An avalanche is a moving mass of snow or snow and ice. A large avalanche can mobilise many thousands of tonnes of debris, easily enough to destroy buildings, forests and other infrastructure such as ski lift pylons. Along with falls they are one of the principal dangers faced by backcountry travellers.
Avalanches can be divided into three main categories:
Avalanches are far from a random event. Most backcountry avalanche accidents are triggered by the victim or a member of the victims party with slab avalanches the main danger, especially during the winter months of January to March in France.
Avalanches occur when the frictional bonds between a surface layer and the underlying bed and surrounding snow are overloaded. Possible triggers are a skier, snowboarder or an outside event such as a falling ice-block or meltwater percolating through the snow pack.