Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 176

resistance to sliding. Densely forested slopes (>1,000 conifer trees per hectare [>1,000
trees per 2.5 acres] on steeper slopes) generally offer good protection from avalanche
initiation (McClung and Schearer 2006). However, avalanches may start above the
timbered zone and destroy strips of the forest in their path (see Fig. 4.21), and many
mountain forests have been cut or destroyed in recent centuries (Aulitzky 1967). Once
this happens, these zones become more vulnerable to avalanching and it is difficult for
the forest to regenerate, since trees in the path of the avalanching are continually dam-
aged or killed (Frutiger 1964; Schaerer 1972; Martinelli 1974; McClung and Schaerer
2006).
WEATHER
Any weather factors that change the mechanical state of the snowpack may also quickly
change the avalanche conditions. The three most important weather factors are new
snow (or rain), wind, and changes in temperature. More than 80 percent of all large
slides occur either during or shortly after storms. The more snow, the more weight ad-
ded to the snowpack, and the greater the stress acting on any weak layers in the ex-
isting snowpack or on the interface between the new snow and the previously existing
snowpack. The rate of snowfall, or the snowfall intensity, is critically important. If the
snow falls slowly enough, the snowpack may be able to adjust to the new snow load,
but rapid snowfall may quickly overload weak layers before they can adjust, thereby
causing avalanches. Rain can also be an important factor, since it adds weight to the
snowpack without any addition in strength; rain falling on a midwinter snowpack often
rapidly initiates a large number of avalanches.
Wind is also a critically important weather factor for avalanches. Wind redistributes
snow onto the lee sides of ridges, gullies, and other terrain features, where it may pile
into thick wind drifts. Even relatively gentle winds (about 15 km/hr [9 mi/hr]) are suffi-
cient to move low-density snow (Tremper 2008). Wind also breaks snowflakes into smal-
ler particles, which then bond together quickly, forming cohesive wind slabs. Areas of
wind deposition are areas where more stress has been added to the snowpack, thereby
creating more unstable conditions.
Temperature is another important weather factor, affecting the mechanical strength
of both falling and accumulated snow. The temperature at the time of snow deposition
affects the snow crystal type, the density, the rate of settlement, and the general cohe-
sion. After deposition, the temperature of the snow layer and of the air above is crit-
ical to the rate of settling, compaction, internal creep, and metamorphosis. In general,
instabilities in the snowpack tend to persist longer when temperatures are cold, and
stabilize more quickly when temperatures are moderate. However, high temperatures
permit melting and the loss of cohesion among snow layers, resulting in wet-snow ava-
lanches. In addition, changes in temperature may affect how easy it is to trigger ava-
lanches, with warmer temperatures decreasing the stiffness of the snow and allowing
stresses to penetrate farther into the snow-pack, thereby facilitating triggering by ski-
ers or snowmobilers (McClung and Schweizer 1999).