The term avalanche usually refers to the sliding of snow and ice, while the term landslide is interpreted to mean the slippage of all other materials (i.e., dirt, rock, etc.). Avalanches usually do not cause widespread destruction to human life or property because the phenomenon is restricted to the high mountain areas of the world, which tend by their nature to be sparsely inhabited.
However, two or three major avalanches have been catastrophic, causing deaths in the thousands; and several more have caused deaths by the hundreds. Also, avalanches are so sudden and devastating in their localized areas that they are a cause of great concern in mountainous regions. Moreover, the increasing popularity of skiing, mountain climbing, hiking and camping has led to a recent rise in the death toll from avalanches in the United States.Snowfalls during a winter season or even over several seasons build up layers of ice and snow. Produced under very difficult conditions, these separate layers settle and become compacted and then become subject to a variety of complex external and internal factors - slope angle, wind, crystal types, stability of the snow layers - all of which may serve to determine the avalanche potential of the snowfield.
About 150 people are killed annually by avalanches around the world. Many victims are killed quickly, either by being struck by debris within the avalanche or by the twisting, wrenching motion of the slide. However, others may die more slowly, from suffocation caused by snow pressure that prevents a buried victim from expanding his lungs or through the inhalation of snow, blocking air passages, or by the depletion of oxygen.
Historically, the greatest threat of snow avalanches has been in the Alps of Europe and the Andes Mountains of South America. For example, on May 31, 1970, at Yungay, Peru, a massive avalanche wiped out the city and killed all of its 20,000 inhabitants.
Destructive avalanches have occurred in Utah.
- Alta, 1867: Some 60 to 65 people were killed in the mining camps and towns near Alta by a series of avalanches. In 1937 Alta became the site of the first U.S. avalanche observations and research center.
- Bingham, Feb. 17, 1926: After more than a foot of heavy snow fell on the slopes near this town, an avalanche of snow, rocks and timber slid down Sap Gulch to bury some 75 people, killing 40.
It is always wise to check local weather forecasts and the Avalanche Center before going to mountain recreation areas or into the back country. When you enter avalanche country, be alert to the following conditions:
- Recent avalanche activity. If you see new avalanches, suspect dangerous conditions. Beware when snowballs or "cartwheels" roll down the slopes.
- Old slide paths. Generally, if an avalanche occurs once in a given area, it will happen there again. Watch for avalanche paths. Look for pushed-over small trees and trees with limbs broken off. Avoid steep, open gullies and slopes.
- Sounds and cracks. If the snow sounds hollow, particularly on a leeward slope, conditions are probably dangerous. If the snow cracks and the cracks continue to form, slab avalanche danger is high.
- New snow. Be alert to dangerous conditions when one foot or more of new snow falls.
- Old snow. When the old snow depth is sufficient to cover natural anchors such as rocks and brush, additional snow layers will slide more readily. The nature of the old snow surface is important. Rough surfaces favor stability; smooth surfaces, such as sun crusts, are less stable. A loose, underlying snow layer is more dangerous than a compacted one. Check the underlying snow layer with a ski pole.
- Wet snow. Rainstorms or spring weather with warm winds and cloudy nights can warm the snow cover. The resulting free and percolating water may cause wet snow avalanches. Wet snow avalanches are more likely on south slopes and on slopes beneath exposed rock.