Avalanches can happen wherever there is snow lying on ground of sufficient
angle. Accidents in recent years in most mountain areas, demonstrate the
truth of this statement. The vastly increased popularity of winter climbing
and hill walking, along with the growth of interest in ski touring means
that greater numbers are at hazard. Sadly, each year adds to the list
of injuries or fatalities. Many of these accidents would have been
avoidable, given greater care or knowledge, or if the victims had even
paused to consider that avalanche hazard might be present.
In making practical assessments of avalanche hazard, there is no substitute
for the instinctive feeling for snow conditions which can be gained only by
years of experience. However, no-one is born with such experience and the
novice or the less frequent winter mountain user, may still enjoy a safe day
out if some basic principles are learned and acted upon.
Having accepted this, you have
greatly reduced your chance of ever being involved in an avalanche.
Remember that experience in itself is no antidote to avalanches and that
"the avalanche does not know you are an expert!"
Snow is deposited in successive
layers as the winter progresses. These layers may have dissimilar
physical properties and an avalanche occurs when one layer slides on another
(Surface Avalanche), or the whole snow cover slides on the ground
(Full-Depth). An avalanche may be Dry or Wet, according to whether
free water is present in the snow. It may be of Loose Snow, when the
avalanche starts at a single point or a Slab Avalanche which occurs when an
area of more cohesive snow separates from the surrounding snow and slides
out. In practice, any snow slide big enough to carry a person down is
This is the most important factor
in determining whether avalanches are likely, and the evolution of the
snowpack is entirely dependent on this. However, as the mountaineer
can study both of these, it is useful to do so.
Many weather variables affect
avalanche release and information can often be gained before setting out.
Readouts from summit weather stations can be beneficial. The
information provided on temperature, wind speed and direction often enables
useful predictions to be made before leaving home. For instance,
if a SW wind of 25mph is indicated with freezing temperatures and snow known
to be lying, then it may be assumed that some avalanche hazard will be
building on NE - facing slopes.
Local advice can often be obtained
regarding recent weather, while forecasts are always available. Remember
that mountain weather is particularly difficult to predict and the likely
influence of unexpected changes in weather, both on your own expectation as
to snow stability should be considered.
When visibility is adequate,
snowpack observation can begin from the roadside. Evidence of recent
avalanche activity, main snow accumulation zones, fresh loading by new snow
and drifting, can often be noted from below.
Observations can continue on the
approach, noting such details as depth of foot penetration, cornice build
up, ease of release of small slabs and the effect which localized wind
patterns may have had on slab formation.
Any suspect slopes which must be
negotiated (bearing in mind that the safest course is to avoid them) may be
tested by digging a snowpit. Pits should not initially be dug on the
main suspect slope, but on small, safe slopes of similar orientation.
There's no need to dig to ground
level, but only down to the first reasonably thick layer of nevé (old
re-frozen snow). The snow layers may then be identified by smoothing
the back wall of the pit and probing with a finger all the way down.
This will help assess the hardness of the layers. The following
features should be looked for:
- Adjacent layers of different
hardness. (difference of more than 2 on a scale of 5).
- Water drops squeezed out of a
snowball made from any layer.
- Layers of ice.
- Layers of graupel (rounded,
heavily rimmed pellets). These act like a layer of ball bearings
in the snowpack.
- Feathery or faceted crystals.
- Layers of loose, uncohesive
- Air space.
- Very soft layers. (fist
Any of the above might be the
source of a dangerous weakness in the snowpack.
These observations may be
supplemented by a shovel test (see Fig 1). For this, a shovel is not
necessary. Your ice axe and gloved hands will suffice.
Having made the snowpit
observations, isolate a wedge shaped block, cutting down to the top of the
next identified layer. If the top layer then slides spontaneously,
clearly a very poor bond exists between the layers. If it does
not, then try to rate the ease with which you can pull the block off by
inserting your shovel/axes/hands behind the block and pulling. Do this
for each suspect layer in your pit. Performing this test many times will
help you to build up a "feeling" for the stability of the layers.
Techniques such as this should
enable you to make an educated hazard assessment. Remember that your snowpit
observations will hold good only for slopes of similar orientation and
altitude to your test pit. You will need to extrapolate for situations
higher up, for instance below cornices, where surface windslab layers may be
much thicker, etc.
An attempt should be made to rate
the slope Safe, Marginal, or Unsafe. Even if a slope is Marginal or
Unsafe, it may be possible to choose a safe route by careful selection.
