By Neal Lineback and Mandy Lineback Gritzner, Geography in the NewsTM
Avalanche! Watch Out!
“Avalanche!” is one of the most feared warnings in the world’s alpine regions. Roaring down steep mountain slopes at speeds sometimes exceeding 200 miles (322 km.) per hour, snow avalanches present enormous danger to anything in their paths.
Heavy, late-November snowfalls on the western U.S. mountains often threaten skiers with avalanches, particularly when the venture “out-of-bounds.” Ski patrols make every attempt to control avalanche potential within ski resort boundaries, but some venturous skiers refuse to abide by the rules. As the 2014 Winter Olympics in Sochi, Russia, approach, we are reminded of very real threats by avalanches in snow-covered mountains the world over.
The most avalanche prone U.S. regions are in the West, specifically the Cascades of Washington and Oregon, the Sierra Nevada of California and the Rockies from Montana to New Mexico. Other outliers include the Black Hills of South Dakota, the Wasatch Mountains of Utah and the mountains near Flagstaff, Arizona.
Although most Americans may never see an avalanche (AV‑ah‑LANCH) or an avalanche path, this powerful physical phenomenon occurs more than 100,000 times per year in the United States, according to the National Geographic Society. About 100 people are buried and 25 are killed each year in this country alone.
The numbers of people killed appear to be rising worldwide, as more and more people reside in avalanche‑prone regions and participate in winter sports, particularly skiing, snowboarding, snowshoeing and snowmobiling. Although avalanches may occur throughout the winter, it is the spring of the year that creates the most dangerous conditions.
Avalanches are “landslides” of snow, although they may pluck trees, rocks and even buildings as they move down-slope. Gravity is the force at work, but other factors, both natural and artificial, trigger avalanches as well.
Although avalanches are complex phenomena, there are generally three conditions that must be met before an avalanche will occur. The first is a steep slope, generally between 30 to 45 degrees, though they sometimes occur on lesser slopes.
The second condition is an unstable snowpack, usually resulting from a layering of successive snowfalls. Spring freeze-thaw cycles tend to lessen the bond between snow layers, causing one layer to slide off another.
The third and possibly most important factor is the local meteorological condition. Most avalanches occur during or shortly following a snowstorm, especially when windy conditions result in snowdrifts accumulating on steep slopes above the timberline.
When snow‑covered mountain slopes reach a point of instability, triggers may be as simple as falling snow, a sonic boom or a skier or snowmobile traversing the slope. Once some small part of the snow begins to move, the changing weight and pressure can loosen the entire mass.
As the snowpack moves down-slope, a turbulent wind is generated out in front of the moving mass, creating a billowing, rolling cloud of snow particles. These winds alone are very destructive.
Slab avalanches, formed when blocks of the snowpack separate as one cohesive mass, are the most dangerous. Not only are they enormously destructive, but humans buried by these avalanches may be crushed or buried so deeply that prompt rescue is impossible.
Avalanche specialists are trained to recognize the conditions and locations of potential avalanches. Once unstable snowpack conditions are identified, it is their job to either keep people out of harm’s way or to eliminate the danger.
For example, avalanche warnings are issued to motorists and winter sports enthusiasts by a number of state and federal agencies in high alpine areas. Despite efforts at minimizing danger to humans, however, an estimated 90 percent of avalanche victims fall prey to slides that they have triggered themselves. One study, in fact, found that most victims were experienced backcountry adventurers rather than novices, indicating greater propensity for risk-taking by experts.
Skiing within the bounds of a resort where avalanche professionals perform “control” work is relatively safe. Ski patrols will close areas where avalanche danger exists.
Sometimes ski patrols will attempt to trigger an avalanche manually. Small explosive charges or, less frequently, artillery shells can be used as triggers to eliminate an avalanche threat.
One of the best ways to predict where an avalanche will happen in the future is to see where one has already occurred. Avalanche paths and runout zones may be visible for years after their occurrence and can be easily mapped.
Many alpine communities have enacted land-use controls that restrict or prohibit building in avalanche paths on mountains and runout zones on the valley floors. Geographic information systems (GIS) are being used to map such hazard dangers.
For most of us, avalanche dangers are far away. But to those who live or make their living in alpine environments, “Avalanche!” is right up there with “Tornado!” and “Fire!” as serious warnings.
And that is Geography in the News.
Source: GITN 604 “Avalanche!,” Maps.com, Mar. 18, 1997.
Co-authors are Neal Lineback, Appalachian State University Professor Emeritus of Geography, and Geographer Mandy Lineback Gritzner. University News Director Jane Nicholson serves as technical editor. The original version of this article was co-authored by David R. Hughes, a geographer currently with the Appalachian Regional Commission. Geography in the NewsTM is solely owned and operated by Neal Lineback for the purpose of providing geographic education to readers worldwide.