Avalanches can occur anywhere in the world where there is snowfall buildup at a particular angle. The Alpine countries of France, Austria, Switzerland and Italy have the greatest number of avalanches and loss of life annually. In the United States, which ranks fifth in avalanche danger, Colorado, Alaska and Utah are the most deadly states.
Avalanches occur when snow falls at a rapid rate down a hill, mountain or other slope. They may be classified as dry or wet, depending on the amount of water present in the snow. They are more common during certain times of the year, particularly December through April.
The three main parts of an avalanche are the starting zone, the avalanche track and the run-out zone. The starting zones are where the snow pile is most volatile. These often are located at the highest point of the slope. The avalanche track is the pathway the snow takes as it travels downhill. There may be a pile of snow at the bottom of the track indicating the end of this path. The run-out zone is where the snowfall comes to a stop.
A variety of conditions affect whether an avalanche occurs. These conditions include the weather and temperature, the steepness of a slope and wind direction. Whether the slope is facing north or south is also a factor, as are terrain, wind direction, vegetation and the condition of the packed snow.