Avalanches explained: How people trigger disasters

"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, the National Geographic said. Countless natural avalanches happen on mountainsides in a normal year, says Doug Chabot of the Gallatin National Forest Avalanche Center. Overthe 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 Glaciologyanalysis 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 Changesupports that finding. For full version go to