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What's behind all the deadly avalanches in Western ski areas?

The heavy snowfall in Western states that's been good news for skiers has come with a price: an unusual number of avalanche-related deaths on resort mountains.

Twelve people have died in U.S. avalanches since mid-December, and 10 in Canada, according to statistics kept by avalanche information centers. In the U.S., three of those deaths were "in bound" (within established ski trails) in Utah, Wyoming and California. That's the most in-bound fatalities since 1976, when three skiers at Lake Tahoe's Alpine Meadows resort perished in an avalanche, the New York Times reports today.

"It’s a war zone," Lanny Johnson, a former patroller at Alpine Meadows, told the Times.

There aren’t reliable stats on avalanches; if they escape detection by witnesses, they're not recorded, Ethan Greene, director of the Colorado Avalanche Information Center, tells ScientificAmerican.com. He notes, however, that while there don’t appear to be more deaths or avalanches overall this season than in the past, the number of in-bound deaths, "is unusual," and the pattern of avalanches across the West "atypical."

Unstable bases on mountain snowpacks from Colorado to Idaho are to blame, says Karl Birkeland, an avalanche scientist with the U.S. Forest Service's National Avalanche Center. The bottom layers of these snowpacks are made up of thin, sugary snow crystals from early-season dustings, covered by ice layers from rain afterwards, he tells us. More recently, big storms dumped heavy snowfall on top of those deeper layers.

If a weak layer is near the top of a snowpack, it's "not such a big deal, but buried, at some point it will be overloaded by a big snow or a big wind storm or a snow-mobiler adding enough force onto the snowpack [that] it starts to fracture," Greene says. "When an underlying plane becomes detached, the whole plate of snow falls off the hill."

The Golden State hasn’t experienced the same weather pattern as mountains to the east, but had heavy snowfall in a short period, Greene says.

Ski patrols cut down on avalanche risk by releasing snowpacks with explosives, Greene says. "Like all natural hazards," he notes, "we can do a lot of work to make the problem of an avalanche very small, but we can't get it to the point where it's 100 percent safe.

"We have a trickier snowpack than in recent years," he adds. "People are willing to ride and ski on terrain that is much more complicated and has lots of places where you can trigger avalanches, rather than a large, open bowl where you have easier terrain. That makes it harder to release every piece of snow that could role down the hill."

For information about how to avoid or survive an avalanche, visit www.fsavalanche.org or www.avalanche.org.

Avalanche by Strange Ones via Flickr

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