An avalanche is a layer of snow that flows down a mountain slope. Only three things are required for an avalanche: a slope between 25 and 65 degrees, a layer of snow with a weak layer beneath and a trigger. The potential for loss of life is underscored by statistics between the Austrian and Italian armies, with over 100,000 casualties from avalanche, which easily dwarfs all other reasons for troops lost in the Alps during World War I.
Sluff avalanches, similar to grains of sand rolling down a dune, by themselves, are rarely hazardous because the amount of material is less dense and often weighs comparatively little compared to the millions of metric tons of fresh water locked up as ice and snow. Slab avalanches, where a compacted chunk of snow breaks away from the snowpack, are extremely dangerous. Both kinds of avalanche also trigger weakened layers further downslope in a cascading effect called “stepping down.” The time it takes for the snowpack to go from zero to eighty miles per hour can be less than 6 seconds.
Contrary to popular myth, loud noises and echoes are not known to start avalanches. Naturally occurring triggers are animal movement, new snowfall on weakened layers, windblown snow on weakened layers, and warmer temperatures, particularly in spring, but summer avalanches do occur in some regions. While comparatively rare, earthquake and vulcanism can also cause the snowpack to shift which adds avalanche to an already dangerous environment.
Human caused triggers, especially from back country recreation, are the ones that will generally make the headlines. While many commercial and highly traveled recreational areas will have monitoring and, in some cases, preemptive demolition, to prevent avalanche accidents, the more adventurous tend to wander into areas that are not so well regulated for this specific hazard. Often, the weight of a hiker, skier or snowmobile is just enough to start the chain reaction that causes so much snow to move downhill on short notice.
In spite of being unable to safely examine a weakening layer under a snowpack, winter recreational forecasts for potential avalanche hazards are very good worldwide. Even so, dozens of people lose their lives to avalanches annually. Staying on top of ridge lines and avoiding the runoff areas with no trees will mitigate the safety risk to some degree. If caught in an avalanche, trying to swim on top of a moving snow pile that is three times less dense than a human body will be extremely difficult.
Compounding the risk to life, beyond the battering, is the fact that the snow sets like concrete shortly after it comes to rest. If buried, this will make breathing difficult and digging out alone, next to impossible. In addition, there's the hypothermia risk and the likelihood that rescue teams are not immediately at hand. The survival statistics, after 45 minutes of being buried under snow, are dismal.