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.

Listen to the QUEST radio story The Science of Snow

"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:

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Kirkwood Mountain Resort.

It's that time of year when people head up to the Sierra Nevada Mountains for some great skiing and snowboarding. This season is looking to be a good one with a handful of big storms dumping huge amounts of snow on resorts and in back-country terrain. Although deep snow cover obscures the view of some of the geology, the views from chair lifts and other vistas while skiing is a fantastic opportunity to think about the geologic evolution of these mountains.

My favorite ski area in the Sierra is Kirkwood Mountain Resort, which is along State Route 88 approximately three to three and a half hour drive from San Francisco and 20-30 minutes from the town of South Lake Tahoe. In addition to great terrain, Kirkwood has some fantastic geology, some of which you can ski right up to and check out in detail.

The rocks exposed at the surface on the mountains of Kirkwood are various volcanic rocks ranging from about 6 million to 15 million years old (depending on exactly where you are). The volcanic deposits at Kirkwood are nothing like the smooth lava flows you might see on the Big Island of Hawai'i. They are more similar to the recent volcanic deposits seen on the flanks of and in areas adjacent to the Cascades volcanoes in northern California, Oregon, and Washington.

Many of the cliffs exposed at Kirkwood during the winter are beautiful volcanic debris flow deposits that have up to boulder-sized chunks of igneous rock within a fine-grained rock. These rocks are interpreted to be the deposits of mixtures of mud, sand, and volcanic rock debris that flowed down the flanks of the now-extinct volcanoes. I forgot the name of the specific trail — please comment if you know — but there is a great run where you can take a short break to catch your breath and walk up to some outcrops of these debris flow deposits.

So, next time you're sitting on the chair lift waiting to take that next run, look around at these beautiful mountains and picture the ancient volcanic landscape that created the terrain you're skiing.

Images: (1) Kirkwood from Mal Parkington / Flickr; (2) Glove Rock from Dean_In_SF / Flickr

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Skiing at Tahoe’s Squaw Valley, the resort with the best report card in the country. Photo: Jeff Engerbretson.

Planning a ski trip this winter? Environmentally minded skiers might want to take a look at the Ski Area Report Card before making travel plans. The recently released Report Card gives every ski area in the country a score, based on the resort’s greenness. The grading criteria encompasses everything from recycling the refuse of mid-mountain lunch lodges to using biodiesel to fuel snowmobiles. California’s ski areas fared well: Tahoe’s Squaw Valley was ranked the greenest in the nation, and Alpine Meadows wasn’t far behind.

One of the ski industry’s biggest environmental sins is snowmaking, which often involves taking water out of streams, or adding low quality water to the watershed. Other environmental impacts include carbon emissions from ski lifts and erosion of steep slopes. The worst offense is developing undisturbed land to expand ski area terrain and build new parking lots and hotels. This can wreck habitat for threatened and endangered species. However, there wasn’t much new ski area development over the past year, primarily because of the slow economy.

According to the Ski Area Citizens’ Coalition (the amalgam of skiers and environmental groups that developed the Report Card), 90% of ski areas in the Western US are on public land administered by the Forest Service. In my opinion, environmentally conscientious ski areas are a good use of public land. I think of skiing as a way to be in nature and enjoy the outdoors—although you could argue that there isn’t much that’s natural about the corduroy-like texture of treeless, groomed slopes. But as I carve through fresh powder, look out over mountain vistas, and watch from the chairlift as voles scurry over the snow, I definitely get that small-speck-in-a-big-beautiful-world feeling. I should disclose that I was a ski bum in Alta, Utah (Ski Area Report Card grade: B) before I blew out both my ACLs and limped off to graduate school.

If you’re concerned about your ski-related carbon emissions, I suggest you live like a ski town local: live close to the mountain, and ski the backcountry. No need for carbon-spewing chairlifts when you can get yourself up the mountain on your own power—but watch out for avalanches.

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Julien Guy: supernova cosmologist

I spent the last week at a conference in the Italian Alps with about 200 skier/cosmologists. Mornings were spent in the conference hall watching 15 or 25 minute presentations. Afternoons were for the slopes. Evenings were back in the conference hall.

The conference started with supernova talks – I was fourth on the list. Being in the field, I had heard most of the results that were presented in the other talks. Ditto the other attendees' perspectives on my talk. However, there were some new and very promising results from the Supernova Factory.

The supernova factory is a LBNL-based research group that focuses on "nearby supernovae". By nearby, I mean only a few hundred million light years away. These supernovae occur in galaxies that are distant enough to be free of the gravity of the Milky Way and our neighboring galaxies but close enough to observe with smaller telescopes.

The supernovae observed by the SN factory are very bright compared to the supernovae I observe with the Hubble Space Telescope. The supernovae are bright enough to make very precise measurements at each wavelength of the supernova spectrum. Just like my earlier post on spectroscopy, the supernova light is imaged after passing through a prism. These images provide very detailed information about the molecules and atoms that are present in the supernova explosion.

The spectroscopic observations also tell us how one supernova may differ from another. The small variations in type Ia supernovae have been a mystery for quite some time. If we can learn the causes of these variations, these supernovae could be come even more useful for measuring distances in space.

There are several models and theories to explain the differences, but none has been extensively tested. A large number of bright nearby supernovae is required to test these models. Hopefully, a project like the supernova factory will provide that sample. In this conference, they only showed a handful of supernovae. All but one of these supernovae was well-behaved, fitting our current models. The last one differed enormously from the others, but the detailed spectroscopic observations lent evidence as to why this may be the case. The data is still being examined, but I am encouraged by the progress necessary if supernovae are to be used to explain the cosmology of our universe.

The presentations over the next five days covered a very large range of topics. Some conference attendees presented ideas that had never occurred to me. One that I found very interesting was an experiment to model the orbital paths of stars around the black hole at the center of the Milky Way. For those patient enough to watch these stars for 15 years, it should be possible to measure the properties of gravity and the black hole itself by looking for deviations in the stars orbits from our current models.

While the talks were very interesting and well-attended, I can't help but comment on the other important side of this conference. That would of course be the skiing. The Europeans really have it right – they chose the site and the schedule with the perfect balance for leisure time. We were only ten miles from the tallest mountain in Europe, within site of the Matterhorn, had perfect snow all week, and had just enough time to enjoy it. I even had a chance to practice my amateur photography on the slopes. Now the next challenge will be to organize a conference in Tahiti!

Kyle S. Dawson is engaged in post-doctorate studies of distant supernovae and development of a proposed space-based telescope at Lawrence Berkeley National Laboratory.