Snow Science Against the Avalanche


On slopes shallow enough to accumulate snow but steep enough for it to be unstable, chaos hides beneath the surface.


One night earlier this winter, the only road out of Alta, Utah, was closed down. At ski lodges, signs warned guests to stay inside or face fines. Already that season, twenty-two feet of snow had fallen, and, the day before, a storm had dropped thirty-three inches; another foot was predicted by morning. The most dangerous time for avalanches is after a rapid snowfall, and three-quarters of the buildings in Alta are threatened by a known avalanche path. A standard measure for danger on roads, the Avalanche Hazard Index, computes risk according to the size and frequency of avalanches and the number of vehicles that are exposed to them. An A.H.I. of 10 is considered moderate; at 40, the road requires the attention of a full-time avalanche forecaster. State Highway 210, which runs down the mountain to Salt Lake City, if left unprotected, would have an A.H.I. of 1,045.


Just before 5 a.m., a small group of ski patrollers gathered at a base by the resort’s main lift. Dave Richards, the head of Alta’s avalanche program, sat in the control room. Maps and marked-up aerial photographs hung on the wall next to what looked like a large EKG—that season’s snowfall, wind speeds, and temperature data plotted by hand. Clipboards on hooks were filled with accounts of past avalanches.


Forty and bearded, with tattoos on his arms, Richards has the bearing of a Special Forces soldier. He wore a vest with a radio strapped to it and held a tin of dipping tobacco, spitting occasionally into the garbage can beneath his desk. He obxts when people say that he works in avalanche control; he prefers the term “mitigation.” Sitting nearby was Jude, his English cream golden retriever, named for the patron saint of lost causes.


Jonathan Morgan, the lead avalanche forecaster for the day, described the snow. He wore a flat-brimmed cap and a hoodie. “Propagation propensity’s a question mark,” he said. “Not a lot of body in the slab. . . . Dry facets, two to three mils,” he continued. “It’s running the whole gamut of crystal types—wasn’t ice, by any means. Rimy, small grains.”


At ski resorts like Alta, large avalanches are avoided by setting off smaller ones with bombs. On the walls above the maps were dummy mortar rounds. Above Richards’s desk were binders marked “Old Explosives Inventory.” The idea, Morgan explained, was to “shoot the terrain we can’t get to.”


Richards started considering their targeting plan. The ski resort is cleared from the top down: first by artillery shells, then with hand charges. Before any shots are fired, paths leading to the mountains are closed. Because not all skiers keep to groomed trails—backcountry adventurers seek out remote areas—the Utah Department of Transportation also checks the roadside for tracks. Sometimes it scours the mountainside with infrared cameras before giving the all-clear.


“So we’ll go fourteen for Baldy?” Richards said. “Doesn’t include a shot seventeen.” Baldy was one of the resort’s mountain faces, at which they planned to fire fourteen shells; seventeen was a spot on its ridgeline.
“Seventeen wouldn’t be the worst idea,” Morgan concurred. “You got a seven in there?”
“When was Baldy shot last?” Richards asked. “Forty inches ago?”
“Yeah, Friday morning.”


Richards and Morgan repaired to the mess hall—dark carpet, pool table, a deer head on the wall—for breakfast. At five-thirty, the ski lift opened. As Richards walked out the door, Liz Rocco, another ski patroller, mentioned that she had prepared some of the hand charges they would be using that morning. “And I will light them, and throw them into the darkness,” Richards said.


We rode the lift up in the moonlight. Snow was falling on the fir trees. Richards spent his childhood at Alta: his father was a ski patroller for thirty-three years, and his mother, who later became a university administrator, worked the front desk at the Rustler Lodge. Richards started his career as a professional skier, then worked as a heli-skiing guide, before joining the patrol full time. “The thing that makes it for me is the snow,” he said. “Working with a natural material that can be—” He paused. “It’s light and fluffy and soft and downy, and it’s everybody’s favorite thing in the world. It’s also one of the most destructive forces in nature. Under the right conditions, that soft, wonderful little snowflake can tear forests out of the ground, throw cars through the air, flatten buildings. And you get to watch that.”

我們在月光下乘坐電梯上去。雪落在冷杉樹上。理查茲在阿爾塔度過了他的童年:他的父親在滑雪場當了33年的巡邏員,他的母親后來成為一名大學管理人員,在拉斯特爾旅館的前臺工作。理查茲以職業滑雪者的身份開始了他的職業生涯,然后擔任直升機滑雪向導,最后全職加入巡邏隊。他說:“對我來說,最重要的事情是雪?!薄芭c一種天然材料一起工作,可以…… ”他停頓了一下,“它輕盈、蓬松、柔軟、有絨毛,它是世界上所有人最喜歡的東西。它也是自然界中最具破壞性的力量之一。在適當的條件下,這種柔軟、美妙的小雪花可以把森林從地面上撕下來,把汽車拋向空中,把建筑物壓平。而你可以看到這些?!?

