As snow falls it freezes and forms layers and an avalanche occurs if these layers become separated and move apart

Where they occur: Mountainous regions throughout the world

Occurrences per year: Up to 1m

Average speed: 20-100mph

Max recorded speed: 245mph

Casualties: Approximately 100 deaths per year

An avalanche occurs when a mass of snow moves rapidly down the side of a mountain. This can happen as a surface avalanche, which is a cascade of powdery snow, or as a full-depth avalanche - when whole slabs of snow and ice, come crashing down. A full-depth, or slab, avalanche is the most deadly, moving at frightening speeds and carrying with it heavy debris like rocks and trees.

As snow falls, it freezes and forms layers. An avalanche occurs if these layers become separated and move apart. This can be caused by climate change, heavy snowfall or loud noises. Avalanches may also be caused by people climbing, skiing or snowboarding on unstable snow. Safety officers in ski resorts analyse the snow’s profile. If they discover that the conditions are favourable for an avalanche, they carry out controlled explosions to loosen the ice and snow.

Whole buildings can be crushed under the force of an avalanche. At Galtür, Austria, in February 1999, 31 people were killed in an avalanche which hit the village at 200mph.