There are two main types of avalanches differing, mainly, in causation and destructive power: loose snow avalanches and slab avalanches. Loose snow avalanches are caused by new snowfall on steep terrain. They are usually not as large, powerful, or destructive as slab avalanches. When new snow falls onto already snow-covered terrain, it may not be able to adhere to the existing snow because of differences in temperature (yes, as we will learn later, snow can be different temperatures) and it slides off. The loose snow suffers a "localized rotational slip" because of the lack of cohesion and slides downhill in an inverted V formation. This means that the avalanche starts in one spot because a physical change causes one portion of the snowfall to change position and, as the snow slides downhill, the avalanche spreads across the hill, widening. Loose snow avalanches are usually small and relatively harmless because they do not involve the existing snowpack, which is precisely what makes slab avalanches so dangerous. (Bosma, Cookman, and Williams, Christine, 1996)

In a slab avalanche, a catalyst causes a crack in the snowpack on a hill, sometimes breaking off entire mountainsides of snow. The, often small, sometimes massive, slab is free of the snow which is holding it onto the steepness of the hill and slides off. Why do the breaks in the snowpack occur only at the spot where a large slab avalanche will start? The answer is that they do not. Breaks in the snowpack are not exactly common, but occur far more frequently than avalanches. Oftentimes the snowpack surrounding a rock (which is warmer than the snow) will pull away and form deep chasms, called crevasses. Crevasses usually remain stable. Actual initial slabs for devastating avalanches have measured up to 100 meters long and 2 meters deep. Since so much snow is involved, nearly everything in the path of a slab avalanche is devastated. At the end of the runout (the area where the steepness of the hill levels off to the point that the snow stops sliding), it is possible for the mass of the snow used in an avalanche to be 100 times greater than that of the initial slab. (Perla, 1976)

What is avalanche research?

Who researches avalanches?

Several different groups analyze avalanches in different contexts. The most direct instances are ski guides at helicopter skiing vacation places or ski patrols at ski resorts. In order to attract skiers and their dollars, ski resorts must provide safe conditions for skiing. At places such as the aforementioned Mike Wiegele Ski Adventures, the entire staff of ski guides work in tandem with the helicopter pilots to pinpoint dangerous areas to avoid and safe areas to enjoy. (Wiegele, 1997) At ski resorts like Copper Mountain in Summit County, Colorado, the ski patrol works with information from the national Weather Service to keep their area safe. (Copper Mountain Ski Patrol Manual, 1995-96)

But there are applications to avalanche research beyond the immediate level. Commercial organizations study avalanches to research the development of new equipment for rescuing and such. Those in the field of glaciology make an occupation out of studying snow and its conditions. Their results are published in scientific journals for the general public and affect modern research just as in any other areas of study.

All these researchers work to reduce the unknown factors which cause such difficulty in avalanche situations. Factors such as where, exactly, avalanches are prone to happen and how humans can react to these type of situation complicate any type of rescue effort. The formula itself for an avalanche rescue is simple: find those in danger and make them safe, but the execution of that equation is never that simple. Factors such as location, weather conditions, and psychological responses to stress always complicate the outcome. Rescue operations for those in the avalanche field are the same for those as in any other area of life-saving. Just like the police officer, the firefighter, and the coast guard, they learn the basics of their field and must incorporate them into whatever new situation they find themselves in.

Perhaps the greatest factor in an avalanche is the snow which both composes and causes them. We can learn a great deal about the likelihood of an avalanche from snow conditions. This makes it the first area of attention in the areas of research and prediction.

