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
Tips:
- 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.