School of snow

After a mild February, winter storms in the forecast for early March will likely draw people out to play in the Sierra—but the mountains after a snowfall are not always a safe place to be. At the Slide Snow School on Mount Rose, instructors teach winter sports enthusiasts to recognize conditions that can lead to avalanches, a hidden danger of winter backcountry recreation.

On a warm morning in late February, a group of seven students gathered in an upstairs classroom in the main lodge at Mt. Rose Ski Area, looking at photos of snow crystals on a projector screen. “The good thing about surface hoar is it’s pretty visible,” said instructor Tom Carter, flipping to a slide of a large, feathery snow crystal, approximately the size of a human fist.

Surface hoar, similar to frozen dew, forms on snow surfaces during clear, calm nights. Once buried under new snowfall, surface hoar acts as a weak layer in the snowpack that can create prime conditions for avalanches.

Avalanches can occur any time there are density differences in the snowpack, according to Mike Ferrari, co-founder of the Slide Snow School and ski patrol director at Mt. Rose. They come in two main categories: Loose snow avalanches, and slab avalanches. Slab avalanches, in which a cohesive block of snow breaks off and slides down a mountainside, tend to be the more dangerous variety.

“Basically, to have a slab avalanche you need to have three things,” Ferrari said. “You need to have a slab—snow that is cohesive enough to stay together. Then you need a weak layer, and you need a bed surface.”

A weak layer can be caused by something like surface hoar, or can simply be a layer of snow with a different density than what lies above and below. The bed surface is the lowest layer. “Once that weak layer fails, the slab slides on the bed surface,” Ferrari explained.

Ferrari spends a large portion of each winter trying to keep people safe in and around the ski area. Part of his job includes avalanche hazard mitigation. “In avalanche terrain, you can never reduce the risk of avalanches to zero,” Ferrari said. “So our goal at the ski area is to reduce the hazard to a low level.”

To do so, he and his team use explosives to purposely set off avalanches. Tools at their disposal include hand charges, a tool called an “avalauncher” that uses compressed air to shoot a projectile into the snow, and an avalanche pipe that shoots a large explosive from above. They also “ski-cut,” skiing through unstable areas to set off small avalanches with their skis.

Each year, an average of 27 people die in avalanches in the United States, according to the Colorado Avalanche Information Center. To stay safe, Ferrari recommends checking conditions with the Sierra Avalanche Center before heading out into the backcountry, taking an avalanche class, and staying off of steep slopes—especially slopes of between 30 and 50 degrees, where most avalanches occur.