A cohesive
layer of snow formed when wind deposits snow onto leeward terrain.
Wind slabs are often smooth and rounded and sometimes sound
hollow.
Wind:
What direction the slope faces with respect to the wind is a
HUGE factor. Yes, it's true! And this takes most people by surprise.
But ignore wind at your own peril. It's easy to ignore wind
too because we human beings are big and heavy creatures and
most of the time, we hardly even notice wind unless it mess
up our hair.
But imagine yourself as a bird where wind is its entire world.
Then imagine yourself as something even smaller and lighter--like
a feather…or, how about new fallen snow? Wind to a delicate
snowflake is like the ocean current to plankton.
So why is wind important? As we have learned, loading (added
weight) causes of most avalanches and the fastest way to load
a slope is by wind. Wind erodes from the upwind side of an obstacle
such as a ridge and it deposits on the downwind side, and wind
can deposit snow ten times more rapidly than snow falling from
the sky.
Wind deposits snow most commonly on the leeward side of upper
elevation prominent terrain features such as ridges, peaks and
passes. We call this "top loading." But wind can also
blow across a slope which we call "cross loading"
and wind can even cause loading when it blows down a slope.
(figure 3-17) Remember that wind can blow from any direction
and thus deposit snow on most any slope. (See Weather chapter
on weather factors affecting wind slab development)
The bottom line: be suspicious of any steep slope with recent
deposits of wind drifted snow.
Typical Wind Slab Locations:
Ridgetop
Loading:
Ridge top winds transport wind from the
windward side of a ridge to the lee slope. Cornices are
often formed on steep ridgelines but may not be present
on rounded ridge tops. Windward slopes may show scoured
snow and or exposed rock or grass.
Side
Loading:
Cross slope winds typically load gullies
and chutes. This may occur at any elevation depending
on winds. Higher areas will typically be scoured. Low
areas may look smooth.
Wind slabs are so dangerous
because:
• As the wind bounces
the eroded snow across the snow surface, it grinds up the snow
into small, dense particles. By the time they finally come to
a rest on lee of an obstacle--where the wind slows down--they
pack into a heavy, dense layer of snow that can easily overload
any buried weak layer.
• When strong wind starts to blow, within minutes,
wind can turn nice fluffy powder into a dangerous wind slab.
When very safe conditions quickly turn into very dangerous conditions,
it easily takes people by surprise.
• Wind slabs
can form in extremely localized areas. Often only a few inches
separates safe snow from dangerous snow. We often hear people
say, "I was just walking along and suddenly the snow changed.
It started cracking under my feet, and then the whole slope
let loose."
How to Recognize
Wind Slabs:
Lucky for us, wind creates easy-to-read textures on the snow
surface and characteristically shaped deposits. No one should
go into avalanche terrain without first learning how to read
these obvious signs. An old avalanche hunter's adage: If you
have developed a good eye for slope steepness and the effects
of wind, you can avoid about 90 percent of all avalanches.
Eroded snow vs.
Deposited Snow
Eroded
snow:
• Looks Like: has a sandblasted, scoured, scalloped,
roughed-up look. • Feels Like: often hard snow and
difficult to negotiate on skis, snowboard or snowmobile.
• Also called: "sastrugi". • What
it means: Weight (snow) has been removed from snowpack and it
usually means that the snow has become more stable than before.
Deposited snow (wind slabs):
• Looks Like: smooth and rounded, lens shaped, pillow
shaped, chalky-white color • Feels Like: "slabby"
i.e. harder snow on top of softer snow. • Sound: often
hollow like a drum--the more drum-like, the more dangerous
• Often notice: • Cracks shooting away from
you--the longer the crack, the more dangerous. • Falling
through a harder surface layer into softer snow below. You can
easily feel this with a ski pole or a snowmobile track punching
through. • Difficult trail breaking . Keep falling
through the slab. • Hardness: can be very soft to
so hard that you can hardly kick a boot into it. •
Also called: pillows, wind slabs, snow transport.
• What it means: weight has been added to the snowpack.
If the weight has been added recently, and it's on a steep slope
without anchors, then it almost always means danger. (photo)
• What you should do when you find a wind slab on a steep
slope: • Stop immediately! Don't go any farther!
• Back off if you're on a big slope and dig down to investigate
how well the slab is bonded to the underlying snow (see Stability
chapter) • Jump on a few safe, test slopes to see
how the snow responds. • If the slab breaks away easily
on your tests, don't cross larger slopes. Go back the way you
came or find another route that avoids wind slabs. •
If you absolutely have to cross the slope (and I can think of
damned few reasons why you HAVE to cross a dangerous slope without
delving into B-movie plot devices), stay on the extreme upper
edge of the wind slab, wear a belay rope tied to a solid anchor,
and hope the crown fracture breaks at your feet instead of above
you.
