An avalanche is a slide of a large snow (or rock) down a mountainside, caused when a buildup of snow is released down a slope, and is one of the major dangers faced in the mountains in winter. An avalanche is an example of a gravity current consisting of granular material.
In an avalanche, lots of material or mixtures of different types of material fall or slide rapidly under the force of gravity. Avalanches are often classified by what they are made of, for example snow, ice, rock or soil avalanches. A mixture of these would be called a debris avalanche.
A large avalanche can run for many miles, and can create massive destruction of the lower forest and anything else in its path. For example, in Montroc, France, in 1999 300,000 cubic metres of snow slid on a 30 degree slope, achieving a speed of 100 km/h. It killed 12 people in their chalets under 100,000 tons of snow, 5 meters deep. The Mayor of Chamonix was convicted of second-degree murder for not evacuating the area, but received a suspended sentence.
During World War I, over 60,000 soldiers died as a result of avalanches during the mountain campaign in the Alps, many of which were caused by artillery fire. However, it is very doubtful avalanches were used as deliberate weapons; more likely they were simply a side benefit to shelling enemy troops, occasionally adding to the toll taken by the artillery. Avalanche prediction is difficult even with detailed weather reports and core samples from the snowpack. It would be almost impossible to predict avalanche conditions many miles behind enemy lines, making it impossible to intentionally target a slope at risk for avalanches. Also, high priority targets received continual shelling and would be unable to build up enough unstable snow to form devastating avalanches, effectively imitating the avalanche prevention programs at ski resorts.