Imagine an avalanche in motion. It gains momentum as it moves down the slope and may speed along at 200 miles per hour. Clouds of snow dust may roll into the sky hundreds of feet. The avalanche may rise and ride on a cushion of air. Frequently an air blast, great enough to collapse buildings, precedes an avalanche.
Avalanches result from the overloading of snow and ice and unstable conditions at depth. Most avalanches happen during or shortly after heavy snowstorms, especially when snow falls at a rate of one inch per hour or more and thick snow buildup occurs. Compounding this buildup is wind-blown snow accumulating on leeward slopes, perhaps to the extent of forming snow cornices, which may break loose and set off an avalanche. Unstable layers, on which a snow-ice mass can slide, include those with surface hoar, sun crusts (a refrozen snowmelt), and depth hoar, whose crystals act like tiny ball bearings
If you can't avoid avalanche country, take several precautions. After a heavy snowfall, let the snow consolidate before setting out on a trip. Avoid avalanche paths, indicated by a lack of trees or where trees lean in the same direction. Your safest routes are on the windward sides of high ridges away from the bottoms of slopes. If you must cross an avalanche path, use clumps of trees and prominent rock masses for protection. Listen for snow settling beneath you and watch for rolling snowballs; they often foretell unstable slopes above you. Tie a red, hundred-foot avalanche cord to your waist; it has metal pieces attached to it that indicate the distance and direction to you should you become buried. Consider carrying a small, pocket avalanche transceiver, turned on. Possible rescuers can home in on your signal.
If caught in an avalanche, your actions can help you survive it. Get rid of any encumbering equipment if you have time, such as skis, snowshoes, ski poles, and pack. Try to stay on top of the sliding mass by a kind of backstroke swimming with your head directed up slope. As the avalanche comes to a stop, raise your arms above your face to trap an air space in the congealing snow.