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Wind slab avalanches

Wind slab avalanches are caused by a cohesive slab of wind-deposited snow overloading the bond to an underlying weak layer or interface. Wind slabs consist of snow crystals broken into small particles and packed together by the wind. These wind-deposits, often referred to as “pillows,” are usually smooth and rounded and sometimes sound hollow. The moving snow as well as the debris can include hard and soft chunks of slab. Wind slabs are created on leeward (downwind) slopes and in cross-winded areas where the winds blow across the terrain. They are commonly found behind and below features that act as a natural wind-fence, such as bands or isolated stands of trees, ridges, ribs, etc.

Wind slabs can often be recognized by the pillowed shape of the deposited snow, sometimes a dull texture to the snow surface, a cohesive feel to the snow, an upside-down feel, or sometimes a hollow drum-like feel or sound.

For wind slabs to form there must be snow falling during the wind event or there must be loose surface snow available for transport. The extent of the wind slab formation depends on the speed of the wind, the duration of the wind event, and the amount of snow available for transport. Minor wind events may form wind slabs only in the immediate leeward of exposed ridge tops. As wind intensity increases, wind slabs will form well below ridge top, and leeward of cross-loading features such as ribs and gullies on slopes oriented parallel to the wind direction. Major wind events will form widespread wind slabs in all open areas and occasionally even in normally wind protected areas (such as open glades below treeline).

View larger image Fracture line of a wind slab avalanche | Photo: C. Campbell

Wind slab avalanches often reach a peak of activity during periods of intense snowfall or wind loading. Wind slabs tend to stabilize within one or two days, but the instability may persist longer in cold temperatures. If a wind slab is deposited on a persistent weak layer, the problem may persist much longer and can eventually become a persistent slab avalanche problem. Harder wind slabs tend to persist longer than softer ones!

Managing this kind of avalanche

For more information on how wind slab avalanches can hurt you and how you can manage the problem see: Good Travel Habits - Reducing the Risk in the Field

Persistent slab avalanches