At the top of the lift, we started hiking. A voice crackled over the radio. “Copy,” Richards said. “Just give me a holler when you pull the trigger.” A moment later, the radio crackled again; Richards ducked and covered his head, and an explosion went off somewhere nearby. We resumed hiking. After a few minutes, we arrived at a two-story shed. A garage door opened onto a pair of hundred-and-five-millimetre howitzer cannons, of Second World War vintage, installed on semicircular tracks. The gun barrels were pointed at the mountaintops. A crew was loading bags of gunpowder into the undersides of artillery shells—enormous bullets, six inches wide and two and a half feet long. Richards wrapped a rag around a large stick and jammed it into a gun barrel, to clean it. “One Sunday morning,” he began singing to himself. “As I went walking . . .”

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The patrollers donned foam earplugs and large over-ear headphones; Richards and his co-gunner walked around one of the weapons, checking locks and bolts. They turned a crank, and the barrel swung toward its first target.
“Zero, zero, two, seven,” Richards yelled—the elevation and the deflection. Two other patrollers confirmed the co?rdinates. “Ready to fire,” Richards said. “Fire!”
He pulled hard on a chain. The muzzle flashed, and a plume of acrid smoke filled the air. There was a high-pitched ringing.


It wasn’t possible to see the mountain, but Richards listened for impact and, a few seconds later, yelled, “Report!” Outside, while the barrage continued, a patroller named Kyle took a small cast-booster explosive out of his pack: it resembled two cans of beans wired together with licorice, the cartoon version of a bomb. He pulled the fuse and tossed it underhand over the cliffside. “That didn’t go where I wanted,” he said. Ninety seconds later, it exploded into a black-and-white cloud of snow dust.


Afterward, the cleaning and stowing of the guns began. When everything was done, it was nearly nine o’clock. Richards prepared to ski back toward the base. During the night, the resort had sent an alx to Alta skiers, telling them to expect between nine and fourteen inches of new snow—some of the best skiing of the season. On the way down, the sun shone on fresh powder reaching up to Richards’s waist. Small cracks shot out from his ski tips as he descended. Piles of snow slid downslope. He paused and, turning his ski pole upside down, began using it as a probe. The pole slid easily into the first foot of snow. Feeling resistance, he pushed harder—and broke through into a hollow. After the snow settled and drifted, there could be avalanches.

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The project of avalanche control in the Alps goes back at least to 1397, in Andermatt, Switzerland, with a law that prohibited logging. Swiss peasants had moved farther into the mountains. Their new farmhouses sat in avalanche paths. It was soon discovered that old-growth trees anchored the snow and kept slides from gathering mass. During the eighteen-seventies, Johann Coaz, the head of the Swiss Forest Service, made records of historical avalanches. He drew up maps of potential disaster zones and designed walls to protect vulnerable settlements; the stones used to build them were hauled up the mountainsides by hundreds of men.


Around the same time, prospectors in the western United States began finding silver ore high in the mountains. At Alta, which began as a major silver camp, miners logged the alpine forests for firewood and to reinforce their tunnels. According to legend, the avalanche danger grew so high that women weren’t allowed to live there in winter. Alta was abandoned in 1927, when the price of silver plummeted, but, in the nineteen-thirties, European-style ski resorts spread across the American West. The first mechanical lift appeared in Alta in 1939.


After the Second World War, some veterans of the U.S. 10th Mountain Division, who had trained for alpine combat, found themselves responsible for snow safety at the resorts. In 1945, Montgomery Atwater, a freelance writer who had fought with the 10th, heard about a snow-ranger job at Alta and applied on a whim. “That Alta was ideally conceived by nature to become the first avalanche research center on this continent and that I was there to take the plunge were mere coincidences,” he later wrote, in “The Avalanche Hunters,” from 1968.


Alta lies at the center of three storm tracks, from Canada, the Gulf of Alaska, and the Pacific. Storm systems accumulate moisture in the Salt Lake and, as they rise into the mountains, release about forty-five feet of snow each winter. Atwater learned that although snow always begins the same way—with a water droplet condensing around a dust mote or pollen to form a six-pointed snowflake—it can take innumerable forms later. Snow acts like both a solid and a liquid: it flows—even a blanket of snow on a hillside is slowly creeping—while maintaining its structure. Scientists consider it to be “warm,” because it is always close to its melting point. This is why, before you make your first snowball of the day, it is hard to know how well it will pack: you are working with a material that is about to change state. It’s like building a bridge with red-hot steel.


