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Weak Layers and Avalanches
Written: 2008.03.03 by: Frank W. Baumann

Persistent Weak Layers (PWL's) are a major cause of large and fatal avalanches. They are particularly dangerous because they can leave the snowpack in a weakened state for a long period of time, trigger-ready to avalanche at any time, even though the overall avalanche danger rating (hazard) may be relatively low. This article provides information to understand and help manage the risk.

[photo]whistleravalanche.jpg[caption]Large avalanche below Whistler peak, triggered by avalanche control, that ran on a persistent weak layer. The skiers are not spread out far enough- ideally, only one person at a time should be in a potential slide path.[/photo] Persistent weak layers (PWL's) are potential sliding layers in the snowpack that remain unstable for an unusually long time. They are responsible for producing many large and fatal avalanches. Persistent weak layers can form in many ways; for example, when temperatures are below freezing and the air is supersaturated (>100%

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