About Snow and Avalanches

Snow

Snow is composed of small crystals of frozen water; the crystals form directly by condensation of atmospheric water vapour around solid nuclei at temperatures below 0°C (32°F). As individual crystals of ice (snow crystals) fall through the atmosphere, they cluster together and form snowflakes. The term snow is also applied to deposits on the ground that may occasionally cover as much as 23 percent of the Earth's total land surface.

Because the quantity and frequency of snowfall vary with the availability of moisture and the prevalence of below-freezing air temperatures, annual rates of snowfall can range from several meters of water-equivalent in places such as the high mountain snow fields of the Pacific Northwest of North America to less than 2 centimeters (0.8 inches) of water-equivalent in the interior of Antarctica.

Formation

Snow crystals forming at temperatures above - 40°C ( - 40°F) generally need foreign particles on which to nucleate. This kind of nucleation is termed heterogeneous. The most common types of nuclei are silicate minerals of terrestrial origin and sea salt. Particles from industrial sources and automobile exhaust may also be important. Nuclei range in size from 10 - 5 to 10 - 2 millimeters. At temperatures below - 40°C water can solidify spontaneously, without the need for nuclei, by a process called homogeneous nucleation.

Avalanches

Avalanches are massive downward and outward movements of slope-forming materials; these masses may range from the size of cars to entire mountainsides. The term avalanche includes movement of snow and ice as well as rock and soil materials and applies only to movements rapid enough to threaten life.

Snow avalanches are caused by the added weight of fresh snow or by gradual weakening of older snow. They are often triggered by the weight of a skier or the impact of small masses of snow or ice falling from above. Snow avalanches are a major danger in high mountain areas.

Two principal types of snow avalanche are distinguished. A loose snow avalanche gathers more and more snow as it descends a mountainside. A slab avalanche consists of more compact, cohesive snow and ice that breaks away from the slope in a discrete mass, much like a block-glide landslide; this type is responsible for the great majority of accidents.

Prevention and Damage Limitation

The avalanche danger of unstable slope accumulations is reduced or prevented through detonation, from either the tossing of grenadelike explosives or the shooting of bazookalike shells into the slope. Structural damage is limited by the construction of various types of fencing and of splitting wedges, V-shaped masonry walls that split an avalanche around a structure located behind the walls.