An avalanche (also called a snowslide or snowslip) is a sudden, drastic flow of snow down a slope, occurring when either natural triggers, such as loading from new snow or rain, or artificial triggers, such as snowmobilers, explosives or backcountry skiers, overload the snowpack. The influence of gravity on the accumulated weight of newly fallen uncompacted snow or on thawing older snow leads to avalanches which may be triggered by earthquakes, gunshots and the movements of animals.
Avalanches are most common during winter or spring but glacier movements may cause ice avalanches during summer. Avalanches cause loss of life and can destroy settlements, roads, railways and forests. Typically occurring in mountainous terrain, an avalanche can mix air and water with the descending snow. Powerful avalanches have the capability to entrain ice, rocks, trees, and other material on the slope. Avalanches are primarily composed of flowing snow, 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. In mountainous terrain avalanches are among the most serious objective hazards to life and property, with their destructive capability resulting from their potential to carry an enormous mass of snow rapidly over large distances.
Avalanches are classified by their morphological characteristics and are rated by either their destructive potential, or the mass of the downward flowing snow. Some of the morphological characteristics used to classify avalanches include the type of snow involved, the nature of the failure, the sliding surface, the propagation mechanism of the failure, the trigger of the avalanche, the slope angle, slope aspect, and elevation.
The size of an avalanche, its mass and its destructive potential are rated on a logarithmic scale, typically of 5 categories, with the precise definition of the categories depending on the observation system or geographic region in which the avalanche occurs.
The nature of the failure of the snowpack is used to morphologically classify the avalanche. To this point, there are two main types of avalanches: loose snow avalanches and slab avalanches, and either type of avalanche can involve dry or wet snow. For this reason, professionals refer to avalanches as "dry loose snow avalanches", "wet loose snow avalanches", "dry slab avalanches", and "wet slab avalanches". The primary distinction between wet and dry avalanches is the presence of liquid water in the snow at the time of avalanche formation.
Avalanche formation requires a slope where snow can accumulate, yet has enough steepness for the snow to accelerate once set in motion by the combination of mechanical failure (of the snowpack) and gravity. The angle of the slope that can hold snow, called the angle of repose, depends on a variety of factors such as crystal form and moisture content.
Some forms of drier and colder snow will only stick to lower angle slopes; while wet and warm snow can bond to very steep surfaces. In particular, in coatstal mountains, such as the Cordillera del Paine region of Patagonia, deep snowpacks collect on vertical, and overhanging, rock faces. The angle of slope that can allow moving snow to accelerate depends on a variety of factors such as the snow's shear strength, which is itself dependent upon crystal form, and the configuration of layers and inter-layer interfaces.
Source:www.wikipedia.com
