The mechanical flaw responsible for slab avalanche failures.

A layer of relatively weak snow sandwiched between layers of stronger snow.  Credit: Sawtooth Avalanche Center

Weak layers are stratigraphic layers in the snowpack that have relatively less strength than surrounding layers. A slab avalanche occurs when a weak layer collapses and fractures under the weight of an overlying slab. Weak layers come in a variety of grain types and sizes, form under a variety of conditions, and can be transient or long-lived. Understanding the spatial distribution and sensitivity of weak layers in the snowpack is key to managing slab avalanche hazards.

The walls of this snowpit were carved to allow light to shine through. The weak layers are easy to see because they are lower density and more porous, thus brighter. Credit: Crested Butte Avalanche Center

A weak layer forming on the snow surface. Credit: Crested Butte Avalanche Center

A weak faceted layer that failed below a slab. Credit: Crested Butte Avalanche Center

A weak layer at the snow surface (graupel) and buried weak layers near the ground, responsible for the collapse and shooting crack shown in the photo. Credit: Crested Butte Avalanche Center