Wind-drifted snow - slab avalanches

Be careful and keep distance to each other in steep (above 30 degrees) terrain with fresh wind deposited snow, especially convex terrain.

If danger Level 3-considerable, avoid steep (more than 30 degrees) areas with fresh wind deposited snow, especially convex terrain.

An avalanche is easier to trigger where wind slabs are thin or soft.

Look for areas where the wind recently has deposited snow, typically behind ridges, ribs and in gullies. Remember that local wind effects will cause variation in where the wind slabs occur.

Characteristics

  • This avalanche problem results from wind-drifted snow.
  • Snow can be transported by wind with or without concurrent snowfall.
  • Avalanches vary in size and hardness depending on wind speed and amount of loose snow available for transport.

 

Spatial distribution

  • Highly variable but typically on leeward sides, in gullies, bowls, near distinct changes in slope angle, behind ridgelines or other wind-sheltered locations.
  • More common above treeline.
  • Avalanches release typically in terrain features steeper than 35 degrees where snow has accumulated due to the wind.

 

Trigger mechanisms

  • Wind-drifted snow builds a soft or hard slab which is particularly prone to support crack propagation.
  • Loading of wind-drifted snow leads to potential fracture in weak layers.

 

Location of weak layer in the snowpack

Usually at or close to the interface to the old snow surface or within the slab layer due to variations in wind speed during storm cycle. If the weakest layer is in the old snowpack another avalanche problem wil be chosen, i.e. Persistent weak layer.

  1. Poor bonding between layers in wind deposited snow
  2. Buried weak layer of new snow

 

Duration

  • Wind slabs can evolve very quickly.
  • The problem lasts typically during the snowdrift event and up to a few days later, depending on temperature and radiation.

 


 

Identification of the problem

  • If not hidden by new snow the wind-drifted snow problem can be recognized with training and good visibility.
  • Consider wind signs and locate deposits.
  • Can often be recogniced by change in the snow consistence and chages in snow hardness.
  • However, it is often hard to determine the age of wind signs and wind signs do not necessarily imply an avalanche problem (e.g., in absence of a weak layer).
  • Typical clues:
    • Wind signs on the surface
    • snowdrift deposits, soft or hard
    • recent avalanche activity
    • shooting cracks 
    • whumps (rarly)

 

Drifting snow in Lofoten. Photo: Ragnar Ekker