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Wet slab avalanches

Wet slab avalanches are caused by a thick cohesive slab of snow losing its bond to an underlying thin, weaker layer or interface after becoming damp, moist, or saturated with water. The slab that initially fails can be very firm or even hard but once moving, the debris generally becomes a dense mushy mass, often composed of large rounded lumps and unaccompanied by a powder cloud. Wet slab avalanches, which are generally slower moving than dryer slab types, tend to flow in channels and are easily deflected by irregularities in the terrain. The deposited debris commonly has channels and ridges on the surface. Wet slabs are often highly destructive due to great mass created by the high water content of the snow.

Extended periods of temperatures above the freezing point (especially if there is no overnight freeze), strong solar radiation, and rainfall are the weather events associated with wet slab formation, and are therefore, typically a late spring phenomenon. This is when temperatures are warm enough, solar radiation is strong enough, and/or rainfall amounts are great enough to produce the thick wet layers required to produce this type of avalanche. The exception may be low elevation areas, especially in maritime climates where extended warm spells and significant rainfall can occur at almost any time of year.

In the early stages of wet slab activity, the problem is often cyclical and persists only for hours as the wet surface layers freeze overnight and thaw the next day. During periods where no overnight re-freezing occurs, however, it is possible for wet slab activity to persist for days.

Snowballing, pinwheeling, and small wet slab or loose wet avalanches are often precursors of wet slab activity. Recent wet slab activity is a reliable indicator of the terrain on which another cycle will occur if the conditions that triggered the previous cycle reoccur. Current wet slab activity is generally a very good indicator for similar terrain.

View larger image Debris from a wet slab avalanche

Glide avalanches

Full-depth wet slab avalanches often start with glide cracks. Like crevasses in glaciers, glide cracks are gaping fissures that form when the entire snowpack slowly slides as a unit on the ground. Glide cracks often don’t produce avalanches but occasionally and unpredictability they release catastrophically as a glide avalanche.

View larger image Glide avalanches | Photo: M. Sulkers

Managing your risk

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

Storm snow avalanches