Wednesday, November 30, 2011

The "Average" Avalanche

Avalanches come in all shapes and sizes.  The average snow avalanche is 2 to 3 feet deep at the fracture line, about 150 feet wide and will fall about 400 feet in elevation.  That's a slide area bigger than a football field!  What's more impressive is that the average avalanche travels at speeds around 50 M.P.H., a little faster than most of us like to ski!  The average time duration of a slide of this size is less than 30 seconds.  This is the size of avalanche that catches and kills most backcountry travelers.

Avalanches can be much larger...some of the largest reported avalanches have involved complete mountainsides of snow, the area of 20 football fields, having a depth of 10 feet at the fracture line and falling over 1½ miles at speeds well over 100 miles per hour!  As impressive as these avalanches are, generally the smaller ones are the killers.  In fact, 50% of avalanche fatalities are killed in slides of less than 100 feet and people have been killed in slides of less than 40 feet.  The bigger ones almost always release from natural causes and do not involve people unless they happen to be in the wrong place at the wrong time. Artificial triggers, such as snowmobiles or skiers are much more effective at releasing small to medium sized avalanches in shallower snowpack, where the stress caused by their weight is enough to cause the snowpack to fail.

Avalanches are a powerful phenomenon.  In 1969 at a ski hill in Washington, steel chair-lift poles were bent in an avalanche.  In a recent Colorado avalanche, a flashlight inside a glove compartment of a buried automobile was found completely packed with snow.  Larger avalanches possess the force to uproot mature forests and even destroy structures built of concrete. The reason, forces in excess of 15,000 lbs. per square foot.

The fastest recorded avalanche occurred in Japan and was measured to be traveling at speeds in excess of 230 m.p.h.

Avalanches come in two distinctly different types...  “Loose Snow” and “Slab”avalanches.

Loose snow avalanches are a cohesionless mass of snow that start at a point and fan out as they run, forming an inverted "V".  This type of avalanche usually involves small amounts of near-surface snow and is not considered a major threat to people.  Do not lose respect for these slides as they have taken lives.

Slab avalanches, on the other hand, start when a large area of cohesive snow fails and slides down the slope.  There is a well defined fracture line from where the snow broke away.  In addition, there may be angular blocks or chunks of snow in the slide, sometimes larger than a refrigerator.   A slab avalanche can involve a range of snow thickness from just near surface layers to an event that includes the entire snow cover down to the ground.

Slab avalanches are almost always caused by additional stress on the hill, such as a snowmobiler or skier. Since slab avalanches cause nearly all avalanche accidents, it is important to understand the conditions within the snowpack that lead to these slab avalanches

Snowstorms and wind-redistribution cause the snowpack to develop in layers. Once a layer has achieved sufficient cohesive strength, the first prerequisite for a slab avalanche has been established.  Weather will help add the second requirement...  a weak layer.  If a weak layer has developed underneath a strong layer, the perfect recipe for an avalanche exists.  If the weak layer fails, the cohesive strong layer above it will fracture and fall away from the stress. Both ingredients - a cohesive layer of snow and a weak layer below - are necessary for a slab avalanche.

When the strong layer fractures, the crack is estimated to shoot across the snowfield at a speed of over 1000 miles per hour.

In our next few posts we will discuss the three main variables that help develop a potentially unstable snowcover:  weather, terrain and snowpack. By understanding these variables, backcountry users will have a better chance of predicting avalanche danger.

For more information on avalanche safety check out the Mountain Rescue Associations public education program @ as well as our Backcountry Skiing & Riding Safety Video  

Courage - Commitment - Compassion
     Mountain Rescue Association 

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