By sampling snow from various areas in mountainsides, conclusions can be drawn about the characteristics of the snowpack. Researchers dig "snow pits" and perform several formal observations to gain insight on the status on the larger snowfield. There are three main characteristics of snow, each create different possibilities for slides. The first is density. The density of the snow is related to the strength of the snowpack (the accumulated snow in an area). When the snowpack is strong, the snow adheres both to itself and the mountainside, which makes it less likely to be affected by catalysts. It takes a substantial catalyst to start a slab avalanche in a high-density snowpack area. The inverse is also true. Areas of low snowpack density tend to be more prone to avalanches. (SD, 1988)

The shape of the actual snow crystal is also helpful, especially in areas where the density of the snow is low. When snow is not strong, not tightly bound together, the individual crystals come free when disturbed by a researcher digging a snowpit. Large snow crystals are less stable, small crystals are more stable. (Armstrong, 1985) Large crystals have long lever arms (the protrusions from the center of a snow crystal) which distance the crystals within the snowpack and can break (after all, it's only frozen water...). Breaks between individual snow crystals, even at such a small level, can cause avalanches, even gigantic ones. Small crystals have shorter lever arms and are, therefore, less likely to break. In the event that a lever arm of a small snow crystal does break, the smaller size minimizes the potential danger. (de Quervain, M., 1954) The shape of the snow crystal is also affected by the third most important property of snow: temperature.

Temperature's affect on snow strength isn't exactly known. Melting snow is very unstable and dangerous, but sudden significant changes in snow temperature rarely lead to avalanches. A researcher will use a thermometer to measure the snowpack's temperature at different levels of the snowpack. Since snow compacts in a manner similar to earth, the snowpack is striated just like geological sites. Even though the insulating characteristic of snow tends to regulate differences in temperature, a snowpack is composed of different layers of snow which can range in temperature.

A drop of water, when placed on a hill, will slide down due to gravity. The same is true for snow at high temperatures. Snow is least stable at high temperatures since water turns from snow or ice form to liquid form at 0ƒC (32ƒF). The following chart sheds light on how snow stays on different levels of grade (the steepness of a hill) at what temperatures. The angle of repose refers directly to the maximum steepness of a hill a specific type of snow can maintain without sliding.

snow type temperature (ƒC) angle of repose

fresh, dry -35 63

fresh, wet -4 close to 90

wet, 24% H2O above 0 2

(Bosma, Cookman, and Williams,1996)

Dry snow is categorized as snow that "will not form a snowball when squeezed"; wet snow "oozes water when squeezed." When wet snow falls (if it is cold enough) it can stick to surfaces that are nearly vertical. Dry new snow is colder and not as adhesive. Wet snow has no ability to remain on much on any type of angle. This seems illogical. Shouldn't wet snow slide while dry snow sticks? Yes and no. Moderately wet snow is the best kind of snow for sticking to hills, but if it gets too wet, it becomes the worst kind of snow at maintaining adherence (the ability to stick) to the grade (steepness) of a hill. Dry snow falls somewhere in the middle of the two. The dangers of differences in temperature are that when there are unstable layers within the snowpack is can cause a great enough disturbance to cause the normally stable layers to give way as well.

The layering effect of the snowpack is often the cause of slab avalanches. Oftentimes, wind will redistribute snowfall and pack down areas that are not accustomed to large amounts of snow. This redistribution by wind forms a cohesive dense layer on top of less-cohesive, less-dense layers, creating an unstable snowpack. A catalyst causes an avalanche when "An external shear load is added parallel to the slope exceeds the shear strength of the weak snow layer. The loading causes fractures which can spread." What this means is that too much of a stress load is put upon an unstable region of snow in such a way that it causes the snowpack to fracture. A shear load is simply the amount of stress a particular type of snowpack can maintain. It must be added parallel to the slope because the fractures in the snowpack must run perpendicular to the surface, cleaving layers of snow at right angles. Non-vertical snowpack breaks do occur, but they are less frequent and rarely cause an avalanche). (Birkeland, Johnson, and Schmidt, 1995)

Avalanches are often caused by melting snow in the spring. As the snow becomes wetter from the rising temperatures, its angle of repose decreases below that of the hill that the melting snow is on and it slides downhill. Sometimes the snow at the bottom of the snowpack can melt at the same rate as that snow directly exposed to the sun, due to the insulating properties of the earth, while the snow between the two melting areas remains cold and solid. This melted snow lubricates the entire snowpack, causing more dangerous and powerful avalanches involving more snow. (Bosma, Cookman, and Williams, 1996)

