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Cornice Fall Avalanches:
Cornices are the fatal attraction of the mountains, their beauty
matched only by their danger. Cornices are elegant, cantilevered
snow structures formed by wind drifting snow onto the downwind
side of an obstacle such as a ridgeline. Similar to icefall
avalanches, the weight of a falling cornice often triggers an
avalanche on the slope below, or the cornice breaks into hundreds
of pieces and forms its own avalanche—or both. Be aware
that cornice fragments often "fan out" as they travel
downhill, traveling more than 30 degrees off of the fall line.
Cornices tend to become unstable during storms, especially with
wind, or during times of rapid warming or prolonged melting.
Each time the wind blows, it extends the cornice outward, thus,
the fresh, tender and easily-triggered part of the cornice usually
rests precariously near the edge while the hard, more stable
section usually forms the root.
Similar to icefall avalanches, cornice fall avalanches don’t
kill very many people. And similar to slab avalanches, the ones
who get into trouble almost always trigger the avalanche, in
this case, by traveling too close to the edge of the cornice.
Cornices have a very nasty habit of breaking farther back than
you expect. I have personally had three very close calls with
cornices and I can attest that you need to treat them with an
extra-large dose of respect. NEVER walk up to the edge of a
drop off without first checking it out from a safe place. Many
people get killed this way. It's kind of like standing on the
roof of a tall, rickety building and walking out to the edge
for a better view. Sometimes the edge is made of concrete but
sometimes the edge is made of plywood cantilevered out over
nothing but air. It feels solid until, zoom, down you go. Check
it out first.
But cornices aren't all bad. You can use cornices to your advantage
by intentionally triggering a cornice to test the stability
of the slope below or to intentionally create an avalanche to
provide an escape route off of a ridge.
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Cornice
Tests:
Squeamish folks or lay-people might think cornice tests are
dangerous but they have been standard techniques among ski patrollers,
helicopter ski guides and especially climbers for decades. Cornices
are the "bombs of the backcountry." First,
make sure no one is below you--very important.
Next, simply find a cornice that weighs significantly more than
a person and knock it down the slope. A cornice the size of
a refrigerator or a small car bouncing down a slope provides
an excellent stability test. The smaller the cornice, the less
effective the test. You can kick the cornice, shovel it or best
of all, cut it with a snow saw which mounts on the end of a
ski pole. With larger cornices you can use a parachute cord
with knots tied in it every foot or so, which acts like teeth
on a saw. Throw the cord over the cornice or push it over the
edge with an avalanche probe. You can saw off a fairly large
cornice in under 5 minutes. It's best to work with small, fresh
cornices and not the large, old and hard ones. You can also
trundle heavy rocks down the slope, which work just as well
as cornices, but they’re often harder to find. This is
also a great way to create a safe descent route during very
unstable conditions. In other words, make an avalanche and use
the slide path to descend.
Caveat:
It doesn't take much imagination to see that knocking cornices
down avalanche paths can be very dangerous. ALWAYS use a belay
rope on slopes with bad consequences and practice your cornice
techniques on safe slopes until you get the techniques worked
out. Cornices have a nasty habit of breaking farther back than
you think they should. Be careful.
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