Av'alanche, a large mass of snow or ice that slides down a mountain. Avalanches are of different forms; those consisting of fine, dry particles of snow driven down the mountain by a strong wind are known as wind or dust avalanches; those which consist of great masses of snow sliding down a slope by their own weight are known as sliding avalanches; those which are detached by heat from the high glaciers are known as glacier or summer avalanches. The sliding avalanche is the most dangerous of all, and consists of vast accumulations of snow set free from above, which increase in force as they descend, overthrowing houses, tearing up trees, burying villages and swallowing up forests, cattle and human beings. Avalanches are sometimes of immense size; two which fell in the Alpine districts of Italy, in 1885, contained 45,000 and 250,000 tons of snow respectively.