At danger level 2-moderate: Keep distance to each other and to release areas. NB, remote triggering is possible.

At danger level 3-considerable: Avoid steep terrain (more than 30 degrees) and in runout zones. NB, remote triggering is likely.

Make very conservative route choices, especially in unknown terrain, after snowfall and if temperatures rise. 

Identifying areas where the weak layer is present might be difficult and requires experience.

Danger signs are whoumpf-sounds, cracking and recent avalanches. However, absence of signs does not mean it is safe.

Persistent weak layer

Characteristics

  • The avalanche problem is related to the presence of persistent weak layers in the old snowpack.

  • These weak layers form in cold weather and typically include buried surface hoar, depth hoar or faceted crystals

  • Mostly human triggered avalanches; natural avalanches are rare, mainly in combination with other avalanche problems

  • Avalanches can become very large


 

Spatial distribution

  • The avalanche problem can be widespread or quite isolated.
  • Can often be found in specific elevations, expositions or terrain features.
  • Can exist in all aspects, but is more frequent on shady, wind sheltered slopes.
  • Will typically release in terrain between 30-40 degrees.

 

Release characteristics

  • Release of avalanche when loading exceeds the strength of the weak layer.
  • Fracture in the weak layer occurs either due to increased slab weight (loading) or to weakening of the weak layer.
  • Remote triggering is typical.

 


 

Possible weak layers

  1. Buried weak layer of surface hoar
  2. Buried weak layer of faceted snow near surface
  3. Buried weak layer of faceted snow above a crust
  4. Buried weak layer of faceted snow beneath a crust
  5. Buried weak layer of faceted snow near the ground

 

Duration

  • Stabilizes very slow. Weak layers can persist for weeks to months; possibly most of the winter season. 
  • Can exist in specific terrain features/pockets even after most other areas have stabilized.
  • Can often have "sleeping" or "hibernating" periods and become active again, typically in spring when the overlying slab becomes soft.

 

Identification of the problem

  • Persistent weak layers are very challenging to recognize.
  • Signs of instability such as whumps are typical but not necessarily present. Lack of such signs does NOT mean it is safe.
  • Typical danger signs:
    • Whupmf sounds
    • Shooting cracks
    • Remote triggering
    • Recent avalanche activity
  • Information on snowpack history is critical and reference to the published avalanche report is important.
  • Stability tests can be helpful to detect the persistent weak layers. However, to interpret the test and to find a good place to dig requires a lot of knowledge. 

 

Soft slab on top of persistent weak layer. Photo: NVE