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Tilt – Listening Tapescript
DESERT FORMATION
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.
Desert formation:
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 areas.
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 highpressure 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.
Most of the non-polar deserts lie within the two trade winds belt. From
USGS
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
found.
Sea 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.
Upwelling 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.