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.

First it is a mechanical phenomenon: 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. 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.
Second it is a natural hazard: 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 avalanches during summer. 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.
Third it is an event: Avalanches cause loss of life and can destroy settlements, roads, railways and forests.
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.
Formation and classification
Most of the time, avalanches are caused by external stress on the snowpack; natural events are not random or spontaneous events. Natural triggers of avalanches include additional precipitation, rapid warming, rock fall, ice fall, and other impulse loads; however, even when environmental conditions are consistent, the seasonal snowpack will evolve over time and develop stresses, often from the downslope creep of the snowpack. Artificial triggers of avalanches include skiers, snowmobiles, and controlled explosive work. The triggering stress usually causes an avalanche at the location where force is directly applied to the snowpack (local trigger), but can in some cases cause avalanche formation at a different location nearby (remote trigger). Remotely triggered avalanches occur when a disturbance is transmitted from one location in the snowpack to another location in the snowpack. Small avalanches sometimes trigger much larger avalanches: for example, a small avalanche may apply significant overburden pressure to the snowpack, disturbing deeper weaknesses, and a larger avalanche may form as a result. This phenomenon is referred to as "stepping down".
A number of the forces acting on a snowpack can be readily determined. For example, there is little problem in calculating the weight of the snow, which provides information about the load on a weak layer. However, other factors are much more difficult to determine. It is very difficult to estimate the shear, ductile and tensile strengths within the snowpack or relative to the ground below. These strengths vary with the hardness of the snow, type of snow crystal, the number of bonds per unit volume, and the strength of contact interfaces between the layers. The thermo-mechanical properties of the snow crystals in turn depend on the local conditions such as ambient air temperature that control moisture transport inside the snowpack. One of the aims of avalanche research is to develop and validate computer models that can describe the evolution of the seasonal snowpack over time. A complicating factor is the chaotic interaction of terrain and weather, which causes significant spatial and temporal variability of the depths, crystal forms, and layering of the seasonal snowpack.
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.
Loose snow avalanches
Loose snow avalanches, most common in steeper terrain, often occur in freshly fallen, low-density surface snow, or in older surface snow that has been softened by strong solar radiation. In loose snow avalanches, the release usually starts at a point and the avalanche gradually widens as it travels down the slope and entrains more snow. The characteristic shape of a loose snow avalanche is usually described as resembling a teardrop. Large, loose snow avalanches may cause slab avalanches.
Slab avalanches
Slab avalanches form frequently in new snow, wind deposited snow, and, less frequently, in old snow, and have the characteristic appearance of a block of snow cut out from its surroundings by fractures. Elements of slab avalanches include the following: a crown fracture at the top of the start zone, flank fractures on the sides of the start zones, and a fracture at the bottom called the stauchwall. The crown and flank fractures are vertical walls in the snow delineating the snow that was entrained in the avalanche from the snow that remained on the slope.
Slab avalanches, which account for around 90% of avalanche-related fatalities, form when the application of dynamic forces causes catastrophic structural failure inside a weakness below a slab of snow. Energy for fracture propagation is provided by gravity as the slab falls onto the weak layer. This cascade of failures causes one layer of snow to delaminate from the layer of snow below, enabling gravity to pull the delaminated slab downhill. Fracture propagation can be widespread, sometimes traveling for hundreds of meters, and in some cases kilometers, and can involve snow depths ranging from 10 centimeters to five or six meters. Avalanches that form when the failure occurs between the base of the snowpack and the ground are known as full depth slab avalanches.
Among the largest and most powerful of avalanches, dry slab avalanches can exceed speeds of 300 km/h, and masses of 10,000,000 tonnes; their flows can travel long distances along flat valley bottoms and even uphill for short distances. A powder snow avalanche is a turbulent cloud of snow and air that forms when an avalanche travels over an abrupt change in slope angle, such as a cliff band. Powder snow avalanches may also form when the powder cloud of a dry slab avalanche continues moving after the core of the avalanche has stopped.
There are two main types of slab avalanches, "soft slab avalanches", and "hard slab avalanches". Both types of avalanches are denoted by debris morphology: the debris from a soft slab avalanche is highly granular, resembling a slurry of snowballs and ice grain paste, and the debris from a hard slab avalanche is angular, often featuring pieces of the original slab that did not break up during descent. Avalanches that descend significant vertical or horizontal distances may create debris that is not suitable for classification purposes.
