'You cannot tell when and where the avalanche might occur'

'You cannot tell when and where the avalanche might occur'

This is Part 1 of a 3-Part Series Avalanches in Oregon

CORVALLIS, Ore. - Avalanches killed 35 people last year in the United States.

So far this winter, 11 skiers, snowboarders and snowmobilers have died in avalanches in the U.S.

To appreciate the power of these deadly forces of nature, it's important to understand how they happen.

"You can't tell, by looking at the surface, you cannot tell when and where the avalanche might occur," said Anne Nolin, an Oregon State University professor considered an expert on snowpack - and avalanches.

She said avalanches are caused by two types of snow: dry and wet.

"A dry snow avalanche sometimes is what we call a slab avalanche, which will start because of weaknesses in the snowpack," Nolin said.

Wet snow avalanches "usually happen in the spring when you've got melting snow. You usually see them later in the afternoon."

The anatomy of an avalanche all depends on the season and location.

But it's the slab or dry snow avalanches that are especially deadly.

"The slab avalanches are very fast moving and they can become airborne," Nolin said, "and those are really dangerous."

Steep slopes are where most avalanches occur. Add fresh snow and wind and the scene is set for danger.

"It's the slopes that are between 35 to 50 degrees," Nolin said.

Another factor: the amount of fresh snow that falls in a 24-hour period.

"It's the layer above the weakly bonded layer that's the thing that's going to slide," Nolin said.

Wind is also a key ingredient.

"The leeward side of the ridges where the wind blows the snow and it accumulates into these big cornices, you might see some of those break off and you might see some development of slab avalanches," Nolin said.

Most avalanches happen in the backcountry areas that are rarely visited.

They can be triggered naturally - or by humans.

"Whether it's you skiing, or snowshoeing or maybe snowmobiling," Nolin said.

And while they aren't extremely common in Oregon, Nolin said they do happen.

"Just because you don't hear about them," she said, "doesn't mean we don't have them."

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