Geography Reference
In-Depth Information
Mountain climate and weather
Hills and mountains can substantially modify regional and local
climate. There tends to be a greater amount of the Sun's energy
received in the form of ultraviolet energy in mountainous areas.
This can be harmful to humans, causing skin cancer if frequent,
high doses are received. Pressure and temperature normally fall
with increasing altitude within the troposphere. This means that
typically it becomes cooler as you ascend a mountain. The higher
and more isolated a mountain the more the air temperature will
resemble that of the free atmosphere rather than the atmosphere
heated a short distance from the Earth's surface.
If there are light winds and clear skies at night, this can allow
the ground to cool forming a strong temperature inversion,
drawing air down the slopes into cooler valley bottoms. If there is
cloud cover or strong winds then this process will be restricted.
During the day if the Sun's energy is strong and slopes are steep
sided, the air can be warmed not only from below but from the
sides of the steep slopes too. This leads to local winds and rising air
up the slopes of some valleys. It is these features that are often util-
ised by thrill-seekers who go hang-gliding or parasailing in hilly
areas.
Precipitation totals tend to increase with altitude. However, this
increase is greatest in mid-latitudes. In the tropics the increase in
precipitation with height is more complex and often stops at
around 1,500 metres. As air moving over the land surface reaches
the mountain range it is forced to rise over the mountains. The
cooling of the air that occurs as it rises reduces the temperature to
the dew point. Further rising air leads to the formation of cloud
and precipitation. The reasons for differences in response in mid-
and low latitudes are not yet clear and research is hampered by the
lack of monitoring data at high altitudes.
If the rising air over mountains has been forced to the dew point
and cloud formation and precipitation has occurred, it means that
when the air falls again, once it passes the mountain range and
warms, then it will be much drier than it was before it encountered
the mountain range. Therefore, in the lee (downwind) of moun-
tains the climate can be significantly drier than on the windward
side of the mountains. Since mountains are regular barriers to air
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