Shouting will not start an avalanche, but a skier crossing the slope can
Films love the yodel that brings down a mountainside, but the pressure of sound is far too feeble, by orders of magnitude, to break a snowpack. The real culprits are fresh snowfall, sunshine, rain and, all too often, people on skis or snowmobiles crossing a slope near the critical angle of 38 degrees.
Most avalanches release on their own during storms, as new snow or wind-blown drifts add weight. Melting driven by sunshine is the second biggest natural cause, followed by rain, earthquakes and falling rock or ice. The physics is simple to state and hard to predict: a snowpack fails when its load outweighs its strength. The load is just the weight of snow, but strength varies grain by grain with size, density, temperature, water content and the bonds between crystals, all of which keep changing.
Slab avalanches are the deadliest kind, responsible for around 90% of avalanche deaths. They happen when a stiff layer sits on a weak one; once a crack starts, it can race outward so that thousands of cubic metres break loose almost at once, leaving a sheer crown wall at the top. At the other extreme, powder avalanches mix snow with air into a turbulent cloud that can top 300 km/h and even run briefly uphill. Wet snow avalanches crawl at perhaps 10 to 40 km/h yet carry such mass that they gouge out boulders and trees.
Terrain sets the stage. Slabs usually start on slopes of 30 to 45 degrees, travel down a track of 20 to 30 degrees and stop in a runout zone gentler than 20. Slopes flatter than 25 or steeper than 60 degrees rarely slide, because snow either stays put or never builds up. Lee slopes behind ridges collect deep drifts and cornices, and convex bulges fail more readily than hollows. Dense forest helps anchor snow; scattered boulders can weaken it from below.
Doug Fesler and Jill Fredston boiled the hazard down to three elements: terrain, weather and snowpack. Their practical rule is blunt: any slope gentle enough to hold snow yet steep enough to ski can avalanche. In the United States about 28 people die in avalanches each winter.
Source: Avalanche
