Avalanches

An avalanche

Avalanches are rapid movements of large masses of snow down a slope that commonly occur in mountainous regions.

Despite what you may have seen in movies and on television, shouting cannot trigger an avalanche, according to the Utah Avalanche Centre. The UAC says that in 90% of avalanche deaths, it was the weight of the victim or someone in their group that triggered the avalanche.

In some countries the authorities monitor avalanche conditions and estimate risk levels. Specialists also intentionally trigger small snowslides with explosives so that the snow does not build up and is brought down the mountain in a controlled way when people are not present.

Image: An avalanche cascades down a mountain (credit: John Terence Turner)

Introduction

An avalanche Avalanches

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Avalanches

An avalanche (also called a snowslide or snowslip) is a rapid flow of snow down a sloping surface. Avalanches are typically triggered in a starting zone from a mechanical failure in the snowpack (slab avalanche) when the forces on the snow exceed its strength but sometimes only with gradually widening (loose snow avalanche). After initiation, avalanches usually accelerate rapidly and grow in mass and volume as they entrain more snow. If the avalanche moves fast enough some of the snow may mix with the air forming a powder snow avalanche, which is a type of gravity current.

Slides of rocks or debris, behaving in a similar way to snow, are also referred to as avalanches (see rockslide). The remainder of this article refers to snow avalanches.

The load on the snowpack may be only due to gravity, in which case failure may result either from weakening in the snowpack or increased load due to precipitation. Avalanches that occur in this way are known as spontaneous avalanches. Avalanches can also be triggered by other loads such as skiers, snowmobilers, animals or explosives. Seismic activity may also trigger failure in the snowpack and avalanches.

Although primarily composed of flowing snow and air, large avalanches have the capability to entrain ice, rocks, trees, and other material on the slope, and are distinct from mudslides, rock slides, and serac collapses on an icefall. Avalanches are not rare or random events and are endemic to any mountain range that accumulates a standing snowpack. Avalanches are most common during winter or spring but glacier movements may cause ice and snow avalanches at any time of year. In mountainous terrain, avalanches are among the most serious objective natural hazards to life and property, with their destructive capability resulting from their potential to carry enormous masses of snow at high speeds.

There is no universally accepted classification of avalanches—different classifications are useful for different purposes. Avalanches can be described by their size, their destructive potential, their initiation mechanism, their composition and their dynamics.

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