Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 115

the reduced wind speed there (Fig. 3.24b). The wind follows a similar pattern across the
rock, with small eddies developing in depressions and to the lee (Liptzin and Seastedt
2009). A mat of vegetation occupies the center depression where wind speeds are lower
(Fig. 3.24c).
Wind is clearly an extreme environmental stress; in many cases, it serves as the lim-
iting factor to life. What may be the two most extreme environments in mountains are
caused by the wind: late-lying snowbanks, where the growing season is extremely short,
and windswept, dry ridges. Both of these environments become more common and more
extreme with elevation, until eventually the only plants are mosses and lichens—or noth-
ing at all. Trees on a windswept ridge may be “flagged” with the majority of branch
growth on the protected lee side (Yoshino 1975). In the extreme conditions within the
krummholz (crooked wood) zone, trees take on a prostate cushion form (see Chapter 7).
The redistribution of snow by the wind is a major feature of the alpine environment
(Hiemstra et al. 2002). The wind speed necessary to pick snow up from the surface and
transport it depends upon the state of the snowcover, including temperature, size, shape
and density of the snow particles, and the degree of intergranular bonding (Liston et
al. 2007). For loose, unbound snow, the typical velocity is about 5 m/s (16 ft/s), while a
dense, bonded snowcover requires velocities in excess of 25 m/s (82 ft/s). Blowing snow
can abrade surfaces, causing erosion to snowcover and flagging trees. Once the wind
velocity lowers, the snow is deposited into dune-like features called drifts. Drifts are
found in the lee eddy of obstacles of all sizes (e.g., trees, ridges, fences; Fig. 3.20). To
control blowing snow, snow fences and other barriers are specially engineered to max-
imize deposition, and carefully placed to reduce the hazard of blowing snow or drifts
(Liston et al. 2007). In mountains, snow redistribution by wind is strongly affected by
meso- and microscale topography and vegetation (Pomeroy et al. 2006; Löffler 2007;
Liptzin and Seastedt 2009). Topographic traps fill in with snow, where it may survive
late into spring or summer because of its depth and because of temperature inversions.
Many glaciers receive a significant component of their accumulation from snow blown
over crests (Pelto 1996).