Geography Reference
In-Depth Information
of airflow over a mountain causes acceleration of the air, while frictional effects cause
a slowing. Frictional drag in the lowest layers of the atmosphere is caused by the in-
teraction of air with individual small-scale roughness elements (i.e., vegetation, rocks,
buildings, or landforms of <10 m dimensions) and by the influence of larger topograph-
ic features and vegetation canopies (Walmsley et al. 1989).
The sharpest gradient in wind speed usually occurs immediately above the surface.
Wind speed doubles or triples within the first few meters, but the vegetation and surface
roughness make a great difference in the absolute velocity (Fig. 3.23). The low-lying
foliage of alpine vegetation does not produce much frictional drag on the wind, so the
wind can reach quite high velocities close to the ground. There is nevertheless a sharp
gradient within the first few centimeters of the surface, and most alpine plants escape
much of the wind (Liptzin and Seastedt 2009). A reciprocal and reinforcing effect oper-
ates here: Taller vegetation tends to reduce the wind speed and provide a less windy en-
vironment for plants, while low-lying alpine vegetation provides little braking effect, so
the wind blows freely and becomes a major factor of stress in the environment. Under
these conditions, the presence of microhabitats becomes increasingly important.
TABLE 3.5
Mean Monthly Wind Speeds during Winter at Selected Mountain Weather Stations
In Order of Decreasing Velocity
Readings were taken above treeline or in treeless areas, but anemometers were located at vari-
ous heights above the ground.
Surface roughness caused by clumps of vegetation and rocks creates turbulence and
hence great variability in wind speed near the surface (Fig. 3.24). In the illustration,
wind speed at a height of 1 m (3.3 ft) above the grass tussock is 390 cm/s, while closer
to the ground it is 50 cm/sec on the exposed side of the tussock and 10 cm/s on the lee
side (Fig. 3.24a). Similar conditions exist with the eroded soil bank, except that wind
speeds are higher on the exposed side and there is more eddy action and reverse flow
on the lee. The restriction of the vegetation to the lee of the soil bank is largely due to
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