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. Over the past five decades, U.S. avalanche deaths have increased to about 30 a year, up from about four a year in the 1950s.

The increase is largely because of the growing popularity of snowmobiles and backcountry skiing, both can trigger avalanches. Slides usually begin on mountain slopes with a 30-degree or steeper grade. Beyond that, it’s all about gravity.

Experts can’t tell for certain exactly how much weight will trigger the collapse of a particular snow bank. Snow layers vary in weakness and when enough weight sits on a weak layer, it is like a pistol with the hammer cocked.

Many deaths result from avalanches inadvertently triggered by the victim. Victims cross a pleasant-looking snowfield atop a slab, then suddenly it collapses, sending overloaded layers of snow downslope.

A particularly dangerous type of avalanche is the “deep slab” variety. The layers of a snowfield start to connect and change over time, melting or stiffening in unpredictable ways. Slabs of snow can be more than seven feet (two meters) thick, as they were in the avalanche seen outside Cooke City, Montana, early this year.

A related kind of avalanche is a “wet snow” slide, where warming spring temperatures or rainfall weakens snow.

In ice avalanches, a huge block of glacial ice called a serac gives way without warning, sending cliffs of ice that pick up snow on their way downslope. The only thing forecasters can do is warn people to avoid ice fields or to spend as little time as possible in them.

While avalanches are hard to predict, there are known risk factors, including the amount of snow, its layers, and the wind direction. Wind-borne snow poses a particular threat due to re-distribution of snow.

Fourteen regional avalanche centers run by the U.S. Forest Service help provide avalanche warnings. The agency also has its own 105-millimeter howitzers to trigger small snow slides. For avalanche warnings and updates, visit

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