Every year, on average, avalanches kill 10 people in Canada. In the past few days, two more people were added to this year’s tragic toll as Michel Trudeau, son of former Prime Minister Pierre Trudeau, and Susanna Donald, a University of Calgary student from Regina, became the latest victims of these deadly snowslides, also known and feared as the “White Death.”
The yearly toll has been increasing in the past few years as more and more people take to the backcountry in winter. Avalanches happen naturally–it’s estimated there are a million of them a year throughout the world’s mountain ranges–but in cases where the avalanches claim victims, 90 percent of the time, experts say, it was the presence of people that caused the avalanche in the first place.
An avalanche is simply a mass of snow sliding down a slope. To create an avalanche, you need only two things: a steep, snow-covered slope and a trigger. But despite that apparent simplicity, it’s very difficult to predict which slopes are at risk of an avalanche–which is why even experienced skiers and hikers with lots of knowledge about the dangers of avalanches sometimes get caught. (It doesn’t help that most avalanches occur on slopes of 35 to 40 degrees, the same angle as the most challenging–and therefore, to some skiers, most appealing–ski slopes at most major ski areas.)
Snow is prone to sliding because it’s made up of ice crystals, and ice crystals can take a variety of radically different shapes, depending on the conditions under which they’re formed. In the mountains, every storm deposits a distinct type and amount of snow. That shape of that snow’s crystals can be further changed by the weather that follows. Snow that is promptly buried by snow from another storm, for example, will change in a different fashion from snow that is exposed to the sun or wind for several days. And even after is has been buried for months, snow continues to change, due to the pressure of the snow piling up overhead..
This means that over time, the snowpack becomes a multi-layered history of the season’s weather–and some of those layers aren’t very stable, because the crystals that form them happen to have shapes that don’t allow them to bond closely together. For example, snow near the warm ground can partially melt, sending water molecules moving upward through the snowpack, where they reform a type of ice called “depth hoar crystals”–crystals which don’t stick together. The result is similar to putting a layer of ball bearings under the snowpack.
Another type of weak layer can form when a layer of snow is exposed to cold, clear weather for a long period of time. Like other surfaces, it collects frost, or “surface hoar crystals,” and these fragile crystals don’t support additional layers of snow well.
Lots of snow resting on one or more weak layers on a steep slope is an avalanche just waiting to happen. All it needs is a trigger; something to disturb one of those weak layers. It doesn’t take much; if just a bit of snow breaks away, it in turn becomes the trigger to break away more snow, creating a chain reaction. The initial trigger can be something as seemingly insignificant as the weight of a single skier, snowboarder or hiker.
Avalanche forecasts are based on the presence of these weak layers, the amount of snow that has fallen on top of them, the current weather, the weather over the course of the season, and more. Forecasters dig pits on slopes to examine the layers of snow, hammer on the snow to see how much force is required to break it loose, and use a variety of explosives, from hand-grenade like charges to artillery shells, to trigger avalanches safely before they become a danger to roads, railways and skiers. (In Bridger Bowl, Montana, for example, a team of experts spends hours every day before the ski slopes open examining slopes and triggering avalanches. Rogers Pass is the focus of similar attention in Canada.) However, forecasts aren’t perfect. So many variables are at work that no one can ever be sure that any particular slope is sure to avalanche–or is entirely safe.
There are two types of avalanches. Loose snow avalanches occur most often after periods of heavy snow, especially if that snow falls on a smooth, older snow surface, such as one that has partially thawed, then frozen, to form a crust. Loose snow avalanches start from a single point, then fan out, incorporating more and more snow as they do so. They typically don’t involve all that much snow and aren’t considered major threats to people or property: even their nickname, “sluffs,” sounds harmless. In a loose snow avalanche, it’s the new snow itself that is the weak layer.
However, the deeper that weak layer is buried, the more likely a much more dangerous “slab avalanche” will occur. In a slab avalanche, the snow breaks loose in a single large plate, which then fractures and slides down the slope. They involve much more snow and typically take up a wider area, which makes them more dangerous to people and property. Not only that, but when they’re triggered by a person, that person is often in the middle of the slab, which, as it breaks apart, engulfs them.
Avalanches descend at speeds anywhere from 80 to 160 kilometres an hour, so outrunning one generally isn’t an option, and with tonnes of snow moving at those speeds, they pack a wallop. Even “small,” 100-tonne avalanches pose serious danger to individuals; larger ones can snap tree trunks like matchsticks and overturn cars, larger ones still can demolish buildings and rail cars, and the very largest have devastated entire towns. On October 25, 1995, for example, an enormous avalanche smashed into the town of Flateyri, Iceland. Experts estimate it began when a slab of snow 400 metres long and four metres thick broke loose after weeks of heavy snow. It smashed into the town at 125 miles an hour and buried it in more than 200,000 tonnes of snow, destroying 17 houses and killing 20 people. These same experts fear an even worse disaster is just waiting to happen in Juno, Alaska, which lies at the foot of seven avalanche paths and many of whose neighborhoods are at risk.
While the avalanche is still sliding, the main danger to victims trapped in one is being smashed into a rock or tree. Disoriented, flung this way and that, there is little they can do to right themselves or struggle to the surface of the snow. Once the snow stops moving, survival depends on prompt rescue, because avalanche debris, heated by friction and puffed full of air as it’s sliding, settles and refreezes once it’s stopped moving, setting as hard as concrete and making it impossible to breathe. A recent study in Nature magazine estimated that 90 percent of avalanche victims are alive after five minutes, but after 30 minutes only 50 percent are. The overall odds of surviving burial in an avalanche are less thatn 50 percent.
That being the case, anyone travelling in the backcountry in winter owes it to themselves to take every precaution, checking and obeying avalanche warnings religiously, learning to judge snow conditions for themselves, and avoiding routes that take them through avalanche country. Special equipment such as long poles designed for probing the snow for buried victims and radio beacons that can help rescuers pinpoint a victim’s location are also recommended.
But all the training, knowledge and equipment in the world doesn’t guarantee safety in avalanche season. Like earthquakes, tornadoes and floods, avalanches are a force of nature, and despite our best efforts, one we do not fully understand and can never hope to control.
