Slide Rules

An avalanche buried their friend -- and a Denver trio is still digging out.

Many metaphors have been used to describe how snow becomes an avalanche. Bruce Tremper, a Utah forecaster, tells people to imagine a dictionary resting on potato chips. Others liken it to snow sitting on a pitched metal roof. The mechanics are the same: A heavy layer of snow lying on top of a steep, slippery surface will slide. In snow science, the slippery part of the equation is referred to as "stability"; the art of forecasting avalanches lies in calculating the slipperiness of the roof.

Once triggered, most avalanches are pretty much the same: Snow, usually (but not always) in the shape of a book-like slab, slides down a slope. That said, different regions of the country have different avalanche risks. On the West Coast of North America, higher temperatures and proximity to the ocean mean that snow is heavy and frequent. During and immediately after heavy snowfalls, avalanches are common as the snow comes to terms with gravity. Yet it doesn't take long for the wet snow to settle and stabilize -- maybe a few hours or a day.

By contrast, Colorado's snow is drier -- the so-called champagne powder -- and less frequent. In a typical year, the mountains will see a heavy snow in November, and then a long period of dry weather. Over time, that initial layer of snow loses stability, turning into dry, sand-like crystals. The relative infrequency of storms, combined with the region's dryness, means that the snow never really gets a good chance to settle.

 
Craig LaRotonda
 
 
Craig LaRotonda
 

As a result, while Colorado sees fewer avalanches during and immediately after a storm than the West Coast does, the danger of avalanches here exists days or even weeks after the snow stops falling. Over the past couple of years, this pattern has been intensified by the state's drought. "Drier years have a weaker, more dangerous snowpack," says Atkins. He likes to say that Colorado's avalanche season "ends around August 31 and starts about the first week of September." But most of the state's snowslides occur between November and April.

Even with perfect avalanche conditions, a trigger is required to set the snow sliding. In ski areas and on high mountain roads, this is done artificially, to control when and where the avalanche occurs. Wind and other natural phenomena can also set off a snowslide.

Yet studies of avalanche accidents have consistently shown that, in nine out of ten deadly avalanches, it was a human that set off the snow. In other words, the odds are overwhelming that if you find yourself "caught" in an avalanche, you -- or one of your party -- started it.


After nearly four decades of working in and around avalanches, Tom Kimbrough is ready to retire. "I figure I'm not dead, so I'd better get out soon," he says. An employee of the U.S. Forest Service, Kimbrough has been caught in avalanches and pulled countless people, both alive and dead, out of snowy tombs.

Now with the Utah Avalanche Forecast Center, he says he has been impressed with the advances in avalanche rescue tools. Take beacons, the transmitters that have become standard equipment in backcountry sports. Worn by responsible skiers and snowmobilers, a beacon emits a magnetic signal that rescuers, using their own beacons, can pick up and use to locate a buried person.

Despite their promise, beacons take a fair amount of training and finesse to use, although a new generation of machines makes pinpointing a buried signal much easier. But even the new beacons aren't foolproof, and recent research shows that the majority of people persist in underestimating the practice required to become proficient in using a rescue beacon. In field studies, Atkins, of the Colorado Avalanche Information Center, says that ski patrollers, professional guides and others who perform search-and-rescue operations for a living can use a signal to locate a buried body in an average of seventeen minutes.

By contrast, the average recreational beacon user takes an average of 35 minutes to find the same body. Subtleties -- such as the fact that, depending on how the rescuer is holding his beacon, he can actually lose a signal the closer he gets to a body -- continue to confuse the occasional user. "Most people who have transmitters," says Atkins, "can't use them quickly enough."

Worse, rescuers don't tell you that even the time it takes a professional to dig out a buried party is not particularly encouraging. For years, searchers had operated under the assumption that if a person had been buried for fifteen minutes or less, his chances of survival were excellent. After that, it was thought that asphyxiation occurred slowly over the next fifteen minutes, with mortality rates shadowing the gentle curve.

A decade ago, however, a team of Swiss scientists looked at avalanche victims in Switzerland between 1981 and 1991 and found that assumption to be wrong. Although the researchers confirmed that a person uncovered in fifteen minutes or less had an excellent chance of survival -- about 92 percent -- his likelihood of living past that time did not decrease gradually at all. Instead, it plummeted dramatically. The study concluded that by the time a victim had been buried for 35 minutes, he would have less than a one in three chance of being revived. (Interestingly, once he has made it that long, the odds of living stay about the same through ninety minutes.)

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