Question

Which condition primarily causes an avalanche to occur?

• A. When the air temperature drops significantly below freezing.
• B. When the weight or "load" on the upper layers of snow becomes greater than the cohesive forces holding the snowpack together.
• C. When strong winds blow snow off the mountain peaks.
• D. When there is a sudden increase in sunlight melting the snowpack surface.

Answer 1

Answer:

B. When the weight or "load" on the upper layers of snow becomes greater than the cohesive forces holding the snowpack together.

An avalanche primarily occurs when the weight or "load" on the upper layers of snow becomes greater than the cohesive forces holding the entire snowpack together.

• Load vs. Strength: The snowpack is a layered structure, and its stability depends on a delicate balance between the downward force of its weight (load) and its internal strength (cohesion and friction between snow grains and layers).

• Weak Layers: Often, there are weaker layers of snow (e.g., from different snowfall events, temperature changes, or crystal types) buried within the snowpack that have less cohesion.

• Triggering Event: When the added load from new snow, wind-blown snow, rain, or an external force (like a skier, snowmobile, or even seismic activity) exceeds the strength of these weak layers or the overall snowpack, a fracture can propagate.

• Gravitational Pull: Once this bond breaks, gravity takes over, pulling the unstable mass of snow rapidly down the slope.