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| av·a·lanche |
n. |
1 A fall or slide of a large mass, as of snow or rock,
down a mountainside. |
WHEN
AND WHERE AVALANCHES HAPPEN?
Although avalanches can occur on any slope given the right
conditions, in the United States certain times of the year
and certain locations are naturally more dangerous than others.
Wintertime, particularly from December to April, is when most
avalanches will "run" (slide down a slope).
While expertise is not a
guarantee that you won't be caught in an avalanche,
it does provide some basic knowledge about how to avoid
avalanche areas, what types of weather and terrain signs
to watch for, and what to do if you are caught in an
avalanche - all information that may save you or other
members of your party.
ANATOMY
OF AN AVALANCHE
An avalanche has three main parts. The starting zone is the
most volatile area of a slope, where unstable snow can fracture
from the surrounding snowcover and begin to slide. Typical
starting zones are higher up on slopes, including the areas
beneath cornices and "bowls" on mountainsides. However, given
the right conditions, snow can fracture at any point on the
slope.
The avalanche track is the path or channel that an
avalanche follows as it goes downhill. When crossing
terrain, be aware of any slopes that look like avalanche
"chutes." Large vertical swaths of trees missing from
a slope or chute-like clearings are often signs that
large avalanches run frequently there, creating their
own tracks. There may also be a large pile-up of snow
and debris at the bottom of the slope, indicating that
avalanches have run.
The runout zone is where the snow and debris finally
come to a stop. Similarly, this is also the location
of the deposition zone, where the snow and debris pile
the highest. Although underlying terrain variations,
such as gullies or small boulders, can create conditions
that will bury a person further up the slope during
an avalanche, the deposition zone is where a victim
will most likely be buried.
AVALANCHE CONDITIONS
Several factors may affect the likelihood of an avalanche,
including weather, temperature, slope steepness, slope orientation
(whether the slope is facing north or south), wind direction,
terrain, vegetation, and general snowpack conditions. Different
combinations of these factors can create low, moderate or
extreme avalanche conditions. Keep in mind that some of these
conditions, such as temperature and snowpack, can change on
a daily or even hourly basis.
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Weather
Avalanches are most likely to run either during or immediately
after a storm where there has been significant snowfall.
The 24 hours following a heavy snowstorm are the most
critical.
Snowfall
Recent snowfall puts extra stress on the existing snowpack,
especially if it does not adequately bond to the pre-existing
surface layer. The extra weight of new snow alone can
cause a slab to break off and fall down the slope, particularly
in storm-induced avalanches.
Temperature
When temperatures rise above freezing during the daytime
and drop back down again at night, melting and re-freezing
occurs, which can stabilize the snowpack. This is particularly
common during the springtime. When temperatures stay
below freezing, especially below zero degrees Fahrenheit,
the snowpack may remain relatively unstable.
Snowpack conditions
Perhaps the most significant factor (but not the only
one) is how the snowpack has developed over the season.
We only see the surface and maybe the top few layers
of snow, but it can be layers of snow several feet deep
that may ultimately determine whether the slope will
fail.
Slope angle
Most avalanches occur on slopes between 30 and 45 degrees,
but can occur on any slope angles given the right conditions.
Very wet snow will be well lubricated with water, meaning
it might avalanche on a slope of only 10 to 25 degrees.
Very dry or granular snow will most likely avalanche
on a slope close to the 22 degree angle of repose. Compacted,
well-bonded layers create a snowpack that can cling
to steeper slopes until a weak layer is created.
Slope orientation
Although avalanches will run on slopes facing any direction,
most avalanches run on slopes facing north, east, and
northeast (also the slope directions that most ski areas
are located on).
Terrain
Paying attention to where you are in the grand scheme
of things can offer clues about avalanche likelihood.
Bowls and gullies are suspect at any time, regardless
of other conditions. During hazardous conditions minimize
the amount of time traveling beneath avalanche starting
zones and never camp in a potential avalanche runout
zone. Even a small avalanche starting high on the slope
can carry down large amounts of snow onto and across
the valley floor. Remember to keep an eye out for obvious
avalanche chutes, where avalanches occur more frequently.
