Geography Reference
In-Depth Information
flow they are also regular locations for heavy precipitation. In fact
on the island of Kauai (part of the Hawaii chain of islands) in the
Pacific, Mount Waialeale receives an amazingly high average of
around 12,500 millimetres of rainfall per year on its wind-facing
side. However, on the leeward side of the mountain, the slopes
only receive 500 millimetres of rainfall per year. Nearby locations
over the ocean where there are no mountains would normally
receive around 640 millimetres of rainfall per year. The warm and
dry wind that blows down lee slopes of hill and mountain ranges is
called a föhn wind . These winds are increased further by the wave
effect of fast moving air being forced over a mountain range, a little
like flowing water over a pebble. There are different local names
for föhn winds depending on where you are in the world, such as
the Chinook in North America or Zonda in Argentina. These
winds can be very important because when they start to flow they
can increase local air temperature by up to 25°C in an hour causing
sudden snow melt or avalanche risk and influencing plant growth.
In fact in Canada, in the lee of the Rockies, temperature rises of
over 20°C have been recorded in just a few minutes as the wind
begins to flow. This can bring plants out of their winter dominancy
only to be damaged when cold weather returns once the föhn
wind has stopped. As the winds are warm and dry they may even
increase fire risk.
Normally surface friction reduces wind speeds by about 30 per
cent. However, exposed peaks and ridges can have higher wind
speeds as there is less surface friction around them and so the wind
experienced in mountains can be different from that in the low-
lands because of the nature of the terrain (and not necessarily
because of the altitude). Furthermore, if wind is funnelled through
gaps or valleys between individual peaks then it can be more
intense at those locations. The winds in mid-latitude mountains are
also influenced by the prevailing westerly winds and these winds
are generally faster at higher altitudes in the troposphere. However,
in the tropical and subtropical trade wind belts, the north-east and
south-east trade winds generally weaken with height. Therefore,
wind speeds can be low on tropical and subtropical mountains.
More precipitation may fall as snow in mountain regions which
can accumulate over time. Box 1.3 describes rain and snow forma-
tion. Often snow melt in the spring can produce large seasonal
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