We think of the snow on a mountain as a solid mass. In reality, it is a layer cake created by serial snowfalls, each layer distinctive and changeable. “The snow cover is never in a state of repose,” Atwater wrote. “It is continually being pushed, pulled, pressed, bent, warmed, chilled, ventilated, churned.” The topmost layer might be evaporating into the night air; at the same time, radiant heat from the ground, or from nearby trees, could be melting the lowest layer. When the temperature differences between the layers are small, snow tends to sinter, or coalesce: the crystals knock off one another’s arms, becoming rounded grains that fuse into a strong, dense snowpack. When the differences are larger—say, between the pack and the ground—snow vaporizes upward and refreezes, creating hollow, cup-shaped crystals. The result is brittle, spiky snow, called depth hoar. (In ice cream, a similar process creates freezer burn.)


Neither settled snow nor weak hoar is dangerous in itself. The problem arises when a dense layer lies atop a weak layer to which it is poorly bonded. Depth hoar is “the eeriest stuff on any mountain,” Atwater wrote; it grows unseen, rotting the snow until it is weak and potted. It is strong in compression but weak in shear. Like a row of champagne glasses slowly loaded with bricks, it can hold a surprising amount of weight until, with the slightest shove, the structure falls apart, creating a slab avalanche.


The word “avalanche” is too graceful for the phenomenon it describes. On slopes shallow enough to accumulate snow but steep enough for it to be unstable—the sweet spot is said to be thirty-nine degrees—the layers will separate, and the slab will crack and slide. Churning violently, the snow reaches eighty miles per hour within a few seconds. A skier who avoids colliding with trees and rocks is likely to be pulled under, then pinned in place by thousands of pounds of snow that harden like concrete. Very few people can dig themselves out; most can’t even move their fingers. Within minutes, an ice mask forms around your face. You asphyxiate on your own exhaled carbon dioxide.


At a test site in the mountains, Swiss scientists have set off avalanches powerful enough to destroy their equipment. Photograph by Yann Gross

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In his book “Staying Alive in Avalanche Terrain,” from 2008, Bruce Tremper, the former director of Utah’s Avalanche Center, offers a taxonomy of avalanches. In slab avalanches—the most dangerous kind—an entire layer releases at once. In storm slabs or wind slabs, the releasing layer falls from above; in wet slabs, a layer lower down is weakened by water; in a persistent slab, it was weak to begin with. A soft slab, composed of powdery snow, tends to break where you stand; a hard slab breaks above you, which is more perilous. Non-slab avalanches are said to be “loose.” In a dry loose avalanche, powder releases in disconnected sloughs. Wet loose avalanches—portended by “pinwheels,” small snowballs that leave streaks as they roll—are slower but stickier, and more likely to bury you if you get caught. Mixed avalanches, which start dry and get wet lower on the slope, have become increasingly common. So have glide avalanches, caused by meltwater seeping in below the snowpack.


Students of tsunamis or volcanoes must wait for nature to deliver their disasters, but an avalanche can be provoked. In the nineteen-fifties, Atwater used a technique now called “ski-cutting.” Two patrollers descended dangerous slopes; while one looked on, ready to stage a rescue, the other skied to a safe point on the far side, picking up enough speed to try and ride through any avalanches he might start. In theory, the slopes that slid were safer because of it; the ones that didn’t were deemed stable enough for everyone else.


It wasn’t practical to ski-cut every hill. Knowing that the Swiss used bombs to combat avalanches, Atwater tapped the Forest Service’s wartime supply of tetrytol, the high-powered explosive; he asked his supervisor whether he could have some artillery, for distant targets. National Guardsmen arrived with a First World War-era French 75. (“What would avalanche research be without war surplus?” he later wrote.) For mid-range targets, too close for artillery but too distant for hiking or skiing, Atwater tried rifle grenades, bazookas, bombs dropped from helicopters, and an air-to-air rocket known as the Mighty Mouse. These methods were too costly, or unsuited to the snow; in the end, a modified ball machine, of the sort used for batting practice, was the most reliable delivery mechanism. Richards’s team still uses Atwater’s “Avalauncher” to shoot about thirty rounds each morning.


