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Avalanches Explained: How People Trigger Disasters

Just a little extra weight on a slab of snow can create a deadly, downhill force of nature.

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Cartoon avalanches start with a snowball merrily rolling downhill, picking up more snow as it travels. That's not how it really works, say avalanche experts, which explains the deadly results of recent avalanches that caught hikers off guard in Nepal.

"Avalanches are a crazy thing," says Karl Birkeland, director of the U.S. Forest Service National Avalanche Center in Bozeman, Montana. "They are natural disasters that we can trigger ourselves."

In Nepal, blizzard-driven avalanches have been blamed for the death of hikers on the popular Annapurna trekking circuit this week. The Tourism Ministry reports that at least 23 people have died from the avalanches and from exposure afterward. (Related: "Dozen Or More Feared Dead in Himalayan Blizzard and Avalanches.")

Instead of a snowball, an avalanche actually starts with a long stretch of snow detaching from its resting spot on a slope as a block or slab. And it's hard to predict when that will happen.

Countless natural avalanches happen on mountainsides in a normal year, says Doug Chabot of the Gallatin National Forest Avalanche Center. Over the past five decades, U.S. avalanche deaths have increased to about 30 a year (from about four a year in the 1950s), largely because of the growing popularity of snowmobiles and backcountry skiing. Both can trigger avalanches.

Despite the increasing number of deaths, a 2000 Annals of Glaciology analysis suggested that global warming will lead to "a slight decrease" in mountain avalanches overall, particularly summer ones, because warmer temperatures mean less snow. A more recent study of historical avalanches in the Swiss Alps published in the journal Climatic Change supports that finding.

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An avalanche races down a 7,000-foot chute on Pik Kommunizma in Tajikistan.


Snow Physics

Simple physics explains most avalanches, says snow scientist Jordy Hendrikx of Montana State University in Bozeman. Slides usually begin on mountain slopes with a 30-degree or steeper grade. Beyond that, it's all about gravity.

"You have a layer of snow on a slope that can only bear so much of a load. When you exceed that load, it collapses," he says. "That is the simple cause of most snow avalanches."

That seems to be what happened in the recent Nepal avalanche. The area had at least three feet of snow, Hendrikx says, and probably much more at higher elevations.

Experts can't tell for certain exactly how much weight will trigger the collapse of a particular snow bank. "Snow is one of the most variable materials we can imagine encountering at everyday temperatures," Hendrikx says.

Because of that variability, some snow layers are weaker than others. When enough weight sits on a weak layer, it is like a pistol with the hammer cocked.

"A pinprick almost can set them off," says Birkeland. A small weight such as a skier or a load of new snow can trigger a weak spot to collapse.

Many people die in avalanches they have inadvertently triggered, Birkeland adds. Victims cross a pleasant-looking snowfield atop a slab, then suddenly it collapses and  sets off more overloaded layers of snow as it heads downslope.

Deep Slabs

A trickier type of avalanche is the "deep slab" variety, Hendrikx says. In this type of avalanche, the layers of a snowfield start to connect and change over time, melting or stiffening in unpredictable ways. Slabs of snow can be more than seven feet (two meters) thick, as they were in this avalanche seen outside Cooke City, Montana, early this year.

These deep slabs detach unpredictably; as they fall, they can fell trees and leave behind clearings in danger zones that winter hikers, years later, may mistake for a nice place to pitch a tent.

A related kind of avalanche is a "wet snow" slide, where warming spring temperatures or rainfall weakens snow, Hendrikx says. Even if overall avalanche numbers drop, the share of these avalanches could increase in the future due to climate change.

Geography also plays a role. "Some landscapes just produce a lot of avalanches, which is something we try to educate people about," he says. "That nice clearing may not have any trees for a reason."

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In one of the worst alpine accidents on Mount Rainier in decades, climbers fell thousands of feet in an avalanche in June 2014.


Ice Slides

Ice slides are another beast, different from snow avalanches. One such ice slide triggered the April 18, 2014, disaster that killed 16 Sherpas on Mount Everest. (See: "Measuring Everest's Monster Avalanche.")

In ice avalanches, Chabot says, a huge block of glacial ice called a serac gives way without warning, sending cliffs of ice that pick up snow on their way downslope.

"They are completely unpredictable," Chabot says, dependent on fractures propagating rapidly through massive ice blocks.

The only thing forecasters can do is warn people to avoid ice fields or to spend as little time as possible in them.

"The Sherpas on Everest who died were maximizing their exposure time, in a sense, to help set up a trail that would let climbers through the ice field faster, minimizing their exposure," Chabot says.

Armed Against Avalanches

While avalanches are hard to predict, there are known risk factors, including the amount of snow, its layers, and the wind direction.

Wind-borne snow poses a particular worry. The weather report might say that six inches of snow fell on a mountain, for example, but wind could actually deposit 12 inches or more on farside slopes.

Fourteen regional avalanche centers run by the U.S. Forest Service help provide avalanche warnings. The agency also has its own 105-millimeter howitzers to trigger small snow slides.

"It's like a stick of dynamite," Birkeland says. "Once it goes off, it's not dangerous. Better we set off one harmless one than let a whole field of dynamite gather together on a mountainside."

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