Snow avalanches are a natural process, occurring perhaps 1,000,000 times per year, world-wide. They are one way for snow on an incline to adjust to the pull of gravity. The vast majority of these slides are not a problem, because an avalanche, in and of itself, is not a hazard. A person (or a person's belongings) has to get involved in order for there to be a problem.
The fact is, that avalanches don't drop from the peaks onto the heads of unsuspecting innocents with the unpredictability of a plummeting meteorite. 95% of people who are caught in avalanches are caught by a slide that was triggered by themself or a member of their party.
I think this is good news! Our behaviour creates the hazard, so we can change our behaviour to avoid problems.
There are many different types of avalanches, but the one that worries us the most is the "slab" avalanche, in which a mass of cohesive snow releases as a unit. This type is easily recognized by its distinct crown and flanks (click for a diagram of the nomenclature associated with avalanches).
Slab and other avalanches can be hard or soft, wet or dry and can be triggered naturally or artificially.
Spotting Avalanche Hazard: The Avalanche Triangle
Some places have avalanches: Switzerland's Alps, Utah's La Sals, Nepal's Himalayas, New Zealand’s Mountains. Some places don't: South Viet Nam, the Mile-High Stadium, your living room (the distinctions are not always so obvious, unfortunately).
Why is this? Avalanches are formed by a combination of three things that together are known as the "Avalanche Triangle". These 3 ingredients may be present in one location but absent 10 feet away. The three legs of the triangle are Snowpack, Terrain and Weather.
There are many tip-offs to the presence of a potential avalanche, including surface clues and active stability tests. You should never trust a single information source--stability evaluation is an ongoing process!
The information from this page originates from www.avalanche.org.