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Montana researchers dig into avalanche mechanics

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The Subzero Science and Engineering Research Facility at Montana State University is worlds apart from Adams' early avalanche studies. In the lab, scientists can simulate almost any kind of weather found in nature by controlling the temperature, humidity, radiation and other wintery factors. Several lab chambers can even simulate varying degrees of cloud cover. The lab also has an array of measuring equipment, including a CT scanner—yes, the same device doctors use to look at your brain—to observe the intricacies of a snow sample.

A typical experiment involves creating or "farming" snow on an inventive contraption designed in-house. The apparatus blows cold air and moisture over a column of strings, there to catalyze the condensation of snow nuclei. When the man-made snow falls into a bucket below, researchers have their base material. The process is representative of how snow forms in the atmosphere.

With the farmed snow, Adams and his students simulate snowpacks seen in the outdoors, complete with different layers and snow consistencies that result from varying weather conditions. The structural engineering side of avalanche science comes into play when these snowpacks are tested under pressure. The goal is to determine which environmental conditions cause the structural changes that make snow weak and prone to sliding.

David Walters is one of Adams' doctoral students at the Subzero Lab who studies a particular kind of snow change called radiation recrystallization. Walters came to the program from a background in ski patrolling, and his experience in the backcountry has helped steer his research in the lab.

Radiation recrystallization can best be described through a hypothetical scenario. Imagine you've had some bluebird days in the backcountry, but things are getting tracked out and you haven't floated on powder for a while. You do a snow dance. A couple of inches fall, but it's still too sunny and warm for the real goods. You dance harder. And then the big one touches down: 22 inches of snow overnight. Whereas you immediately imagine face shots of powder and shredding freshies, Walters sees something more.

PHOTO BY CHAD HARDER
  • Photo by Chad Harder

When the sun beats down on snow, it creates a temperature gradient in the snow profile, meaning some depths of snow are colder or warmer than others. In a typical scenario, the snow below the surface can actually become warmer than the surface itself. The difference in temperature is enough to change the shape of the snow crystals. Hexagonal jewel-like snowflakes morph into mini-daggers of ice in a process called radiation recrystallization, better known as surface faceting.

"This is a sneaky layer because it's so thin," Walters says as we look at a 3-D image of a snow core sample on his computer. "If I know there's been sunny weather for a while before a storm, I'll be looking for this."

Looking at a recrystallized layer up close shows a network of tenuous ice fibers loosely bonded together in a haphazard form overtop denser snow and ice crystals. The uppermost part of the core sample is the most porous, which makes it structurally weak.

This change is nothing to fuss over if it stays on the surface of a snowpack. But if another couple of inches of snow falls on top of it, avalanche-savvy skiers will be on the lookout for signs of unstable snow. This weak layer, now buried, suddenly has a lot more weight to support, which can put its breaking strength to the test.

Radiation recrystallization was once thought to be mostly prevalent in Colorado and Utah's high Rocky Mountains where solar radiation is traditionally more intense. Input from the local ski community, combined with Adams' interest in the phenomena, literally brought it under the microscope at the Subzero Lab.

"I think we've really established that [radiation recrystallization] is much more common in [Montana]," Adams says.

Unfortunately, identifying the phenomena is not as simple as our hypothetical scenario, where a formula of conditions creates a weak layer.

"How do you incorporate all the right inputs, and which are the most important parameters?" Adams asks. "That's where the real research is—trying to put into context what the governing concepts for radiation recrystallization are."

Radiation recrystallization is hardly the only snow metamorphism being studied at the Subzero Lab. The researchers are also unraveling the mechanics of surface hoar and depth hoar, two other weak layers familiar to backcountry skiers under the Big Sky. Overall, the lab's capabilities make it unlike any other in the world, and it attracts visiting snow scientists from countries like Switzerland, France and Japan.

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It's one thing to understand how snow shape-shifts in a lab, but transferring that knowledge to the field is an equally important component of research at the Subzero Lab.

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