Avalanche researchers call it the white dragon, for when the snowpack is disturbed and snaps awake like an angry beast, it can cut a path of devastation as deadly as anything in the medieval imagination. Like dangerous animals, avalanches have their favorite haunts. Whether harmless dribbles or million-ton masses more than a mile wide, avalanches tend to schuss down the same runs year after year. They are particularly fond of open slopes, bowls or gullies just above the timberline, where no trees anchor the snow, says Dale Atkins of the Colorado Avalanche Information Center. Also, avalanches can begin only on slopes pitched at an angle greater than 20 degrees, with some 90 percent occurring on slopes of 30 to 45 degrees. (On steeper slopes, snow can't accumulate.) But many things about the white dragons aren't predictable, and are therefore the object of intense research by geologists, engineers and physicists from Scandinavia to the Alps to the American Rockies. The driving question is how terrain and weather combine to awaken the dragon.
The first rule of avalanches is a no-brainer: the heavier the snowfall, as the Alps experienced in February, the greater the chance of the snow's giving way. After that generalization, the science gets trickier, so it helps to think of the snowpack as a multilayer cake. If you stack one layer atop another, and then tilt the plate, the layers will hold together until the plate is moderately tilted. Now spread sticky icing between the layers. The top layer sticks to the bottom even if you tip your creation practically on its side. But if you scrape off the icing and substitute applesauce, you have set up the conditions for a culinary avalanche: tilt the layers even slightly, and the top slides off the bottom. In physics-speak, the shear load along the slope due to the pull of gravity is greater than the shear strength of cake and filling.
So it is with the layers of snow making up a pack: some are sticky and some have no cohesion at all. "What holds the pack together against the downward pull of gravity is the bonding of the snow," says Don Bachman, executive director of the American Association of Avalanche Professionals. Dense, wet snow--the snowman-building kind--forms strong bonds. Dry snow--the skiers' delight--doesn't. But knowing which kind of snow fell doesn't indicate the likelihood of an avalanche, because snow undergoes near-constant metamorphosis in a pack. The greatest driving force for change is the difference in temperature between the bottom (warmer, next to the ground) and top (colder, due to frigid air). "When you have such a temperature gradient," says Bachman, "the snow undergoes changes." In particular, snow near the bottom gives off water vapor (yes, even though it is frozen) that moves toward the cold surface. As the vapor passes through colder layers, it condenses onto the snow crystals above. The crystals grow. As they do, their geometry changes: they become sharp and angular, producing what's called sugar snow. These so-called faceted crystals have all the bonding strength of applesauce. "The angular crystals have no cohesiveness," says Bachman. "If you have a slab of snow perched on a layer of faceted crystals, additional snowfall can produce enough stress that nothing will hold that snow on the mountain."
The chance of these weak snow layers forming depends on weather, and Europe this season had the avalanche-making kind. "They had a dry start to the season," notes Atkins. A shallow snowpack has a greater temperature gradient (there is a shorter distance between the warm ground and the cold air), "so a layer of sugar snow formed," says Atkins. "When the February storms rolled in, the snow was falling on a very weak base."
All that the weak layer needs to give way is a trigger. A trigger can be any added weight, such as a skier. But a more common trigger is wind and snow. Avalanches are most common during or soon after a heavy snowstorm, when weak underlying layers are suddenly covered with more weight than they can hold back. When this occurs the top layer slips off the bottom in what is called a slab avalanche, the most powerful and deadly kind. The deeper the weak layer, the bigger the avalanche.
The art in avalanche science is determining when the snowpack is in danger of giving way. To do this, researchers typically dig a pit and isolate a column of snow, examining it for a layer made of faceted crystals. They may also give the snowpack a stress test: they either tap the isolated column with a shovel or drop nylon sacks stuffed with 10 pounds of snow onto it from different heights, says Kirk Birkeland of the Gallatin (Montana) National Forest Avalanche Center. That reveals how much more weight the snowpack can support.
Just because the hazard is high doesn't mean disaster is inevitable, though. In some avalanche zones, the ski patrol and highway engineers set off small avalanches by lobbing explosive charges by hand or firing 75mm and 105mm recoilless rifles into the snowpack. Frequent minor avalanches prevent the buildup of unstable snowpack that can lead to large ones.
But detonating a controlled avalanche makes sense only if it will run out far from buildings and people. Where villages nestle at the base of a slope, as they do throughout the Alps, even an intentional avalanche can be destructive. In Switzerland, Austria, France and Italy, mitigation therefore means physical barriers. Fences and nets in the avalanche "starting zone" hold in place snow that would otherwise give way. Some steep slopes in Austria's Paznaun Valley are almost covered with the barricades, made of logs and steel beams. Deflecting dams in the run-out zone divert the avalanche away from villages. But February's Alpine snows were so intense that they buried many of the fences and nets, allowing avalanches to start--and grow to such sizes that deflectors were approximately as effective as a child's sand-castle wall against a tsunami.
The worst may be yet to come. The world is warming, perhaps due to the greenhouse effect (in which the burning of coal, oil and gas releases heat-trapping gases into the atmosphere). In a greenhouse world, precipitation falls erratically, with long dry spells followed by deluges. That pattern increases the instability of the snowpack and therefore the risk of avalanches. This winter, for instance, Austria received only light to moderate snowfall for most of the season. That wasn't enough to trigger even small avalanches, so the snowpack slowly built, resting atop a loose foundation of sugar snow. The heavy snows and raging winds of February made it come apart. With climate change, says Birkeland, more regions in more years may see that deadly pattern. The white dragon, in other words, will not sleep any time soon.