Avalanche Hazards

These basic concepts are based on ideas presented by Jill Fredston and Doug Fesler in their book Snow Sense: A Guide to Evaluating Snow Avalanche Hazard.

There are four factors that contribute to avalanche hazard.  They are: terrain, weather, snowpack and humans.  They can be addressed by answering these questions:

Is the terrain capable of producing avalanches?
Most human triggered avalanches occur on slopes between 30 and 45 degrees steep.  Avalanche terrain consists of any snow covered slope that is steeper than 25 degrees, or any slope, which has a slope steeper than 25 degrees immediately above it.

Is the weather contributing to the avalanche hazard?
Most avalanches occur during or shortly after snowstorms or periods of strong wind.  Rising temperatures during or shortly after a snowstorm or temperatures above freezing can increase the avalanche hazard.

Could the snow slide?
The snowpack is comprised of layers.  Avalanches occur when a slab, which is a cohesive layer of snow, buries a weak layer.  If enough weight is added to the snowpack so the weak layer fractures and the fracture propagates through the slab, an avalanche will release.  Slopes with deposits of wind transported snow are dangerous because the deposited snow adds weight and the wind drifted snow often forms slabs.  All steep, leeward slopes should be approached with extra caution. 

Recent avalanches provide an obvious clue that the snow could slide.  Other signs of snowpack instability include collapsing (often accompanied by a whoomph sound) or cracking of the snow around your feet or snowmachine.

Are you willing to get good information about current avalanche conditions and can you make good riding decisions based on that information?

Avalanche information for many areas in Montana can be obtained from these sites:

Bozeman, Big Sky, West Yellowstone, and Cooke City:  www.mtavalanche.com

Flathead and Kootenai National Forests:  www.glacieravalanche.org

Lolo, Bitteroot and Clearwater National Forests:  www.missoulaavalanche.org

If you are caught in an avalanche your best chance of survival depends on your partners locating and digging you out within 15 minutes.  This means everyone in your party should carry and know how to use an avalanche rescue beacon, shovel and probe pole.  Also, only one person at a time should be on any slope steep enough to produce an avalanche.  All other folks in the party should watch that person from a safe location.

Recognizing and reducing avalanche hazard will help you and your friends enjoy the great mountain riding opportunities that exists in Montana.

Article provided by: Ron Johnson, Avalanche Specialist, Gallatin National Forest Avalanche Center