Preventative measures are employed in areas where avalanches pose a significant threat to people, such as ski resorts and mountain towns, roads and railways. There are several ways to prevent avalanches and lessen their power and destruction; active preventative measures reduce the likelihood and size of avalanches by disrupting the structure of the snowpack; passive measures reinforce and stabilize the snowpack in situ. The simplest active measure is by repeatedly traveling on a snowpack as snow accumulates; this can be by means of boot-packing, ski-cutting, or machine grooming. Explosives are used extensively to prevent avalanches, by triggering smaller avalanches that break down instabilities in the snowpack, and removing over burden that can result in larger avalanches. Explosive charges are delivered by a number of methods including hand tossed charges, helicopter dropped bombs, Gazex concussion lines, and ballistic projectiles launched by air cannons and artillery. Passive preventive systems such as Snow fences and light walls can be used to direct the placement of snow. Snow builds up around the fence, especially the side that faces the prevailing winds. Downwind of the fence, snow buildup is lessened. This is caused by the loss of snow at the fence that would have been deposited and the pickup of the snow that is already there by the wind, which was depleted of snow at the fence. When there is a sufficient density of trees, they can greatly reduce the strength of avalanches. They hold snow in place and when there is an avalanche, the impact of the snow against the trees slows it down. Trees can either be planted or they can be conserved, such as in the building of a ski resort, to reduce the strength of avalanches.
To mitigate the effect of avalanches, artificial barriers can be very effective in reducing avalanche damage. There are several types. One kind of barrier (snow net) uses a net strung between poles that are anchored by guy wires in addition to their foundations. These barriers are similar to those used for rockslides. Another type of barrier is a rigid fence-like structure (snow fence) and may be constructed of steel, wood or pre-stressed concrete. They usually have gaps between the beams and are built perpendicular to the slope, with reinforcing beams on the downhill side. Rigid barriers are often considered unsightly, especially when many rows must be built. They are also expensive and vulnerable to damage from falling rocks in the warmer months. In addition to industrially manufactured barriers, landscaped barriers, called avalanche dams stop or deflect avalanches with their weight and strength. These barriers are made out of concrete, rocks or earth. They are usually placed right above the structure, road or railway that they are trying to protect, although they can also be used to channel avalanches into other barriers. Occasionally, earth mounds are placed in the avalanche's path to slow it down. Finally, along transportation corridors, large shelters, called snow sheds, can be built directly in the slide path of an avalanche to protect traffic from avalanches.
Terrain management - Terrain management involves reducing the exposure of an individual to the risks of traveling in avalanche terrain by carefully selecting what areas of slopes to travel on. Features to be cognizant of include not under cutting slopes (removing the physical support of the snowpack), not traveling over convex rolls (areas where the snowpack is under tension), staying away from weaknesses like exposed rock, and avoiding areas of slopes that expose one to terrain traps (gulleys that can be filled in, cliffs over which one can be swept, or heavy timber into which one can be carried).
Group management - Group management is the practice of reducing the risk of having a member of a group, or a whole group involved in an avalanche. Minimize the number of people on the slope, and maintain separation. Ideally one person should pass over the slope into an area protected from the avalanche hazard before the next one leaves protective cover. Route selection should also consider what dangers lie above and below the route, and the consequences of an unexpected avalanche (i.e., unlikely to occur, but deadly if it does). Stop or camp only in safe locations. Wear warm gear to delay hypothermia if buried. Plan escape routes. In determining the size of the group balance the hazard of not having enough people to effectively carry out a rescue with the risk of having too many members of the group to safely manage the risks. It is generally recommended not to travel alone, because there will be no-one to witness your burial and start the rescue. Additionally, avalanche risk increases with use; that is, the more a slope is disturbed by skiers, the more likely it is that an avalanche will occur. Most important of all practice good communication within a group including clearly communicating the decisions about safe locations, escape routes, and slope choices, and having a clear understanding of every members skills in snow travel, avalanche rescue, and route finding.
Risk Factor Awareness - Risk factor awareness in avalanche safety requires gathering and accounting for a wide range of information such as the meteorological history of the area, the current weather and snow conditions, and equally important the social and physical indicators of the group.
Leadership - Leadership in avalanche terrain requires well defined decision-making protocols that use the observed risk factors. These decision-making frameworks are taught in a variety of courses provided by national avalanche resource centers in Europe and North America. Fundamental to leadership in avalanche terrain is honestly assessing and estimating the information that was ignored or overlooked. Recent research has shown that there are strong psychological and group dynamic determinants that lead to avalanche involvement.
Control measures: In many areas, regular avalanche tracks can be identified and precautions can be taken to minimise damage, such as the prevention of development in these areas, the construction of avalanche sheds over existing roads and railways and the use of tunnels for new road and rail links. Avalanches cause danger when their path cannot be predicted and are a major hazard for skiers and mountaineers.

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