Technical information, news, research, and opinion on avalanches, snow safety, and winter backcountry travel.

Monday, November 23, 2009

Types Of Instability

Don't you, forget about me—Simple Minds

NOTE: Backcountry avalanche forecasting is composed of four interconnected elements: goal, people, awareness, and uncertainty. The type of instability is linked to awareness and uncertainty—maintaining awareness and managing uncertainty are central tools for helping people successfully accomplish the goal of backcountry avalanche forecasting.

It's important to outline the two key classes of instability: temporary instability in new snow and persistent instability in old snow. Safe winter backcountry travel often involves dealing with both types of instability to some degree. Persistent instabilities are harder to trigger but much more difficult to manage. Temporary instabilities are much easier to trigger and easier to manage. Since it doesn't take very much snow to bury, injure, or kill a person, both types of instability can be equally dangerous.

Table 1.1. The two types of instability.
Short-Term InstabilityGenerally found in new snow ( storm snow ). Instability expressed by loose snow avalanches and soft slab avalanches. Generally does not surprise professionals; definitely surprises recreationists. Instability at surface is easy to find. Large, new-snow slab avalanches may trigger secondary avalanches that involve long-term instability. Recency is critical factor; sensitivity to trigger is high. Hazard is obvious.
Long-Term InstabilityGenerally found in old snow. Instability expressed by wet or dry hard slab avalanches. Surprises professionals and recreationists alike. Instability far below surface is difficult to evaluate. Produces large primary avalanches. Sensitivity to trigger is unknown; recency is much less unimportant. Hazard is unclear.

Combinations Of Short-Term And Long-Term Instability
Absolutes are rare in chance-based activities such as backcountry avalanche forecasting. Even the type of instability is rarely a choice between one or the other. For the purposes of backcountry avalanche forecasting, instability is separated into weaknesses and triggers. The actual type and distribution of these weaknesses is usually a mix of long-term and short-term instabilities.

Figure 1.1. Combinations of long-term and short-term instability. The typical state of instability is often a blend of both, and largely defined by recent weather and the parameters of the snowpack.

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