Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 127

crests (Fig. 3.29). The rotor flow moves toward the mountain at the base and away from
it at the top (Doyle and Durran 2002). It is marked by a row of cumulus clouds but,
unlike ordinary cumulus, they may contain updrafts of 95 km (60 mi) per hour. The po-
tential of such a wind for damage to an airplane can well be imagined. The height of
the rotor clouds is about the same as that of the crest cloud or foehn wall. The rotat-
ing motion is thought to be created when the lee waves reach a certain amplitude and
frictional drag causes a roll-like motion in the underlying air (Fig. 3.29; Scorer 1961,
1967).
Several other kinds of turbulence may be associated with lee waves, particularly
when the wave train produced by one mountain is augmented by that of another situ-
ated in the right phase relationship. In some cases, they cancel each other; in others,
they reinforce each other. Wind strength and direction are also important, since a small
change in either can alter the wavelength of two superposed wave trains so that they
become additive and create violent turbulence (Scorer 1967; Neiman et al. 2001).
Microclimates
In addition to the climatic characteristics reviewed above, it should be emphasized
that there are substantial variations in climates over very short distances within moun-
tains. Mountain environments are exceedingly spatially complex in terms of vegetation
types and structures, geology, hydrology, soils, and topography. All vary in composition
(i.e., plant species, canopy characteristics, or rock types), and variations occur across a
range of slopes and aspects. The climate over each of these surfaces, or microclimate,
can differ significantly because of variations in net radiation, soil and air temperature,
humidity, precipitation accumulation (amount and form), soil moisture, and winds
(Pomeroy et al. 2006; Löffler 2007; Daly et al. 2010).
Large differences in temperature, moisture, and wind can be found within a few
meters, or even centimeters (McCutchan and Fox 1986). The thin atmosphere at high el-
evation means that surfaces facing the sun on a clear day can warm dramatically, while
shaded surfaces remain cold (Germino and Smith 2000). Other effects may arise accord-
ing to valley orientation with respect to the mountain range, valley cross profile, and
the effect of winds and cold air drainage. The effect of aspect in generating slope winds
can exceed the influence of elevation on wind velocity and temperature (McCutchan and
Fox 1986).
The mosaic of microclimates determines local variability in ecosystem processes, for-
cing plants and animals to a high degree of specialization and adaptation (Körner 2003).
The distribution of vegetation zones and individual species often follow the distribution
of microclimates (Fig. 3.8; Löffler 2007; Daly et al. 2010; see Chapter 7).
Vegetation creates its own microenvironment by creating shade and windbreaks
(Fig. 3.24). In association with the wind regime is a recurring pattern of snow accumu-
lation in the lee of obstacles. These snowdrifts add to soil moisture during the melt sea-
son, and protect trees from freezing in winter (Pomeroy et al. 2006). The distributions
of snow and vegetation are closely linked. Plant species in these areas exhibit distribu-
tions related to the duration of snowcover as well as relief (Körner 2003; Erschbamer
2007; Löffler 2007).