People in the Mountains - Mountain Geography: Physical and Human Dimensions - page 501

FIGURE 10.9 Chamonix-Mt. Blanc is an example of a medieval agricultural settlement transformed
into a modern year-round tourist and service center and expanded into areas of floods, snow ava-
lanches, icefalls, and debris flow hazards. (Photo by J. S. Gardner.)
PRECIPITATION
As discussed in Chapter 3, mountains increase precipitation amounts in windward loc-
ations (rainsheds) through the orographic (barrier) effects imposed by topography on
moving air masses. In leeward locations, an opposite effect occurs (rainshadows). A pat-
tern of windward wet/leeward dry is present at many scales across mountain systems.
Also, precipitation tends to increase with altitude, though, in some higher ranges, such
as parts of the Himalaya, this increase reaches a maximum below the summits and de-
clines further upward. Such very general patterns may be altered at a smaller scale
by more localized pockets of atmospheric instability that produce high-intensity rain or
snow events in otherwise drier locations. A further complexity is that precipitation takes
several forms in mountain environments—rain, snow, sleet, hail, rime, hoar frost, fog,
and mist—and the relative importance of each varies geographically, examples being
snow at high altitudes in the Cascade and Coast Ranges in the North American Cor-
dillera, monsoon rain in the central and eastern Himalaya, and fog/mist at lower and
mid-altitude in the south-central Andes (garua). Precipitation also is variable through
time, most obviously in the seasonality of wet, dry, snow, and so on. Seeming less regu-
lar are longer-term variations in precipitation from year to year or over many years. A
good example of such variations is in the tropical Andes and the adjacent Pacific coast