(male voice) This is Duane Friend with
University of Illinois extension.
There are areas on the Earth near
large mountains that are
very dry or desert-like.
These dry areas are called rain shadow areas.
What causes a rain shadow?
When wind blows up a mountain,
the air is moving into
an area that has lower air pressure,
making the rising air expand and cool.
When air cools, its capacity to hold
moisture in the form
of water vapor decreases.
Water vapor is the gaseous form of water.
As air continues to rise up the mountain,
it cools to the point it's at
its maximum capacity to hold water vapor.
When air is at its maximum capacity,
it is at 100% relative humidity.
Further cooling will cause
excess water vapor to
condense or turn into liquid water.
This forms clouds.
If enough condensation occurs,
rain or snow will fall out of
the clouds. As long as
the air continues to rise up the mountain,
condensation, clouds and possibly
rain or snow will occur.
The side of the mountain
where wind is blowing
up the mountain is called the windward side.
Many areas of
the world experience large amounts
of rain or snow where
air is forced up mountains.
The western parts of
Oregon and Washington states are
good examples of this process.
As air flows down the other side,
or the lee side of the mountain,
it starts to warm.
The warming is caused
by the air getting squeezed
together as it moves
downward into higher air pressure.
Warmer air can hold more water vapor,
so condensation does not
occur when air is sinking.
If there is no condensation,
there are no clouds
and therefore no rainfall.
Dry or desert conditions prevail
on the lee side
and are known as the rain shadow area.
Eastern parts of Oregon and Washington are
examples of where the rain shadow occurs.
While Laurel Mountain
in Western Oregon receives
over 119 inches of yearly precipitation.
Bend, Oregon on
the lee side of the mountains,
receives only about 12 inches
of precipitation a year.
Large differences in climate will be
experienced depending on which side
of the mountain you happen to be on.