Atwater worked with Ed LaChapelle, who had done a stint at the Swiss Avalanche Institute, to create a “snow study plot”—a clearing where they could measure snowfall and take samples of the snowpack at regular intervals. They tracked the snow’s rate of accumulation and weight in water, discovering that weight mattered far more than depth: when placed atop a layer of hoar, a foot of fluffy powder was less dangerous than three inches of dense slush. Wind, they learned, could deposit many feet in just a few hours; pillows of windblown snow looked tranquil but were deadly. Studying how snow settled, Atwater wrote, “We saw things going on within that placid-appearing mass which no man had seen before—or even suspected.” He concluded, “There are apparently random plastic flows and currents within the snow cover whose causes and effects were unknown, and still are.”


In 1805, the Irish hydrographer Sir Francis Beaufort developed a scale for measuring wind speed at sea by observation. Later, it was adapted for use on land. In his book “Defining the Wind,” from 2004, Scott Huler argues that the descxtions accompanying the scale, which were written anonymously, should count as literature. At Beaufort 0, the wind is “calm; smoke rises vertically.” At Beaufort 3, a gentle breeze, one sees “leaves and small twigs in constant motion.” At Beaufort 5, a fresh breeze, “small trees in leaf begin to sway; crested wavelets form on inland waters.” The poetic descxtions connect subjective impressions to obxtive reality. A near-gale—a Beaufort 7—is defined by “whole trees in motion; inconvenience in walking against wind.” See and feel those things, and you know that the wind is between thirty-two and thirty-eight miles per hour.

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Atwater devised an analogous guide to snow. His language is evocative, but there’s less authority in the descxtions. “Unstable damp snow is tacky,” he wrote. “It slithers out from underfoot and rolls away in balls or slips blanketwise. . . . Well settled snow has good flotation and makes a clean, sharp track.” Snow is less forthcoming than the wind. Its chaos hides beneath the surface.


One crisp, bright morning in February, I walked along a brook just outside the center of Davos, toward the headquarters of the Swiss Institute for Snow and Avalanche Research. In Davos, the train from the valley potters up through wooded hills, picking up locals in ski boots; the S.L.F., as the institute is now known, occupies a squat building a few minutes from the train station. A small exhibit in the lobby explains the history of snow and avalanches in Switzerland.


In 1951, while Atwater was experimenting with explosives, Switzerland experienced the worst avalanche season in its recorded history. Ten feet of snow fell in ten days. About a hundred people were killed; villages that had survived avalanches for centuries were destroyed. The S.L.F., which was founded in 1942, suddenly became an institution of national import.


Henning L?we, the forty-six-year-old head of the institute’s Cold Lab, wears an earring in his right ear; before taking up the study of snow, he received a Ph.D. in theoretical condensed-matter physics. Dressed in jeans, black Nikes, and a worn fleece shirt, he led me inside the lab, where computers sat beside refrigerated rooms with three-inch-thick steel doors. The lab’s goal, he explained, was to find out what the wetness or heaviness or hoariness of snow really meant, on the level of its crystals. “We are connecting physical properties of snow to structure,” L?we said. He picked up a palm-size cube that looked elaborately hollowed out, like a plaster mold of a termite’s nest. A twenty-millimetre-wide sample of snow had been taken from the crown of an avalanche—the pit that’s left when a slab releases—scanned with X-rays, and then 3-D-printed in plastic, at high magnification: the layer cake, under a microscope. The weak, bottom layer was composed of what looked like large popcorn kernels. The top layer, which had settled, was a tight tangle, like instant ramen. “You start to shear this thing”—L?we made a chopping motion where the two layers met—“it’s ninety-nine per cent sure that this will break there.”


Snow science has come a long way since Atwater’s experiments at Alta. The basic process by which newly fallen snow crystals sinter into a cohesive slab can now be seen in slow motion: it resembles the way ice cubes in an empty glass fuse together. The process of recrystallization—the re-separating of the cubes—was more mysterious. L?we opened a closet, and pulled a cylinder from a shelf marked “Snowbreeder 3.” The device allows scientists to observe a snow sample while applying varying degrees of heat and pressure. At his computer, L?we played a time-lapse video of “snow metamorphism” in the Snowbreeder. “In the beginning, it’s typical snow, it’s round-grained snow, the crystals are small,” he said. Then heat was applied from below. The lower crystals began evaporating their moisture to the crystals above, which used it to grow downward. “We see that, here, a facet’s growing. There, a facet’s growing,” he said, pointing. This was hoar—the snow becoming spiky, brittle, weak. “Seeing something is always the beginning of understanding,” he said.

