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Avalanche types

There are various types of avalanche, and these tend to occur under different conditions and present different degrees of hazard.

Powder or loose snow avalanches typically occur during, or right after, a snowfall. Usually the failure begins at a point and spreads out down the slope in an inverted V shape. They are generally small in size, but in the confines of a gully they can be unpleasant and are sometimes large enough to knock a climber off.

A slab avalanche

Sgor an Lochain Uiane and An Garbh Choire from Ben Macdui (Cairntoul/Braeriach Amphitheatre)

A slab avalanche, on the other hand, occurs when a cohesive slab of snow slides on a weak layer. They are the most common, least predictable and, therefore, the most dangerous type of avalanche. This type of snow, called windslab, is formed when wind-transported, and therefore damaged, snow crystals accumulate in sheltered areas such as lee slopes. Windslab consistency can vary from soft to hard, depending mainly on the speed of the wind that transported the snow. Hard slab, in particular, is very deceptive, as it can be firm enough to walk on without sinking in and so feel safe to the unwary. However, it does tend to have a dull, chalky appearance and may squeak or creak when walked on or probed with an axe. Although most common on lee slopes, particularly below cornices, it can build up in unexpected places, even on windward slopes.