Avalanches are common in high mountains everywhere; perhaps as many as 250,000 major snowslides occur every year, though not all of them destroy life or property.
Avalanches and glaciers are entirely different phenomena. While glaciers are composed of thick, slow-moving masses of rock-hard ice, avalanches are fast-moving masses of loose snow.
Snow on a slope is always in delicate balance. With successive falls, tiers are built up over the ground, something like the layers of a cake. As temperature and weather conditions change, these layers settle, pack down, melt and refreeze. New snow, for instance, may fall atop a previous layer that has become a crystalized mass of ice. The bond between the two tiers is tenuous. Any small disturbance can start the top layer of snow sliding over the icy crystals underneath.
Mountaineers divide avalanches into two categories: "ground" and "dust."
Dust avalanches commonly occur when fresh heavy snow fails to cling firmly to older layers. The danger is considered greatest when ten or more inches of new snow have piled up over a period of 24 to 48 hours. Pulled downhill by the force of gravity, the top layer starts sliding. The heavy mass picks up speed quickly and gathers more snow and weight as it moves. In just a few seconds, down a long slope, it reaches fantastic speeds - often more than 200 miles per hour. Dust avalanches also create enormous air pressures in front of the fast-moving wall of snow. Such a powerful "wind" can uproot many acres of trees 250 acres of sturdy, 100-year-old trees in the Alps without snow from the avalanche itself actually touching them.
Ground Avalanches-slides of wet snow-seldom achieve more than 60 miles per hour, but also can be destructive. This type of avalanche is common in the spring when snow starts to melt. The snow in ground avalanches tends to roll up into balls as it moves, gathering up debris such as earth, uprooted trees and boulders along the way in a mass that may weigh a million tons. Forces of as much as 100 tons per square yard have been measured on obstacles in the paths of such avalanches.
While avalanches are an inevitable characteristic of mountainous areas, they have been causing more death and destruction than any time in history prior to the late 20th century due to our greater interest in mountain-based sports and activities, such as skiing and other winter sports and winter homes and resorts being built on land where avalanches have been crashing down for centuries.
But unrestricted development is not the only cause of avalanche deaths. Careless skiers start more than half of all reported avalanches and account for many of the casualties.
Snow scientists still have great gaps in their knowledge of avalanches and how to protect against them. Thousands of baffling questions remain to be answered. How can snow be artificially stabilized? What specific snow surfaces are sensitive to specific weather conditions? How can protective trees best be grown in avalanche areas?
Avalanche Safety Rules
1. Even without a warm-up of temperature or wind, a foot of fresh snow presents a likely peril on any ski slope steeper than 25 degrees. Eighty percent of all avalanches start during or immediately after snowstorms.
2. As temperatures go up, avalanche odds increase. Snow begins to melt, becomes loose and slides start easily and frequently.
3. Whatever the weather-fair or foul-check with local authorities before beginning any ski excursion and believe them.
4. Whenever possible, when skiing in unknown territory, choose a route through heavily timbered slopes and long ridges. It is best, too, to avoid crossing steep slopes. If it has to be done, go as close to the top as possible. When crossing a slope in a group, never bunch together. Form a single line and space out so that only one person at a time is in any possible danger.
5. Always carry Beacons - known as "beepers", peeps (pieps), ARVAs (Appareil de Recherche de Victimes en Avalanche, in French), LVS (Lawinen-Verschütteten-Suchgerät, Swiss German), avalanche transceivers, or various other trade names, are important for every member of your party. As a backup you may also want to pack an old-style avalanche cord, a 100-foot red nylon string that is tied to you. If you are caught in a slide, there's a chance that part of the cord will remain on the surface to guide rescuers if your Beacons fail for some reason.
6. When someone in your group is caught in an avalanche, mark the spot where he was last seen. Use a ski or ski pole as well as a Beacon and make a note of the GPS coordinates. Start searching immediately and, if your party is large enough, send someone for help. Keep up the search, if possible, until trained aid and dogs arrive.