This article originally appeared at the Gaia GPS blog and is republished here with permission. – Ed.
Beyond the boundaries of ski resorts, untouched powder and the promise of adventure beckon skiers, snowboarders, snowmobilers, mountaineers, and hikers into the backcountry. However, traveling in the backcountry is not without risk—25-30 people die in avalanches each year in the United States alone. While avalanches are undoubtedly hazardous, they tend not to occur randomly. Backcountry travelers can largely avoid avalanches with careful route planning and terrain management.
Traveling in avalanche terrain is an inherently dangerous activity. You should not travel in avalanche terrain until you and your partners undergo avalanche safety training and are competent in the use of avalanche safety gear (beacon, shovel, probe). Maps serve as a supplement to avalanche safety training, not a replacement.
This article covers how to identify avalanche terrain by observing: Aspect and Elevation; Topography; Terrain Traps; and Avalanche Paths.
A Short Primer on Avalanche Terrain
If you aren’t familiar with identifying avalanche terrain, the Avalanche Canada online terrain identification tutorial is a great place to start. Our article gives you some examples of how you can identify and analyze avalanche terrain on a map. While a map is an important planning tool, keep in mind that it can be hard to completely understand terrain solely from a 2D map. You can supplement your understanding of a route with photographs from guidebooks or online sources like PowderProject and with 3D tools like Google Earth. And, most importantly, continually evaluate terrain as you travel, as subtle details may not be completely conveyed by the map.
Aspect and Elevation
Avalanche hazard often varies by aspect and elevation. Many avalanche forecast centers use an avalanche danger rose to forecast the distribution of the different avalanche problems at different aspects and elevations. The danger rose, in conjunction with your map, is a good place to start when planning a tour. Use the forecast to help you pick a safe objective for the day, rather than choosing a location first.
Here is an example from the Utah Avalanche Center forecast. Below 9500 feet, the avalanche hazard is “low,” a subjective categorization meaning human-triggered and natural avalanches are unlikely. Keep in mind that conditions can change rapidly, avalanche forecasts are imperfect, and accidents and fatalities have occurred on “low” hazard days. Elevation and aspect are only one part of mitigating avalanche risk.
Aspect can give you important clues about danger from wind loading and sun exposure. If the winds have been coming from the west, you may be more concerned about wind slabs on easterly aspects. On sunny days, solar radiation can contribute to wet loose avalanches on solar aspects. And, particularly in continental and intermountain snow climates, persistent slabs may be present on all aspects.
When planning your tour, note the elevation on the map by reading the contour intervals and compare it with the danger rose. Or, better yet, create a route and look at the elevation statistics.
When avalanche expert Bruce Tremper introduces terrain management in his must-read book, Staying Alive in Avalanche Terrain, he leads with a quote from Canadian avalanche specialist Karl Klassen. Klassen says, “The snowpack is a capricious and erratic acquaintance who you never get to know very well. The terrain is a steady and predictable friend that you can always depend on.”
Caveats riddle avalanche textbooks and avalanche safety classes. Snow science has fewer hard rules than backcountry skiers would like, and discussions of slope angle are no different. Avalanches are dynamic, conditions dependent, and can occur on a wide variety of slopes. These inconsistencies demonstrate why terrain management should always be used in conjunction with snowpack evaluations, weather observations, and avalanche forecasts. That said, while by no means a foolproof strategy, backcountry travelers who want to avoid avalanches significantly minimize risk by sticking to slopes under 30°.
Slope Angle and Avalanche Correlation: Because slope steepness directly correlates to avalanche activity, savvy backcountry travelers pay close attention to slope angles. Over 90% of avalanches start on slopes between 30° and 45° degrees. A 30° slope compares to a blue run at a ski area, while 45° slopes are found on double black diamond runs. In other words, prime skiing terrain is also prime avalanche terrain. One key detail is that most avalanches start on slopes between 30° and 45° degrees. Avalanches can be triggered remotely and run into lower angle terrain, so it is important to have a thorough understanding of runout zones.
Slope Angle Maps: Gaia GPS’ Slope Angle layer can help you identify avalanche terrain. Slopes are color-coded with their approximate steepness, with orange and red denoting terrain between 30 and 45 degrees.
