DESERTS are usually
very, very dry. Even the wettest deserts get less
than ten inches of precipitation a year.
Some
deserts are caused by what is called the "rainshadow"
effect. As air moves up over a mountain range,
it gets cold and loses the ability to hold moisture
-- so it rains or snows. When the air moves down
the other side of the mountain, it gets warmer.
Warm air can hold lots of moisture, so it doesn't
rain as much, and a desert is formed.
