Types
of Avalanches:
Slab Avalanche:
If you’re looking for the killer then this is your man.
This is the White Death, the Snowy Torrent, the Big Guy in the
White Suit. Dry slab avalanches account for nearly all the avalanche
deaths in North America.
A “slab” is a cohesive plate of snow that slides
as a unit on the snow underneath. Picture tipping the living
room table up on edge and a magazine slides off the table. Now
picture you standing in the middle of the magazine. The crack
forms up above you and there you are, there’s usually
no escape and you’re off for the ride of your life.
The bonds holding a slab in place typically fractures at 350
kilometers per hour (220 miles per hour) and it appears to shatter
like a pane of glass. It’s typically about the size of
half a football field, usually about 30-80 centimeters (1-3
feet) deep and it typically reaches speeds of 30 km/hr (20 mph)
within the first 3 seconds and quickly accelerates to around
130 km/hr (80 mph) after the first, say, 6 seconds. Dry slab
avalanches can lie patiently, teetering on the verge of catastrophe,
sometimes for days to even months. The weak-layers beneath slabs
are also extremely sensitive to the rate at which they are stressed.
In other words, the rapid addition of the weight of a person
can easily initiate the fracture on a slope that would not have
avalanched otherwise. A slope can lay in waiting like a giant
boobie trap--just waiting for the right person to come along.
The crack often forms well above the victim leaving little room
for escape. Does any of this sound dangerous to you? |
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Loose Snow Avalanche:
Loose snow sliding down a mountainside is called a loose snow
avalanche. Small Loose snow avalanches are called "sluffs".
Loose snow avalanches usually start from a point and fan outward
as they descend, and because of this they are also called “point
releases.” Very few people are killed by sluffs because
they tend to be small and they tend to fracture beneath you
as you cross a slope instead of above you as slab avalanches
often do. The avalanche culture tends to minimize the danger
of sluffs, sometimes calling them "harmless sluffs."
But, of course, this is not always the case. Houses have been
completely destroyed by "harmless sluffs," and if
caught in one, it can easily take the victim over cliffs, into
crevasses or bury them deeply in a terrain trap such as a gully.
Most of the people killed in sluffs are climbers who are caught
in naturally-triggered sluffs that descend from above--especially
in wet or springtime conditions.
Sluffs can actually be a sign of stability within the deeper
snow when new snow sluffs down without triggering deeper slabs. |
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Icefall Avalanches:
When glaciers flow over a cliff they form the ice equivalent
of a waterfall—an icefall. Falling blocks of ice create
an avalanche of ice, which often entrain snow below it or triggers
slabs. Especially in big mountains, icefall avalanches can be
large and travel long distances. Despite this, icefall avalanches
kill few people compared to dry slabs that people trigger themselves.
Most of the deaths from icefall avalanches occur to climbers
in big mountains who just happen to be in the wrong place at
the wrong time.
Icefall avalanches occur more or less randomly in time. However,
in warmer climates, more ice tends to come down in the heat
of the day than at night. Also, on a longer time scale, glaciers
tend to surge, meaning that they actually have very slow waves
that travel through them that produce a surge of movement for
a few days to a month, followed by less movement for several
more days or even months. For instance, sometimes an icefall
seems very dormant for several months, then suddenly, it produces
lots of activity for several days to a month.
But besides these exceptions, icefalls are fairly random--pretty
much a roll of the dice when traveling under an icefall. The
best way to deal with icefall avalanches, of course, is to avoid
traveling on them or beneath them. And when you choose to travel
beneath them, do so quickly. At the risk of being too obvious--never
camp under icefalls. But sometimes bad weather prevents climbers
from seeing icefall hazard when they set up camp, or bad weather
forces them to camp in the wrong spot. Many accidents with icefall
avalanches happen this way. |
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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. 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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Wet Avalanches:
Most avalanche professionals make a hard separation between
wet snow avalanches and dry snow avalanches, because wet and
dry avalanches are so different. You forecast for wet and dry
avalanches very differently, much of the mechanics are different,
they move differently, and it's only natural for us to think
of them as two separate beasts altogether. But really, there’s
a continuum between wet and dry avalanches. For instance, there
are damp avalanches, and often, large, dry avalanches start
out dry and end up wet by the time they get to the bottom because
either the energy of the descent heats the snow up or they travel
into a region of warmer snow. Like dry snow avalanches, wet
avalanches can occur as both sluffs and slabs.
Wet avalanches usually occur when warm air temperatures, sun
or rain cause water to percolate through the snowpack and decrease
the strength of the snow, or in some cases, change the mechanical
properties of the snow. Once initiated, wet snow tends to travel
much more slowly than dry snow avalanches--like a thousand concrete
trucks dumping their load at once instead of the hovercraft-like
movement of a dry avalanche. A typical wet avalanche travels
around 15 to 30 km/hr (10 or 20 mph) while a typical dry snow
avalanche travels 100 to130 km/hr (60 or 80 mph)--big difference.
Wet slides are also harder for a person to trigger than a dry
slide. Because of these two facts, wet avalanches don’t
account for nearly as many avalanche fatalities as dry snow
avalanches. But they’re certainly not insignificant. They
still account for a sizeable percentage of avalanche fatalities
in maritime climates, especially to climbers. Wet slides can
also do quite a bit of damage to property or to forests and
often cause significant hazards on highways. |
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Glide Avalanches:
Glide occurs when the entire snowpack slowly slides as a unit
on the ground, similar to a glacier. Don't mistake glide for
the catastrophic release of a slab avalanche that breaks to
the ground. Glide is a slow process, that usually occurs over
several days. Glide occurs because melt water lubricates the
ground and allows the overlying snowpack to slowly "glide"
downhill. Usually, they don't every produce an avalanche but
occasionally they release catastrophically as a glide avalanche.
So the presence of glide cracks in the snow do not necessarily
mean danger. It's often difficult for a person to trigger a
glide avalanche but at the same time it's not smart to be mucking
around on top of them and especially not smart to camp under
them.
We tend to find them in wet climates and when they occur in
dry climates they do so in spring when water percolated through
the snow or sometimes during mid winter thaws.
When do they come down? Like an icefall, they come down randomly
in time--when they're good and ready--not before. You would
think that they would come down during the heat of the day or
when melt water running along the ground reaches its maximum.
But oddly enough, they tend to release just as often with the
arrival of cold temperatures following melting as during melting
itself. It's hard to play a trend with glide avalanches. They
come down when they're good and ready and it's impossible to
tell when that is. Just don't spend much time underneath them. |
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Slush Avalanches:
An oddity in most of the avalanche world, slush avalanches usually
occur in very northern latitudes such as the Brooks Range of
Alaska or in northern Norway. They're unusual because they occur
on very gentle slopes compared with other avalanches, typically
5-20 degrees and they rarely occur on slopes steeper than 25
degrees. A typical slush avalanche occurs in impermeable permafrost
soil, which allows water to pool up, and occurs during rapid
saturation of a thin, weak snowpack. When water saturates the
snowpack, it catastrophically looses its strength and the resulting
slush often runs long distances on very gentle terrain. Once
again, very few people are killed by slush avalanches possibly
because so few people live in high latitude permafrost mountains.
But they can certainly be dangerous to people camped in the
wrong spot or structures built in the wrong locations. |
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