自阿特沃特在阿爾塔的實驗以來,雪的科學已經有了長足的進步?,F在可以在慢動作中看到新落下的雪的晶體融結成粘性板塊的基本過程:它類似于空杯子中的冰塊融合在一起的方式。再結晶的過程——冰塊的重新分離——更加神秘。洛維打開一個壁櫥,從一個標有“Snowbreeder 3”的架子上拿出一個圓筒。該設備允許科學家在施加不同程度的熱量和壓力的同時觀察雪樣。在他的電腦前,洛維在Snowbreeder中播放了一段“雪的變質”的延時視頻。他說:“一開始,它是典型的雪,它是圓形顆粒的雪,晶體很小?!比缓髲南旅媸┘訜崃?。下面的晶體開始向上面的晶體蒸發它們的水分,上面的晶體利用它向下生長?!拔覀兛吹?,這里,一個切面正在生長”,他指著那里說道。這是白霜——雪變得尖尖的、脆脆的、非常脆弱。他說:“所見總是理解的開始。

The scientific study of snow layers has refined our understanding of avalanches. In 2008, a study published in Science by a group of Scottish and German materials researchers modelled how, when one part of a heavy layer of snow collapses onto a weak layer, it can produce a wave. Their model explained a curious observation from the field: skiers occasionally trigger deadly avalanches above or below them, even when standing on flat slopes. The weak layer, it turns out, behaves like the coils in a mattress: apply force in one place, and it spreads all over the bed. The concept is now a cornerstone of avalanche-safety education, where it is known simply as “remote triggering.”


Snow research also has applications beyond avalanches. Spinning his keys around a finger, L?we led me through the cold rooms. In one, a humidifier generated tiny clouds of perfect, lab-grown powder; in another, snow from the Arctic, Finland, and Iceland had been carefully preserved. Scientists are studying how snow’s crystal structure determines its color, or “albedo,” which, in turn, affects its ability to act like a giant mirror and mitigate global warming.


In an upstairs office with mountain views, Perry Bartelt, a gray-haired research engineer, works on Rapid Mass Movement Simulation, or ramms—software for simulating avalanches. The week before, an avalanche in Turkey had killed half a dozen people; dozens more died during the rescue, when the mountain avalanched a second time. Turkish researchers had rushed data from both slides to Bartelt. ramms calculated that the first avalanche had hit the bottom of the slope with five times the force needed to knock down a building. Its core had the density of wood.

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Using a terrain map, ramms predicts the path and the power of an avalanche. Its central innovation is its ability to treat an avalanche as a “granular shear flow,” using statistics to average out the activity of millions of interacting grains. Imagine a box of cereal, full of flakes and marshmallows; now pour it out. Some bits will fly straight, carried by their own momentum. Others will catch on the surface they’re sliding down. Many flakes will shake against one another, breaking up and settling below the intact marshmallows. (In granular flows, small things sink beneath bigger ones.) ramms seeks to predict the outcome of this churn.

使用地形圖,快速大規模運動模擬軟件預測了雪崩的路徑和力量。它的核心創新是它能夠將雪崩視為 "顆粒剪切流",利用統計學來平均化數百萬相互作用的雪粒的運動。想象一下一盒麥片,里面裝滿了片狀物和棉花糖;現在把它倒出來。一些碎片會直接飛起來,被它們自己的動力帶著。其他的會被它們滑下去的表面抓住。許多片狀物會相互搖晃,碎裂并沉淀在完整的棉花糖下面。(在顆粒狀的流動中,小東西會在大東西下面沉下去。)快速大規模運動模擬軟件試圖預測這種攪動的結果。

The software was validated on historical avalanches—especially on data about whether trees had been knocked down, and, if so, how old they were. “Trees are wonderful mechanical sensors,” Bartelt said. If an avalanche takes down a seventy-year-old stand of trees, you know that the avalanche has a return period of at least seventy years. Fine-tuning the model would require more precise data, which are hard to come by. Gathering this information would require taking readings inside, or under, an avalanche.


For this purpose, the S.L.F. maintains an avalanche test site in the Vallée de la Sionne—a steep, mountainous area about two hundred miles from Davos. Hearing the phrase “test site,” one might imagine a bunny slope. Actually, it is an enormous mountain, improbably reserved for science.

為此,雪崩研究所在雄恩峽谷維持著一個雪崩試驗場——這是一片離達沃斯約200英里的陡峭山區。聽到 “試驗場“這個短語,人們可能會想到一塊平緩滑雪坡。但實際上,它是一座不可能是為科學保留的巨大的山。

The site’s chief scientist is Betty Sovilla, a hydraulics engineer. When we met at S.L.F., she was wearing red-frxd glasses, a black cardigan, jeans, and red boots. “ramms is a very simplified model,” she said. The goal of the test site was to develop a more realistic version, by correlating detailed measurements of the snow cover with the avalanches it created. She was particularly interested in glide avalanches: there were more of them every year, but they were elusive. “You cannot predict when they are released,” she said. “This is really the avalanche of the future.”


