Falling masses of snow and ice, avalanches pose a threat to anyone on snowy mountainsides. Beautiful to witness from afar, they can be deadly because of their intensity and seeming unpredictability.

Humans trigger 90 percent of avalanche disasters, with as many as 40 deaths in North America each year. Most are climbers, skiers, and snowmobilers. Learning about avalanches, and the conditions that cause them, can help people recreate more safely in the backcountry.

The recipe for an avalanche may seem simple: a mountain slope and a thick layer of snow. But Simon Trautman, an avalanche specialist at the U.S. Forest Service’s National Avalanche Center and Northwest Avalanche Center in Washington, says it’s actually a trifecta that causes avalanches: terrain, snowpack,