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Avalanche Notes for Skiers
Rob Collister
Avalanches are easily the most significant hazard the off-piste
skier is likely to encounter. Every year, roughly a hundred people
die in avalanche accidents in the European Alps and many more are
injured or badly frightened. A high proportion of these incidents
involve skiers and climbers and all too many stem from ignorance.
These notes relate specifically to Europe. Most avalanche books
available in English are based on North American conditions and
research. In general the weather in the interior ranges of North
America (e.g. the Rockies) is colder than in the Alps and as a
result the snow pack is more complex and hazards longer lasting.
With this in mind, I deliberately do not elaborate on crystal
shape and size, or on the processes of rounding, sintering and
facetting (sometimes described as ET and TG metamorphism),
concentrating instead on what a skier can observe with eyes and
ears, without delving beneath the surface of the snowpack.
Ice Avalanche
It is useful to start by distinguishing between ice avalanches and
snow avalanches. Ice avalanches are created by the collapse of
ice-cliffs, or seracs, due to the downward flow of glaciers and
the pull of gravity. They often set off snow avalanches on their
descent, but are immensely destructive in their own right. Skiers
or climbers in the path of a serac fall are unlikely to survive.
Although slightly more likely to occur in warm weather, they are
basically unpredictable and can occur at any time, albeit
infrequently.
Always keep an eye open for hanging ice above you and never linger
in the vicinity of seracs.
Snow Avalanche
Snow avalanches are classified as comprising either dry snow or
wet snow. Both wet and dry snow avalanches are further classified
as being either loose snow (point release) or slab avalanches.
Wet Snow
Wet snow can be created by strong sunshine, a warm wind like the
fohn, or rain. It is inherently weak and, once released, can flow
on surprisingly gentle slopes. Wet snow avalanches often release
in the vicinity of rocks that have been warmed by the sun and
radiate heat into the surrounding snow pack. Snowballs that grow
larger as they roll down a slope are a warning sign that surface
snow layers are very wet.
Dry Snow
Snow that is cold and dry and unaffected by wind can be as much as
95% air. As a result, when released as an avalanche, it quickly
forms a turbulent cloud which can become airborne, attaining
tremendous speeds and travelling long distances. Fortunately, such
avalanches only occur after exceptionally heavy snowfall or in
combination with a huge serac collapse.
Loose snow avalanches
This type of avalanche, be it of dry or wet snow, starts from a
single point and fans out as it picks up more snow. They are very
common after any new snow and are a normal part of the settling
process. When they are small and comprise only the surface layer
they are known as sluffs. Provided there is not a great depth of
new snow (less than 30cm), they are not normally a major hazard in
themselves. Nonetheless, a sluff can easily knock skiers off their
feet and the existence of a terrain trap below can render an
otherwise innocuous slide lethal. Typical terrain traps are
cliffs, boulders, trees, or a gully or hollow into which the snow
can build up to great depth on top of an avalanche victim.
Slab avalanches
Slabs fracture along a line that may be anything from a few feet
to hundreds of yards wide. Usually, this fracture is initiated by
the shearing of the bond between two layers within the snowpack,
most commonly new snow and the older snow beneath. In the case of
wet snow, water trickles down through the snowpack until it
reaches an impervious layer which could be an icy crust or could
be the ground. The water continues to flow downwards but now it is
flowing between the two surfaces, destroying the bond between
them. If the bottom surface is the ground and it is suitably
smooth, like a rock slab or long grass, a full-depth avalanche may
result.
However, the most common type of slab, and the most common cause
of accidents, is that created by wind.
Wind-slab
Wind-slab is snow that has been transported by the wind and
deposited on a lee slope, as opposed to wind-crust which is the
hardening of the surface of the snow by wind blowing over it.
Wind-crust is usually no more than a few centimetres deep and is
not a hazard in itself, except in skiing terms as breakable crust;
wind-slab on the other hand, can form layers many feet thick and
is only as safe as the bond which attaches it to the layer below.
Wind-slab can be soft or hard, or somewhere in between. At one
extreme, when the snow has been transported by light winds of 10
mph or less, it feels like powder and is enjoyable to ski; at the
other extreme, after strong winds of 40 mph or more, the original
snow crystals have been so fragmented and they have packed down so
densely when the wind decelerates, that the edge of a ski or boot
barely grips. Both types, however, are similar in that they form a
homogeneous layer which, until it has had time to form bonds with
the layer below, is extremely unstable and likely to break away in
a mass at the slightest provocation.
Because it is so dense, hard slab takes longer to bond to the
layer beneath than soft slab. The more similar two layers are the
quicker they will bond together firmly. Soft slab on top of soft
old snow will stabilize more rapidly than the same slab lying on a
crust. This is due to the way in which individual crystals attach
to each other, a process known as sintering. Temperature, also,
plays an important part in sintering, speeding it up when warm.
(but still sub-zero) and slowing it down when cold, till at
-40°C, it ceases altogether. Because of this, North and East
facing slopes which receive less sun remain unstable for longer;
yet because of the prevailing SW to NW winds in the Alps, it is
these same slopes that receive most snowfall and are most prone to
wind-slab.
It is important to remember that it does not have to be snowing
for wind-slab to form. Snow blown by the wind can form slabs very
quickly in sheltered spots even when the sky is dear and the sun
shining. Remember also that sheltered hollows, ideal for slab
formation, can occur anywhere on a mountain, not just on the lee
side of a ridge crest.
Danger signs to take heed of are the squeaky sound of skis or
boots on hard slab, cracks shooting ahead of skis, snow breaking
away in compact blocks and the scary whoomph of snow settling
under one. Angle of slope is critical in determining whether a
specific slope is hazardous. Slabs rarely release on slopes under
30° and most commonly between 35° and 45° - steep
ground for skiers. The most hazardous point on any slope is on a
roll, or convexity, where the pull of gravity creates the greatest
tension. However, in very unstable conditions it is quite possible
to release a slab from an easy-angled slope well below the
fracture line, bringing the whole slope down on top of one.
Finally, it is all too easy to forget that temperature becomes
colder, winds stronger and snowfall heavier as you go up the
mountain. Even if the snow is falling vertically in the valley, it
is rare indeed for there to be no wind at all on the tops; and
conditions even at the halfway station may bear little relation to
what is happening at the ridge crest hundreds of metres higher.
Conclusion
Weak layers within the snowpack such as depth hoar or buried
surface hoar can and do occur in Europe but seem to cause fewer
accidents than in North America. In the Alps, the majority (though
by no means all) of avalanche accidents:
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are triggered by the victims, they are not "acts of God";
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occur during or the day after a snowfall;
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involve surface layers of soft slab;
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are on North or East facing slopes;
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are on slopes of 30° - 45°;
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occur when the avalanche hazard is at Category 3 on a five-point
scale. This is partly because most people have the sense to keep
clear of avalanche terrain when the hazard warning is 4 or 5,
but it also reflects a common misapprehension that Category 3 is
relatively safe. In fact, the official description of the level
of risk at 3 is "considerable".
If you decide that conditions are stable enough to ski on, or
beneath, slopes steeper than 30°, do carry a transceiver and
shovel and be practised in using them. Develop an awareness,
through using ski poles or a clinometer, of what a 30° slope
looks and feels like. And ski those steeper pitches one at a time.
Treat the mountains with respect, and have fun!
© Text Rob Collister 2002
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