Avalanche
From ArticleWorld
Avalanche is a sudden flow of a large mass of snow or ice down a slope or cliff, sometimes at speeds exceeding 160 km/hr (100 mph). Such flows can be destructive of life and property. Avalanches are most common on slopes exceeding 30°, frequently when a deep snow falls suddenly and does not have a chance to cohere, or when a thaw undercuts a blanket of older snow. Pellet-like snow (graupnel) is also more prone to avalanche than a fall of ordinary snowflakes. Flows of wind-packed slabs of snow can be especially hazardous.
Avalanches are set off by a combination of factors, including temperature, shearing of creeping snow masses, and sudden vibrations, including loud noises. Snow patrols in mountain areas reduce the hazard by detonating strategically placed explosives that cause smaller, less destructive flows. A landslide is a similar massive movement of rock and soil.They rank alongside earthquakes, flash floods, sharks and lightning strikes as deadly, unpredictable, unstoppable forces that can strike at random to maim and kill unsuspecting holidaymakers.
A million tons (1,016,050,000 kilograms) of snow rumble eight miles (13 kilometers) downhill, kicking up a cloud of snow dust visible a hundred miles (161 kilometers) away.
This is not a scene from a disaster movie—this describes reality one day in April 1981. The mountain was Mount Sanford in Alaska, and the event was one of history's bigger avalanches. Amazingly no one was hurt, and luckily avalanches that big are rare.
An avalanche is a moving mass of snow that may contain ice, soil, rocks, and uprooted trees. The height of a mountain, the steepness of its slope, and the type of snow lying on it all help determine the likelihood of an avalanche.
Avalanches begin when an unstable mass of snow breaks away from a mountainside and moves downhill. The growing river of snow picks up speed as it rushes down the mountain. Avalanches have been known to reach speeds of 245 miles an hour (394 kilometers per hour)—about four times as fast as the speediest downhill skier.
This winter in the western United States alone, about 100,000 avalanches will tumble down mountainsides. In the United States and Canada crashing walls of white will bowl over about 300 people. Most will be skiers, snowboarders, or snow mobilers who set out to have fun. Many will be buried by snow. Most will survive; some will not.
At the other end of the spectrum, the rationalist view of avalanches would have it that they can be controlled, their risk managed or even eliminated by a combination of technology and common sense.The truth lies somewhere between these extremes. For the most part avalanches are creatures of habit - they start under fairly predictable conditions and they follow the same tracks year after year. As long as you follow some basic rules, you need never put yourself at risk.
Most avalanche accidents happen when people ignore these basic rules. The majority of avalanche deaths among skiers and snowboarders are caused by the skier or snowboarder themselves or someone in their group.
Every so often, however, freak weather conditions change the rules. Either so much snow falls, or it falls under such unusual weather conditions, that normally safe slopes become avalanche-prone. These are the big killers, which come down unexpectedly on villages from slopes which have no history of avalanching.