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The science behind the slide

Broadcast Date: May 24, 1989

Rogers Pass in British Columbia, surrounded by steep-sided mountains, is one of the most avalanche-prone places on earth. So it's here that Bob McDonald, host of CBC Television's Wonderstruck, has come to explain the science behind the unpredictable natural hazards. What may look like a fluffy white snow cloud can weigh a million tonnes and take lives in a flash, he says. The slopes are kept safe with the help of a cannon that triggers controlled, harmless slides.

The science behind the slide

• The Canadian Encyclopedia defines an avalanche as "a mass of snow, ice and other materials moving rapidly down a mountain slope." The most dangerous type is called a "slab" avalanche. It happens when a weak layer of snow cannot support a heavier layer — or slab — above it. A "point" avalanche involves loose snow spreading downhill from a single point into a V shape. Both types can be dry, moist or wet.

• A snow-covered slope is stable when the bond between the snow crystals is stronger than the tug of gravity trying to pull it down the slope. What usually makes it unstable is when different types of snow with different densities are piled on top of each other. A layer of heavy, dense snow on top of weaker types of snow is an avalanche waiting to happen.

• Hoarfrost, the crinkly sheet of ice crystals that sometimes forms on top of snow, is particularly dangerous when it gets buried on a steep slope. Called "depth hoar," the buried frost is weak and loosely packed and prone to give way when layers of heavier, densely packed snow are pressing down from above.

• Other contributing factors include:
- The degree of slope. Most avalanches happen on slopes at angles of between 25 and 40 degrees.
- Ground cover. Rocky slopes are most susceptible to slides.
- Air temperature. Warmer weather is thought to make snow less stable. However, most avalanches that injure people happen on clear days with little or no snowfall and little wind.

• The biggest slab avalanches can reach speeds of up to 150 km/h — much too fast to outrun or out-ski — and carry more than 100,000 cubic metres of snow, ice and debris with enough force to destroy a village or forest.

• Between 1970 and Sept. 30 2004, some 347 Canadians were killed by avalanches, an average of 10 per year, according to the Canadian Avalanche Association. The average rose to almost 16 per year between September 1994 and September 2004, when there were 159 fatalities. The figures should be viewed in the context that only a small fraction of Canadians venture into areas where there is an avalanche risk.

• The winter of 2002-2003 was the deadliest in 30 years, when 29 people lost their lives to avalanches. Experts blamed the spike on a warm winter caused by the El Nino weather effect as well as the increased popularity of "extreme" snowboarding and skiing on remote backcountry slopes where there are no avalanche control programs.

• While backcountry recreation is often blamed for the increase in avalanche deaths, much of the rise reflects the increase in the population in Canada. According to the Canadian Avalanche Association, the number of avalanche deaths per capita has actually been going down in recent years.

• In the winter of 2001-2002, almost 143 people worldwide were killed by avalanches, according to the Colorado Avalanche Information Centre. The hardest-hit country was the United States but in long-term averages the countries with the most fatal snowslides are France, Austria and Switzerland. The Alps account for more than 80 per cent of the accidents.

• The avalanche control program in Glacier National Park near Revelstoke, B.C., is carried out by a crew of Parks Canada staff and a 14-member Armed Forces artillery unit permanently based at Rogers Pass. The unit moves and fires the 105-mm howitzer gun, a relic of the Korean conflict. Controlled avalanches in Canada are also triggered by explosives dropped from helicopters and even by hand on the slopes.

The science behind the slide

Medium: Television

Program: Wonderstruck

Broadcast Date: May 24, 1989

Guest(s): Sylvain Hebert, Dave Skjonsberg

Reporter: Bob McDonald

Duration: 6:12

Please contact the Writers Guild of Canada at 416-979-7907 Ex. 5236 if you are able to identify the writer of this clip.

Last updated:
March 19, 2007