A Mind-Bending Avalanche Animation That Could Save Your Life

March 20, 2019 0 By JohnValbyNation

On the surface, a so-called slab avalanche doesn’t make sense. Take a good look at the rendering above. The snowman represents a skier or snowshoer venturing where they shouldn’t. It barely disrupts the snow, but uphill the accumulated powder suddenly snaps and flows down the hill, devouring our brave test subject.

Avalanches are awesome, in the classical sense of the word. In the most severe cases, tens of thousands of tons of snow accelerate from zero to 80 miles per hour in five seconds, obliterating pretty much everything but rock. Their violence boggles the human mind.

In particular, it boggles the minds of scientists trying to model that behavior. When an avalanche lets loose, its path of destruction depends on everything from the environment to the consistency of that snowpack. Icefall avalanches are essentially waterfalls of snow off a cliff, while glide avalanches move slowly, kind of like glaciers. But now, thanks in part to groundwork laid by the animators of Disney’s Frozen, researchers have simulated in awesome detail the moment a slab avalanche shears from a mountain. The rendering isn’t just hypnotic—it may well help develop better avalanche warning systems to keep humans away from one of nature’s most fearsome phenomena.

While our snowman doesn’t seem to make much of a fuss when it lands, its disturbance has in fact triggered a weak layer of snow hidden under the surface. “It's buried below the more cohesive and denser snow slab, so it's really like a sandwich,” says snow physicist Johan Gaume, of the Swiss Federal Institute of Technology, who developed the simulation. “You have this weak layer, it's very fragile. It's like a house of cards, basically.”

The weak layer doesn’t form because of some weird snowfall event, but because of temperature. When snow first falls at the beginning of the season, it’s landing on relatively warm ground. The temperature outside, though, can be considerably lower. This creates a temperature gradient that leads to the formation of an extremely porous layer of crystals—it’s made up of 80 percent air. More snow falls on top of this, forming that denser snow slab. So when the snowman makes contact with the bottom of the hill, it triggers the cascading collapse of the hidden weak layer all the way up the hill, where the slab releases and turns into an avalanche. (In the animation above, that weak layer is in red. You can watch it travel up the hill in slow motion.)

Scientists already knew this is what causes a slab avalanche, but Gaume is the first to build a modeling system to simulate the collapse of the weak layer. To do it, he began with a system that Disney animators have used to model snow, called a material point method, a kind of physics simulator that factors in things like gravity and deformation. These animators, though, weren’t necessarily interested in comprehensively modeling those physics. “Basically, the Disney model was done to make snow look good,” says Gaume. “It was physically based, but it was not validated with real data.”

So Gaume ventured into the field. As you can see in the GIF above, the team cut into snowpack, exposing a cross section, which they stuck black tracking dots on. They then sawed through the snowpack at the weak layer, which collapsed as a high-speed camera watched the dots. (Longer arrows mean more movement of the snow in that particular spot.) “We managed to really reproduce everything,” says Gaume. Things like the propagation speed of the crack, “and also all the displacement of these black dots, with a very good precision.”

Now that they had a better understanding of how the weak layer collapses, they could marry this real-world data with the powerful snow-modeling system behind Frozen. We’re talking 30 million particles in motion—which, to be clear, don’t represent individual snowflakes. “A particle is just a position, a velocity,” says UCLA applied mathematician Joseph Teran, coauthor with Gaume on a new paper detailing the work. For each frame in the rendering, their algorithm determined the position of each particle. “Now physics tells us how all those particles move to frame number two.” Each particle shifts down the hill based on the laws of physics for each of the 24 frames per second in the simulation.

As you can see, the slab avalanche begins as a solid, but then turns into more of a study in fluid dynamics. “Once the slab releases,” says Gaume, “all the individual parts of the slab that are breaking, they're getting into collisions with each other, they're compacting. It's really the collective behavior of all these particles that make it behave like a fluid.” The snowman looks more like he’s getting swallowed up by a wave of water than by snow.

His loss is our gain, though. The findings could help forecasters better warn of avalanches. At the moment, researchers usually need to go out and sample snow to find that weak layer, giving them an idea of the likelihood of an avalanche in a particular area. What that doesn’t give them, though, is the magnitude of a potential avalanche. “We can really identify the volume of snow that is released, and that's the crucial parameter when you do avalanche forecasting,” says Gaume. “You want to know the size of your avalanche.”

And this also means even more realistic animations in movies, yeah? Maybe, but it could be a bit too much information for Hollywood. “The paper is concerned with a phenomenon you don't see,” says Alexey Stomakhin, who works with material point methods, but who wasn’t involved in this new work. “This is about realism. It's important for people who care about safety in the mountains, but people in movies maybe care more about the avalanche itself rather than how it actually forms.”

Sacrificial snowmen: awesome for science, awesome for avalanche safety, maybe awesome for Hollywood.