How do avalanches occur?

By Kevin Byrne, AccuWeather staff writer


For those who frequently take to the great outdoors in the winter, oftentimes in remote, wide open terrain where there is copious amounts of fresh snow, understanding the dangers of avalanches and how they form is essential.

An avalanche is a flow of snow that moves down the slope of a mountain and frequently forms in two different ways: slabs or loose snow.

A loose snow avalanche is an avalanche that releases from a point and spreads downhill. Small loose snow avalanches are also known as sluffs and tend to start from a point and fan outward as they descend, according to the National Avalance Center (NAC).

avalanche formation 3/9/2018

The wind scours a snowy ridge in the Indian Peaks Wilderness, as seen from Eldora Mountain Resort, near Nederland, Colo., Wednesday, Feb. 26, 2014. (AP Photo/Brennan Linsley)


Slab avalanches tend to be more dangerous to people, according to Karl Birkeland, director of the NAC.

"Slab avalanches are when you have a denser or at least a stronger layer of snow over a weaker layer and that strong layer tends to break over the slope, almost like a pane of glass,” Birkeland said. “And then all that snow slides down at the same time.”

Avalanches can be triggered by human activity, such as skiing or snowboarding. However, there are many natural causes as well.

Natural avalanche triggers include new snowfall on existing snowpack, a significant temperature change, breaking of snow cornices and even earthquakes.

Wind also plays a significant role, as it can redistribute snow from windward terrain to the leeward side of a mountain.

"Cohesive layers of snow that make up slabs are often the result of wind action," said AccuWeather Senior Meteorologist Jim Andrews.

"Drifted snowpack, especially cornices, is typically coherent. Such slab-prone snowpack can happen during storms or afterwards," he said.

Avalanches tend to be either wet or dry and usually require separate weather factors to form.

Wet avalanches form when there's rapid warming above the melting point. That allows a lot of water to move through the snowpack and weaken it, which can create dangerous avalanche conditions, Birkeland explained.

Dry avalanches are usually the product of wind drifted snow or the loading of new snow. These can also flow faster than a wet avalanche which usually moves around 10-40 mph, per the NAC.

Once an avalanche gathers significant momentum down a mountainside, it can gather serious speed. Birkeland said some have been tracked moving as fast as 100 mph.

avalanche formation 3/9/2018

A sign warns out of bounds skiers at the beginning of an unpatrolled area, on avalanche-prone National Forest land adjacent to Corona Bowl, known for its extreme skiing, at Eldora Mountain Resort, near Nederland, Colo., Wednesday, Feb. 26, 2014. (AP Photo/Brennan Linsley)


Avalanches kill more people in national forests each year than any other natural hazard, according to the NAC. About 25-30 people are killed annually while enjoying recreational activities in the forests.

For the 2017-2018 winter season, there have been 18 avalanche fatalities in the U.S., according to the Colorado Avalanche Information Center.

Almost all of the avalanche fatalities that occur in the U.S. occur when people are in the backcountry, according to Birkeland.

Within ski areas, ski patrols regularly attempt avalanche mitigation work such as using explosives to reduce avalanche danger and shutting off access to terrain that is deemed hazardous. Yet, there are still people that will take risks and ignore boundaries to ski in areas with fresh, untouched snow.

“People will step outside those boundaries and feel like it is still the ski area even though they’re now in the backcountry just because they can see the ski area,” Birkeland said. “And it's a completely different ballgame out there because you don’t have professionals assessing the avalanche danger and opening and closing terrain.”

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Experts emphasize the need to take precautions before heading out to enjoy winter recreational activities in the backcountry.

Birkeland recommends first gathering the forecast conditions for avalanche-prone areas on websites like avalanche.org, which can then direct you to the nearest avalanche center in your specific region of the country.

Getting the proper safety equipment is also vital. Avalanche rescue gear includes a beacon and a shovel or probe so you help your companion dig out of an avalanche or be tracked by rescuers. There are also avalanche airbags that can help someone remain at the surface of the snow rather than getting buried if caught in an avalanche.

Those who are unfamiliar with avalanche-prone terrain should consider taking a class on the subject. Know Before You Go is a website that promotes free educational resources where users can learn the basics about avalanche safety.

It is recommended that folks travel in these areas with a partner.

"Then you should only expose one person to the avalanche danger at a time while the other watches from a safe location," Birkeland said. "This is the only way that an effective avalanche rescue can be initiated if someone is caught."

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