The temperature of snow also changes the speed and direction snow in an avalanche will travel once sliding. This information is important because, as will be shown later on, engineers are designing structures which direct sliding snow down a hillside like cattle through a cattlegaurd. Theoretically, too, knowing what type of snow is sliding towards you from behind will let you know which direction to travel to escape its path, but, who, while being pursued by an avalanche has time (or peace of mind) to think about such things? Dry sliding snow travels much faster than wet snow because it does not soak into the new snow, or earth, it encounters. Dry snow avalanches can slide at speeds from 25 to 75 meter per second, whereas wet snow slides as speeds between 5 and 30 meters per second. The quickly moving dry snow smashes through and over whatever objects are in its way: skiers, trees, even the terrain. Dry sliding snow forges its own path regardless of the mountain topography-- they travel in straight paths directly down the fall line of the hill. The slow moving, wet snow slides conform the shape of the terrain they are sliding over. Wet snow slides follow the curves in the gullies and valleys of the terrain. (Bradley, Brown, Williams, 1977)

During a snow dig, a researcher uses a shovel to dig from the surface of the snowpack to the terrain underneath. They then catalogue the characteristics of the different snow layers in a line perpendicular to the angle of repose. Using a powerful magnifying glass, researchers examine the shape and structure of the snow crystals. They also use a sensitive thermometer to gauge the differences in temperatures in the snowpack layers. They gauge the density of the snowpack by how strong the snowpack is, how easily it falls apart when crumbled in their fingers, and are rated as "good", "fair", and "poor." All the results are kept in a logbook which the surveying researcher will bring back to the command center. This data is then compared with the samples from similar and surrounding areas to ascertain the structure and stability of the snowpack in a region in general. (Copper Mountain Ski Patrol Manual, 1995-96)

During a snow dig, a researcher uses a shovel to dig from the surface of the snowpack to the terrain underneath. They then catalogue the characteristics of the different snow layers in a line perpendicular to the angle of repose. Using a powerful magnifying glass, researchers examine the shape and structure of the snow crystals. They also use a sensitive thermometer to gauge the differences in temperatures in the snowpack layers. They gauge the density of the snowpack by how strong the snowpack is, how easily it falls apart when crumbled in their fingers, and are rated as "good", "fair", and "poor." All the results are kept in a logbook which the surveying researcher will bring back to the command center. This data is then compared with the samples from similar and surrounding areas to ascertain the structure and stability of the snowpack in a region in general. (Copper Mountain Ski Patrol Manual, 1995-96)

An extensive knowledge of the terrain underneath the snowpack plays an important role in prevention and avoidance. Researchers who work in specific areas over lengthy periods of time will tend to know which areas are prone to avalanches and when. Steep grades are dangerous when temperatures rise. Areas that are exposed to wind can develop cornices which can fall and create avalanches. Flat regions remain safe season-round. These are the types of observations which are specific to each site but can be based upon the traditional information surrounding where avalanches usually occur. A good basic knowledge of the terrain is the best defense because those that know the region know the dangerous spots. These areas can be avoided, it sounds silly, but it remains a slogan to the Copper Mountain Ski Patrol: "you can't be in an avalanche if you're not there." (Copper Mountain Ski Patrol Manual, 1995-96)

The employees of Copper Mountain, just like those of any other major ski area, are required to learn a great deal about the terrain. First-year patrollers must pass three exams which test them on dangerous areas for avalanches and safe spots where others can go in case of avalanche activity. This makes them aware, first of all, of the areas to mark off where skiers should not go because of avalanche danger when snow conditions suddenly change. These signs are not always obeyed, though, and their second terrain knowledge benefit is that in the event of an avalanche, they will be able to make decisions in life saving much quicker than if uninformed. This is rather important because time remains the most critical element in life saving. If caught in an avalanche, your chances for survival are nearly 80% if you re discovered within the first hour of the rescue effort (they'll probe for you with long poles), and less than 10% after three hours. (Bosma, Cookman, and Williams, 1996)

