Feature December 17, 2015

When Mountains Come Down

Coming to Grips with Avalanches

In early February of 1996, three days before he was killed in a massive avalanche near Paradise Peak west of Ketchum, Jim Otteson said to me, “We’re only ever as strong as our weakest link.” Otto, as he was known, was sizing up the Sun Valley Ski Patrol. But it also seemed, in his unassuming way, he was speaking to the broader responsibility he felt to look after those less able than he, which was just about everyone. They were words to live by. As it happened in the context of the avalanche that killed him, they were also sadly prophetic.

Coming to an understanding of avalanches is a process of accretion; no one perspective is sufficient. For instance, one might appreciate the abstract beauty of an entire slope of snow breaking free of a mountain. To witness an avalanche is to glimpse forces at work beyond what most imaginations can conjure. It is undeniably thrilling, but it is not the full picture.

Then, one might come to know avalanches through the prism of quantitative science, a different kind of beauty. After all, it is the physics of thermodynamics, weather and gravity that define the phenomenon. Parsing all of that can consume a lifetime. And still, the best avalanche forecasters in the world learn to live with a maddening number of unknowns.

Perhaps the most difficult piece of the puzzle to assimilate is what avalanches engender in us: an amalgamation of fear, loss and vulnerability. Live in the mountains long enough and avalanches become personal on some level, whether peripherally or in a devastating way. As hard as it is to accept, this is an ugly truth of avalanches that can and does keep people alive.

Why Snow Slides

The science of avalanches should be intuitively obvious to most; it’s really gravity turned on its side a bit. Think of high school physics and the “block of ice on an inclined plane” problem. Like everything else, snow on a slope is acted upon by gravity. Gravity is always pushing snow down the slope—how forcefully depends on just two things: the mass of that snow and the steepness of the slope. More mass—that is, a bigger load of snow, or wetter, heavier snow—increases the force, pushing the “block” of snow down the slope. Likewise, a steep slope creates a greater component of force on that block of snow than does a gentler slope. The only force holding the block of snow in place is friction between it and the mountain. If the weight of the snow builds such that the force of gravity overcomes the resistance, or friction, holding it in place, the snow will slide.

TERRAIN: Avalanche hazard begins with terrain, namely, its steepness. The majority of human-caused avalanches occur on slopes between 33 degrees and 45 degrees. The shape of a slope will have an effect as well—convex slopes tend to be more prone to human-caused avalanches than are concave slopes. Finally, densely packed trees and large boulders can sometimes reduce avalanche hazard by anchoring layers of snow that sit on weaker layers below. Photo: Wyatt Caldwell.

The rub is that a snowpack is not a single block. It builds over time, comprising many layers, each a product of a snowstorm or a weather event of some sort. A midwinter snowpack might have seven or eight layers, each unique in character. Depending on the weather when they were formed, those layers may be strong or weak, and may or may not bond to each other.

Terrain, Weather, Snowpack

Leaving aside for now the complication of human activity in the triggering of avalanches, experts typically focus on three factors in trying to understand avalanches: terrain, weather and snowpack. Terrain is the simplest to assess since it is a constant. Again, the steeper the terrain, the greater is the force of gravity pushing snow down the slope. The large majority of human-caused avalanches occur on slopes between 33 and 45 degrees, with the most common slope angle being 38 degrees. (The top of Lookout Bowl on Bald Mountain is approximately 34 degrees.) Above 45 degrees, snow tends to avalanche naturally in small doses so the load doesn’t build up. Also, fewer people, in general, are on slopes that steep.

In addition to angle of incline, there are other characteristics of a slope, such as its shape and the presence or absence of anchors that play a role in whether snow slides or not. All things being equal, snow on a concave slope—say Easter Bowl on Bald Mountain—is less likely to slide than is snow on a convex slope like Inhibition. Likewise, large trees, bushes and rocks can serve as anchors on a snow slope, inhibiting big slides.

Weather and the snowpack are a little tougher to sort out since they are both dynamic. Simon Trautman, an avalanche specialist at the National Avalanche Center, doesn’t see a big distinction between the two. “If you think about it, the avalanche phenomenon is really just an extension of the weather,” he explained. “The weather is the carpenter that creates the snowpack.”

