When Does an Avalanche Occur?
An avalanche occurs
when a mass of snow which has built up on a mountainside 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 a passing vehicle, the movement 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 thunderous roar, 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.
Many avalanches are small slides of dry powdery snow that move as a formless mass. These “sluffs” account for a tiny fraction of the death and destruction wrought by their bigger, more organized cousins.
Disastrous avalanches occur when massive slabs of snow break loose from a mountainside and shatter like broken glass as they race downhill. These moving masses can reach speeds of 80 miles (130 kilometers) per hour within about five seconds. Victims caught in these events seldom escape.
Avalanches are most common during and in the 24 hours right after a storm that dumps 12 inches (30 centimeters) or more of fresh snow. The quick pileup overloads the underlying snowpack, which causes a weak layer beneath the slab to fracture. The layers are an archive of winter weather: Big dumps, drought, rain, a hard freeze, and more snow. How the layers bond often determines how easily one will weaken and cause a slide.
Storminess, temperature, wind, slope steepness and orientation (the direction it faces), terrain, vegetation, and general snowpack conditions are all factors that influence whether and how a slope avalanches. Different combinations of these factors create low, moderate, considerable, and high avalanche hazards.