In February, five people were killed in separate avalanches across B.C. and Alberta. That same month, more than a dozen people were killed in California and Utah, including a particularly deadly avalanche that claimed the lives of nine. In Europe, from Andorra to Slovakia, the season has recorded 125 deaths from avalanches so far.
These tragedies highlight the ever-present risk of backcountry recreation, even as some of those killed were experienced and well-equipped for avalanche dangers. But they also highlight the challenges to knowing when an avalanche will strike.
CBC News spoke with three experts to find out how avalanches form, why they’re hard to predict and whether climate change will make them more dangerous.
What makes an avalanche?
An avalanche is “a mass of snow moving at a visible speed,” said Simon Horton, research officer and forecaster at Avalanche Canada.
Ethan Greene, director of the Colorado Avalanche Information Center, adds that this movement is generally down an inclined slope and follows a basic premise: “You need a very strong layer over a very weak layer in order to produce an avalanche.”
And then you need a trigger, some kind of stress on the layers to kick it off.
How and when these ingredients are added is critical — and you can see that when cutting into a mountain snowpack.
And while it may look uniform, Greene says snow and ice are very dynamic.
“The crystalline structure of the snow is always changing,” Greene said. “Sometimes, it’ll change to make things stronger. Sometimes, it will change to make that layer weaker.”
Nicolas Eckert, a scientist who studies mountain risks with Université Grenoble Alpes and the national research organization INRAE, says the snowpack gradually transforms with each successive precipitation or temperature event.
“Each layer evolves,” Eckert said. “That can create conditions which can be more or less favourable to avalanches.”
Are there different kinds of avalanches?
Avalanche types can differ, from loose, not very cohesive sluffs to heavy, water-logged chunks, each with different triggers.
“We call them avalanche problems because there’s these common patterns that form that weak structure and those patterns have similar distribution over terrain,” Horton said.
Greene and Horton consider ‘slab’ avalanches as the most dangerous group, which are cohesive pieces of snow that break off the hillside.
Why are they hard to predict?
Despite knowing a lot about the structure and formation of avalanches, figuring out exactly when they happen can be difficult.
“Each event is somehow different,” Eckert said. He just visited the site of a recent avalanche in the Queyras massif in the French Alps. Data gathering has improved in the last decade, he says, but the main difficulty is that snowpacks are different under your feet — even a short distance away.
“Every specific snow profile is somehow different. You make a snow profile at one point and then 100 metres away … it’s not completely different, but it’s different,” Eckert told CBC News from near Grenoble.
Horton, at Avalanche Canada, calls forecasting a mix of art and science, sometimes relying on interpretation and past patterns. And while knowing the structure of a snowpack is one thing — knowing how it will react is another.
“You can have kind of a layered structure that seems that has potential to be concerning,” Horton said. “Depending on how the next weather pattern comes into play, whether the storm is cold or warm, it could go different ways.”
Even a strong layer needs to have the right conditions to fracture into an avalanche.
“You need enough cohesion in the slab to drive this kind of collapse failure that happens,” Greene told CBC News from Leadville, Colorado. “If the slab is too weak, it’s not going to be able to contribute to the crack propagating over a wide distance very quickly.”
How does climate change affect avalanches?
As the planet warms from humans burning fossil fuels, experts say there is likely an influence on avalanches.
“Exactly how is a harder thing for us to say,” Greene said. Because avalanches are so dependent on the type, intensity, location and sequence of weather events, figuring out climate change’s exact role is tricky.
“There’s all of these subtleties in there that [make] trying to connect all of those dots to something larger … like climate change is really quite difficult.”
Eckert, who studies the climatic factors involved, says total precipitation is important for avalanches, but snow is more important.
“Overall, when temperature increases, logically, snow amounts are decreasing,” Eckert said. He says low elevations will see less snow. But, at higher elevations, where temperatures are low, there might be more snow.
The erratic weather patterns and unseasonable patterns that climate change brings could contribute to forming weaker snowpack layers. But whether that increases risk to people who go off-track is also a human question, experts say.
A fourth person has been killed in an avalanche in British Columbia this season. As the CBC’s Alanna Kelly reports, the recent death happened in an area frequently visited by people in the backcountry.
“There’s a lot of positive benefits of spending time out in these beautiful landscapes,” Horton told CBC News from Kimberley, B.C. “So it’s just really important for people who want to do these activities to have done a few things before they go out.”
Beyond the latest local forecasts, the right gear and avalanche safety training, Eckert says people need to examine themselves, too. For example, setting very clear and objective rules within a group dynamic.
“Who has the responsibility? Who can decide when you need to go back? How is the group able to react to unpredictable situations? There are many questionnaires of this kind which can be used to try to reduce risks,” he said.
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