A World of Storms

A snowpack is rarely a homogeneous mass; it builds in layers over time. As storms roll in over Sun Valley’s Bald Mountain, for instance, each has a unique character in terms of air temperature, associated winds, and water content of its precipitation. Because warm air holds more water than does cold air, warm storms drop snow that has relatively more water per volume (high density snow) and so add a lot of weight to the snowpack. However, wetter snow also tends to bond well to itself (good snowball material) and to the layer it lands on. Cold storms lay down light, fluffy snow that has less water per volume and so less weight, but that also doesn’t bond as well to the existing snowpack (generally). Try to make a snowball with dry, light snow and it simply falls through one’s hands. It’s why skiing fresh, dry snow is relatively easy and fun—the snow is essentially frictionless.

The Wild Card

One wild card with storms and their effect on avalanche danger is wind because it can transport huge amounts of snow onto slopes. The rate of loading caused by wind can be up to ten times that of the snowfall itself. However, storms are spinning counterclockwise (in the Northern Hemisphere) as they advance, so the wind direction, along with where snow is transported, often changes during a storm. Typical storms on Bald Mountain will come in with winds from the south, then shift to the east or west, then leave with northern winds.

CHANGING SNOWPACK: When a snowpack is exposed to a temperature gradient across it—relatively warm temperatures at ground level and colder temperatures on the surface—heat flows to the surface, modifying snow crystal structure along the way and creating “faceted” crystals. These crystals tend to be weak and don’t bond well to other layers. Faceted crystals on the surface (above) are referred to as “surface hoar.”

EVALUATING THE SNOWPACK: Avalanche forecasters and ski patrolmen use a number of field tests to evaluate the snowpack at different elevations and aspects. Ultimately, they are trying to identify weak layers, how easily they fail and whether a given failure propagates through the snowpack.


Avalanche forecasters and ski patrolmen go to great lengths to track strong winds because they not only move snow but also grind it up into tiny particles, ultimately enabling the particles to bond tightly together and form hard slabs of snow. The slabs themselves may be relatively strong and cohesive, but they can be deadly if they happen to lie on top of a less stable layer, or on one to which the slab doesn’t bond well.

To illustrate the complexity of predicting what weather ultimately does to the avalanche hazard, Trautman explained, “You might come to work in the morning when it has been blowing 50 [mph]. It’s not as easy as saying the hazard is high because it’s been blowing all night. There’s a fine line with wind speed between the perfect loading rate and that when slabs don’t form because the snow just blows up into the air [and eventually evaporates].”

The Only Constant Is Change

To complicate matters more, the layers of a snowpack, once in place, are not static. They change in time, depending on the temperatures above and below them. A particularly vexing problem in the Continental climate of the Wood River Valley is the fact that snowstorms are often followed by clear, cold weather. In this situation, heat moves from higher temperatures at the bottom of the snowpack (the ground is generally at about 32 F in winter) to the much lower temperatures at the surface of the snowpack and air above, which can be many degrees below freezing.  This transfer of heat causes snow crystals in any given layer of snow to change: they elongate, become more angular and, importantly, less able to bond to each other or other layers. The technical term for these crystals is “faceted snow.”

‘Champagne Glasses’

When a layer of snow becomes faceted, it becomes more fragile. Bruce Tremper, in his book “Staying Alive in Avalanche Terrain,” describes such snow as a layer of “champagne glasses” propping up stiffer and heavier layers above—visualize layers of plywood. Generally, this depth hoar, as it is known, becomes and remains a weak link buried under multiple layers. If it is shocked with a force—the load of new snowfall, the weight of a skier, or the shockwave of an explosive—it will fracture. The champagne glasses break and the fracture can propagate along that layer in almost any direction at speeds averaging 260 feet per second. Once that happens, it’s as if layers of plywood are resting on a slope of crushed glass. Gravity wins out, and the snow begins to slide.

TESTING STABILITY: One way to test the stability of a slope is to shock it with a concussive force. Ski patrolmen will deploy explosives, use ski-cutting techniques and sometimes break off large cornices to elicit a response from the snowpack. Some ski resorts, like Jackson Hole in Wyoming, have installed fixed Gazex systems that deliver an explosion of oxygen and propane to known avalanche starting zones. Top photo: Eric Kiel / Bottom photo: Tal Roberts

Mitigating Risk

Skooter Gardiner, director of snow safety for the Sun Valley Ski Patrol, starts his annual winter job long before he is officially on the payroll, tracking the first few snowfalls that stick to the ground, which can be in October. “Oftentimes we get a weak layer early—old snow that sits, rots and dries out, then we’ll get fresh snow on top of it,” he said. “So, once we get up there, it’s a matter of digging around in the snow, trying to identify where that weak layer or layers exist. And it changes around the mountain; there’s spatial variability on Baldy between the north-facing and south-facing slopes.”

