Describing Snow Instability by Failure Initiation, Crack Propagation, and Slab Tensile Support

Snow instability is a generic term describing the propensity of a snow slope to avalanche. In need of a concise mechanics‐based concept we suggest a framework based on failure initiation, crack propagation, and slab tensile support. Following these three steps we modeled three metrics from mechanical data, which we derived from snow micropenetrometer signals. Verifying the metrics with field measurements confirmed that slab thickness and weak layer strength typically influence failure initiation, elastic modulus and weak layer fracture energy largely control crack propagation, and slab thickness and tensile strength provide the required tensile support. For all three metrics, considering slab layering was essential. Validation with signs of instability showed that the most accurate model includes all three steps – suggesting that snow instability can be described by failure initiation, crack propagation, and slab tensile support. Further validation is needed to assess the framework's potential for operational use. Plain Language Summary Snow avalanches threaten people and infrastructure in all snow‐covered mountain regions. Avalanche mitigation includes avoiding avalanches in space and time. The latter requires avalanche forecasting, that is, predicting snow instability or the probability of a snowpack to avalanche. As properties of the mountain snowpack vary spatially and evolve with time, modeling approaches are better suited than point observations to assess snow instability. However, for simulating snow instability in snow cover models quantitative measures are needed, preferably reflecting the processes leading to slab avalanche release. Dry snow slab avalanche release can be interpreted as a sequence of fractures, including failure initiation and crack propagation. We follow these steps and complement our recently developed metrics describing failure initiation and crack propagation with a criterion for the tensile support of the slab. We identify the snow properties that are most relevant to drive the three processes and validate the metrics with independent field observations. The validation confirmed that snow instability is best interpreted as a combination of failure initiation, crack propagation, and slab tensile support. Our new model to describe snow instability on mountain slopes is now ready to be assessed in operational use for avalanche forecasting. Key Points Fracture processes relevant for dry snow slab avalanche release were modeled wi