An avalanche is a natural process in which snow responds to the pull of gravity. Avalanches occur regularly on mountains around the world, and are harmless, unless someone happens to be in the way. They tend to run down the same pathways every year, and danger zones are usually well known.

Avalanches are born from a weakness in the snow. Snow is a shape-changer, depending on prevailing temperature and weather conditions. Snow begins life as a fluffy six-armed crystal flake, but while it's laying on the ground, as part of a snowpack, changes occur. During mild weather, water vapors can slide down the arms of a flake and refreeze at its center. If this happens, the individual flakes develop strong bonds and form a solid and cohesive mass. But when it is cold, water vapors can slip to the bottom of the snowpack, forming angular crystals. These crystals tend to weaken the snow and undermine it from below.

Sun and light rain can also produce thin surface crusts, which make it difficult for new snow to bond securely. Rain weakens the bonds in the snow and increases its mass. But when rain freezes, it can strengthen and bind the snow. Hoar frosts, which are flat frozen crystals, can also form on the surface of the snow in extremely cold weather, creating a slippery layer when covered by new snow.

Instabilities in the snowpack can be triggered by the wind, a heavy storm, a change in temperature, or the weight of a person. They are most common on slopes between 30 to 45 degrees. A rule of thumb: if a slope is good for skiing, it can avalanche. Nearly all avalanches are triggered by their victim, or someone in the victim's party.

Generally, there are two types of avalanches: sluffs and slabs. Sluffs, or loose snow avalanches, are most common. They occur when loose, light snow tumbles down a mountain, and usually begin at a single point, growing wider and wider as they gather snow during the descent. Slab avalanches tend to be more deadly. They occur when a large slab of snow is released from its moorings and slips down a mountain slope. The slab is a strong layer of snow laying on top of a weaker layer of snow. When the weak layer fractures, the slab begins to avalanche, and can travel down the mountain at speeds of up to 80 miles per hour. A major avalanche, one that runs for 1,000 feet or more, will often develop a cloud of airborne snow crystals that ride above the tumbling snow.


The best hope of surviving an avalanche is to stay near the surface. For years, people were advised to wriggle out of their backpacks and try to "swim to the surface." Newer research suggests you are better off keeping your pack and covering your face with your hands to create a breathing space. Two thirds of avalanche victims perish from suffocation. The remaining one third expire after being flung against boulders and trees at high speeds.

The bigger you are, the more likely you will end up near the top. Large particles in the avalanche have a tendency to rise towards the surface, and avalanche victims are often found buried under just two feet of snow. Unfortunately, avalanche debris is dense and only 50% air -- as opposed to fresh snow, which is 80% air -- and tends to set like concrete. Once the flow stops, you should try to dig to the surface. If you are wearing a radio transmitter, as is widely recommended, your chances of being found and rescued are much greater. However, the avalanche survival statistics are still quite grim: only one avalanche burial victim in four lives to talk about the experience.