EARTH SCIENCE SERVICES > ENVIRONMENTAL ADVISORIES > HYDROLOGICAL ADVISORIES > AVALANCHE FORECASTS
An avalanche is caused when a build up of snow is released down a slope, and is
one of the major dangers faced in the mountains in winter. In an avalanche,
lots of material or mixtures of different types of material fall or slide
rapidly under the force of gravity. Avalanches are often classified by what
they are made of, for example snow, ice, rock or soil avalanches. A mixture of
these would be called a debris avalanche.
A large avalanche can run for miles, and can create massive destruction of the
lower forest and anything else in its path. Avalanches occur when the load on
the upper snow layers exceeds bonding forces (bonding to layer beneath, support
from anchors such as rocks and trees, stress support from top or bottom of
slope). Critical load may be exceeded naturally by adding new snow or by rapid
loading, by falling ice, cornices and similar means. The bonding forces within
a snowpack are affected by temperatures (e.g., a lubricated melt layer, or a
fragile crystal layer) before, during and after snowfall. Avalanches are also
triggered by humans - because of the additional weight, kicks during skiing
(e.g. during jumps) or intentionally by explosives, slope-cuts and other means.
An Avalanche Advisory reports on conditions favorable for an avalanche and will
provide locations in which an avalanche is more likely to occur.
Adapted from: http://en.wikipedia.org/wiki/Avalanche