Avalanche Safety – Slope Angle

Terry O’Connor, MD

A large problem with prime avalanche slopes is they are highly enticing to ride. This presents a challenge to all who are trying to have fun, yet ride safely in avalanche terrain.

Recognizing  what slopes are prone to avalanching is an important part of making safe backcountry decisions. One of the best ways to assess the risk of avalanche in the terrain you travel in is to know the angles of the slopes you are riding.

Avalanches happen when four ingredients are present: a slab, a weak layer, a trigger and a slope angle steep enough to slide, generally between 25-45 degrees. Not all slopes are steep enough to avalanche and some are too steep to regularly form slabs.  The most common slope angles on which avalanches occur is between 36-38 degrees

 

It is important to note that not all avalanches start on slopes with these precise angles. If a gentle slope of 25 degrees or less is connected by a persistent slab to a larger, steeper slope it is still possible to trigger a slide from below without ever getting on the steepest part of the slope. This is known as remote triggering and is a common way that riders get into avalanches in the backcountry when the hazard is high.

The easiest way to assess slope angle is to use a small tool called and inclinometer. These useful devices can be found online or at local backcountry touring stores.

BCA inclinometer in use indicating this slope in the prime range for risk of avalanche.

Or alternatively you can utilize a number of simple apps available on your mobile device, like iHandy Level pictured above

 

Although a basic inclinometer can measure a slope angle with a small amount of error, they do pose a few problems. The main one is that you have to venture onto the slope to get an accurate reading. One technique to overcome this issue is to measure angles on small slopes that appear to have a similar aspect and angle as the slope you plan to ride. Practicing with an inclinometer frequently and knowing what slopes angles are prone to avalanching is a great way to reduce your exposure to avalanche terrain.

When it doubt, remember that we frequently are wrong in ‘guesstimating’ slope angles. Check your inclinometer, and practice, practice, practice. .

For more on navigating and recognizing avalanche terrain check out this link from friends at the American Avalanche Institute.

 

REFERENCES

Schweizer, J., and J. B. Jamieson, Snow cover properties for skier triggering of avalanches,Cold Reg. Sci. Technol., 33,207–221, 2001

Tremper, B., Staying Alive in Avalanche Terrain, 284pp., The Mountaineers,Seattle, Wash., 2001.