Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 117

are the same processes that give rise to convection clouds over mountains during the
day and provide good soaring for glider pilots and birds. At night, when the air cools
and becomes dense, it moves downslope and downvalley under the influence of gravity.
This is the flow responsible for the development of temperature inversions. Although
they are interconnected and part of the same system, a distinction is generally made
between slope winds, and larger mountain and valley wind systems (Fig. 3.25).
SLOPE WINDS
Slope winds consist of thin layers of air, usually less than 100 m (330 ft) thick. In gen-
eral, the upslope movement of warm air during the day is termed anabatic flow, and the
downslope movement of cold air during the night is referred to as katabatic flow, or a
gravity or drainage wind. The upslope flow of air during the day is associated with sur-
face heating and the resulting buoyancy of the warm air (Rucker et al. 2008). The wind
typically begins to blow uphill about half an hour after sunrise and reaches its greatest
intensity shortly after noon (Fig. 3.25a). By late afternoon, the wind abates, and within
half an hour after sunset reverses to blow downslope (Fig. 3.25c). Katabatic winds in
the strict sense are local downslope gravity flows caused by nocturnal radiative cooling
near the surface under calm, clear-sky conditions, or by the cooling of air over a cold
surface such as a lake or glacier (Whiteman and Zhong 2008). The extra weight of the
stable layer, relative to the ambient air at the same altitude, provides the mechanism
for the flow. Since slope winds are entirely thermally induced, they are better developed
in clear weather than in clouds, on sun-exposed rather than on shaded slopes, and in
the absence of overwhelming synoptic winds. Local topography is important in directing
these winds; greater wind speeds will generally be experienced in ravines and gullies
than on broad slopes (Defant 1951; Whiteman and Zhong 2008).
Downslope winds form better at night and during the winter, when radiative cooling
dominates the surface energy system (Whiteman and Zhong 2008). The downslope flow
of cold air is analogous to that of water, since it follows the path of least resistance and
always tends toward equilibrium, although water has a density 800 times greater than
air (Porch et al. 1989). Even with a temperature difference of 10°C (18°F), the density
of cold air is only 4 percent greater than warm air; and unlike the rapid flow of water
because of gravity, the displacement of warm air by cold air is a relatively slow pro-
cess (Geiger 1965). Katabatic winds begin periodically when the layer of air just above
the surface cools, then slides downslope (Papadopoulos and Helmis 1999). The cycle
is repeated when the radiative cooling rebuilds the downslope pressure gradient. This
pulsating downslope flow depends on the temperature difference between the katabatic
layer and valley temperature (Whiteman and Zhong 2008). Surges of cold air, termed
“air avalanches,” are commonly observed on slopes greater than 10° (Geiger 1965).
A final steady velocity will be achieved once a certain temperature has been reached
(Papadopoulos and Helmis 1999). Further down a drainage basin, a steady velocity will
be reached, maintained, and increased throughout the night as individual slope winds
accumulate down basin in a fashion similar to tributaries in a stream (Porch et al. 1989).