How do deserts form?
If you look at the general atmospheric circulation and
topography, you will find that most desert areas
are downwind of some fairly large mountain ranges or
long expanses of land. Mountains lift the air, causing
condensation and precipitation that "dries" the air.
As the air drops down off of the mountain range it naturally
warms, thereby causing increased evaporation of water from
the land that it passes over. That keeps the land dry.
Northern and sub-central Africa are a little different.
These areas have been stripped of their vegetation by
man and animals, thereby reducing transpiration from
plants and making evaporation from the soil easier.
This has combined with long expanses of land and long
distances from water bodies (moisture sources) to cause
a large desert area.
Then there are plateaus of land of high elevation that
are dry, such as the Tibetan plateau. The precipitation
rains and snows out in the mountains around them leaving
little moist air to flow over the plateau, leaving it dry.
So, there are various causes of deserts. Actually,
Antarctica is a cold desert. It is so cold that moisture
quickly condenses out into ice crystals, leaving some
snow, but almost no water vapor in the air.
I hope that this information helps you.
Argonne National Laboratory
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Update: June 2012