One morning, Pierre Huguenin, a forty-nine-year-old mountaineer and snow scientist, drove me to the site in a white Mitsubishi Pajero. “You see the flakes. You see the crystals,” he said, gesturing out the window. There had been a storm the previous night. He stopped the car where the road ended, and we changed into snowshoes.


Outside, there was about a foot of pristine powder. I stooped and ran my hand through it. Bone-dry, it was the pure bright white of confectioner’s sugar, with the texture of sea salt. Huguenin pulled out his phone. The avalanche forecast for the area had us covered in orange. “We are in the third degree,” he said—the risk category in which the most avalanche deaths occur in the Alps, equivalent to the American “considerable.” He pulled out two avalanche beacons—transmitters that would relay our location to rescuers—and set them to Send. We strapped them under our jackets.


“My job before working at the S.L.F. was at a cement plant,” Huguenin said, as we set out. (He was an engineer there.) “It was so loud.” Now we could hear the river as we walked. Beneath the blue sky, ours were the only tracks. After twenty minutes, the site came into view: a broad, bare mountainside, eight thousand feet high. Between two couloirs—the main avalanche paths—a half-dozen chalets huddled near a small wood.


“They are not allowed to live here in the winter,” Huguenin said. Two days earlier, there had been a naturally occurring glide avalanche at the site. I asked whether it had been dangerous. “You would be dead,” he said. “No chance.”


The site was built in 1997; in the winter of 1999, the snow was the heaviest it had been since 1951—perfect conditions for an experiment. Using explosives dropped from a helicopter, the S.L.F. triggered three avalanches in the course of a month. They were so massive that they destroyed most of the institute’s equipment. If you had been skiing on the mountain during the last avalanche, you might have heard a soft exhalation: air releasing from a crack in the slab. Upslope, it would have looked as though someone had slit the mountain’s forehead. Now its face was falling off; the break, nine football fields across, was as deep as eleven feet in places. Blocks of snow would begin leaping up prettily, breaking like roiling water. In the quiet, you might feel something lapping at the back of your legs before being swept off your feet.


The slide generated a powder cloud nearly two hundred feet high. It seemed to move in slow motion, like dry ice billowing, but it levelled the trees. Underneath, the core was formed by four hundred thousand tons of snow. Huguenin asked me to visualize the test peak, two kilometres distant, and the peak of the mountain on which we stood as the two sides of a half-pipe. With a deep roar, he said, the avalanche had run through the valley like a skateboarder, with enough speed to climb the other side.


“It came all the way up there?” I asked, pointing to the top of our peak, three hours’ hike away.


“Yup, and there is a trail there. One of the wards was on it. The guy at that time saw a huge amount of snow jumping the top here”—he motioned toward the ridgeline above us—“and falling on the other side.” As the snow poured over the ridge, the warden could hear tree trunks snapping like matchsticks. “He really thought he was going to die,” Huguenin said. The experiment, which destroyed much of the forest, didn’t go over well with the locals.


Huguenin and I continued walking. To our left, a Soviet-looking bunker poked out of the hill. It was two stories tall; in the 1999 experiment, it had been covered by thirteen feet of snow. To reach the observers buried inside, a crew had to cut a vertical tunnel with a chainsaw. Near the bunker, an array of continuous-wave radar antennas, designed to measure the flow at the avalanche’s core, craned toward the peak. Huguenin pointed to “obstacles” on the slope—pressure and velocity sensors mounted on concrete-and-steel structures. Against the mountainside, the largest obstacle, a sixty-foot-tall pylon studded with flow-measurement devices, looked like a toothpick.


Avalanche country is like bear country. The threat hardly ever comes, but it defines the place, and lends it its grandeur. Outside the bunker, the mountains rose around us; flat clouds gathered in a distant valley like steam. We had lunch: bread, cheese, chocolate. The snow was warming in the sun. Scooping it up, I found that, instead of seeping through my fingers, it now formed a perfect snowball—metamorphism within a matter of hours. I thought of how plants observed in time lapse seem to move with animal purpose. I imagined the crystals in this newly fallen snow sintering and crackling with life.

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From where we were sitting, we could see the glide avalanche from two days earlier. It was hard to get a sense of scale. Huguenin handed me his binoculars. Through them, I saw chest-high boulders of snow. Without them, the avalanche was a scratch on the mountainside.


