Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 172

FIGURE 4.19 Loose snow avalanches (sluffs) above Snowbasin Ski area in Utah, U.S.A. Such ava-
lanches are typically small and harmless, often occurring during or shortly after storms. Note the
large overhanging cornice on the ridgeline above. (Photo by K. Birkeland.)
Factors Influencing Avalanche Occurrence
Snow is a highly variable material. It occurs under different environmental conditions,
displays vastly different mechanical characteristics as its temperature and micro-struc-
ture change, and is susceptible to constant modification once on the ground (McClung
and Schaerer 2006). Modern avalanche forecasters use both their practical experience
and their scientific knowledge of snow and avalanches to predict the probable occur-
rence of avalanches in time and space (LaChapelle 1980; McClung 2002). Though many
of the tools used for avalanche forecasting have improved (e.g., remote weather sys-
tems, geographic information systems [GIS], and some computer models), most ava-
lanche forecasters still use techniques pioneered over 50 years ago (Seligman 1936; At-
water 1954; Perla and Martinelli 1976; McClung and Schaerer 2006). Ultimately, there
are three main ingredients needed for avalanching: (1) favored terrain, (2) unstable
snow (which is a product of both the weather and the existing snowpack), and (3) a trig-
ger.