A sign in B.C.'s Glacier National Park tells visitors a trail is closed, days after seven teens on a high-school ski excursion at the park were killed in an avalanche in February 2003. A sign in B.C.'s Glacier National Park tells visitors a trail is closed, days after seven teens on a high-school ski excursion at the park were killed in an avalanche in February 2003. (Adrian Wyld/Canadian Press)

A single snowflake is a unique, beautiful crystal. But when millions of flakes are added together on a steep mountainside and gravity has its way, the snow can become a destructive force: an avalanche.

Deadly Canadian avalanches:

Three people were killed and another person seriously injured after an avalanche on Mount Gerald, north of Golden, in eastern B.C. on Feb. 19, 2011.

Two brothers died when they were swept away by a wall of snow in the Burstall Pass area of Peter Lougheed Provincial Park, in Kananaskis Country on Jan. 16, 2011.

A snowmobiler died in an avalanche near Merritt, B.C. on Dec. 28, 2010.

Two men from Alberta and more than 30 others were injured when a wall of snow comes down on about 200 snowmobile enthusiasts in the backcountry area known as Turbo Bowl for the annual Big Iron Shoot-Out snowmobile event, on Mar. 13, 2010.

Eight men from Sparwood, B.C., died when caught by avalanches while snowmobiling in the backcountry near Fernie, B.C., on Dec. 28, 2008.

Seven Calgary-area Grade 10 students died Feb. 1, 2003, when a wall of snow swept down on them near Revelstoke, B.C, while on a backcountry skiing trip organized by their school.

On Jan. 20, 2003, four Canadian and three American skiers were killed when an avalanche struck on the Durrand Glacier in B.C.

The remote northern Quebec village of Kangiqsualujjuaq was devastated when an avalanche slammed through the wall of a school gymnasium during New Year's Eve festivities in 1998, killing five children and four adults.

Michel Trudeau, the youngest son of former prime minister Pierre Elliott Trudeau, is among the country's most prominent avalanche victims. He drowned after he was swept into Kokanee Lake by an avalanche while backcountry skiing with friends in November 1998.

An avalanche or snowslide is the rapid movement of snow down a mountainside. Most occur on slopes of 25 to 50 degrees — the same slopes favoured by many skiers, snowboarders and snowmobilers.

The most hazardous type is called a dry slab avalanche. When a weak layer in the snow cover can no longer support new layers of snow above, the crystalline structure collapses, sending the top layers down the slope.

A dry slab avalanche, says Bruce Temper, director of the Forest Service Utah Avalanche Center, is like a dinner plate sliding off a table. The victim is caught in the middle of the slab.

On average, dry slab avalanches travel from about 100 to 130 kilometres per hour — too fast to outrun.

Avoidance is key

The result is that, all too often, these snowslides have tragic consequences. Canada has averaged 14 avalanche deaths a year over the past decade, according to the Canadian Avalanche Centre.

The centre has also identified some trends about this country's avalanche victims: They tend to be male backcountry skiers or snowboarders in their 20s, according to data collected by the centre for the years 1984-2003.

The majority of avalanche incidents during those years took place during January, February and March, between the hours of noon and 2 p.m. Almost half of the incidents took place in the interior mountain ranges of British Columbia.

Since avalanches travel so fast, avoidance is key. Skiers, boarders, snowmobilers and hikers should check advisories before heading out.

Although it sounds obvious, the presence of an avalanche is the best sign of more to come.

Adventurers should also learn how to look and listen for signs of collapsing snow and follow safe travel techniques such as climbing one at a time and wearing a beacon.

Canadian avalanche warning scale

Danger level Colour Probability and trigger Recommended action
Low Green Natural avalanches very unlikely. Human-triggered avalanches very unlikely. Travel is generally safe. Normal caution advised.
Moderate Yellow Natural avalanches unlikely. Human-triggered avalanches possible. Use caution in steeper terrain on certain aspects.
Considerable Amber Natural avalanches possible. Human-triggered avalanches probable. Be increasingly cautious in steeper terrain.
High Red Natural and human-triggered avalanches likely. Travel in avalanche terrain is not recommended
Extreme Deep red Widespread natural or human-triggered avalanches certain. Travel in avalanche terrain should be avoided and confined to low angle terrain, well away from avalanche path runouts.

Source: Canadian Avalanche Centre

Storms, thaws increase risks

The risks of an avalanche increase during major snowstorms and periods of thaw. Wind can gather up snow and deposit it 10 times faster than snow falling from storms. That makes wind the most common weather-related cause of avalanches. They can also be triggered by human activity on the snow pack, even remotely triggered on a gentle slope far down the mountain.

Many factors influence the production of an avalanche, including:

  • Slope angle.
  • Snow factors, such as the size of grains, slab thickness and density.
  • Terrain, such as presence of trees.
  • Wind speed and direction.
  • Aspect with respect to sun

Avalanche forecasters try to take these and other factors into account in their models. But predicting an avalanche can sometimes seem more art than science.

A television reporter stands next to a snow-covered truck in Kangiqsualujjaq, Que., on, Jan. 2, 1999, a day after nine people were killed and 25 injured in an avalanche. A television reporter stands next to a snow-covered truck in Kangiqsualujjaq, Que., on, Jan. 2, 1999, a day after nine people were killed and 25 injured in an avalanche. (Paul Chiasson/Canadian Press)There are different patterns of snowfall and temperatures in different mountain ranges. Rapid variations in weather can make predictions more difficult and the physical area of research is limited.

The University of British Columbia Avalanche Research Group says most forecasting models are based on weather data for mountain ranges, rather than individual slopes.

Early models assumed avalanches flowed like frictionless fluids, but geophysicist David McClung of the University of British Columbia in Vancouver said that friction does exist, given that avalanches can stop on a steep slope and the deposits show signs of particles scraping each other.

McClung's new model of flow includes the friction between particles flowing around curved valleys. By incorporating new factors such as the friction, researchers hope to improve their models.

Part of the problem is models aren't very strong at predicting what's happening down in the ice crystal layers, where the weak spots might be forming. Instead, forecasters dig pits to look for signs of weak layers.

Time-consuming and expensive tests have to be done frequently while the snow pack develops so that forecasters can see patterns emerging.