An avalanche is a natural process in which snow responds to the pull of gravity. Avalanches occur regularly on mountains around the world, and are harmless, unless someone happens to be in the way. They tend to run down the same pathways every year, and danger zones are usually well known.
Avalanches are born from a weakness in the snow. Snow is a shape-changer,
depending on prevailing temperature and weather conditions.
Snow begins life as a fluffy six-armed crystal flake, but while
it's laying on the ground, as part of a snowpack, changes occur. During mild weather, water
vapors can slide down the arms of a flake and refreeze at its
center. If this happens, the individual flakes develop strong
bonds and form a solid and cohesive mass. But when it is cold,
water vapors can slip to the bottom of the snowpack, forming
angular crystals. These crystals tend to weaken the snow and
undermine it from below.
Sun and light rain can also produce thin surface crusts, which
make it difficult for new snow to bond securely. Rain weakens
the bonds in the snow and increases its mass. But when rain
freezes, it can strengthen and bind the snow. Hoar frosts, which
are flat frozen crystals, can also form on the surface of the
snow in extremely cold weather, creating a slippery layer when
covered by new snow.
in the snowpack can be triggered by the wind, a heavy storm,
a change in temperature, or the weight of a person. They are
most common on slopes between 30 to 45 degrees. A rule of thumb:
if a slope is good for skiing, it can avalanche. Nearly all
avalanches are triggered by their victim, or someone in the victim's
Generally, there are two types of avalanches: sluffs and slabs.
Sluffs, or loose snow avalanches, are most common. They occur
when loose, light snow tumbles down a mountain, and usually
begin at a single point, growing wider and wider as they gather
snow during the descent. Slab avalanches tend to be more deadly.
They occur when a large slab of snow is released from its moorings
and slips down a mountain slope. The slab is a strong layer
of snow laying on top of a weaker layer of snow. When the weak
layer fractures, the slab begins to avalanche, and can travel down the mountain at speeds of up
to 80 miles per hour. A major avalanche, one that runs for 1,000
feet or more, will often develop a cloud of airborne snow crystals
that ride above the tumbling snow.
best hope of surviving an avalanche is to stay near the surface.
For years, people were advised to wriggle out of their backpacks
and try to "swim to the surface." Newer research suggests you
are better off keeping your pack and covering your face with
your hands to create a breathing space. Two thirds of avalanche
victims perish from suffocation. The remaining one third expire
after being flung against boulders and trees at high speeds.
The bigger you are, the more likely you will end up near the
top. Large particles in the avalanche have a tendency to rise
towards the surface, and avalanche victims are often found buried
under just two feet of snow. Unfortunately, avalanche debris
is dense and only 50% air -- as opposed to fresh snow, which
is 80% air -- and tends to set like concrete. Once the flow
stops, you should try to dig to the surface. If you are wearing
a radio transmitter, as is widely recommended, your chances of being found and rescued are much greater. However, the avalanche survival statistics are still quite grim: only one avalanche burial victim in
four lives to talk about the experience.
-- By Micah Fink