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Where are rain shadow
regions formed?
A region is termed as a rain shadow region when it receives
relatively low rainfall as compared to its neighboring areas. They
are formed in the valleys near mountains or mountain ranges. Rain
shadow areas form when the passage of wind is obstructed by a
mountain and the wind is forced to change direction and move
upwards.
What leads to the
formation of a rainshadow region?
As we all know, when air rises upwards, it expands and cools. If
the air cools to a certain extent then clouds may form and it may
even result in snowfall or rainfall. When the air has reached the
top of the mountain and begins its descent on the other side, it
gets warmed. This occurs owing to compression. As a result, the
clouds that may have formed while the air rose, get dissipated.
Consequently, the rainfall on this side – the downwind side of the
mountain – gets lower rainfall (as compared to the upwind side) and
is therefore called a rain shadow area.
The type of
precipitation produced when moisture laden air is lifted upwards is
known as orographic precipitation. Cooling of air due to descent
results in the formation of orographic clouds, which are the sources
of snowfall or rainfall on the upwind mountain slopes. Some amount
of this precipitation may be carried over to the downwind side along
with the descent of wind and is called ‘spillover’.
The warm dry winds that
descend mountain slopes are known by different names in different
parts of the world. In the US and in Canada they are the chinook
winds
and in Europe they are called foehns.
The effect of rain
shadows on the surrounding area.
Less rainfall in a particular area is bound to influence the
geographical make up of the region. Many rain shadows produce
deserts. Examples of such deserts are the Atacama Desert, the result
of easterly winds blowing over the Andes Mountains, and the
California’s Death Valley that has resulted from westerly winds
blowing over the Sierra Nevada mountains.
The predominant
direction of the wind is instrumental in causing a rain shadow
area. Sion (Switzerland), a city situated in a deep valley in the
Alps, remains in a rain shadow regardless of the direction of the
wind as it has mountain ranges on all sides.
If a region is humid
then rain shadows merely have the effect of causing less
precipitation than neighboring regions. In India, when the
southwest monsoons lash the Western Ghats, they give a lot of rain
in the upwind side. The amount of rainfall can go to as high as 245
inches – recorded at Mahabaleshwar at the crest of the Western Ghats.
Whereas on the leeward side, in the resultant rain shadow region,
the amount of rainfall goes down to as little as 25 inches per
year. Cherrapunji has an annual rainfall of 450 inches whereas the
Bramhaputra valley to its north experiences a rain shadow effect.
The presence of the Himalayas on the other side however ensures
sufficient rainfall in the area. |
