Avalanche!

Snowpack Instability

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

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