When the area gets into a real storm cycle, Gardiner’s job tends to run around the clock. He’ll be tracking the weather through the day and into the evening, particularly in light of the fact that grooming operators will be driving around at night and potentially at risk. And with a mid-mountain restaurant serviced by a gondola open at night—wind is always on Gardiner’s mind. At least once during the day he’ll communicate with Scott Savage, director of the Sawtooth Avalanche Center, to see what kind of snow stability he has been finding in areas nearby.

During big storms, Gardiner tends to “sleep with one eye open.” He will check remote web cams on Baldy to check snowfall amounts and gather wind readings a couple of times in the night. By 4:30 a.m., he’s on the phone with Ski Patrol Director Mike Lloyd to make the decision as to whether they’ll call an “early morning,” the express goal of which is to evaluate and, if possible, mitigate potential avalanche hazards.

“Early mornings” entail notifying the Forest Service, then calling in to work early a number of entities: the mountain manager, the lift department, and the entire ski patrol. Before lifts are running, the lead explosives patrolman drives a snowmobile to the top and begins preparing hand charges—2- and 4-pound explosives of pentolite, a mixture of PETN and TNT—while the rest of the patrol helps shovel out lifts needed to get to the top.

In the patrol’s morning meeting, Gardiner will summarize the weather overnight and brief patrolmen on areas—aspects and elevations of terrain—he thinks might be unstable. Teams of between two and six patrolmen gear up with shovels, probes, avalanche transceivers, explosives and fuse igniters, then fan out to a number of carefully choreographed routes in the Warm Springs and River Run drainages of the mountain. Each team will use a combination of explosives and ski cutting (patrolmen making quick, diagonal traverses) in likely trigger points or starting zones for a given route to test the snow’s stability. Once all of the teams have cleared their areas safely and have returned to the top, the entire patrol meets to discuss results and to decide whether the terrain should be opened. Then they will head out to do routes in the upper and lower bowls and Seattle Ridge.

Like all snow safety experts and avalanche forecasters, Gardiner has learned to live with uncertainty. “We want to touch everything, make sure we’re not missing anything. However, we’ll always go in with a hypothesis or a game plan. And when things aren’t happening according to the plan, then maybe it’s time to pull back, reevaluate and ask, ‘what did I miss?’” Rich Bingham, the former snow safety director at Sun Valley who is entering his 49th year on the Baldy patrol, put it this way, “The unexpected is the hardest thing we have to deal with. But you have to expect the unexpected.”

Gardiner is also a realist. “While we can throw a bomb without really putting our personnel at risk, it’s simply a test to see how the snowpack reacts to a concussional force,” he said.  “We have to always remember that it’s a test, not a control measure.” In essence, it’s one data point among many on a very big and spatially diverse mountain. For this reason, Gardiner is above all else concerned about the safety of the patrol personnel because “they are putting themselves at risk first. And you never really know what you’re dealing with.”

CONTROLLED AVALANCHES: One of the objectives of avalanche mitigation efforts is to reduce the potential for big avalanches by creating smaller avalanches in a controlled setting. The efforts also serve to disrupt unstable layers buried in a snowpack.

According to Bingham, one decidedly low-tech but effective tool the patrol uses to reduce avalanche risk is skier compaction. “One of the reasons we have fewer problems on Baldy these days is that over the last dozen years or so, we’ve taken a really aggressive posture about opening things early, before they are actually in prime condition—just to disrupt the weak layers before they get buried. It has made all the difference in the world,” he said.

The Human Factor

Even with all of the variability of weather and the snowpack, the most unpredictable factor in the avalanche equation is human decision making. While good decision-making is certainly important within resort boundaries, it is even more crucial in backcountry skiing where there are no efforts to reduce avalanche hazard. Even on Baldy, there is a great deal of unpatrolled, out-of-bounds terrain accessible from the top of the mountain. This is not unique to Sun Valley—Jackson Hole has a similar situation—however, as Trautman pointed out, “You can duck a rope on Baldy … and it looks a lot like the ski area, but there’s some very dangerous avalanche terrain out there. To the layperson, it may not look dangerous … but it’s kind of upside-down. It gets steeper towards the bottom, which is weird, and it’s very tight with lodgepole pines. Tiny little avalanches can kill people back there.”

Savage and his team at the Sawtooth Avalanche Center prepare avalanche forecasts for over 4,600 square miles of terrain. Speaking to the Baldy “sidecountry” issue, in which resort skiers can so easily access the backcountry, Savage observed, “With equipment advances, people become expert skiers much more rapidly than they used to. But their avalanche and decision-making skills, or mountain experience, is not commensurate with their skiing ability. You see people in much more serious terrain with, generally, a lower level of mountain and avalanche knowledge, compared to people in the past.” Savage also cited the effect of the GoPro camera in elevating risk exposure. He noted that people will take on much more risk than they otherwise would if they know they are being filmed.

