Avalanches can descend in a dry (powder snow avalanche) or wet to moist (wet snow avalanche) form.
Their enormous speed (powder snow avalanche) and mass (wet snow avalanche) mean that they develop very high forces (pressure and suction). Once an avalanche has started, it is virtually impossible to stop it until it reaches the bottom of the valley. Large avalanches can destroy forests and buildings within seconds. They pose a threat to people both inside and outside buildings.