Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 124

Pacific Ocean, which is considerably warmer in winter than the continental polar air
characteristic of the Great Basin and High Plains.
BORA, MISTRAL, AND SIMILAR WINDS
Like the foehn, these winds descend from mountains onto adjacent valleys and plains
but, unlike the foehn, they are cold. Compressional heating occurs, but it is insufficient
to appreciably warm the cold air that blows from an interior region in winter across the
mountains to an area that is normally warmer. These winds and others like them are
basically caused by the exchange of unlike air across a mountain barrier.
FIGURE 3.28 Diagrammatic representation of classical development of a foehn (Chinook) wind. Tem-
peratures at different locations are based on the assumption that air at the base of mountain on
windward side is 10°C (50°F). By the time the air has undergone the various thermodynamic pro-
cesses indicated in its journey across the mountains it reaches the base on the leeward side at
18.1°C (64.6°F). (A. J. Bach.)
The “bora” is a cold, dry north wind on the Adriatic coast of Dalmatia. It reaches
its greatest intensity in winter, originating from high-pressure, cold continental air
in southwestern Russia that results in air movement southward across Hungary and
the Dinaric Alps (Durran 1990). Ideal conditions for the bora exist when a southerly
wind has brought exceptionally warm conditions to the Adriatic coast, and relatively
large temperature and pressure differentials exist between the coast and the interior.
Under these conditions, the cold continental air may move down the pressure gradient,
steepened by the presence of the mountains, with extraordinary violence (Yoshino
1975). It frequently reaches gale force, especially when channeled through narrow val-
leys and passes. The bora has been known to overturn haywagons, tear off roofs, and
destroy orchards. It is even claimed that it once overturned a train (De La Rue 1955).
The “mistral” occurs in Provence and on the French Mediterranean coast (Jansa
1987). It is caused by the movement of cold air from high-pressure areas in the north
and west of France toward low-pressure areas over the Mediterranean between Spain
and Italy in the Gulf of Lyon. It is as violent as the bora, or more so, since it must pass