Resource: How Do Avalanches Form?

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Avalanches are one of nature's sudden and destructive forces. Often occurring without warning, an avalanche can shift hundreds of thousands of tons of snow quickly and with deadly speed. In avalanche-prone countries, about 100 to 200 people die each year from avalanches. Today, snowmobilers account for the highest percentage of avalanche deaths in the U.S., followed by skiers, snowboarders, climbers, and others involved in backcountry recreational activities.

A snow pack is made of both strong and weak layers. Strong layers are usually composed of small, angular snow crystals that interlock to create strong bonds. Weak layers tend to be composed of granular snow crystals that bond poorly. Weak layers prevent strong layers from bonding with one another, thereby creating an unstable situation. When a weak layer lies atop a strong layer, there is no problem. But when the situation is reversed, and the overlying weight becomes too much for the weak layer to support, it collapses and the snow above it slides downhill.

Typically, weak layers give way quickly, producing small avalanches. However, when frozen melt-water called "melt crust" forms between the snow crystals, it permits the weak layer to hold longer. When a weak layer remains in place for a long time and more and more snow accumulates above it, the size of a potential avalanche increases.

The slope's gradient (or angle), directional aspect, and topographic features all help determine whether its snow will slide. Of these factors, gradient is the most important. Almost all avalanches occur on slopes between 35 and 45 degrees. Precipitation, wind, and temperature also influence the stability of a snow pack. Of these, wind is the most common cause of avalanches. Windblown snow can collect 10 times faster than falling snow, eroding snow from windward slopes and moving it to downwind slopes. The additional weight of wind-driven snow subjects the weak layers below to additional stress and is often enough to precipitate a collapse.

By looking at the layers within a snow pack and applying what they know about the response of snow crystals to changing environmental conditions, avalanche researchers can detect weaknesses where collapse might occur. Data about past avalanches, including location, frequency, and prevailing weather conditions, can further aid their predictions.

To learn about one fishing village's experience with a deadly avalanche in Iceland and the safety plan devised in response to the event, check out Avalanche Town.

Create a model to investigate the complexities of snowpack formation in this NOVA classroom activity.

Print Questions for Discussion

• What causes snow crystals to form unstable layers?
• What information does an avalanche forecaster need in order to predict the possibility of an avalanche?
• What kinds of field observations helped the avalanche specialists reconstruct the events that led to the near-fatal avalanche described in the video?