Geography Reference
In-Depth Information
MOUNTAIN AND VALLEY WINDS
The integrated effects of slope-generated flows produce mountain and valley winds,
blowing longitudinally up and down the main valleys, essentially at right angles to the
slope winds (Yu et al. 2009). They are all part of the same system, however, and are
controlled by similar thermal responses. The valley wind (blowing from the valley to-
ward the mountain) is interlocked with the upslope winds, and both begin after sunrise
(Buettner and Thyer 1965; Rucker et al. 2008; Fig. 3.25b). Valley winds involve greater
thermal contrast and a larger air mass than slope winds, however, so they attain great-
er wind speeds. In the wide and deep valleys of the Alps, the smooth surfaces left by
glaciation allow maximum development of the wind. The Rhône Valley has many areas
where the trees are shaped by the wind and flagged in the upvalley direction. Moun-
tain winds (blowing from the mountains downvalley) are associated with the nocturnal
downslope winds and can be very strong and quite cold in the winter (Porch et al. 1989;
Fig. 3.25d).
As with slope winds, a circulation system is established in mountain and valley winds.
The return flow from aloft (called an anti-wind) can frequently be found immediately
above the valley wind (Defant 1951; McGowan 2004), as shown by a study near Mount
Rainier, Washington, using weather balloons (Fig. 3.26; Buettner and Thyer 1965). This
wind system beautifully demonstrates the three-dimensional aspects of mountain cli-
matology: Next to the surface are the slope and mountain-valley winds; above them is
the return flow or anti-wind; and above this is the prevailing regional gradient wind
(McGowan and Sturman 1996a; Rampanelli et al. 2004). During clear weather, all of
these may operate at the same time, each moving in a different direction.
OTHER LOCAL MOUNTAIN WINDS
An important variant of the thermal slope wind is the glacier wind, which arises as the
air adjacent to the icy surface is cooled and moves downslope due to gravity. The gla-
cier wind has no diurnal period but blows continuously, since the refrigeration source
is always present. It reaches its greatest depth and intensity at midafternoon, however,
when the thermal contrast is greatest. At these times, the cold air may rush downslope
like a torrent. During the day, the glacier wind frequently collides with the valley wind
and slides under it (Fig. 3.27). At night, it merges with the mountain wind that blows in
the same direction (Defant 1951). Glacier winds have a strong ecological effect, since
the frigid temperatures are transported downslope with authority, and the combined ef-
fect of wind and low temperatures can make the area they dominate quite inhospitable.
In a valley with a receding glacier, these winds can entrain the unconsolidated till, sand-
blasting vegetation and rocks (into ventifacts) downvalley (Bach 1995).
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