VANCOUVER -- No one should second-guess the decision by leaders of the doomed backcountry ski expedition to head out on the slopes Saturday, the director of the Canadian Avalanche Association said yesterday.

"I would have gone up there, too, and felt entirely confident in that decision, based on my training and experience from tromping around the mountains almost every day in wintertime for the past 30 years," Clair Israelson said.

"And I would have been wrong."

Mr. Israelson said it appears that a very rare series of events triggered the huge snow slide that claimed the lives of seven Alberta high-school students.

"They were simply in the wrong place at the wrong time."

He and other avalanche experts blamed what they called "persistent weak layers" in the mountain snowpack.

The snow responsible for the layers well below the surface had fallen as long ago as November, Mr. Israelson said.

He said similar weak snow layers likely caused the Jan. 20 avalanche 65 kilometres west of Saturday's tragedy that also killed seven backcountry skiers. But, in that case, Mr. Israelson speculated, the skiers themselves may have triggered the deadly cascade of snow.

The most recent avalanche was almost certainly a natural event, he explained, set off by a small slide high up in the mountains. As it came down, the slide picked up snow and became heavy and fast enough to start a huge second slide that engulfed the 17-member ski expedition.

"It did dig deep and brought out the entire winter's snowpack lower on the path," Mr. Israelson said. At the time, the group was skiing in a valley, well below recognized avalanche terrain.

Officials said the size of the powerful slide was rated at 3.5 out of 5.

According to the avalanche classification system, a size 3 avalanche can destroy small buildings and break trees, while a size 4 slide is typically 2,000 metres long, 10,000 tonnes in weight and powerful enough to wipe out buildings, large trucks, railway cars and up to four hectares of forest.

Oddly, avalanche experts say a lack of snow in the mountains this year, rather than an abundance of snow, has contributed to the "persistent weak [snow] layers" blamed for both recent avalanches.

"This has been one of the most unusual winters I've seen in 31 years," said Dave Skjonsberg, Parks Canada's avalanche-control manager.

"It's been characterized by very low snowfall, 60 per cent of what we normally have in this area."

The low snowpack makes it more likely that weak layers of snow will remain unstable for longer periods of time.

But Mr. Israelson said conditions Saturday where the skiers were would have seemed safe. The weather was good, recent storms were over, and the latest avalanche cycle appeared over as the fresh snow stabilized.

He said both tragedies indicate the need for the avalanche-awareness industry to improve its grasp of the implication of persistent weak snow layers.

"We need to better understand the science of snow and the science of available forecasting so we can get better."

Mr. Israelson said Canada is the only country with snow and mountains that receives no government funding for avalanche research.

Last year, the B.C. government withdrew financial support for the avalanche association's twice-weekly bulletin, which now operates at a deficit.

"Primarily, our support comes from the private sector and public donations. That's not good enough," he declared.