Vegetation
Many avalanches start above the tree line, making high-elevation
mountains especially risky. Although forests help stabilize
the snowpack, if an avalanche starts above tree line,
it can cut its own path, or chute, through the trees
below. |
How to determine if the snowpack is safe
There are several ways to gauge snowpack stability. Keep any
eye out for any cracks shooting across the surface, or small
slabs shearing off. These are signs of weakened snowpack.
Also, listen for "hollow" or "whumping" noises as you walk
or ski. This indicates that there is a weaker layer underneath,
leaving the surface layer more prone to collapse.
Tips for avalanche survival
Before crossing a slope where there is any possibility of
an avalanche, fasten all your clothing securely to keep out
snow. Loosen your pack so that you can slip out of it with
ease and remove your ski pole straps. Make sure that your
avalanche beacon is on and switched to "transmit" rather than
"receive." Cross the slope one at a time to minimize danger.
If you are caught in an avalanche
Yell and let go of ski poles and get out of your pack to make
yourself lighter. Use "swimming" motions, thrusting upward
to try to stay near the surface of the snow. When avalanches
come to a stop and debris begins to pile up, the snow can
set as hard as cement. Unless you are on the surface and your
hands are free, it is almost impossible to dig yourself out.
If you are fortunate enough to end up near the surface (or
at least know which direction it is), try to stick out an
arm or a leg so that rescuers can find you quickly.
If you are in over your head (not near the surface), try
to maintain an air pocket in front of your face using your
hands and arms, punching into the snow. When an avalanche
finally stops, you will have from one to three seconds before
the snow sets. Many avalanche deaths are caused by suffocation,
so creating an air space is one of the most critical things
you can do. Also, take a deep breath to expand your chest
and hold it; otherwise, you may not be able to breathe after
the snow sets. To preserve air space, yell or make noise only
when rescuers are near you. Snow is such a good insulator
they probably will not hear you until they are practically
on top of you.
Above all, do not panic. Keeping your breathing steady will
help preserve your air space and extend your survival chances.
If you remain calm, your body will be better able to conserve
energy.
Rescuing a victim
Begin looking for clues on the surface (a hand or foot, piece
of clothing, ski pole, etc.), beginning with the point where
they were last seen. As you move down the slope, kick over
any large chunks of snow that may reveal clues. Since equipment
and items of clothing may be pulled away from a victim during
an avalanche, they may not indicate their exact location,
but can help determine the direction the avalanche carried
them. Mark these spots as you come across them. Be sure that
all rescuers leave their packs, extra clothing, etc., away
from the search area so as not to clutter or confuse search
efforts.
Once the victim is found, it is critical to unbury them
as quickly as possible. Survival chances decrease rapidly
depending on how long a victim remains buried. Treat them
for any injuries, shock, or hypothermia if necessary.
If you lost sight of the victim early during the avalanche,
or if there are no visible clues on the surface, mark where
the victim was last seen. Look at the path of the snow and
try to imagine where they might have ended up. For those wearing
avalanche transceivers, switch them to "receive" and try to
locate a signal.
For those using probes, begin at the point the victim was
last seen at. Or if you have a good idea of where they were
buried, begin in that area. Stand in a straight line across
the slope, standing shoulder to shoulder. Repeatedly insert
the probes as you move down slope in a line. Pay particular
attention to shallow depressions in the slope and the uphill
sides of rocks and trees, since these are terrain traps where
they may have been buried.
| Following is a list of quick
checks you can make throughout the day: |
| 1. |
What have the weather conditions been
over the past few days? Recent heavy snows? |
| 2. |
Can you observe any wind loading on the slopes? |
| 3. |
Do you have a good sense of the snowpack? Have
you performed any snowpit or shear tests? |
| 4. |
Have you noticed many fracture lines, heard "whumping"
or cracking sounds, or hollow noises in the snowpack? |
| 5. |
Are you keeping an eye on the orientation and
steepness of the slopes as you cross them? |
| 6. |
Are you lingering in gullies, bowls, or valleys? |
| 7. |
Noticed any recent avalanche activity on other
slopes similar to the one you are on? |
| 8. |
If a slope looks suspect, are there alternative
routes? |
Information
taken from the National Snow and Ice Data Center.
http://nsidc.org/snow/avalanche
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