weaklayer

A weak layer is a layer within a snowpack that has comparatively low shear strength relative to the layers above or below it. This weaker plane can act as a potential failure point when the overlying snow is subjected to additional load, such as new snowfall, wind deposition, or warming.

Weak layers commonly form through metamorphism and bonding processes that reduce cohesion between snow grains. Depth hoar crystals develop in cold, dry snow under strong temperature gradients, creating a porous and weak layer. Buried surface hoar, faceted grains, or wind-packed layers with poor bonding can also behave as weak layers. Wet or melting conditions can produce crusts that bond poorly to deeper layers, forming another type of weak plane.

Typical weak-layer types include depth hoar, buried surface hoar, faceted grain layers, and crusts with weak bonding. The exact strength of a weak layer depends on grain type, bonding, temperature, and stress history. The spatial distribution of weak layers within the snowpack is often uneven, leading to localized stability variations.

Role in avalanches: A weak layer can serve as the fracture plane for slab avalanches when the load exceeds the shear strength of the layer, causing the overlying slab to slide off. The likelihood of release depends on snowpack structure, weather history, and current loading conditions. Forecasting relies on identifying potential weak layers and assessing how they interact with overlying snow.

Identification and testing: Snowpack analysis, including snow pits and stability tests, helps evaluate weak-layer presence and strength. Field indicators include crack formation, a visible drop in strength when tapping or cutting, and a sudden failure along a plane within the snowpack. Common tests include the Extended Column Test and other shear or compression tests.

See also: avalanche, snow science, snowpack.