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| Avalanche - Types |
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| There are three principal types of avalanche - powder, wet snow and slab. |
| Powder Avalanches |
These generally start from a single point, growing as they progress.
Ever-increasing
volumes of powder snow are thrown into the air, flowing down the slope as
a dense cloud. Speeds can reach up to 400 kph (250 mph) and the shock wave
of the approaching avalanche can flatten everything in its path.
Although
rare, especially in Europe, this type of avalanche is extremely dangerous,
battering or choking its victims to death.
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| Wet Snow Avalanches |
In spring, or whenever it is subjected to elevated temperatures, snow transforms
rapidly. As a result its cohesion and strength is reduced; wet snow avalanches
frequently result.
These generally start from a single point, spreading
as they slide. Their progress is generally slow, unless they encounter steeper
terrain or are funnelled into a gully.
Due to the density of snow carried,
these avalanches can be extremely destructive. Once they stop, they set
like concrete; buried victims are unable to move or breathe.
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Photo: ifyouski.com -
A wet snow slide
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Photo: Jess Stock, Stock Shot
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| Slab Avalanches |
Around 75% of all avalanche incidents involving skiers are due to slab avalanches. The most common cause is wind; snow is carried over ridges and dropped on lee slopes, building a thick, unstable layer of dense snow, called wind slab.
This type of avalanche, however, can occur whenever layers of snow do not cohere strongly, for instance if the intervening surface is smooth, damp or granular.
The reason that slab avalanches are so dangerous is that the instability is invisible to the naked eye. It may be buried under a layer of fresh, inviting powder. But given the right trigger, such as the tracks and weight of a skier, the top layers of snow fracture right across the slope and begin to slide over the layers below.
Victims find themselves standing on or below the moving slab, with very little chance of escaping as it breaks up and carries them down the slope.
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