by Doug Fesler and Jill Fredston*

Doug Fesler and Jill Fredston, both avalanche specialists, were Co-Directors of the non- profit Alaska Mountain Safety Center, Inc and the Alaska Avalanche School prior to their retirement. They submitted this article to the Avalanche Center back in 1999.

Highmark, the challenge of riding a snowmachine as high as possible on the side of a steep, snow-filled mountain is great fun that requires skill. But if you climb a slope from the bottom without first assessing the snow stability, you are playing a game of Russian roulette which has already killed too many snowmachiners. It's unlikely that you'd cross a busy highway blindfolded. The bottom line is that when you're traveling in avalanche country, you need to take off the blinders and think like an avalanche. If not, the Snow Dragon may thrash you. Here's why:

Avalanches are not random events. They occur as a result of the interaction between terrain, snowpack, weather, and, often, people. More than 95% of the avalanches catching backcountry recreationists are triggered by people. Most of the time, it is our weight on or near steep slopes that causes the problem.

Each snowstorm and wind event during the winter deposits a layer of snow. Over time, these layers change. Some become stronger in response to settlement or compaction, while others grow weaker. Potential for slab avalanches, the kind of avalanche dragon catching most people, exists when relatively strong, cohesive snow known as a slab overlies weaker snow or is not bonded well to the underlying layer.

Slabs can be made up of snow that is new or old, hard or soft, wet or dry. Weak layers come in a variety of types and thicknesses. Frequently sugary and fragile, they can be formed in shallow snowpacks or cold, clear weather, often weeks or months before the slope avalanches. The weak layer that killed the snowmachiner near Eureka Lodge began forming at the beginning of the winter.

Slabs fail initially as a unit and then break up into smaller blocks as the snow moves downslope. A slab avalanche becomes possible when the strength of the snowpack is nearly equal to the stress applied to it. Actual failure does not take place until the stress exerted equals the snowpack's strength.

Think of a pencil you're trying to break in half with your hands. Suppose that the breaking strength of the pencil requires 5 pounds of force. If you apply 4.9 pounds of force, the pencil becomes unstable. It bends, it creaks, but it doesn't break. Only when you apply 5 pounds of force or more, will it break in two. In order to cause the snowpack to avalanche, the amount of stress exerted on it must increase or its strength decrease, or both.

So what causes stress? The steeper the slope angle, the greater the stress on the slab, especially along the interface between the slab and weak layer underneath it. A slab is like a huge dinner plate lightly glued to a table. If you tilt the table on edge, the stress on the boundary regions of the plate increases. If you keep tilting the table steeper and steeper, the plate eventually slides off. This happens more easily on a smooth surface than a rough one. Avalanches need steep slopes in order to fail. Prime-time slopes are steeper than 30 degrees, especially slopes with angles in the high 30 s to mid 40 s range, the same ones highmarkers seek to test their skills. You don't have to be riding on a steep slope to cause failure, you just have to be connected to it. Five out of the six snowmobilers who died in Turnagain Pass a few weeks ago were traveling on relatively gentle terrain connected to steeper slopes above. The sixth was highmarking.

The second thing that causes stress is load such as the weight of new snow, rain, and wind-transported snow. The snowpack is only able to absorb a certain amount of stress and only at a limited rate of speed. All things being equal, the snowpack is better able to adjust to three feet of snow falling over several weeks than to three feet of snow falling overnight. Avalanches are more likely to occur in response to sudden changes such as heavy loading or intense warming than to changes which happen slowly.

Now, add yourself to the formula. Will the snowpack be able to adjust to the weight of a 600 or 700 lb. snowmachine and rider pounding up the hill? From the Snow Dragon's perspective, it may be time to pounce especially if you hit a trigger spot. These are localized areas of sensitive snow, places where buried weak layers are waiting like ticking time-bombs for someone to tip the balance on the scale, or places of increased stress. Trigger spots are typically found in areas of thinner snowcover, buried bushes or rocks, or below steep roll-overs. But steep slopes can be triggered from anywhere, when conditions are ripe.

The moment a snowmobiler hits a trigger spot, a chain reaction is set off which results in the immediate release of a tremendous amount of stored elastic energy -- like releasing a giant, stretched rubber band. The slab fails at all of its boundaries and the dinner plate slides off the table. Suddenly, the whole slope is in motion and the word "avalanche" begins to have a new meaning for the person caught. After years of avalanche accident investigations and discussions with the families of avalanche victims, we remember the words of an unknown ex-Viet Cong soldier poet every time we respond to the scene of an accident: "A bullet fired, for whatever reason, is first, a bullet to the heart of a mother."


* Doug Fesler and Jill Fredston, both avalanche specialists, were Co-Directors of the non- profit Alaska Mountain Safety Center, Inc and the Alaska Avalanche School.

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