Avalanches: The Big Slide

Watching snow fall is a backcountry joy--unless it's barreling 80 mph down a mountain and you're in its path.

High above us, a block of snow the size of a compact car tumbles down Mt. Washington, slowly at first, then gathering speed and dislodging more chunks of snow as it moves down the mountain. We stare in awe for a few long seconds, and then someone yells, “We’d better move!”

Like a flock of spooked mountain sheep, we scatter out of the avalanche’s path to safer ground. Maybe we should have been better prepared, since avalanches are not uncommon in this area known as the Gulf of Slides. But we’re also in New Hampshire, and that illustrates an important lesson: Avalanches aren’t just high-mountain, Western phenomena. From early winter until late spring, they pose a serious risk to hikers, skiers, and snowshoers everywhere from the high mountains of the Sierra to the rolling Appalachians to a local riverbank with a steep slope. In fact, avalanches catch more than 100 people in this country every year and claim more victims than hurricanes or earthquakes. And part of the reason is that you don’t need to go deep into the backcountry to find yourself in avalanche terrain. The Gulf of Slides, for instance, is just a few miles from the Appalachian Trail’s popular Pinkham Notch trailhead.

While slides can be deadly, fear of falling snow shouldn’t keep you out of the mountains in winter. By knowing how, where, and when avalanches occur, you can avoid the dangerous areas.

Avalanche Anatomy

Layers of snow tell the story of the season’s storms and the density and consistency of each snowfall. When these layers fail to bond, one layer of snowpack slides atop another. A person crossing above or below a snow-loaded slope can cause a layer to break and trigger a slide–an avalanche.

The steepness of the terrain is also a factor-avalanches need slopes of 25 to 55 degrees. Anything less rarely slides, and anything steeper doesn’t accumulate enough snow. Slopes of less than 30 degrees–the steepness of most ski resort black-diamond trails–rarely carry the powerful snowslides, while 38-degree slopes, such as those found on a mountain, gully, bowl, or cirque, are the most common avenues for avalanches.

To estimate slope angle and keep yourself in safe terrain, use a clinometer or compass that is equipped to measure slope angle. To assess a slope’s angle before you even leave for the trail, use a ruler to measure the distance between topo lines on 7.5-minute maps. In places where two or more contour lines fall within 1U16 inch, the slope is 33 degrees or more and in the avalanche danger zone.

Forecasting A Slide

Forewarning. Before heading into snow country, check the avalanche risk for your destination. A regional forecast center is a good place to start. Most mountainous areas have a hotline for current conditions, but you still need to use your head when you’re out there. Frequently consult a detailed topo map (such as a USGS 7.5-minute quad) so you don’t travel downhill from potential avalanche slopes that may be hidden by clouds or terrain.

Wind. Watch for sudden changes in the wind. Winds of 15 mph or greater can transport, or “wind load,” snow to the edges of slopes and ridges, creating potentially dangerous slabs. Wind-loaded ridgelines often drift into recognizable cornices, which are overhanging features that resemble breaking waves. Avoid traversing on or beneath cornices. The windward side of a slope usually has more compacted snow and is a safer option.

Weather. During storms, the fresh snow hasn’t had a chance to settle and bond with the old snow beneath it. Wait at least a day after a storm to travel in avalanche-prone territory. If snow starts falling heavily while you’re in the mountains (especially at a rate of an inch or more per hour), or the wind starts blowing hard, get to safer ground.

Aspect. Use your compass to determine the direction, or aspect, of the slope. Southerly inclines get more sun, which in winter means that the snow melts and refreezes into a more compact block, making it less likely to slide. In spring, however, the warming sun increases the chances of a loose-snow avalanche.

Northerly slopes compact more slowly, making them less stable in midwinter and prone to slab slides. However, these slopes tend to be more stable in late winter and spring.

The avalanche hazard can vary tremendously among different aspects on the same day; never assume that because one aspect is safe, others are, too.

Slide Smarts

If you wander onto an unstable slope and hear a heart-stopping “whump” on a nearby slope that’s steeper than 25 to 30 degrees, or see cracks in the snow, get to safe terrain immediately. Either backtrack or follow the least-angled terrain available. If this unsettling noise occurs while you’re hiking a trail that’s not on a slope, take it as a warning to stay off tilted terrain.

Recognize and understand the snow-covered terrain and you can avoid a slipup in avalanche-prone areas.

  • Traverse above a slope, such as along a broad, open ridgeline, or below a slope in a valley. Bear in mind if you choose the latter that a slide may fill up the valley if an avalanche is triggered on the hillside above you.
  • Avoid traveling directly up- or downhill of another person. Ninety-five percent of avalanche victims trigger the slide that catches them, so stay clear of others’ paths.
  • Cross gullies and other avalanche-prone terrain one person at a time, and watch the hikers crossing ahead of and behind you in case they trigger a slide. The first person to cross isn’t always the one to trigger an avalanche.
  • Unbuckle your pack’s hipbelt and sternum strap when in questionable areas. Then you can ditch the pack quickly and use your arms to “swim” through the snow if an avalanche occurs.
  • Keep 100 to 200 feet of distance between hikers in avalanche-prone country, and never rope up. If a slide occurs, fewer people will be swept up in it. Resist the temptation to group together for conversation or comfort during stormy periods.
  • Carry a compact snow shovel, probe (or probe ski poles), and an avalanche transceiver-and have an expert teach you how to use them (see our Gear pages, for a review of our favorite transceiver). An ice axe and crampons generally aren’t necessary in the areas where avalanches are likely to form.

If you do get caught in a slide:

  • Try to get to the side of the avalanche before it accelerates, then grab onto a tree or rock outcrop. Or, swim atop the snow. When you feel the slide slowing down, punch your arm upward so others can find you. Not sure which way is up? Drool a bit; gravity will pull it down your face.
  • Clear an air pocket in front of your face if you’re buried. Don’t wait for the snow to stop sliding, because it will set like concrete almost immediately.
  • See Snow Smarts, for more tips on safe travel across four seasons of snow. Order a reprint by calling (610) 967-8296, or read the article at www.backpacker.com/article/0,2646,1851,00.html. Resources Check these sources for more on avalanche safety. To order the books, go to www.backpacker.com/bookstoreSnow Sense: A Guide to Evaluating Snow Avalanche Hazard, by Jill A. Fredston and Douglas S. Fesler (Alaska Mountain Safety Center, 907-345-3566; $8.95).The ABC of Avalanche Safety, by E. R. LaChapelle (The Mountaineers, 800-553-4453; $6.95).Winning the Avalanche Game (videotape, Utah Avalanche Center, 801-524-5304; www.avalanche.org~uac; $29.95). Westwide Avalanche Network, www.avalanche.org. Information about avalanches and safety courses, and links to regional forecasting centers across the country are available here. Cyberspace Snow and Avalanche Center, www.csac.org. This site provides avalanche conditions for 12 countries, plus photos, firsthand accounts, a Rutschblock Test reference card, lists of classes, and more. In the Rocky Mountains, try the American Avalanche Institute, (307) 733-3315; www.avalanchecourse.com. In the Northeast, check out Chauvin Guides International, (603) 356-8919; www.chauvinguides.com. Mountain Savvy offers numerous courses on Mt. Hood and other peaks, (503) 780-9300; www.mountainsavvy.com.

When you buy something using the retail links in our stories, we may earn a small commission. We do not accept money for editorial gear reviews. Read more about our policy.