Avalanches have three ingredients -- snow, a sloped surface and a trigger. A weak layer within the snowpack, caused by ice, surface or depth hoar, faceted crystals or graupel also contributes to the process. If the weak layer is near the surface, it causes a sluff -- a cascade of loose, powdery snow in an inverted "V" shape down the slide of the mountain. Sluffs are like sand rolling down a dune, and they usually cause minimal damage to people and property.
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If the weak layer is deeper in the snowpack, it can cause a slab avalanche, which is far more dangerous. In a slab avalanche, a strong, cohesive layer of the snowpack slides down over a bed layer of snow, like thawing snow sliding down a car's windshield. Sometimes, the entire snowpack breaks free from the mountain and slides over the ground.
The strength of a slab avalanche depends on the properties of the slab and the depth of the weak layer, also called the failure layer. Hard, cohesive slabs create very large chunks of solid slow, while softer slabs create smaller blocks. Slabs of wet snow cause generally slower avalanches than dry slabs, but they typically hit obstacles with more force.
Avalanches usually start on mountain slopes that are at a 25 to 60 degree angle to the ground. Slopes less than 25° generally aren't steep enough to produce avalanches, and slopes steeper than 60 degrees usually sluff their snow constantly, giving slabs little chance to develop. Most avalanches begin on 35 to 45 degree slopes.
Most slab avalanches take place on leeward, rather than windward, slopes. They can have a natural trigger, like a sudden change in the weather, a falling tree or a collapsing cornice -- an icy overhang of wind-driven snow near the ridge. In spite of what movies and cartoons depict, the trigger is almost never a loud noise. In most fatal avalanches, people create the trigger. Once they begin, they have three segments:
- A starting zone, often above the tree line and near the ridge, where the slab breaks away from the rest of the snow.
- A track, or the course the avalanche follows down the mountain. You can often see avalanche tracks even in the summer because of missing trees.
- A runout, where the sliding snow and debris eventually comes to a stop.
When the snow stops, it compacts and sets up like concrete. This is what makes avalanches so dangerous to skiers, hikers and snowmobilers -- they generally cannot dig themselves out and must wait for rescue.