AVALANCHE

large mass of rock debris or snow that moves rapidly down a mountain slope, sweeping and grinding everything in its path. An avalanche begins when a mass of material overcomes frictional resistance of the sloping surface, often after its foundation is loosened by spring rains or is rapidly melted by a foehn (warm, dry wind). Vibrations caused by loud noises, such as artillery fire, thunder, or blasting, can start the mass in motion. Large rock avalanches (rockfalls) have dammed rivers and buried towns. They are commonly composed of bedrock fragments a few centimetres (an inch or so) in diameter and include much soil and dust. Rock avalanches are thought to ride on a cushion of compressed air that allows them to travel long distances. A debris avalanche usually occurs in unconsolidated earth materials when weakened by moisture. Some snow avalanches develop during heavy snowstorms and slide while the snow is still falling, but more often they occur after the snow has accumulated at a given site. One of the causes of snow avalanches is the slow formation of depth hoar (hexagonal cuplike ice crystals that begin to form at ground level) under the snowpack. Depth-hoar crystals develop in loose array from the evaporation of the original snow particles and the simultaneous vapour deposition of larger, denser ice crystals near the ground; thus a zone of weakness occurs within the snowpack near the ground, the particles of which act as a lubricant when the upper layers of the snow start sliding down the mountain. The wet snow avalanche is perhaps the most dangerous because of its great weight, heavy texture, and tendency to solidify as soon as it stops moving. The dry type is also dangerous because its entraining of great amounts of air makes it act like a fluid; this kind of avalanche may flow up the opposite side of a narrow valley. Avalanches can carry a considerable amount of rock debris with the snow. Considerable attention is given to the dangers presented by avalanches to mountain climbers, skiers, travelers, and residents of mountainous terrain. Much of the pioneering research and the testing of protective devices was conducted in Switzerland at the Snow and Avalanche Research Institute. Despite the construction of avalanche-protection works in most parts of Switzerland, destructive avalanches still occur following periods of intense snowfall. In some older Swiss towns, buildings in open areas likely to be swept by avalanches are built like the prows of ships to divert the flowing snow. In the United States, avalanche danger is monitored by the Forest Service of the U.S. Department of Agriculture because most of the ski areas in mountainous regions are located within national forests. Rangers, specially trained in avalanche forecasting and control, monitor the ski resort areas and mountain slopes where avalanches might endanger highways or other structures. Much of the control at present consists of lobbing explosives into the upper reaches of the avalanche zones, intentionally causing the snow to slide before accumulations become very great. Most large ski areas subject to the danger are now continuously patrolled by avalanche experts.