Which grain type is considered persistent in snowpack stability assessments?

• A. Depth hoar
• B. Round grains
• C. Melt-freeze crust
• D. Granular snow

Answer: (A)

In snowpack stability, persistent weak layers are layers that stay weak over time, through multiple weather cycles and loading events. Depth hoar fits this description because it forms at the base of the snowpack when a strong temperature gradient drives extensive metamorphism, creating large, faceted grains that bond very weakly. This coarse, low-strength layer tends to persist through the season, and when new load or warming occurs, it can fail and trigger deep slab avalanches. The other grain types come from more typical metamorphism and generally bond more strongly, so they don’t establish the long-lasting weakness that depth hoar does. Melt-freeze crusts can influence stability in some situations, but they aren’t the classic, persistent weak layer that avalanche practitioners focus on.