An avalanche is a large mass of snow that moves quickly down a mountain. It breaks loose from the area around it and collects more and more snow as it pushes downward.

Avalanches occur in mountainous regions, for example the Alps, Himalayas or the Rocky Mountains. They can reach speeds of over 100 km an hour and destroy forests and villages that are in their way. They can block roads and train tracks and make areasunreachable from the outside. Avalanches kill over a hundred people in Europe and North America every year, burying them below tons of snow.

Although the danger of avalanches is high throughout the winter months there are weather and snow conditions in which avalanchesoccur more often. The composition of snow is an important factor. Once the snow is on the ground temperature changes its structureso that different layers of snow can form on the slope.

Avalanches occur on slopes that have an angle of 30 to 60 degrees. On slopes that are not so steep there is not enough gravity to let the snow break off from its surroundings. On steeper slopes the snow usually breaks off more quickly.

Avalanches can be triggered by many factors. In some cases an increasing amount of new snow can start an avalanche. Strong winds can transport snow to the leeward side of the mountain. There, the snow stays soft and does not bind with other layers. In other cases,skiers, snowboarders or other alpine tourists can trigger an avalanche. 

As snow falls it builds up layer upon layer.  After a certain time the layers harden and bond with one another. If they don’t the snow profile becomes unstable and soft layers can slide on top of harder ones.