Avalanches can be caused by a variety of factors, including terrain, slope steepness, weather, temperature, and snowpack conditions. (Larger image not available)
—Credit: Copyright Richard Armstrong, NSIDC
An avalanche is a rapid flow of snow down a hill or mountainside. Although avalanches can occur on any slope given the right conditions, certain times of the year and certain locations are naturally more dangerous than others. Wintertime, particularly from December to April, is when most avalanches tend to happen. However, avalanche fatalities have been recorded for every month of the year.
Anatomy of an avalanche
All that is necessary for an avalanche is a mass of snow and a slope for it to slide down. For example, have you ever noticed the layer of snow on a car windshield after a snowfall? While the temperature remains low, the snow sticks to the surface and does not slide off. After the temperature increases, however, the snow will sluff, or slide, down the front of the windshield, often in small slabs. This is an avalanche on a miniature scale.
Of course, mountain avalanches are much larger and the conditions that cause them are more complex. A large avalanche in North America might release 230,000 cubic meters (300,000 cubic yards) of snow. That is the equivalent of 20 football fields filled 3 meters (10 feet) deep with snow. However, such large avalanches are often naturally released, when the snowpack becomes unstable and layers of snow begin to fail. Skiers and recreationalists usually trigger smaller, but often more deadly avalanches.
An avalanche has three main parts. The starting zone is the most volatile area of a slope, where unstable snow can fracture from the surrounding snow cover and begin to slide. Typical starting zones are higher up on slopes. However, given the right conditions, snow can fracture at any point on the slope.
The three parts of an avalanche path are the starting zone, avalanche track, and runout zone. (Larger image not available)
—Credit: Betsy Armstrong
The avalanche track is the path or channel that an avalanche follows as it goes downhill. Large vertical swaths of trees missing from a slope or chute-like clearings are often signs that large avalanches run frequently there, creating their own tracks. There may also be a large pile-up of snow and debris at the bottom of the slope, indicating that avalanches have run.
The runout zone is where the snow and debris finally come to a stop. Similarly, this is also the location of the deposition zone, where the snow and debris pile the highest.
Several factors may affect the likelihood of an avalanche, including weather, temperature, slope steepness, slope orientation (whether the slope is facing north or south), wind direction, terrain, vegetation, and general snowpack conditions. Different combinations of these factors can create low, moderate, or extreme avalanche conditions. Some of these conditions, such as temperature and snowpack, can change on a daily or hourly basis.
For more information see Snow Resources.
Education Resources: Online map tools, video, printed, and printable materials. Photographs, animations, and more.
Cryosphere Glossary: Find terms and definitions relating to snow and ice.
Cold Links: Search for Earth system education materials.
Is there fresh snow at the ski hill? Visit NSIDC Scientist Drew Slater's site for SNOTEL station snow data in the Western U.S., updated hourly.