|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
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 important.
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
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 grains.
- Air space.
- Very soft layers. (fist penetrates easily)
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 avoided:
- During snow storms or heavy drifting
- Immediately (24-48 hours) after these.
- During heavy thaw or sudden temperature rise.
When walking above cornices, take care to
give them a wide berth. Fig. 2 below, shows the possible fracture line.
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.
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 provide security.
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 rims.
TOP SIX FACTORS
- Visible avalanche activity. If you see
avalanche activity on a slope where you intend to go, go somewhere else.
- 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
- Feels unsafe. The "seat of the
pants" feeling of the experienced observer deserves respect.
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
- 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
- If hard slab, try to remain on the top of a
- Get rid of gear, sacks, skis etc.
- Try to roll like a log off the debris.
- 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 danger.
- Make a QUICK SEARCH of the debris surface.
- 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 arrives.
- 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 long time.