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.
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.