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Types of Avalanches

Wet Avalanche
Definition: A flow of snow, ice, rock, and other material that occurs as a result of thawing.
Context: These occur mostly late in the snow season when the snowpack is deep and the thaw is just beginning.

Slab Avalanche
Definition: A mass of snow, ice and possibly other material caused when a large slab of snow breaks free from the layers beneath.
Context: A slab avalanche is the most deadly. The weight of a skier is enough to break a slab.

Powder or Loose Snow Avalanche
Definition: A mass of loosely packed snow that begins with a piece of falling rock or ice. Small loose snow avalanches are called "sluffs".
Context: The largest and most destructive is a powder avalanche. A piece of falling ice or rock starts a mass of loose snow sliding down the mountain.

Cornice Fall Avalanche
Definition: Cornices are snow structures formed by wind drifing snow onto the lee (downwind) side of an obstacle, such as a ridgeline. The weight of a falling cornice breaks into hundreds of pieces and forms its own avalanche.
Context: Cornice fall fatalities are a significant problem in big mountains, they often break off farther back than expected.

Ice Avalanche
Definition: Falling blocks of ice create an avalanche, which often entrains snow below it or triggers slabs.
Context: Glaciers flowing over cliffs form icefalls and most common in big mountains.

Glide Avalanche
Definition: A type of wet avalanche, they occur when the entire snowpack slowly slides as a unit on the ground (similar to a glacier).
Context: Glide avalanches are a slow process which usually occur over several days, weeks or even months. Caused by melt water libricating the ground and allowing the overlying snowpack to slowly "glide" downhill.

Slush Avalanche
Definition: A sluch avalanche occurs when a cold dry snowpack suddenly becomes saturated with water. It then catastrophically looses strength and the resulting slush often runs long distances on gentle terrain.
Context: Common in northern latitudes such as Alaska and other high latitiude permafrost regions.

Avalanche Causes:

What factors cause an avalanche to occur?

An avalanche occurs when the stress (from gravity) trying to pull the snow downhill exceeds the strength (from bonds between snow grains) of the snow cover. There are four ingredients of an avalanche:

1. a steep slope
2. a snow cover
3. a weak layer in the snow cover
4. a trigger

How frequently to avalanches occur?

Avalanches are most likely to occur when danger increases with major snowstorms and periods of thaw. Thousands of avalanches are reported to Avalanche Centers in the U.S. in an average winter. More than 80% of these fall during or just after large snowstorms. The most avalanche-prone months are, in order, February, March, and January. Avalanches caused by thaw occur most often in April. What is the typical setting or circumstances in which avalanches occur? About 90% of all avalanches start on slopes of 30-45 degrees; about 98% of all avalanches occur on slopes of 25-50 degrees. Avalanches release most often on slopes above timberline that face away from prevailing winds (leeward slopes collect snow blowing from the windward sides of ridges.) Avalanches can run, however, on small slopes well below timberline, such as gullies, road cuts, and small openings in the trees. Very dense trees can anchor the snow to steep slopes and prevent avalanches from starting; however, avalanches can release and travel through a moderately dense forest.

What are the dangers associated with the hazard?

Loss of life and injury Property damage Blocked highways, roads, and trails.

Avalanche Safety

What can be done in case of an emergency?


Surviving avalanches can depend on luck; therefore, it is always better to avoid them in the first place. Remember that only 1 of 3 victims buried without a beacon survives. If you are caught, first try to escape to the side, or grab a tree or rock. If you are knocked down, get rid of your poles, skis and or snowshoes, and a heavy pack. Swim with the avalanche to try to stay on top and avoid trees. When the avalanche slows down, reach the surface or make an airpocket.

Avalanche victim survival depends on rapid location and extrication by companions. Victims generally do not have enough time for successful rescue by outsiders. Groups must posses self-sufficient rescue skills and equipment (beacon, probe, & shovel).
Call park incident command (rangers, etc.) in a park unit.
Contact local emergency personnel (911).

What can be done in terms of prevention?

Avoidance is the best policy. Education. Planning prior to activity in the area. Education and understanding. Interpreters work in partnership with the scientific community to ensure that complex information can be conveyed accurately, and in a form that is comprehensive and relevant to non-specialists.

updated on 06/14/2007  I   http://www.nature.nps.gov/geology/hazards/avalanche.cfm   I  Email: Webmaster
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