Here is an important
fact: Nearly all avalanches that involve skiers are triggered
by the victims or by someone they are skiing with! This is important
information since knowing this fact gives us a valuable clue
about to what we need to do to avoid getting caught. More on
that later, but first some basics:
Skier involved avalanches don't just happen by accident. Three
variables interact to determine the possibility or likelihood
that an avalanche might occur, they are represented by the "Avalanche
Triangle" at left and they are:
Terrain: the slope must be steep enough to avalanche.
Snowpack: the snow must be unstable enough to avalanche.
Weather: Weather is another important variable. Changing
weather can quickly increase instability.
These three variables are often present
in a sort of delicate balance. Add skiers and the possibility
that the balance may be altered and an avalanche triggered increases
dramatically. Keeping in mind the three variables, the first
question we need to ask before beginning any climb or jumping
into a bowl or chute is this, is this avalanche country? Is
this slope capable of taking off?
This is the Terrain part of the
Avalanche Triangle and it is the first step in hazard evaluation.
Avalanches occur on slopes steeper than 25 degrees and usually
less than 50 degrees. Steep slopes tend to see their snow
sluff off before it can accumulate and under 25 degree slopes
just aren't steep enough.
The vast majority of avalanches, particularly
the most dangerous slab variety, occur between 30 and 45 degrees,
but slabs occasionally occur on slopes less than 30 degrees.
Unfortunately the most desirable slopes for skiing are right
in that 30 to 45 degree prime slide zone. It's also important
to remember that the snowpack is interconnected. A skier can
be in the flats and have a wind loaded 35 degree slope above
come crashing down. Being aware of the slope angle you are on,
and the slope angles above you, is key. Carry and learn how to
use a slope meter. With practice you will be able to accurately
gauge slope angles through observation, but a slope meter is
the best way to learn this important skill.
The next question to ask yourself, and
the second part of the Avalanche Triangle, is "what's the
snowpack doing?" The snowpack's layers build throughout
the winter with each new weather event. Layers are formed when
new snow falls, when a rain crust forms, or when wind or temperature
acts on the snow. There are almost always strong layers and weak
layers within a given snowpack. The stronger layers are usually
denser, made up of smaller, rounder, grains of snow that are
packed tightly and are well bonded to each. Weak layers are often
less dense layers of granular snow that did not bond well. These
layers are often called "sugar snow" because of their
loose, grainy appearance. The weak layers can serve to prevent
the strong layers from bonding to each other, for this reason
knowing what's going on in the snowpack and where the weak layers
are hidden is another key to safe skiing in avy terrain.
Sometimes a weak layer is supporting a
dense upper layer and all it takes is the presence of one or
more skiers on top to upset the balance between stress and strength.
When the snow is stable the strength is greater than the stress.
Fortunately, this is most often the case otherwise snow would
never stay on a hillside. But sometimes the balance between stress
and strength is almost equal and we have an unstable snowpack.
Add additional stress such as a rapid load of precipitation,
a sudden increase in temperature, wind blown snow, or a skier,
and an avalanche could be triggered.
Skiing safely in the backcountry and making
correct assessments of risk is greatly enhanced by the ability
to recognize unstable snow.
The third part of the triangle and another
important question to ask yourself is this, is the weather
contributing to an increase in snow instability and an increased
likelihood of a slide occurring? There three weather factors
that can affect snow stability, precipitation in the form
of rain or snow, wind, and temperature.The type
of precipitation and at what rate it falls are equally as important
as the amount. In general, the more rapidly the precipitation
falls the less time the snowpack has to adjust to the additional
stress and the more likely is to avalanche. Watch out when it's
dumping! You do not need precipitation to increase the avalanche
hazard. It can be blue sky for days, but if the wind is
blowing snow around the hazard is probably increasing. Wind can
redistribute large amounts of snow by scouring windward slopes
and rapidly loading leeward or downwind slopes. Take note of
wind speed and direction to help you anticipate which slopes
may be loaded and avalanche prone. Beware wind loaded slopes.
When the sun comes out and the temperature
rises, the snowpack will usually settle more rapidly, and become
denser and stronger. However, if the warming is rapid, the snowpack
can become very wet and unstable. Pay attention to temperatures
during your ski tour. In a cold snowpack, unstable conditions
often persist longer because the settlement and strengthening
process is slowed. Thus a shaded slope and a sunny slope in the
same area can have different conditions.
Now that you understand some of the basics, pay a visit
to the United States Forest Service's excellent "A Day In
The Backcountry" slide
guide . Then take National Avalanche Center's Interactive
Backcountry Tour. Here they will create a scenario for you
and you will be asked to make the correct calls along the way.
The above was adapted from US
Forest Service National Avalanche Center information.