Persistent Weak Layers

/Persistent Weak Layers
Persistent Weak Layers 2017-10-20T13:18:52+00:00

Sometimes within the snowpack weak layers are developed and preserved. These are formed by big and sharp snow crystals, capable of collapse when overloaded and cause a slab avalanche. These unstabilities can persist weeks.


Tactics: Be conservative when choosing terrain

Duration: days to weeks


Typical signs:

  • Weak layer within the snowpack
  • Alarm signs (whumpf sounds)

Typical distribution:

  • Regions / zones with relatively shallow snow cover
  • Terrain transitions (e.g. from flat to steep or from ridge to bowl)
  • Rocky outcrops
  • Often found on north facing slopes


  • Simple snowpack tests often useful
  • Difficult to recognise
  • Pay attention to bulletin information
Foto: Montse Cuixart

Favourable conditions:

  • Deep snowpack
  • Similar layers
  • Critical layers (often soft layers) more than 1 meter below the snow surface

Unfavourable conditions:

  • Shallow snowpack
  • Large differences between layers, e.g. hardness and grain size
  • Critical layers (often soft layers) in the upper meter of the snowpack

Snowpack tests

Avaluating the snowpack becomes more important when unstability signs are not obvious

  • Penetration Depth (with or without skis): Allows the identification of weaker base layers in a relatively thin snowpack. Thinner layers are not easily recognised by this method.
  • Pole Test: Allows the identification of differing hardness between layers.
  • Snowpack Tests (e.g. column or block tests): Allows the identification of weak layers and an estimation of their strength.
  • Weak layers are often soft and coarse grained.