Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 116

FIGURE 3.23 Wind velocity with height above a tundra surface. Note how wind speed increases with
distance above the ground, one reason why alpine plants grow so close to the ground. (From
Warren-Wilson 1959: 416.)
There are two overall groups or types of winds associated with mountains. One type
originates within the mountains themselves. These are local, thermally induced winds
given distinct expression by the topography. The other type is caused by obstruction
and modification of winds originating from outside the mountain area. The first type is a
relatively predictable, daily phenomenon, while the second is more variable, depending
on the vagaries of changing regional wind and pressure patterns.
LOCAL WIND SYSTEMS IN MOUNTAINS
Winds that blow upslope and upvalley during the day and downslope and downvalley
at night are common. Albrecht von Haller, author of Die Alpen, observed and described
these during his stay in the Rhône Valley of Switzerland from 1758 to 1764. Since then,
many studies have been made on thermally induced winds (see Barry 2008). The driv-
ing force for these winds is differential heating and cooling, which produces air density
differences between slopes and valleys, and between mountains and adjacent lowlands
(McGowan and Sturman 1996a). During the day, slopes are warmed more than the air at
the same elevation in the center of the valley; the warm air, being less dense, moves up-
ward along the slopes. Similarly, mountain valleys are warmed more than the air at the
same elevation over adjacent lowlands, so the air begins to move up the valley. These