What minimum grain size difference between a weak layer and its surrounding layers is considered significant for stability assessment?

• A. Greater than 5 mm
• B. Less than 0.5 mm
• C. Equal to 1 mm
• D. Greater than 1 mm

Answer: (D)

Differences in grain size across a snow layer boundary affect how well the layers bond and transfer load. When the weak layer stands out by more than about 1 millimeter from the surrounding snow, the contact between grains changes significantly, leading to poorer bonding and a weaker interface that can fail under stress. That size contrast often means the layer won’t sinter together as effectively or interlock with adjacent grains, so it becomes a potential weak plane where shear can occur, increasing avalanche risk.

Smaller differences tend to behave more like a continuous, well-bonded system, so they’re less likely to act as a distinct weak layer, which is why the threshold is set around 1 millimeter. In practice, this guideline helps you quickly identify layers that warrant closer testing or management, while acknowledging that it’s a heuristic rather than a hard physical boundary.