Making Decisions

Ian McCammon is a snow science expert and researcher who has studied decision-making in avalanche accidents. In his often-cited 2002 paper, “Evidence of Heuristic Traps in Recreational Avalanche Accidents,” McCammon explored how “heuristics,” or rules of thumb, influenced the decisions of avalanche victims. Rules of thumb are decision-making shortcuts that, as McCammon put it, “guide us through routine but complex tasks such as driving or shopping. Because we use them so often, [they] tend to operate at the threshold of consciousness.”

McCammon studied the “familiarity,” “social proof,” “commitment,” and “scarcity” heuristics. The familiarity heuristic is when one believes one’s actions are the right ones simply because he or she has done it before. For example, who hasn’t heard someone say, “I’ve skied this slope before, it didn’t slide then, so it must be ok”?  The social proof heuristic refers to our tendency to justify actions based on the fact that other people are doing it as well. The commitment “trap” is our inclination to keep doing something simply because we had planned to do it. Finally, the scarcity heuristic has to do with our tendency to pursue something because a resource, such as powder snow, is in short supply.

Studying over 598 avalanche accidents, McCammon found that when any of these factors were present, people exposed themselves to much higher risk than they did when they weren’t present. He also concluded that, in these situations, even victims with avalanche training ignored obvious clues to potential avalanche danger, which, as he wrote, is a classic characteristic of “heuristic, single-piece-of-evidence-type decision making.” Finally, he determined that it was likely that these decision-making traps actually contributed to causing avalanche accidents.

Guys like Trautman and Savage spend a great deal of time and effort trying to evaluate big swaths of terrain with good but limited data. Their goal in producing avalanche advisories is to reach a diverse group of users. These, Savage pointed out, might include a 13-year-old skiing Baldy, a 75-year-old snowmobiler in the Fairfield area, or an experienced skier touring near Galena Summit. Each season Savage receives over 300 field observations from local ski professionals. He uses remote weather station data and daily excursions into the backcountry to evaluate the snowpack. Still, Trautman noted, “An advisory is not really data. It is an idea, a forecaster’s picture of what’s happening on a given day. We believe that the best way to increase backcountry safety is to give people the best possible information that allows them to make decisions that work for them.”

MAKING DECISIONS: In studying avalanche accidents, researcher Ian McCammon found that when people fell into “heuristic” traps—decision-making shortcuts—they exposed themselves to much greater avalanche risk than they otherwise would have. Photo: Tal Roberts

February 10, 1996

The day Jim Otteson was killed was an otherwise spectacular day in the Smoky Mountains. Otteson was guiding skiers during a day of helicopter skiing. He was the first of his group to leave the safety of the ridge and ski down a line that had been previously skied several times that day. The avalanche broke above him and carried him down 1800 vertical feet. Two fellow guides found and uncovered Otteson within 15 minutes, which was remarkably fast given the conditions. Barring trauma, the probability of surviving a full snow burial has been documented to be greater than 90 percent for the first 18 minutes. However, several people—guides, ambulance personnel, myself and eventually the staff in the hospital emergency room—tried to revive him for over 40 minutes. We couldn’t bring him back.

The helicopter pilot for the skier group, who was parked at the base of three similar drainages, later reported seeing avalanche debris first exiting the second drainage over from Otteson’s. Then he witnessed avalanche debris exiting the drainage next to Otteson’s, and finally the one Otteson was in. Given this series of events, the scenario many came to subscribe to was that a natural event two drainages over from Otteson caused a weak layer to fail (the champagne glasses broke), and the failure propagated over the ridges to Otteson’s slope. It was a case of the snowpack only being as strong as the weakest link.

Any sort of scrape with an avalanche will spook a person, most likely forever. As uncomfortable as that can be, in the end, it might be a good thing. No doubt there’s a tremendous amount of technical knowledge a person can acquire about snow science. But sometimes it’s not enough. Sometimes that vestige of fear and humility, unease in a person’s eyes, may be the one piece of critical data that’s missing.

This much is certain: when the strongest of the strong, the most competent and capable are taken, as Jim Otteson was, it gives one pause.

Living with and among avalanches, it seems, demands a certain reverence and attention. As Flannery O’Connor, who knew something about reverence, once wrote: “The life you save may be your own.”

This article appears in the Winter 2016 Issue of Sun Valley Magazine.