The Three A's of Avalanche Terrain

Angle Aspect and Altitude Key Components of Snow Slopes That Slide

Mar 21, 2009

Wind is an Important Modifier of the Snowpack, Lisa Auer

An understanding of avalanche terrain is one step towards hikers, climbers and ski tourers developing decision making skills for safe travel in the backcountry.

An important aspect of Avalanche Hazard Management, as safe snow travel is known in the industry, is preparation. On any planned backcountry itinerary, the three terrain factors - angle, aspect and altitude can tell you whether a given slope is likely to produce an avalanche. This is information that can be gleaned in advance in the comfort of a living room - from a topographic map.

Angle of Snow Slope

The main determinant of whether a slope can avalanche is its angle.

Angles greater than 60 degrees - Sluffs and spindrift (wind blown powdery snow) avalanches occur throughout a snow fall, preventing these steeper slopes from accumulating snow.
On slopes between 35 and 45 degrees - slab avalanches are most common (the steepness of expert runs at ski resorts).
At angles of less than 20 degrees - very wet slides are possible.

For skiers, snow mobilers, hikers, all mountain enthusiasts alike; always be aware of what is above the path of intended travel. All the snow on a slope is connected. While avalanches don't usually initiate on slopes of less than 25 degrees they can still be triggered from below on a low angle slope. It's important to avoid standing in a potential run-out zone, the end of an avalanche path where the snow and debris piles up.

Aspect of Slope to Wind and Sun

Aspect refers to a slope's orientation to the wind and the sun.

Wind is the single most important modifier of the snow pack. There doesn't always need to be a fresh snowfall to increase the avalanche hazard.
Lee (downwind) slopes collect more snow due to drifting and snow being carried from the windward to the lee side of ridges often creating wind slab (a cohesive layer of snow that is a common cause of avalanches). This is called wind loading.
Cornices (overhanging and unsupported coifs of snow) that often form at the crest of mountain tops and along ridge lines are a strong indicator of the presence of wind slab.
Windward slopes tend to be stripped of snow and therefore are safer to travel on.
Radiation from the sun influences the snow stability. In winter it tends to be a stabilizing influence, melting and refreezing - breaking down hazardous snow layers that form in cold conditions.
In spring, intense solar radiation can have the opposite effect; acting as trigger of loose snow slides especially on sunny aspects in the afternoon.

Altitude of Snow Slope

Snow conditions and stability will change with elevation due to variations in snowfall, temperature and wind.
With a deeper snow pack there's more snow available to avalanche.
Avalanche hazard does not always increase in the high country.
A shallow snow pack (where less snow has fallen) on shady slopes can sometimes create an avalanche hazard.
It's easy to imagine for example the sliding layer created when fresh snow falls at lower elevations on a smooth, icy slope. A rain soaked slope on the lower half of a mountain will turn to ice in below freezing temperatures. This ice layer can be preserved under new snow for a long time.

There are various resources at the disposal of back country enthusiasts for ascertaining snow conditions and avalanche hazard. These include avalanche bulletins, weather reports and topographic maps on which one can mark out a proposed skiing, hiking or climbing route and get an idea of the terrain to be encountered.

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The copyright of the article The Three A's of Avalanche Terrain in Winter Sports is owned by Lisa Auer. Permission to republish The Three A's of Avalanche Terrain in print or online must be granted by the author in writing.