One is unlikely to encounter an avalanche on the bomb-cleared trails of a ski resort like Alta. Avalanche accidents happen far more often in the backcountry, where skiers search for what the First Nations author Richard Wagamese called “the great white sanctity of winter.” In a recent survey, more than half of backcountry skiers said they had triggered an avalanche; a quarter said they’d got caught in one. It’s telling that the standard kit separating them from resort vacationers consists of a beacon, a probe, and a shovel.


I grew up skiing at small mountains in the Laurentians, just north of Montreal. Well groomed and popular, they were often scraped to ice. It was only a few years ago that I went with a friend to a large ski resort in Colorado. One day, we travelled to a remote part of the mountain. There had been fresh snow that morning, and I whooped as I dropped in, not another soul in sight. The snow felt like a cloud underfoot; falling evoked the childhood joy of jumping in leaves. Carving slow curves, I recognized the feeling of discovery: I was writing my name on the mountain. I also understood, for the first time, how powder and silence lure skiers into the backcountry.


To some extent, backcountry skiers can rely on avalanche forecasts. At the Utah Avalanche Center (motto: “Keeping You on Top”), forecasters make daily field observations (“+” means fresh snow; “.” round grains; “?,” depth hoar), integrating them into uncannily specific recommendations: “It remains possible to trigger a wind slab avalanche. . . . This snow will feel upside down and stiff.” Different kinds of terrain are assigned levels of danger, on a one-to-five scale; colorful diagrams with cartoon icons show which parts of the mountain—above the treeline, say, or southern aspects—are to be avoided.


Some experts worry that such diagrams give skiers a false sense of security. My sixty-seven-year-old godfather, Richard, happens to be the most experienced backcountry adventurer I know; a snowboarder for decades, he has logged more than a hundred thousand vertical metres in the past two years, in Kashmir, Antarctica, and other places. In the backcountry, he relies not just on forecasts but also on guides, to whom he attributes extraordinary diagnostic powers. Before taking a group out, a guide might dig a small column out of the slope. He’ll examine the layers, sussing out weakness, assessing the look of the crystal grains. Then he’ll tap the top of the column with his hand ten times, bending from the wrist. If the column survives, he’ll do it again, bending from the elbow; finally, he’ll do it from the shoulder. His interest is in when the column collapses, and how. Once, on a slope that seemed risky, a guide told Richard’s group that, whatever they did, they must follow, one by one, to the right of his line. Each skier followed in turn, carefully staying to his right. As Richard descended, a layer of snow unsettled beneath him, a few feet to the left of the guide’s tracks, and sent a wave across the bowl. The slope fell like a sheet.


One way to avoid avalanches is to ski shallower slopes. Slopes of around twenty-five degrees are perfectly enjoyable; steeper ones are only marginally more fun. And yet it’s hard for skiers to hold back. “The tricky part is controlling our lust,” a forecast reads. After a student of his died in an avalanche, Jordy Hendrikx, a professor at Montana State University, shifted his focus from geophysical research to behavioral science. (“Understanding how a crystal grows is not enough to change the current fatality profile,” he told me.) In one long-running study, he had a large group of backcountry skiers log their activity with a G.P.S.-enabled app. He found that experts chose steeper terrain, as did all-male groups, especially younger ones. (“Quantifying the obvious,” he has said.) When Tremper published his book, in 2008, he reported that, although a third of those who used the backcountry in Utah were women, women accounted for only 3.3 per cent of fatal accidents.


In the early two-thousands, when no amount of snow science seemed to be improving outcomes, the study of “human factors” that contributed to avalanche accidents became popular. Tremper lists six common “heuristic traps” that lead to avalanche fatalities: doing what is familiar; being committed to a goal, identity, or belief; following an “expert”; showing off when others are watching; competing for fresh powder; and seeking to be accepted by a group. The Swiss pocket guide for backcountry skiers is full of technical information about slabs and slope angles, but it also includes the advice “Don’t give in to temptation!”


New pilots are said to be most accident-prone right after their hundred- and-fiftieth hour; that’s when self-confidence peaks. Dave Richards, the Alta avalanche director, told me that, for many skiers, danger is highest right after the completion of an avalanche-avoidance course. The backcountry is what behavioral scientists call a “wicked” environment for learning: it gives you no negative feedback until it kills you.


