Avalanches Explained: How People Trigger Disasters

Recently 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. 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.

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.

Read more:
https://news.nationalgeographic.com/news/2014/10/141017-avalanche-explainer-snow-science/