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Avalanche ABCs

It's that time of year to bone up on snow stability and snow safety issues. Skiing has started for the 2010 and 2011 season and while the snow pack is still shallow, such conditions won't be lasting. In the next few days, major snow deposition is forecasted. Then a few days later, temperatures will rise dramatically. We may see relatively high hazard here Central Washington, first as the new snow piles up on the early-season snowpack, then perhaps again as temperatures rise and bonds within the snow weaken.

Being avalanche aware and avalanche savvy is important to life whether you're a backcountry skier, snowshoer, or mountainee. So whether you're a new to these sports or an old hand with a leaky memory (like most of us), read the avalanche rules below before you head out to enjoy this winter's snow.

First, to better understand some of the rules that follow, here are some terms to understand. Snow cover or the snowpack refers to all the snow ground with its different layers. The start zone or release zone is the area on a steeper slope where an avalanche actually stars; the path or track is the route the avalanche takes flowing downhill, and the deposition zone or runout zone is where the avalanche piles up at the bottom. Little, loose snow slides that run no more than 150 feet are called sluffs and massive slides where an entire slope releases its entire snowpack is called a climax avalanche. Surface avalanches involve upper layers of the snowpack sliding over lower layers below and full-depth avalanches slide all the way to the ground. Loose snow avalanches are cohesion-less slides that start at a point and then flow downhill picking up larger quantities of snow, while slab avalanches involve upper layer(s) of snow that have cohesion and that slide as a plate on a clearly identified gliding surface or bed surface and which are often aided by slippery snow crystals or water that forms a lubricating layer. The upper place where an avalanche broke free is called the fracture line or crown surface and the lines confining the sides of slab avalanche are the flank surfaces. 

1) Triggers. To get started, avalanches need the trigger of added weight or general disturbance of the snowpack. Skiers add weight and slice the snowpack so they trigger avalanches both ways. Other triggers include: new snow or rain adding weight to the snowpack, cornices falling on a slope, snow falling off cliffs onto slopes, snow falling off trees onto slopes, temperature change affecting the bonds between snow flakes, wind transporting more snow onto a slope, bombs thrown onto slopes…

2) Precipitation. A simple rule of thumb: 80 to 90 percent of wintertime avalanches occur during or within 24 hours of significant storms--whether the precipitation comes as snow or rain. 'Significant' is an imprecise term but storms depositing an inch or more of water equivalency (e.g., 1 inch of rain, 10 inches of 10% snow, 20 inches of 5% snow) to the snowpack in under 24 hours should be treated with great concern. Measurable rain on top of snow is very hazardous because the upper layers of snow will be heavier than the supporting layers... and because water may percolate down to a crust and lubricate a sliding surface. Meanwhile, snow accumulating at the rate of an inch an hour or more for over 6 hours becomes quite unstable. Such snows haven't had time to settle or stabilize so the triggers of more snow or human disturbance can easily make the schibang slide on steeper slopes.

3) Temperatures.
There are a number of variables related to temperature that can make slopes perilous or safe. Storm cycles where the temperature rises during the storm create less stable conditions because heavier, more-cohesive snow sits on top of lighter less-cohesive snow. Cycles where the temperature drops during a storm are usually much more stable because the heavier, denser snow creates a good base for the lighter, less-cohesive snow. Also, temperature rises during the day often destabilize bonds in the snowpack and/or lubricate snows sitting on top of a crust. In spring the southfacing snows getting lots of sun can quickly become unstable as they soften into a sludge. Cold nightime temperatures, especially in spring, allow the snowpack to set up and stabilize.

4) Read the terrain. Snow is dynamic and unpredictable -- even experts are rarely completely certain about how the snow on a given slope will behave. Terrain on the other hand is quite static and, if you learn how to read it properly, you can get out skiing (even when there’s considerable hazard) and protect yourself against the uncertainties of the snow through intelligent use of the terrain. There are a number of terrain features you need to learn how to read like identifying the pitch where avalanches are likely, recognizing the path an avalanche will take, recognizing how the terrain may channel avalanches or shed them. You must also understand how a slope’s aspect (the direction it faces), its substructure (grassy or rocky), and its degree of forestation contributes to its safety or hazard. Through good route selection and good terrain selection you stay above, below, or to the side of avalanche-prone slopes.  

