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Causes of aridity:
High atmospheric pressure Hadley cells
Deserts lie under the falling limb of the Hadley cell,
carrying warm dry air from the equator & high
pressure = little chance of rainfall.
The Hadley cell forms heavy clouds which become
saturated and cause rain hydrating rainforests along
the equator however when it runs out deserts form.
Air temp: warmer the air, 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.
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.
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 of continents have very little rainfall simply because
the air has become much drier. This effect is best seen on large continents such as
Australia, 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.
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.