There are three basic kinds of avalanches. Each depends on the right mixture of air temperature, ground temperature, the steepness of the terrain, the kind of snow that has fallen, and how the snow changes on the ground.
On January 19, 1951, more than 18 inches of snow piled in the village of Andermatt in Switzerland. Suddenly, a booming whoosh of air came racing down into the valley, blowing one luckless individual and the roof of his house 100 feet in 45 seconds. When the villager dug himself out of the snow, all he could see was a mound of snow 30 feet deep burying his house. He had just lived through a wind avalanche. Four more airborne avalanches swept through Andermatt that same day. The last one was preceded by an enormous, thunderous crack before it came down the mountain. The snow destroyed an army barracks and pushed four-ton cannons hundreds of years from their initial position.
Airborne avalanches are a mixture of granular and crystalline snow, the kind of loose powder that skiers love. However, loose powder can either settle into ordinary snow or avalanche. When it avalanches, the snow becomes airborne instead of flowing along the ground. Its light weight allows it to lift in the air, so that clouds of feathery snow can roar down slopes at more than 200 miles per hour. As it moves down mountainsides, pressure builds up in front of the snow mass. So great is this force that the blasts of air can move heavy objects and blow roofs off buildings. Survivors say that the air pressure seemed so great they felt their lungs would burst.
In 1962, another airborne wind avalanche caused the greatest destruction to timber ever seen in Switzerland. 250 acres of 100-year-old trees were knocked down like broken toothpicks, cut off 50 feet above the ground. Others were torn out of the ground or snapped off at the base. A forest with the tops of its trees blasted off is a sure sign that a wind avalanche has passed through.
Slab avalanches are caused by build-ups of old snow that has been packed down and settled for a long time. Hard-slab avalanches start when the top layer of snow breaks up, causing car-sized chunks of ice to come down a mountain at 30 to 50 miles an hour.
Sometimes, hard deposits of snow are carved into waves or cornices by the wind, forming a sort of wave as thick as 30 feet that hangs up to 20 feet out over the edge of a cliff. These can be deadly to skiers, who think they are on safe, firm snow. However, the weight of just one person can trigger an avalanche. With a loud crack, the slab can break loose and bury a person in seconds.
Wet-slab avalanches occur commonly on bright, cloudless days in spring when snow is softened by rain. As the bonds between water molecules in the snow weaken, an entire slope can begin to slide. Wet-slab avalanches move much slower, creeping along at 5 to 10 miles per hour, picking up boulders, trees, and earth along the way. These accumulate into a dirty wall of debris where the mass stops.