The Inaccuracy of Slope Angle Maps: Keep in mind, slope shading is only “a depiction of reality.” The map will differ from the actual terrain. Avalanche start zones like convexities or small pockets of steepness can escape detection. The error can be as high as 4 degrees. The difference between 26 and 30 degrees, or 30 and 34 degrees, can be a significant increase in risk exposure – check out this distribution chart of avalanches by start zone slope angle.
In January 2019, a tragic avalanche accident occurred during an avalanche training class in Colorado. One contributing factor was that the slopes the group traveled on were a few degrees steeper than what the slope-shading estimated. To understand more about how nuanced—and impactful—slope-angle readings can be, read the full accident report. For an even deeper discussion, see Jeff Deems’ presentation about how these map overlays are made from the 2019 Colorado Snow and Avalanche Workshop or the article “Digital Mapping: Do You Know What Your Map Knows?” in September 2020 issue of The Avalanche Review.
Additional Tools for Measuring Slope Angle: Slope angle maps are best used for a big picture overview of terrain. To assure that you avoid 30°+ slopes, use a map in conjunction with a hands-on tool, like a slope meter, for a more accurate slope assessment. That said, measuring by hand isn’t infallible either—to get an accurate slope reading with an inclinometer, you may have to expose yourself to the start zone or the slide path.
Understanding topography helps differentiate more dangerous, complex terrain from simpler, safer terrain. Identifying start zones, ridges, and gullies can help you choose safe travel routes. Keep in mind that ridges are generally safer places to travel.
Terrain traps magnify the consequences of being caught in an avalanche. Being washed over a cliff or into a stand of trees increases the chance of being injured or killed due to trauma—in addition to the risk of being buried under the snow. Trauma causes up to 30% of avalanche fatalities. Gullies and abrupt transitions to flat zones cause the snow to pile up higher, meaning a victim could be buried more deeply, decreasing the chance of a successful rescue.
Carefully scout your intended line for terrain traps with topo and satellite layers. Cliffs may be visible on satellite layers, and will show up on topo maps as contour lines stacked very close together.
Use satellite layers to identify known avalanches paths that you may want avoid on your tour. While avalanches can happen in a wide variety of terrain, satellite maps often reveal signs of obvious avalanche paths including open slopes, sparse trees, and funnel-shaped terrain.
Utilize the Satellite Topo layer to help identify ridges and drainages, which are sometimes confusing on satellite images.
After identifying an avalanche path, it’s important to have a clear understanding of avalanche runout angles. While start zones are generally over 30°, the momentum of a slide may carry avalanche debris down gentle slopes, across flat expanses, and even up inclines, so be sure to give yourself adequate distance from the path. Keep in mind that in very unstable conditions, avalanche paths might exceed the normal runout—taking out old trees or buildings and redefining the path.
Supplement your analysis of avalanche paths with data and maps from avalanchemapping.org, which provides an atlas of common avalanche paths in popular backcountry skiing areas.
Key points to keep in mind before you venture into avalanche terrain:
• Get avalanche safety gear. Practice sufficiently with your beacon, shovel, and probe before you head into the field.
• Get proper training. Using maps is a supplement to education, not a replacement. Visit avalanche.org to find an avalanche education course. Start with the free avalanche awareness and education series “Know Before You Go.”
• Read the avalanche forecast. From Colorado to Utah, Montana to Alaska, avalanche centers provide detailed local forecasts that outline aspects of concern and relevant avalanche problems. When you are online or within cell range, you can use the Avalanche Forecast layer to access the detailed local forecast. Pull up the map to see what the danger rating is for the area you plan to ski that day. From the Avalanche Forecast map, click on the shaded area to be able to access a link that will take you directly to the local avalanche center’s website for more information. Be sure to read the entire forecast from the avalanche center from the link provided on the map.
• Supplement the weather forecast with Snow Forecast overlays to see what snow is expected to roll in during your ski tour. Check the Snow Stations (Daily) map before your trip to get a daily report on the amount of new snow that has fallen in the last 24 hours and what the average water density reading is for the new snow in the area you plan to visit.
• Always travel with a trustworthy partner. Beacon, shovel, and probe are useless if you don’t travel with a partner who knows how to use them.
• Plan routes at home. Planning routes at home is always a smart idea: you can use the desktop version of Gaia GPS, create routes or import tracks from a friend, and print out maps just in case your phone dies when you’re in the field. You can also plot decision points, make backup plans, and determine exit strategies. For more on backcountry route planning, click here.
Top photo: Nikolay Hristov/Unsplash