John Seibert is a whitewater kayaker, a small-plane pilot in Alaska, and for the past 35 years a back-country skier. He is a risk-taker, but he's no fool.
The 54-year-old geophysicist has attended many avalanche safety courses and learned what an avalanche-prone mountainside looks like. So as he skied across a steep slope in British Columbia's Selkirk Mountains in January with 20 companions and a professional guide, he said, "There was nothing that I saw on that slope that even set off the slightest alarm bell in my head."
The first alarm was a boom that resounded like a shotgun blast. The snow shuddered, Seibert said, and "a second later the entire slope started moving."
The accelerating avalanche threw him onto his back, and swept him downhill in a torrent of snow. "I was doing these big, broad backstrokes, doing everything I could to keep my head above this mess," he said. "I knew once it stopped it would harden up real quick. I just absolutely wanted to have my head above it when it stopped."
Seibert survived. But seven of his companions died - broken or suffocated beneath the avalanche. They were among the latest in a rising toll of deaths and injuries in North American avalanches.
Experts attribute the increase to explosive growth in the number of people who ski, snowmobile and helicopter into "avalanche terrain" - remote areas with deep snow and steep slopes. There, they say, too many fall prey to psychological traps that lead them into danger.
"Avalanche accidents are not accidents of slope angles, snowfall, winds and weak layers in the snowpack," said Dale Atkins, a researcher at the Colorado Avalanche Information Center who surveyed avalanche survivors. "Human factors are the primary cause of avalanche accidents."
In the United States this winter, 21 winter sports enthusiasts have died in avalanches. In Canada on Feb. 1, seven high school students died in a snow slide near Revelstoke, British Columbia.
Last year's 35 avalanche deaths in the United States were the most in at least 17 years, up from a low of six in 1989, according to the Colorado Avalanche Information Center.
Scientists say avalanche-prone slopes are angled between 25 and 45 degrees. Weather events build or alter the snowpack to create a strong layer of dense snow atop a weaker layer of light snow or ice crystals. One expert compared it to a brick lying on potato chips. When something disturbs the brick, it starts to slide.
At resorts where snow safety is monitored closely, managers release slides by firing shells at the slope from a howitzer. In more remote areas, snow slabs can be set loose by temperature change or the added weight of new or blowing snow.
Karl Birkeland, an avalanche scientist with the U.S. Forest Service in Montana, said the rise in avalanche fatalities can't be tied easily to climate change.
"With avalanches, we're talking more about short-term weather events that can be big players," he said. The variables of snow, wind, temperature and humidity that prime a mountain for avalanches are highly localized and change quickly, he said.
Researchers are testing sensitive new machines that can probe mountain snows for hidden weaknesses more precisely than people armed with shovels and poles. They're developing computer models that will translate such detailed weather and snow cover data into better avalanche forecasts.
Most victims trigger their own demise when their movement or weight cracks the snow slab they're crossing. As it slides, the snow is ground up and warmed. When it stops, it congeals like concrete, entombing and suffocating its victims.
"It went from liquid to solid instantaneously," Seibert said of the snow that nearly killed him Jan. 20. He came to a stop buried to his neck.
Of 13 people swept down the mountain, he said, 10 were trapped in the snow, eight of them fully buried. Only three survived. Among the dead was snowboarding champion Craig Kelly, 36.
"We can expect more incidents," said Knox Williams, director of the Colorado avalanche center. Wider skis that improve efficiency in deep powder, more supportive boots, and snowmobiles with more effective tracks are giving more people access to avalanche terrain.
Most are good athletes and are well-prepared, he said. They consult avalanche hot lines and Internet advisories. They know how to dig snow pits to reveal dangerous layering, and recognize risky slopes and weather. Many carry snow probes, shovels and radio beacons.
In the past 10 or 12 years, "most avalanche victims have had at least some level of avalanche awareness training," Atkins said. But "it can be a recipe for getting in trouble."
"As our experience and preparation increase, our perception of the hazard decreases," he said. "It gives us a false sense of confidence."