Avalanche!

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.

Terrain Slope

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.