenance to plants and animals before
returning to the sea through ground water, streams and rivers.
Theoretically, desert can be created by removing
the cloud cover. It is believed that a cloud contains less than an inch (2.5cm) of
precipitable moisture fails to produce rain.
| Deserts occurs in 2 broad belts,
at 20-30o north and south of the Equator, along the Tropics of Cancer and
Capricorn. Desert areas develop under the influence of the quite permanent high-pressure
| The earth rotates around its axis
at about 1676 km per hour at the equator but at nearly zero speed at the poles. Hot air at
the equator rises and spreads north and south before cooling and it condenses and releases
its moisture over the tropical zones. An equatorial zone of low atmospheric pressure is
created. The two tropical zones are at high pressure. Nearer to the poles are 2
low-pressure belts of cold. The polar regions have descending air and high pressure. As
the denser air sinks towards the ground to the 2 subtropical high-pressure belts, wind is
created (easterly trades), which is hot and completely lacking of moisture, blowing across
the Sahara, the Middle East and the North America.
| As air
rushing into the low pressure regions rises, cools and then drops, these regions get most
of the rainfall and vice versa in the high pressure regions, where deserts can be
temperature influence the water extraction rate and global pattern of currents. Far from
the desert belts, established in the polar regions, cold currents move to the equator and
may come up against the edges of continents.
from the very cold ocean depths adds masses of cold water to them. Wind blowing to the
land over the cold water is cold and can only carry little moisture, bringing fog and mist
but not rain.
| Winds that
travelled a long distance lost their moisture (for example, the winds reaching Gobi and
the interior part of Sahara). Mountain barriers also extract water. (for example, the
comparatively low eastern highlands of Australian behind the Queensland coast) The
dehydration process from the wet mountain side to the dry desert side often occurs over a
horizontal distance of less than 161km (100 miles). That' s why some wettest and driest
area can be quite close to each other, separating by mountain barriers.
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