It's been a harsh winter across the US. Snow has blanketed the Sierra Nevada, where the snowpack is well above normal. Lots of snow means good skiing, but it also means an increased danger of avalanches.
Avalanches aren't something most skiers and snowboarders have to think about. That's because ski areas take preventative action.
On the backside of Squaw Valley Ski Resort, two ski patrollers drop into a black diamond run known as Granite Chief. Below them are mounds of fresh, untouched powder – more than seven feet deep.
The patrollers are throwing explosive charges onto the slopes to trigger smaller, less dangerous avalanches. Booms ring out across the mountain.
"The Sierras are known for getting tons of snow really quick," says Will Paden, the avalanche forecaster at Squaw Valley Ski Resort. "We're constantly trying to start the avalanches so that we don't let the snow pack build up to be too deep."
Paden says on a day like today, they'll use more than a thousand pounds of explosives to make the ski area safe. But the job isn't over when the snow stops falling. The snowpack is constantly changing.
"One day could be perfect powder and then that afternoon the wind can pick up and put wind crust on top of that perfect powder and make it difficult skiing," says Paden.
Avalanche forecasting is even more technical. "We had a lot of riming in this snow and some graupel events."
To translate that, you have to go inside the snowpack.
On a slope outside of Truckee, Brandon Schwartz uses a shovel to cut a cross-section in the snow. As a forecaster with the non-profit Sierra Avalanche Center, Schwartz has dug thousands of avalanche pits like this one.
An avalanche near Echo Summit in Lake Tahoe.
Credit: Travis Feist"We can feel the different hardness of all the layers that have formed in the snow that's fallen over the last two to three days," says Schwartz.
Schwartz is looking for weak layers of snow, which is where avalanches begin. He pulls out a saw and slices through the snow to isolate a one foot wide column. Then he places his shovel on top. "And we'll just start to load on top of it first with ten taps just from my wrist, just from lifting my wrist and letting gravity pull my hand down."
Those taps simulate what a little weight would do to the snowpack, either from more snow falling or from a skier.
Schwartz points to where the snowpack has broken away along a straight line. "So we got a pretty significant crack all the way across the column here. Definitely a difference in strength there and that's what makes up the layers of snow pack and when we have these layers of different characteristics then we start to get some of the ingredients for a slab avalanche."
Schwartz and his team travel into the backcountry every day to assess the avalanche danger in the Tahoe region. Of the 36 people who died in avalanches across the United States last winter, almost all of them were in the backcountry. A large storm like this one means today the danger is high.
But what makes some snow weaker than other snow?
"Once we have snow on the ground, a whole bunch of really interesting things happen. You think of the snow as being rather static, but it's not at all," says Jeff Dozier, an environmental scientist at the University of California-Santa Barbara who studies how snow impacts California's water supply.
Dozier says to understand what's happening, you have go all the way down to the level of a snowflake.
Check out the different types of snow crystals, as seen under an electron microscope:
Once the snow falls, the snow crystals will start to stick together. As they sit there, the crystals grow rounder and bond together. "And if you shovel snow, you see this. If you shovel snow when it's new, you can stick the shovel in the snow and you can lift it. You shovel snow when it's old, it's hard to break that block of snow loose from its neighbor."
When a lot of snow falls quickly like it does in the Sierras, this bonding process may not happen fast enough to support the snowpack, which leads to avalanches. The warmer a snowpack is, the faster it bonds. But if it's colder, sometimes a different kind of crystal grows.
"Typically the temperature at the base of the snowpack – this is gonna be around zero degrees C. But on a very cold night, the temperature at the surface say might be -20 degrees C," says Dozier.
That difference in temperature can create another shape of crystal – a faceted crystal. "They're sort of angular. They don't bond together very well."
These crystals look like grains of sugar and they create weak layers deep in the snowpack. A better understanding of snow crystals could help avalanche forecasters. Dozier says it could also help water managers trying to anticipate the snowpack melt in the spring, an event that's critical to the state's water supply.
Avalanche forecaster Brandon Schwartz in the field: