Avalanches and Avalanche Formation

Important Note: As with other topics presented here about snow, weather and avalanches, this short outline of avalanches and avalanche formation is quite rudimentary and painted with a very broad brush. It is not intended as a source of operational information about the subject. Please consult any of the selected references or links to gain more knowledge about this incredibly complex and interesting topic.

Snow Avalanche Summary

Snow avalanches (release of a mass of snow from steeper slopes) are a natural consequence of gravity, weather, terrain and the snowpack that is created by the weather when it interacts with terrain. This natural consequence results when applied stress on some grains or layer overcomes the strength of individual grain bonding, or shear strength along or between layer boundaries. In the case of slab avalanches, this loss of shear strength between layers is also combined with tensile stress overcoming tensile strength within a particular layer. Such forces may result in layer or snow grain failure (loss of strength of those bonds or layer without release) and subsequent fracture (downhill motion of grains or areal masses of snow).

There are many types of avalanches (AKA slides), including loose and slab, either of which can involve part of the underlying snowpack or the entire snowpack itself (ground or full depth slides) Furthermore, it is often helpful to further differentiate avalanches as either wet or dry or mixed.

Avalanches can also result from release by artificial means, i.e., human interaction, application of control measures (typically explosive forces) or other natural events (e.g., earthquake). The terrain over which snow is sculpted and draped introduces stress concentrations in the overlying and highly malleable snowpack, which itself undergoes further modifications because of ensuing weather. The snowpack is continually and further altered by subsequent weather, sometimes rounding and strengthening, and at other times growing, faceting and weakening. All in all, snowpack structure and its associated stability or instability offer avalanche professionals and back country recreationists a challenging issue to resolve or at least understand on a daily (or more often) basis.

Weak layers that may result in avalanche release may be storm produced or develop both on or near the snow surface (surface hoar, radiation recrystallization) or develop at depth (facets, depth hoar)…and these weaknesses may exist only for a short period (hours to days) or may persist for much longer, perhaps even an entire winter.

Terrain

If not for terrain and slopes steep enough to increase stresses on the overlying snowpack and result in release, snow avalanches would not be a threat. However, once snow falls and is distributed on uneven terrain and subjected to the unbalanced distribution of applied force that such topography produces, its own layering and associated stress concentrations provide plenty of instability just waiting for the appropriate trigger to fail and fracture.

When venturing into snow covered avalanche terrain, slope angle, slope aspect (or exposure), and slope shape become important considerations, as all have strong implications on local snowpack stability, and the potential for avalanche release. While slope steepness changes of a few degrees may not seem like much to the unaware traveler, these slight changes may produce significant differences in gravity driven stresses within the snowpack and its resultant avalanche danger. While in most instances avalanche activity decreases as slope angle drops below 25 to 30 degrees, that activity increases markedly in the neighborhood of 30 to 45 degrees where gravity induced stresses are enough to bring the snowpack into a state of tension (the elastic response of snow), in which the snowpack may need little or no disturbance to slide. Above about 45 to 50 degrees, sufficient gravity forces typically exist in the accumulating snow to produce numerous smaller slides, thus constantly reducing the tension and stress in the snow as it builds up.

Similar powerful effects also apply to the changing potential for avalanches due to slope aspect and shape. In the case of slope aspect, this potential variability is driven by the effects of wind (wind producing highly variable snow deposition or scouring) and sun (solar effects); and with slope shape the stress variations are driven by local stress concentrations that develop near terrain changes like rolls (slope convexities). So, it’s not just the fact that the snowpack is draped over terrain, it’s the way that such snow distributions occur that we’re concerned about. And to explore more about the how and why and when of avalanche danger, please consult some of the references below to help expand your knowledge. And of course, there’s the weather that produces the snowpack in the first place, and continually changes it throughout the winter. That said, being avalanche aware becomes a lifelong learning experience…though well worth it when you consider that your life is the one at risk.

Avalanche Education Resources

Recommended references (books)

Online learning

Avalanches and Avalanche Formation

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Snow Avalanche Summary

Terrain

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Avalanche Education Resources

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