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Recently, there was a huge avalanche in Colorado that crossed a highway in the Arapaho National Forest. When someone is buried underneath the snow, there are issues of trauma and asphyxiation, but what is less frequently discussed in the medical literature is the actual formation of avalanche conditions. What follows below is a very condensed version of some information written in the forthcoming 5th edition of the textbook Wilderness Medicine in a chapter entitled “Avalanches,” authored by Knox Williams, Dale Atkins, and Colin Grissom.
Although snow cover appears to be nothing more than a thick, homogeneous blanket covering the ground, it is one of the most complex materials found in nature and goes through significant changes in relatively short periods of time. All snow crystals are made of water molecules, but local environmental conditions control the type and character of snow found at a given location. At a single site the snow cover varies from top to bottom, resulting in a complex layered structure.
In general, thicker layers represent consistent conditions during one storm, when new snow crystals falling are of the same type, wind speed and direction vary little, and temperature and precipitation are fairly constant. Thinner layers, perhaps only millimeters in thickness, often reflect conditions between storms, such as the formation during fair weather of a melt-freeze crust, a period of strong winds creating a wind crust, or the occurrence of surface hoar, the winter equivalent of dew. Delicate feather-shaped crystals of surface hoar deposited from the moist atmosphere onto the cold snow surface overnight offer a beautiful glistening sight as they reflect the sun of the following day. However, they are very fragile and weak, and once buried by subsequent snowfalls, they may be major contributors to avalanche formation.
One property of snow is strength, or hardness, which is of great importance in terms of avalanche formation. The arrangement of the ice skeleton and the changing density produce a wide range of conditions. The denser the snow layer, the harder and stronger it becomes, as long as it is not melting.
Wind can alter the shape of new snow crystals, breaking them into much smaller pieces that pack very closely together to form wind slabs. These in turn may possess a density 5 to 10 times that of new delicate flakes falling in the absence of wind. These processes occur at different times and locations at the surface of the snow cover and are buried by subsequent snowfalls, so a varied layered structure results.
After snow has been deposited on the ground, the density increases as the snow layer settles vertically or shrinks in thickness. Because an increase in density equals an increase in strength, the rate at which this change occurs is important with respect to avalanche potential. Snow is highly compressible because it is composed mostly of empty air pocket within an ice skeleton of snow crystals. Under very cold conditions, the original shapes of the snow crystals are recognizable after they have been in the snow cover for several days or even a week or two. As temperatures warm and approach the melting point, such shapes disappear within a few hours to a day.
Averaged over 24 hours, snow temperatures generally are coldest near the surface and warmest near the ground at the base of the snow cover. Warm air contains more water vapor than does cold air; this holds true for the air trapped within the snow cover. The greater the amount of water vapor, the greater the pressure. Therefore both a pressure gradient and a temperature gradient exist through the snow cover.
Depth hoar is of particular importance to avalanche formation. It is very weak because there is little or no cohesion or bonding at the grain contacts. Depth hoar or temperature-gradient snow layers can be compared to dry sand. Each grain may possess significant strength, but a layer composed of grains is very weak and flimsy because the grains lack connections. Thus depth hoar is commonly called “sugar snow.” In the cold, shallow snow covers of a continental climate, such as that of the Rocky Mountains, a gradient of this magnitude is common within the first snow layers of the season. Therefore a layer of depth hoar is frequently found at the bottom of the snow cover, and the resulting low strength becomes a significant factor for future avalanches.
There are two basic types of avalanche release. The first is point-release, or loose snow, avalanche. A loose snow avalanche involves cohesionless snow and is initiated at a point, spreading out laterally as it moves down the slope to form a characteristic inverted V shape. A single grain or a clump of grains slips out of place and dislodges those below on the slope, which in turn dislodge others. The avalanche continues as long as the snow is cohesionless and the slope is steep enough. This type of avalanche usually involves only small amounts of near-surface snow.
The second type of avalanche, the slab avalanche, requires a cohesive snow layer poorly anchored to the snow below because of the presence of a weak layer. The cohesive blanket of snow breaks away simultaneously over a broad area. A slab release can involve a range of snow thicknesses, from the near- surface layers to the entire snow cover down to the ground. In contrast to a loose snow avalanche, a slab avalanche has the potential to involve very large amounts of snow.
As the initial crack forms in the unstable snow, elastic energy is released, which in turn drives the crack further, releasing more elastic energy, and so forth. The ability of snow to store elastic energy is essentially what allows large slab avalanches to occur. As long as the snow properties are similar across the avalanche starting zone, the crack will continue to extend, allowing entire basins, many acres in area, to be set in motion within a few seconds.
photo by Colin Grissom
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