Avalanche safety begins with recognizing when a snowy slope is capable of failing. Most deadly avalanches are slab avalanches, where a cohesive plate of snow breaks loose and slides on a weaker layer below. Students, hikers, skiers, and snowmobilers need to read snow, terrain, and weather together because danger often builds before a slide is visible.
Good decisions in avalanche terrain depend on noticing warning signs early and avoiding exposure before an emergency happens.
A dangerous snowpack often has strong layers sitting on weak layers, much like a heavy board resting on loose sugar. New snow, wind loading, rapid warming, and recent avalanches can all increase stress on the snowpack. Slopes between about 30° and 45° are the most common starting zones for slab avalanches, especially when connected to steeper terrain above.
Recognizing cracking snow, hollow sounds, cornices, and wind-drifted pillows helps travelers choose safer routes and reduce risk.
Key Facts
- Most slab avalanches start on slopes of about 30° to 45°.
- Avalanche risk = hazard x exposure x vulnerability.
- A weak layer plus a loaded slab creates the basic structure for a slab avalanche.
- Recent avalanches are a strong warning sign that similar slopes may also slide.
- Rapid loading from new snow, wind-drifted snow, or rain can quickly raise avalanche danger.
- Companion rescue is most urgent in the first 15 minutes after burial because survival drops quickly with time.
Vocabulary
- Slab avalanche
- A slab avalanche is a slide where a cohesive layer of snow breaks away and moves downhill as a sheet.
- Weak layer
- A weak layer is fragile snow inside the snowpack that can collapse and allow stronger snow above it to slide.
- Snowpack
- The snowpack is the full stack of snow layers on the ground, formed by storms, wind, temperature changes, and time.
- Slope angle
- Slope angle is the steepness of a slope measured in degrees from horizontal.
- Wind loading
- Wind loading is the buildup of extra snow on certain slopes when wind transports snow and deposits it in drifts.
Common Mistakes to Avoid
- Judging danger only by how the snow looks on the surface is wrong because weak layers can be hidden deep inside the snowpack.
- Assuming a slope is safe because others crossed it is wrong because each person can add stress and trigger a weak layer after earlier travelers passed.
- Ignoring small cracks, whumpf sounds, or recent slides is wrong because these are direct signs that the snowpack may already be failing.
- Traveling under a steep slope while avoiding the slope itself is wrong because avalanches can run far into flatter terrain below the starting zone.
Practice Questions
- 1 A group reaches three slopes with measured angles of 24°, 34°, and 48°. Which slope is in the most common slab avalanche starting range, and why?
- 2 A storm drops 30 cm of new snow, and wind adds another 20 cm of drifted snow to a leeward slope. What is the total added load depth on that slope in centimeters?
- 3 You hear a hollow whumpf sound and see shooting cracks spreading from your skis near a 35° slope. Explain what these signs suggest about the snowpack and what safer decision the group should make.