How Does an Avalanche Originate & Occur?

According to the Canadian Avalanche Centre, about 150 people are killed each year by avalanches, mostly in the backcountry, beyond the bounds of ski resorts. Avalanches are unpredictable and misunderstood natural phenomena, caused when inconsistencies in the snow send it tumbling down the mountainside. Participants in winter sports often trigger avalanches, so understanding how they work is vital to winter sports safety.

  1. Snow Instability

    • The cause of most avalanches is snow instability. Bruce Tremper of the Utah Avalanche Forecast Center in Salt Lake City explains that snow forms layers as it falls, and some of these layers become warmer or more unstable than others. Furthermore, the top layer may melt and refreeze, forming a hard slab of ice. If the layer of snow beneath this slab is weak, then the slab may begin to slide, triggering an avalanche. According to both Tremper and the Canadian Avalanche Centre, slab avalanches are the most dangerous, sending chunks of ice the size of a car hurtling down the mountain at speeds of up to 100 miles per hour.

    Slope Conditions

    • Slopes do not need to be steep for an avalanche to occur.

      The mountainsides do not have to be particularly steep for an avalanche to occur. According to Tremper, a 30-degree slope may be steep enough to trigger an avalanche. The Canadian Avalanche Centre reports that most avalanches occur on rocky, 30- to 40-degree slopes, at or above the tree line.

    Human Factors

    • The victims of avalanches often cause them. In the great majority of avalanche accidents, human actions play a role in triggering the avalanche. The weight of even one person can be enough to break a fragile layer of snow. If the next layer above it is an ice slab, that damage may be enough to start unstable snow sliding down the mountain, carrying the slab with it. Tremper adds that many people barely avoid disaster, “kind of like playing soccer on a minefield and you didn’t weigh quite enough to set the thing off.”

    Avalanche Myths

    • Although the weight of a single person can begin an avalanche, it is a myth that shouting or snowmobile motors produce enough noise to start an avalanche. While an explosion at close range may produce strong enough sound waves to trigger an avalanche, human voices and motors aren't loud enough. It's also unlikely that you'd be able to outrun, outride or out-ski an avalanche, especially since the victim usually also causes it.

    Safety

    • Involve all members of the party, even beginners, in decision making.

      According to the Canadian Avalanche Centre, 86 percent of avalanche victims survive. The Centre recommends risk avoidance as the number one strategy for survival on a mountain. While the organization recommends that experienced people should lead a mountaineering group, everyone should watch for signs of instability and feel empowered to insist upon reassessment of conditions at any time. The Centre's website cautions against believing that good weather automatically means safe conditions. Furthermore, avoiding fatigue and focusing on safety, not goals, will make sound judgment more likely while on a mountain.