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Wet-snow avalanches

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What?

Characteristics

The avalanche problem is related to weakening of the snowpack due to the presence of liquid water. Water infiltrates the snowpack due to melt or rain.

Expected avalanche types

  • Wet‐snow slab avalanches        
  • Wet loose snow avalanches     
  • Mainly natural avalanches

Where?

Spatial distribution

When sun is the main cause, distribution of the problem is mostly depending on aspect and elevation. All aspects are affected in the event of rain on snow.

Position of weak layers in the snowpack

Anywhere in the snowpack

Why?

Release characteristics

Wet‐snow slab avalanches:

  • Weakening of pre‐existing weak layers in the snowpack or ponding at layer interfaces
  • If rain, there is also additional loading on weak layers

Wet loose snow avalanches:

  • Loss of cohesion between snow crystals

When?

Duration

  • Hours to days
  • Rapid loss of stability possible
  • Especially critical as water infiltrates for the first time deeper down, once the snowpack has warmed up to 0 °C.
  • Natural avalanches might be more likely at certain times of the day, in particular in the afternoon (unless rain is the dominating factor).

How to manage?

Identification of the problem in the field

The wet snow problem is usually easy to recognize. Onset of rain, snowballing, pin wheeling and small wet slabs or loose wet avalanches are often precursors of natural wet‐snow slab avalanche activity. Deep foot‐penetration is another sign of increased wetting.

Travel advice

In the presence of a sun crust the conditions after cold nights with clear skies are usually favourable in the morning due to freezing. After warm nights with overcast skies the problem often exists already in the morning. Normally rain on fresh snow creates this problem almost immediately. Good timing and trip planning are important. Consider avalanche runout zones.

Typical accident

Fründenhütte refuge, Kandersteg (BE), 6 June 2010