Weather remains the most unpredictable of the conditions surrounding avalanche prediction. Ski areas like Mike Wiegele have conferences with guests and guides every evening to warn everyone of the possible dangers and for specific avalanche areas and characteristics of potential avalanche sites so as to avoid any accidents. Ski areas like Copper Mountain use computers to download constant weather information from the national weather service so as to anticipate conditions. Knowledge of the pre-existing snowpack comes into play here, for researchers need to know onto what kind of snow the new snow is being added. Periods of drastic change in climate pose the most danger for avalanches. Heavy snowfall can cause buildup which is too much for certain grades; quickly warming temperatures can cause snow to slide much sooner than anticipated. Both past and future snow conditions and temperatures are crucial to avalanche activity. (Bosma, Cookman, and Williams, 1996)

Avalanche prediction and research is an area of scientific pursuit which has a profound effect on a small few. To the average person it is of little concern, being swept away by an avalanche, but any arena of scientific study that is directly linked to the saving of human life is definitely a candidate for consideration. Future studies are moving into fields of fluid and granule dynamics, as well as in deflector design to make them less noticeable and expensive, while maximizing efficiency. A new invention, the avalanche air-bag proves to be an interesting future development. Unfortunately weighing up to seven pounds, this device can be activated with a pull cord, like a parachute, inflating a large, bright orange, balloon around the victim. The fully inflated balloon improves buoyancy during the slide, lessens impact, lessens snow compacting on the skier, and can provide an airspace to avoid suffocation. They remain prototypes, however, for their tendency to go off when not planned. Just like most areas of avalanche prediction and research, it requires much more investigation and capital to explore properly. (Curtin, 1997) However, the avalanche remains one of nature's most stunning displays of power, just hope never to experience one in person.

Armstrong, R.L. 1985. "Metamorphism in a subfreezing, seasonal snow cover: The role of thermal and vapor pressure conditions." PhD Dissertation, Department of Geography, University of Colorado, pp. 175.

Avalanche Rescue Seminar Handbook, 22nd Annual Avalanche Rescue Seminar (Dec. 7&8, 1996) sponsored by the Summit County Rescue Group and the Colorado Search and Rescue board.

Birkeland, Karl; Johnson, Ron; and Schmidt, Scott. "Near-Surface Faceted Crystals: Conditions Necessary for Growth and Contribution to Avalanche Formation," from Southwest Montana University, U.S.A. (http: //www.csac.org/Professional/pap.html)

Bosma, Dulci; Cookman, Betty; and Williams, Christine. "Snow Avalanches: Hazard Planning and Avoidance." Michigan Technological University in Houghton, MI 49931 (http: //www.geo.mtu.edu:80/hazards/avalanche/avalindex.html)

Bradley, C.C., R.L. Brown, and T. Williams. 1977. "On depth hoar and the strength of snow." Glaciology, pp. 145-147.

Copper Mountain Ski Patrol Manual, 1995-96 version (revised).

Curtin, Dave. "A Degree of Danger" Gazette Telegraph, Feb. 27th, 1997.

Daffern, T. 1992. "Avalanche safety for skiers and climbers." Cloudcap: Seattle, Washington, pp. 192.

de Quervain, M. 1954. "Snow as a crystalline aggregate." Translation 21, U.S. Army Snow, Ice and Permafrost Research Establishment, pp. 7.

Mears, Arthur I., "Design Criteria for Avalanche Control Structures in the Runout Zone," USDA Forest Service General Technique Report RM-84, June, 1981, pp. 1-28.

Perla, Ronald I and Martinelli, M. Jr., "Avalanche Handbook" USDA Forest service Agriculture Handbook 489, July, 1976.

SD, "All You Ever Wanted to Know About Snow," Nature, v336, Nov. 24, 1988, pp. 330-331.

Slickrock gallery: Avalanche Reports Website (http://www.aros.net/~slickroc/ava.html)

Stratton, J. 1977. "Development of upper level temperature gradient crystals" and "Upper level temperature gradient avalanche cycle - February 1-2, 1977";. Short, unpublished papers that were circulated among Utah avalanche workers in the late- 1970s and 1980s by John Stratton, Snowbird Snow Safety.

A personal interview with Mike Wiegele, January, 1997.