The plane, or relative location within the snowpack, where an avalanche fractures.
Paying attention to failure interfaces can help inform what kind of avalanche characters exist and which weak layers are active. The failure interface is sometimes referred to as the “bed surface.”
- Storm snow (S): The avalanche releases on a layer within the new or recent storm snow. These weak layers often form from density differences within the storm snow due to changes in wind speed, temperature, or snowfall rate during a storm, and they typically stabilize quickly (within a day or two).
- New/old interface (I): The avalanche releases at the interface of new or recent storm snow and an older layer, sometimes called the storm interface. These weak layers, such as crusts, facets, or surface hoar, often form on or near the snow surface during the days or weeks that precede a storm.
- Old snow (O): The avalanche releases within the old snow below the most recent storm and storm interface. These weak layers, such as crusts, facets, surface hoar, or depth hoar, are commonly persistent weaknesses that have been buried by several generations of storms.
- Ground (G): The avalanche releases at the ground, on glacial ice, or firn. This interface can present as facets or depth hoar at the base of the snowpack. Full-depth glide avalanches also fail at the ground.