Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 121

FIGURE 3.26 Graphic representation of slope and valley winds. The view on the left is looking upval-
ley at midday. Slope winds are rising along the slopes, while the valley wind and anti-wind are
moving opposite each other, up and down the valley. The illustration on right provides a vertical
cross section of the same situation, viewed from the side. The valley wind and anti-wind essen-
tially establish a small convection system. The regional gradient wind is shown blowing above the
mountains. If the regional wind is very strong, of course, it may override and prevent develop-
ment of the slope and valley winds. (Adapted from Buettner and Thyer 1965: 144.)
Another famous local wind in mountains is the Maloja wind, named after the Maloja
Pass in Switzerland between the Engadine and Bergell Valleys (Hann 1903; Defant
1951; Whipperman 1984). This wind, which blows downvalley both day and night, res-
ults from the mountain wind of one valley reaching over a low pass into another valley,
where it overcomes and reverses the normal upvalley windflow. This anomaly occurs in
the valley with the greater temperature gradient and the ability to extend its circula-
tion into the neighboring valley across the pass. Thus, the wind ascends from the steep
Bergell Valley and extends across the Maloja Pass downward into the Engadine Valley
to St. Moritz and beyond. Other examples of local winds could be given, since every
mountainous country has its own peculiarities, but those mentioned suffice to illustrate
their general nature.
MOUNTAIN WINDS CAUSED BY BARRIER EFFECTS
As mentioned earlier in this chapter, mountains can act as barriers to the prevailing
general circulation of the atmosphere. The barrier effect introduces turbulence to the
winds, increasing and decreasing speeds, changes directions, and modifies storms (re-
viewed earlier). Once the wind passes over the mountain crest, however, it will do one
of two things: flow down the lee side, or stay lifted in the atmosphere (Durran 1990;
Mass 2008). Most commonly, the wind will fall down the lee side under the influence of
gravity. These surface winds are sometimes collectively termed fall winds, but they are
known by a variety of local terms because they have long been observed in many re-
gions downwind from mountains and have distinct weather phenomena associated with
them. When the winds leave the surface in a hydraulic jump, they often travel through
the atmosphere in a wave motion, producing unique cloud forms.