A cohesive layer of snow formed when wind deposits snow onto leeward terrain. Wind slabs are often smooth and rounded and sometimes sound hollow.
What direction the slope faces with respect to the wind is a HUGE factor. 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. 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:
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.
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
• 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.