Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 178

Slabs, weak layers, and bed surfaces often occur in the snowpack, but the snow is
not always unstable. For unstable conditions, the stress on the weak layer must exceed
its strength at some location. The gravitational force of the preexisting overlying slab,
plus any added weight from new or windblown snow, causes the stress on the weak lay-
er. When this exceeds the strength of the weak layer locally, cracks initiate within the
weak layer. After growing to a critical size (on the order of the slab depth), the cracks
rapidly propagate around the slope and, if the slope is steep enough, an avalanche will
release. The mechanics are further complicated by the highly spatially variable nature
of the snowcover (Schweizer et al. 2008; Birkeland et al. 2010).
TRIGGERS
Snow on a slope that is unstable enough to be triggered is called conditionally stable.
This situation is particularly dangerous since adding a person on skis, a snowboard, or
a snowmobile to the slope may result in an avalanche (Heierli et al. 2011). The most
common triggers for natural avalanches include new or windblown snow, though fall-
ing cornices are also important in some areas. Rapidly changing air temperature has
occasionally been blamed for avalanche release, but its importance has not been con-
vincingly demonstrated. Explosive blasts can also be used to trigger avalanches as a
mitigation measure (see below). Loud sounds have been implicated as avalanche trig-
gers, and there was an ancient regulation in Switzerland against yodeling during the
avalanche season (Allix 1924). However, research suggests this is highly improbable.
Even enormous sonic booms are only capable of triggering avalanches in rare cases of
extremely unstable snowpack (Martinelli 1972).
The Avalanche as a Hazard, Avalanche Victims, and Avalanche Rescue
Once humans become involved in the avalanche equation, we have a hazard. It is ironic
that, despite our greater scientific understanding of avalanches and our considerable
investment in their prediction and prevention, the number of accidents continues to in-
crease, primarily because more and more people, especially recreationists, go to the
mountains during the winter (Fig. 4.24). This is graphically illustrated by an analysis of
avalanche accidents in the United States in a series of volumes called the Snowy Tor-
rents (Williams 1975; Williams and Armstrong 1984; Logan and Atkins 1996), as well
as the more recent Canadian volume Avalanche Accidents in Canada (Jamieson et al.
2010).
In the United States, avalanche fatalities have increased greatly over the last few
decades, for a number of reasons. Fatalities increased sharply in the late 1970s as ski
equipment improved and backcountry skiing became more popular. A second sharp in-
crease in fatalities occurred in the 1990s as ski and snowmobile equipment improved,
with the five-year average becoming nearly 30 deaths per year. Since the mid-1990s,
snowmobilers as a group have overtaken skiers and climbers in leading avalanche fatal-
ities (Fig. 4.25). Snowmobile technology has improved greatly since the 1990s, allowing
riders to access more avalanche-prone terrain more quickly after storms (CAIC 2012).
Avalanche rescue can be viewed in terms of two distinctly different sets of responses
and personnel involved. The first response is the immediate attempt at recovery by the