A slab of snow rests on a buried weak layer on a slope. Gravity resolves into a shear component along the slope and a normal component pressing into the weak layer; release happens when shear stress exceeds the weak layer's shear strength.
τ = ρ·g·h·sin(θ)·cos(θ) (driving shear stress)
SI = τ_strength / τ (stability index)
SI < 1 → weak layer fails → avalanche releases
- Slope angle — most avalanches release between 30°–45°; steeper slopes increase shear stress.
- New snow load — thicker/heavier slab increases both driving stress and consequence size.
- Weak layer strength — represents buried surface hoar or depth hoar; lower strength means easier failure.
- Trigger Release — manually initiates failure (simulating a skier's trigger) once conditions allow.
Real application: avalanche forecasters compute exactly this stability index from snowpit tests to set public danger ratings before backcountry travel.