Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 125

through the natural constriction between the Pyrenees and the western Alps (Defant
1951). The mistral was known in ancient times. The Greek geographer Strabo called it
“an impetuous and terrible wind which displaces rocks, and hurls men from their chari-
ots” (De La Rue 1955: 32). Its effects extend throughout Provence and may be felt as far
south as Nice. Like the bora, the mistral poses a major problem for fruit production, and
great expenditures of human labor have gone into constructing stone walls and other
windbreaks to protect the orchards (Gade 1978). The greatest wind velocities occur in
the Rhône Valley, where wind speeds of over 145 km (90 mi) per hour have been recor-
ded.
Although the bora and mistral are the most famous, similar cold, dry winds occur in
many mountain areas (Forrester 1982). The “bise” (breeze) at Lake Geneva between the
Alps and the French Jura is of the same type, and numerous examples could be cited
from the large mountain gaps and passes of Asia. “Helm” winds blow down from the
Pennine Chain in north-central England, often creating rolls of clouds. “Sno” winds fill
the fjords of Scandinavia during winter, and the “oroshi” blows near Tokoyo (Yoshino
1975). In North America, the “northers” of the Gulf of Tehuantepec, Mexico, are a sim-
ilar phenomenon (Brennan et al. 2010). Another example is the exchange of the cold
air during winter between the east and west sides of the Cascade Mountains along the
Columbia River Gorge (the Coho wind) and the Fraser River Valley (Mass 2008). This is
most pronounced when an outbreak of cold arctic air moves southward and banks up
against the east side of the Cascades, causing great temperature and pressure contrasts
between the cold continental air and the relatively warm Pacific air. At these times, cold
air is forced through these sea-level valleys at high velocities and brings some of the
clearest and coldest weather of the winter to the cities of Vancouver, British Columbia,
and Portland, Oregon (both located at valley mouths). At other times, the cold air may
force the warm coastal air aloft and produce locally heavy snowfall. The most dominant
feature, however, is the cold and ferocious wind that leaves its mark on the landscape
in the brown and deadened foliage of needle-leaf trees, and the strongly flagged and
wind-shaped trees on exposed sites.
FIGURE 3.29 Air current over mountains, showing superposition of lee-wave trains. The mountain
ridge (indicated by dashed line of mountain form) produces a certain wave pattern (dashed
streamline) and the other mountain (solid line) produces a different wave pattern (continuous
streamline). Together, the mountains have the effect of creating an obstacle (indicated by the con-
tinuous line). In the upper diagram the wavelength is such that the wave trains cancel out; in the
lower diagram the amplitude is doubled. Since the wavelength is determined by the flow of air