Geoscience Reference
In-Depth Information
The effect is confirmed by precipitation maxima in or just
to the leeward of ranges, followed by downwind rain
shadows. Tropical coastal mountains such as Kauai
(Hawaii) and the Dorsale range (Cameroon) experience
among the highest global precipitation rates at 11,000-
12,000 mm yr -1 . British Columbian and Alaskan coast
ranges receive 2,500-5,000 mm yr -1 , falling to 500 mm and
1,250 mm respectively within 100 km downwind. Much
of this precipitation falls as snow by virtue of their altitude
and latitude, and regional snowlines also reflect rain
shadows, rising from 1·6 km on the west side of southern
British Columbia coast ranges to 2·9 km inland and 3·1
km in the eastern Canadian Rockies. In Europe snowlines
rise from 1·7 km in Scandinavia to 3·3 km in central
Europe and hover just above semi-permanent snowbeds
at 1·3 km on Ben Nevis and the Cairngorms in Britain.
Global snowlines range from sea level in polar areas to 4·5
km in moist equatorial regimes such as the Ecuadoran
Andes, rising above 6 km in the dry Andes and Tibet
( Figure 24.3 ).
growth. Substantial local variations are imparted by
mountain aspect, slope and topography, when slope angle
and orientation in relation to the sun's movement through
the sky and local winds become important. Mountains
do not simply experience broad meteorological trends
influenced by altitude and latitude but play an active part
in creating their own weather and climate. By acting as
heat and moisture sources they emphasize the contrast
between mountain and free atmospheres at any particular
altitude and create mountain atmospheric zones ( Figure
24.4 ). Reference was made earlier to the impact of
mountains which penetrate the mid to upper troposphere
planetary wind belts. They, in turn, steer weather systems
which influence the latitude of maximum orographic
effect (polar front jetstream belts) and mountain arid
zones (trade wind belts). We are concerned now with
progressively more local impacts.
Mountain macroclimate
Mountain barriers disturb air flow in a zone less than 2
km thick, often with a downwind plume, where they
modify the regional climatic character, especially
cloudiness, precipitation and regional winds. The general
pattern of orographic cloud and precipitation, associated
with instability, may be varied by seasonal or transient
Mountain climate
So far we have looked at systematic effects, with important
consequences for regional snowline elevations and plant
m
9,000
8,000
7,000
6,000
5,000
ANDES (W)
4,000
3,000
2,000
1,000
0
90°
60°
30°
30°
60°
90°
NORTH
SOUTH
Highest snow line
Lowest snow line
Highest maximum timberline
Lowest maximum timberline
Highest summits
$SSUR[LPDWHJHRHFRORJ\EHOW
Alpine and
cryonival belt
Forest-tundra
ecotone
Montane
forest belt
Snowfields
Forest belt
Figure 24.3 Pole-to-pole cross-section of the principal elevations and geoecological belts of alpine mountains.
Source: Modified from Ives and Barry (1974)
 
 
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