What Are Avalanches Caused By?

Types of Avalanches

• Avalanche types are determined by three general characteristics. The first characteristic is the depth of the slide. Surface avalanches are created when one layer of the snowpack slides over another layer. A full-depth avalanche occurs when the entire snowpack slides over the ground. The second characteristic is the moisture content of the snowpack. Dry avalanches travel very quickly, 60 to 80 miles per hour, while wet avalanches travel much more slowly, around 20 miles per hour. Finally, avalanches can be loose, starting from one point and building, or slabs, where large areas separate and slide out.

Condition of the Snowpack

• The first component of the avalanche triangle is the condition of the snowpack. Newly fallen snow that does not adequately bond with the older layers is a recipe for disaster. New snowfall that exceeds 1 foot possesses the greatest danger, due to its weight. Snowfalls of between 6 inches to 1 foot pose a moderate risk. Snowfalls of less than 6 inches rarely result in avalanches. The existence of depth hoar, a course, grainy snow crystal, is also a dangerous indicator for the potential of an avalanche. This layer of snowpack bonds very poorly with other layers. Layers of graupel, round pellets of ice, also elevate the risk for an avalanche. These tiny pellets, similar to larger hailstones, act like ball bearings underneath layers of the snowpack.

Nature of the Terrain

• The second component of the avalanche triangle is the terrain. Three features are of primary importance: slope, surface and profile. Most avalanches occur on slopes of between 30 and 50 degrees. Slopes of less than 30 degrees rarely produce avalanches, while slopes over 50 degrees produce slides frequently enough to limit snowpack accumulation. Slick surfaces, such as rock slab, raise the potential for full-depth slides, while rough terrain and thick vegetation limit the potential by stabilizing the snowpack. A convex profile presents the highest potential for avalanches, while uniform or concave slopes reduce the risk.

Status of the Weather

• The final ingredient in the avalanche triangle is weather. Avalanches are most likely to occur during or within 24 hours of new snowfall. Wind is also a key component. Wind typically erodes, or scours, the windward side of slopes, while depositing this snow on the leeward side. This wind-loading of lee slopes increases the risk of avalanche on these slopes. Sun exposure can also contribute to avalanche formation. During the mid-winter, north-facing slopes are more likely to produce dry avalanches, while wet avalanches are more likely to occur on slopes with southern exposures during the spring.