A database maintained by the Colorado Avalanche Information Center contains aviation-style tick-tock accounts of avalanche fatalities. In January, 2019, a group of skiers taking a backcountry avalanche course went out with their instructor for a day in the field. The skiers followed a methodical, rigorous plan. At predetermined waypoints, the group assessed the conditions; they dug a snow pit, testing a snow column for strength. Their plan for the day included slope angles for all the terrain they might encounter. But they didn’t measure the steepness in the field themselves, and one particular slope that they believed to be no more than twenty-nine degrees was actually thirty-two degrees. As the second of six skiers proceeded downward, the other four, waiting above, sidestepped in order to see his progress more clearly. The slope avalanched twice—the first one remote-triggered the second—and the second skier was buried.


Two skiers turned their beacons to Search, monitoring their screens. They assembled their tent-pole-like probes, jamming them into the ground until they struck the buried skier. It took more than twenty-five minutes to shovel the victim out. The report, which identifies “a Persistent Slab avalanche problem,” is longer than most, at pains to explain why this group—so well informed and meticulous—could still be caught.

兩名滑雪者將他們的信標轉向搜索模式,監測他們的屏幕。他們組裝了他們的帳篷桿一樣的探測器,把它們塞進地面,直到它們擊中被埋的滑雪者。他們花了超過二十五分鐘才將受害者鏟出來。這份報告指出了 “持續的板狀雪崩問題”,它比大多數報告都要長,不厭其煩地解釋為什么這群人——消息如此靈通、如此一絲不茍——仍然會撞上雪崩。

On my first night at Alta, I stayed at one of the lodges. Since the road had closed, the cheap dorms filled up, four to a room. One man, Bill, forty-five years old, took a bottom bunk. A week earlier, he’d been in an avalanche—small, he said, and soft-slab. I asked him what it was like. “Manageable, and managed,” he said. He’d realized that the slope had the potential to slide, but he knew what to do if that happened, so he skied it anyway. “I did a couple tomahawks,” he said—tumbling end over end for three hundred feet, then standing up. Was he shaken? He thought about it. Actually, he said, he was serene. “Manageable, and managed,” he repeated, from his bed.


Toward the end of my time in Switzerland, I spent the day with Stefan Margreth, S.L.F.’s chief civil engineer. Easygoing, he wore a pink-and-red winter hat. At the institute, Margreth is the spiritual descendant of Johann Coaz: he carries Switzerland’s avalanche-hazard maps in his head. Margreth sometimes uses ramms to model avalanche risk. “It’s a great honor that he even uses the program,” Bartelt, its creator, said.


Many Swiss towns have building restrictions based on avalanche-hazard maps. “Everyone in the Swiss mountains knows their red zones and blue zones,” Margreth told me. We drove to St. Ant?nien, a tiny farming village an hour outside Davos. The threat of avalanche there is so great that, in storms, residents wear beacons while tending their farms. Margreth helped design or approve nearly every avalanche-mitigation measure in town: a huge concrete wedge on the upslope side of the elementary school; vast lines of steel girders high in the starting zones; houses built into the sides of hills, so that snow slides right over them.


After the winter of 1951, a party from the federal government in Bern travelled to St. Ant?nien to discuss the question of resettlement. The townspeople wanted to stay. “The Swiss mentality is to let people live in the mountains,” Margreth said. Taxpayers spent millions of dollars on mitigation measures; roads running up the mountain had to be built just to transport construction equipment. I asked Margreth why people had moved to St. Ant?nien in the first place. “The good places had been taken,” he said, smiling. In Switzerland, even the mountains are crowded.


A few years back, Margreth was contacted by the emergency-programs manager and avalanche forecaster for the city of Juneau, Alaska. Several neighborhoods were in the runout zones of slide paths; it was probably the most significant avalanche problem in the United States. Could anything be done? Even if tens of millions of dollars were spent on mitigation, the houses could not be completely protected; their destruction was more or less inevitable. Margreth suggested that the city buy the owners out and keep people from building new homes. So far, this has proved politically impossible; the city of Juneau, which had already bought a few empty lots in the area, has invested in warning systems and road-protection protocols.


“Sometimes you need accidents,” Margreth said. Atwater, in his book, suggests that “people need a good scare not less than every three years. Otherwise they begin to think that avalanche hazard is a figment of someone’s imagination.”


They can seem absurd to us, these people living at the base of steep hills. Don’t they know they’re idling in the face of disaster? The feeling was in the air in Switzerland, though not because of avalanches. As we walked on the road toward the edge of town, we saw diners enjoying themselves at sidewalk tables. “It’s much too warm for a February day,” Margreth said, in the winter sun. It had been three years since the team at the test site performed an experiment. Not enough snow had fallen.


原創翻譯:龍騰網 轉載請注明出處