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The snows of winter bring many hazards to humans and wildlife. In urban areas, these hazards are usually manifested as transportation- or power-transmission-related hazards. But in mountainous terrain, another danger arises: the avalanche.
In simple terms, an avalanche is a mass of snow moving down a slope through gravitational pull. While most avalanches are small, unnoticed events in snowy mountain country, some produce monstrous snow torrents that can bury villages and roads, push locomotives off their tracks, snap trees and kill any human or animal caught in its path. An avalanche's size and destructive potential depend on several factors including: the amount of snow available, the terrain and its vegetative cover, the steepness of the slope, and the instability of the snowpack. Weather factors also play key roles in the initiation and ultimate severity of an avalanche.
Snowflakes and Snow Packs
When snow forms in clouds, the snow crystal's shape depends on the temperature and humidity of the surrounding air (for more, see Snowflakes). While individual crystals and massed snowflakes fall, they undergo alterations depending on wind, temperature and moisture conditions of the surrounding air. Once settled on the surface, the crystals undergo further changes. They are not only affected by the air to which they are exposed but also by sunshine, additional precipitation and the weight of snow and ice above them. As the snow crystals alter, melt and reform within the snowpack, their ability to stick to one another changes. Over time, some layers of snow adhere well to older layers beneath and newer ones. But for others, the bond between layers is tenuous at best, and the potential for movement between layers increases with time.
If a snowpack lies over a flat surface, we have no danger for avalanche. But if one forms on a mountain slope, trouble brews. Avalanches can occur on slopes ranging from 10 to 60 degrees of tilt (steeper slopes usually lose much of their snow before accumulating sufficient depth to be of concern). The most hazardous avalanches begin on slopes between thirty and forty-five degrees. A good rule of thumb is that if a slope is steep enough for good skiing, it poses an avalanche threat.
Slope shape can also influence the probability and extent of an avalanche. Convex slopes, those that bow outward, are less supportive of snowpacks and more readily produce avalanches. Concave slopes, those that bow inward like a trough, generally have more stable snowpacks. The underlying terrain cover also has a major influence on avalanche potential. Smooth rock faces and grassy meadows make good sliding surfaces for an avalanche. Slopes with many shrubs and small trees or with boulder fields can anchor a snowpack, particularly when the elements are close together and numerous, at least until the cover becomes buried. Dense stands of trees offer good snow anchors in avalanche-prone terrain.
With snow lying on a slope, the greatest concern is the snowpack's stability, or rather, its degree of instability. Without some weakness within the snowpack, it will not move under normal environmental conditions. A look at the nearest snow-capped peak shows us that snow and slope are not always sufficient to start an avalanche. The stability of a multilayered snowpack generally depends on how strongly the various layers cling to each other.
Over the course of a snow season, the snowpack writes a layered history of sequential snowfalls, immigrant blown snow, and the weather conditions between the accumulations. Changes to the snow crystals within the layers alter the environment of the whole. Usually, one weak layer arises somewhere in the snowpack, and it is on this boundary where the slide will likely begin. Avalanche-control experts look for this potential smoking gun within a snowpack because the weak layer's depth determines the severity of the avalanche threat.
If the weakness occurs high in the snowpack, the threat can be relieved by small slides known as sluffs. These begin over a very small area, collecting additional snow as they move downslope, and widening as they slide. Sluffs do not involve much snow and thus are seldom a major problem. In fact, they often relieve the danger of future, larger avalanches.
A multi-layered snowpack remains in place as long as the forces that keep its layers together remain greater than any accumulated stresses, such as gravity, that work against that adhesion. In many cases, snowpacks will resist all natural and human-imposed stresses and remain in place until melting eliminates the snowpack. But often only a delicate balance holds off an avalanche, and all that is missing is a trigger to get the snowball rolling. That trigger may be additional snowfall, a change in temperature, wind pressures, or pressures on the top layer exerted by animals, skiers, snowmobilers, or hikers. In the extreme, a loud sound can jar the surface loose.
When a weak layer is buried beneath other deep, more-cohesive snow layers, a large slab avalanche threatens. Slab avalanches begin when a large slab of snow breaks away and begins moving downslope. Being large snow masses at the start, they can easily trigger other large slabs to begin moving, thus starting a chain reaction that can grow until a roiling river of snow surges downhill. These avalanches not only move snow but often have the force to rip trees and boulders from the ground. The avalanche continues until it has used up all its energy. Slab avalanches are the most common killer avalanches and destroyers of property.
Another avalanche type is the cornice avalanche. A cornice is a mass of snow stretching out from a ridge or cliff line, often superbly sculptured like a frozen water wave, usually formed when snow blown by prevailing winds from slopes on one side of the ridge are deposited in the lee. At times, cornices seem to defy gravity as they hang suspended in space. Though they are often very dense and hard surfaced, they maintain an extremely fragile hold on the mountain. When a cornice loses its precarious hold, it can trigger surrounding snowfields to avalanche as it moves downslope. Deadly cornice avalanches are often triggered by skiers, hikers, or snowmobilers underestimating the cornice's fragility as they pass over or under the hanging cornice.
Each year, avalanches claim more than 150 lives worldwide, but that number has increased over the past few decades as more people travel to backcountry mountainous regions for recreation. In the past decade, avalanches in Canada have claimed an average of 13 lives per year, but at least 28 died in avalanches during the 2002-03 winter season. The United States recorded 229 avalanche deaths between 1995 and 2002, with 35 during 2001-02.<
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