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What causes avalanches, and can you survive them? - Simon Trautman
What causes avalanches, and can you survive them? - Simon Trautman
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0:06
In February 2012, pro skier Elyse Saugstad was carving through the slopes
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of the Cascade Mountains
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when a crack formed in the snowpack above her.
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Before she was fully engulfed by a 5,000-ton wall of snow,
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Elyse heard “AVALANCHE!” just in time to react with a lifesaving maneuver.
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So, how could she survive this crushing snow surge?
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Avalanches are the rapid descension of snow, ice, rock, or a combination of these
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down a slope.
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They can be destructive natural disasters and personal tragedies:
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in the United States alone, avalanches kill an average of 27 people annually.
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These alpine nightmares don’t just awaken out of nowhere;
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they need three specific conditions.
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First, the mountain needs to have the right slope,
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typically between 30 and 45 degrees.
1:01
This is shallow enough to allow snow to build up on the slope,
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yet steep enough to provide enough potential energy for movement.
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Second, they typically need a layer of sturdy snow
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deposited over a weak or unstable layer,
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or what’s sometimes called “something over nothing.”
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Take the Cascade Mountain avalanche,
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where recent heavy storms deposited a nearly 1-meter-deep slab of fresh snow.
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Below this sat a thin layer of surface hoar.
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These are delicate, feathery ice crystals
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that form when water vapor in the air rapidly freezes on the surface of snow,
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almost like dew in the wintertime.
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Finally, avalanches need a trigger,
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something to collapse the weak layer and put the snow in motion.
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A trigger can be natural, such as heavy snowfall, strong winds, rainfall,
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or even other avalanches.
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Or they can be human-caused, like the surface impact of skiers or snowmobilers.
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Since fractures can propagate quickly through the snowpack,
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triggers can originate hundreds of meters away—
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upstream, downstream, or even adjacent to the avalanche’s path.
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Once in motion, how quickly an avalanche moves depends on the slope’s steepness,
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its exposure or vegetation cover, terrain roughness,
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and the type and amount of snow involved.
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For example, dry, light, and loosely bonded snow tends to travel fast,
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forming a powder cloud as it moves.
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In contrast, wet snow avalanches,
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caused by water moving through the snowpack,
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travel more slowly but are much denser and can be particularly destructive.
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Larger avalanches can travel more than 160 kilometers per hour,
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breaking trees, destroying buildings, and burying roadways.
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In 1970, one particularly massive avalanche
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consisting of 50 to 100 million cubic meters
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of glacial ice, snow, mud, and rock
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reached speeds of over 300 kilometers per hour.
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This avalanche proved to be the deadliest in history,
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engulfing an entire Peruvian town and killing 18,000 people.
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Avalanches aren't always this large,
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but even smaller ones can be dangerous for anyone caught in their path.
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As snow rushes downhill, it can behave like both a solid and a liquid,
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making its movement difficult to predict.
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At the fastest-moving front,
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surface snow moves faster than the snow beneath,
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pulling victims deep into the snowpack and scattering them across wide areas,
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making rescue difficult.
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Once an avalanche slows, the snow loses its fluid-like properties and compacts,
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leaving victims immobilized and unable to escape on their own.
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For these reasons, the best way to survive an avalanche
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is to avoid one altogether.
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Thankfully, at most ski resorts, parks, and even highways,
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avalanche workers mitigate risk by continuously monitoring the weather,
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stability of the snowpack, and human activity.
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If a threat is detected,
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they'll close terrain
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and may even intentionally trigger a controlled avalanche
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with explosives or artillery.
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However, there are regions where avalanche risk is not actively mitigated,
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known as the backcountry.
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Here, you are responsible for managing your own risk.
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Local avalanche centers provide avalanche forecasts,
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and anyone entering the backcountry should understand them,
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be properly trained, and carry essential rescue gear.
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And ultimately, in high-hazard terrain,
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there's no way to completely eliminate risk.
4:44
The group at the Cascade Mountains consisted of highly experienced skiers.
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Yet the area was very exposed, and following a large storm,
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the avalanche danger forecast was elevated.
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In the end, three skiers lost their lives.
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But not Elyse, as she was able to deploy an inflatable airbag.
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These devices are imperfect and a last line of defense,
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yet can keep a victim closer to the surface,
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improving their chance of rescue should disaster strike.
