An avalanche occurs when a mass of snow which has built up on a mountain side begins to slip and finally to fall. There can also be avalanches of earth, stones, rock and ice, but usually the word is used to describe a rapid fall of snow.
Snow builds up to great thickness on steep slopes, especially if the surface is not smooth. Even a very small disturbance may set it in motion. The vibration of passing vehicle, the moment of a man or animal, the fall of a tree branch or even a sound can cause thousands of tons of snow to crash down a mountainside
The speed of an avalanche varies enormously, but some have been estimated to move at about 200 miles an hour. A big avalanche hurtles down the side of the mountain with a thousand roars, crushing or sweeping away anything in its path.
The swiftly moving mass of snow pushes the air in front of it with such violence that it fans out sideways as well as driving directly ahead. This wind sometimes reaches a force almost equal to that of a tornado. This great wind is often a more powerful force of destruction than the avalanche itself.
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