These properties may all metamorphose in time according to the local humidity, water vapour flux, temperature and heat flux. It varies in detail with properties of the snow grains, size, density, morphology, temperature, water content and the properties of the bonds between the grains. However, the strength of the snowpack is much more difficult to determine and is extremely heterogeneous. The load is straightforward it is the weight of the snow. However, if the snow has sintered into a stiff slab overlying a weak layer then fractures can propagate very rapidly, so that a large volume of snow, that may be thousands of cubic metres, can start moving almost simultaneously.Ī snowpack will fail when the load exceeds the strength. Īvalanche initiation can start at a point with only a small amount of snow moving initially this is typical of wet snow avalanches or avalanches in dry unconsolidated snow. Contrary to popular belief, avalanches are not triggered by loud sound the pressure from sound is orders of magnitude too small to trigger an avalanche. Artificial triggers of avalanches include skiers, snowmobiles, and controlled explosive work. Other natural causes include rain, earthquakes, rockfall and icefall. The second largest cause of natural avalanches is metamorphic changes in the snowpack such as melting due to solar radiation. Most avalanches occur spontaneously during storms under increased load due to snowfall and/or erosion. Note the granular characteristic of the debris in the foreground that results from the slab breaking up during descent. Multiple crown fracture lines are visible in the top-middle of the image. 8.1 Impacts on avalanche type and frequencyġ5 cm deep, soft slab avalanche triggered by a snowboarder near Heliotrope Ridge, Mount Baker in March 2010. 7.6 United States classification for avalanche size.7.5 Canadian classification for avalanche size.7.3 North American Avalanche Danger Scale. 3.2 Snowpack structure and characteristics.Avalanches can be described by their size, destructive potential, initiation mechanism, composition, and dynamics. There are many classification systems for the different forms of avalanches, which vary according to their users' needs. In mountainous areas, avalanches are among the most serious natural hazards to life and property, so great efforts are made in avalanche control. They are most frequent in winter or spring, but may occur at any time of year. They are also different from large scale movements of ice.Īvalanches can happen in any mountain range that has an enduring snowpack. Though they appear to share similarities, avalanches are distinct from slush flows, mudslides, rock slides, and serac collapses. If an avalanche moves fast enough, some of the snow may mix with the air, forming a powder snow avalanche. After being set off, avalanches usually accelerate rapidly and grow in mass and volume as they capture more snow. Primarily composed of flowing snow and air, large avalanches have the capability to capture and move ice, rocks, and trees.Īvalanches occur in two general forms, or combinations thereof: slab avalanches made of tightly packed snow, triggered by a collapse of an underlying weak snow layer, and loose snow avalanches made of looser snow. Īvalanches can be set off spontaneously, by such factors as increased precipitation or snowpack weakening, or by external means such as humans, animals, and earthquakes. The terminus of an avalanche in Alaska's Kenai Fjords.Īn avalanche is a rapid flow of snow down a slope, such as a hill or mountain.
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