The Anatomy of an Avalanche

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Kevin P. Casey / Reuters

People enjoy the snow in the skiing area close to where three skiers died in an avalanche near Stevens Pass in the Cascade Mountains, Washington.

Avalanches have already claimed five lives in Washington and Montana since Sunday, as unstable weather conditions have given way to slides of snow. In Washington, two separate events claimed the lives of four people. In one case, a group of about a dozen highly experienced skiers got caught up in an avalanche they triggered, claiming the lives of three of them. One person survived with the use of an avalanche airbag. Earlier the same day, a snowboarder perished after an avalanche sent him over a cliff. On Monday a snowmobiler was killed in an avalanche near Kalispell, Mont., highlighting the dangers of unstable weather conditions in the West’s mountain ranges.

An avalanche occurs when unstable snow conditions let loose and cascade down the mountain. In 90% of all avalanche accidents, either the victim or someone in the victim’s party triggers the event, according to the Utah Avalanche Center. In Washington on Sunday, says the Northwest Weather and Avalanche Center, a “regionally unstable upper snow pack” coupled with high winds likely resulted in the triggering of multiple fatal avalanches.

(MORE: Pro Snowboarder Saved in Avalanche Thanks to Emergency Airbag)

The most common form of avalanche is called “dry slab,” which is similar to a dinner plate sliding off a table, the type of avalanche that occurred in both the recent Washington avalanches. In this event, a cohesive plate of snow slides over weaker snow below. Within five seconds after the snow “fractures” away, the avalanche can reach speeds up to 80 miles per hour.

But what triggers these fractures of snow? (It isn’t loud noises, like movies want you to believe.) Snow doesn’t take kindly to rapid change, especially a weak layer of snow succumbing to too much additional weight. If multiple feet of snow get dumped on a “weak layer” within hours, the weight alone can spark the fracture. But so can the immediate weight of a skier or snowmobiler. The weight of a person can be enough to shatter the underlying slab “like a pane of glass,” putting the victim in the middle of the flowing slab with little or no opportunity for escape.

In other circumstances, wind shifting snow loads quickly, rain dissolving the bonds of snow or rapid warming can all account for avalanche conditions. For this reason, weather events that contain high winds, powerful rains or unseasonably strong sun can all lead to an increased risk of avalanches, with or without the aid of backcountry recreationalists. And the steeper the slope, the more likely the avalanche dangers.

The size of the avalanche is largely determined by how much new snow has packed onto the unstable snow beneath. In Washington, most of Sunday’s avalanches had crowns less than two feet tall, indicating recent snow layers. In these specific cases, the new snow fell on top of “hoar frost,” a layer of weak crystalized ice, which contributed to the avalanches.

Once caught in an avalanche, it isn’t the rushing debris that generally proves fatal, but the loss of oxygen. Avalanche debris contains up to 70% air, so those buried within an avalanche typically have 15 minutes to be dug out before they start to succumb to carbon dioxide poisoning.

With the West’s ever-shifting weather patterns contributing to the number of avalanches, the U.S. trails only France and Austria with the most avalanches each year, each hosting between 17 and 21 percent of the one million annual avalanches.

MORE: Survivor: Wash. Avalanche ‘Horror Story’

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