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Causes of Aridity
Atmospheric high pressure zones (Hadley Cells)
The Hadley cell is an atmospheric circulation pattern in the tropics that produces winds called the tropical
easterlies and the trade winds. The air in these cells rises at the equator and sinks at the medium latitudes.
Deserts lie under the falling limb of the Hadley cell, carrying warm dry air from the equator and high
pressure which means that there is little chance of rainfall. The Hadley cell forms heavy clouds which
becomes saturated and causes rain, hydrating rainforests along the equator
however the areas that this rain does not reach, deserts are formed. The
warmer the air, the more water vapour held and a higher relative humidity
deserts have a low relative humidity because the air has low amounts of
water vapour due to high evaporation rates.
(Trade winds direct the prevailing wind in a somewhat constant direction
East across the tropical regions of the globe.)
E.g. The Sahara Desert in Africa.
Distance from oceans
As an air mass moves over a continent it will lose moisture as precipitation. Equally the air will take up very
little moisture due to the low evaporation rates over land surfaces. This means that areas in the centre on
continents have very little rainfall simply because the air has become much drier.
It is thought that during the last ice age, conversion of water to ice resulted in larger continental areas. This
extreme continentality is thought to have facilitated the spread of deserts during the ice age.
E.g. This effect is best seen on large continents such as Australia (Simpson Desert), North America and Asia.
Several deserts lie along western coasts where, due to the action of circulating wind currents, there is
upwelling of cold sea water. This cools the passing air masses, reducing the amount of water that the air
can hold, so limiting the amount of precipitation which can be held.
E.g. The Atacama Desert on the west coast of South America and the Namib Desert on the west coast of
Africa have formed in this way.
Prevailing winds carry the evaporated water vapour from the
sea, but due to the obstruction of the mountains the air has
to rise cooling it in the process. As it cools it precipitates,
losing the moisture and over the mountain range sinks and is
heated pushing warm dry air over the deserts carrying no
precipitation or moisture to hydrate the soil.
E.g. The Patagonian Deserts, where the Western Ghats and
the Andes intercede.