Many avalanches are
cornice-triggered. In general, climbing below cornices should be
- During snow storms or heavy
- Immediately (24-48 hours) after
- During heavy thaw or sudden
When walking above cornices, take
care to give them a wide berth. Fig. 2 below, shows the possible
On most hills, avalanche hazard can
be avoided by sensible choice of route.
Slope angle. Most large slab
avalanches run on slopes between 25 and 45 degrees. This range
includes the average angle of coire backwalls and approach slopes to crags.
Ground surface. Smooth ground
such as rock slab is pre-disposed to full-depth avalanches. Rough ground
such as large boulders will tend to anchor base layers in position, making
avalanches less likely. Once these boulders are covered, however,
surface avalanche activity is unhindered.
Slope profile. Convex slopes
are generally more hazardous than uniform or concave slopes. The point
of maximum convexity is a frequent site of tension fractures, with the
release of slab avalanches. (Fig. 3)
Ridges or Buttresses are better
choices than open slopes and gullies when avalanche conditions prevail.
The crests of main mountain ridges are usually protected from avalanche,
while in climbing situations, rock belays on ribs and buttresses can often
Lee Slopes should be avoided after
storms or heavy drifting. Their location will obviously vary according to
wind direction, but will include the sheltered side of ridges and plateau
TOP SIX FACTORS
- Visible avalanche activity.
If you see avalanche activity on a slope where you intend to go, go
- New snow build-up.
More than 2 cm/hr may produce unstable conditions. More than
30cm continuous build-up is regarded as very hazardous. 90% OF
ALL AVALANCHES OCCUR DURING SNOWSTORMS.
- Slab lying on ice or neve,
with or without aggravating factors such as thaw.
- Discontinuity between
layers, usually caused by loose graupel pellets or airspace.
- Sudden temperature rise.
The nearer this brings the snow temperature to 0 degrees C, the
higher the hazard, even if thaw does not occur.
- Feels unsafe. The
"seat of the pants" feeling of the experienced observer deserves
It is rarely essential to negotiate
an avalanche-prone slope. It is usually possible to find another
way, or retreat. 90% OF ALL AVALANCHES INVOLVING HUMAN SUBJECTS
ARE TRIGGERED BY THEIR VICTIMS. If it is essential to
proceed, the following should be borne in mind:
- Solo travelers in avalanche
terrain run particularly grave hazards.
- Skiers are in greater danger
than walkers - the lateral cutting action of skis readily releases
unstable snow. All off-piste skiers should use avalanche transceivers
and have them SWITCHED ON before leaving base. They should
carry collapsible probes and shovels. Climbers and walkers should
also consider the use of these items.
- Direct descent or ascent is
safer than traversing.
- Go one at a time - the others
should closely observe the progress of the person on the suspect slope.
- Close up clothing. Wrap
scarf or other item around mouth and nose.
- Belay if possible. This is
rarely feasible on wide, open slopes.
In most avalanche situations, any
defensive action is very difficult. Movement relative to the debris is
often impossible. However, some of the following may be useful.
- Try to delay departure by
plunging ice axe into the undersurface. This may help to keep you near
the top of the slide.
- Shout. Others may see you.
- Try to run to the side, or jump
up slope above the fracture.
- If hard slab, try to remain on
the top of a block.
- Get rid of gear, sacks, skis
- Try to roll like a log off the
- Swimming motions sometimes help.
- As the avalanche slows down, you
may be able to get some purchase on the debris. Make a desperate effort
to get to the surface, or at least get a hand through.
- Keep one hand in front of your
face and try to clear/maintain an air space.
- Try to maintain space for chest
expansion by taking and holding a deep breath.
- Try to avoid panic and conserve
energy. Your companions are probably searching for you.
If you witness an avalanche
- Observe the victim's progress
and if possible mark the point of entry and point at which last seen.
- Check for further avalanche
- Make a QUICK SEARCH of the
- LOOK for any signs of the victim.
- LISTEN for any sounds.
- PROBE the most likely burial spots.
- Make a SYSTEMATIC SEARCH,
probing the debris with axes or poles.
- Send for help.
- KEEP SEARCHING until help
- REMEMBER, YOU ARE THE BURIED
VICTIM'S ONLY REAL CHANCE OF LIVE RESCUE. Although survival chances
decline rapidly with duration of burial, they do not reach zero for a