he Avalanche Handbook defines an avalanche as a falling mass of snow which can contain rocks, soil or ice. Avalanches are caused when an area of snow loses adhesion and slides down the mountain side, possibly carrying you with it or engulfing you in its path. Two different forces are at work here. The force of gravity acting on the mass of snow trying to pull it down the mountain, and friction which is preventing it from sliding away. Since gravity is constant, the trigger for an avalanche is either a reduction in the amount of friction (typically caused by an increase in temperature) or an increase in the force, (typically caused by fresh snowfall or wind blowing new snow onto an already laden slope) or (and unfortunately for us) the presence of a skier or climber on the slope.
Avalanches come in various different flavours, depending primarily on the quantity and type of snow released. There are a number of different systems of classification, most of which go beyond the scope of this guide, but the basic differences are laid out below. The key distinction is between a loose slide (where the snow does not bind to itself) and slab (where it falls as a block).
What is common to all types of avalanche is the extraordinary destructive power they can unleash. It may seem surprising that something as delicate as snowflakes can cause so much trouble, but the largest avalanches can bring down over 100,000 tonnes of snow, travel up to 3 kms and can easily flatten large trees and destroy buildings. Most fatalities are caused by dry slab avalanches which travel at around 100-125 kph. Even the slowest - loose wet slides - still travel at around 30 kph. The high mountain airborne powder avalanche is a particularly deadly beast reaching speeds of over 300kph and preceded by a wind that can flatten ancient forests. Not easy to ski out of despite what you see in the movies!
For all types of avalanche, the risks are essentially the same: - Burial and suffocation (65% of deaths): the greatest danger is being buried by the avalanche and suffocating. The snow in an avalanche can set like concrete when it comes to a rest, and the victim or victims can be buried several metres down. Digging the victim out fast is essential.
- Collision with obstacles (25% of deaths): the power of an avalanche can break bones and limbs. Walkers, climbers and skiers have also been killed when an avalanche (which might otherwise have had only limited effect) has swept them over a cliff or into a crevasse.
- Hypothermia and shock (10% of deaths): victims who survive the initial trauma and avoid being completely buried can still die of cold injury in the aftermath of the accident.
Overall survival rates are poor. Fewer than half the victims fully or partly buried by an avalanche survive to tell the tale. The rest of this section tells you how to avoid being one of them.
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