5) Pitch or steepness. The large majority of human-triggered slides release on a portion of a slope where the pitch is between 30 and 45 degrees. The majority of these avalanches, meanwhile, release on slopes between 35 and 40 degrees in steepness. Slides have been known to release on slopes as gentle as 15 degrees but this is very rare. In the Cascades, if the slide hazard is moderate or considerable, you can usually ski slopes whose maximum angle is under 30 degrees in relative safety. We say ‘relative’ because there are few absolutes about avalanches. This rule is more suspect in continental snowpacks (like the Rockies) whose wintertime snowpack often has layers temperature-gradient snow that will slide at lower angles.  Tip: It takes a lot of experience to properly estimate slope angle. Curt Haire recommends making a simple, cheap clinometer by attaching a weighted string to a cheap, D-shaped plastic protractor. Position the straight part of D-shaped protractor parallel to the slope’s surface and the weighted string becomes a plumb line that will read the angle.

6) Avoid the release zone. The release zone is the most dangerous part of a slope and is often at or near the steepest part of the slope. Most people caught in avalanches disturb the release zones of an easily recognized slide path on either the ascent or the descent. However, when conditions are quite unstable, you don’t need to be at the release zone to start an avalanche. By disturbing bonds in the snowpack below the release zone, you may weaken the entire slope and allow the snow in the release zone to spring free. Consequently, when there’s considerable or high avalanche hazard, knowing what’s above you is important – are you close enough to a release zone to destabilize it? If so, will you be in the path of the descending snow?  

7) Groundcover. Ground cover and the ground structure tell a lot about where slides are likely and how large they may be. Good anchors on a slope, like a talus field of big boulders, can help stabilize and anchor lower parts of the snowpack. This greatly reduces the likelihood of full-depth avalanches and usually means your analysis of the snow can focus on the upper layers of the snow. Grass and heather don’t provide good anchorage and can contribute to deeper, more catastrophic slides. Likewise well established forests tell you a lot about where slides are more or less likely. Slide paths where the trees are mowed down are far more worrisome than old growth forests. This is not to say you can be completely relaxed in the trees--surface slides (both loose snow and slabs avalances) can start and run through forests and these can be exceedingly dangerous if they wrap you around a tree. Nonetheless areas with heavy tree coverage have far less wind-affected snow and slab formation than open slopes, temperatures are warmer and radiation is reduces so hoar frost conditions are reduced, and the trees themselves help anchor the snow. Furthermore in the Cascades, temperatures frequently approach freezing so the snow coating the trees after a storm frequently warms up, falls off, and thumps down on the snowpack. This compacts, settles, and stabilizes the forest snowpack.

8) Observing slide activity. One of the best indicators of avalanche hazard is freshly fallen avalanches (observed as a fracture line where the avalanche started and/or as rubble at the bottom of the slide). Note the aspect (i.e., the direction the slope faces) and elevation. Think about the weather and wind conditions that contributed to these slides. Avalanches that you actually see or that recently released are among the best evidence that similar slopes could behave the same.  

Photo by Kaj Bune

9) Pit analysis. Snow pits (shovel tests, hasty pits, etc) are such a tiny sampling of a variable medium and slope inconsistencies that professionals now consider a pit analysis to be highly unreliable. Pits add information to your decision making but don’t put too much stock in the information from one or two pits when it comes to risking life and limb. Links to videos at the end of this article show you how to conduct two different tests of the snowpack.

10) Winter versus spring skiing. Winter snow is complicated because it is composed of many different layers with different crystals, different densities, and different temperatures. Once the warmer, longer days of spring arrive, heat and water attack the snow pack. The snow crystals throughout the snowpack start rounding off and temperatures throughout the snowpack start to equalize. With these warm temperatures, the snowpack transforms into an extremely dense, mass that, on steep slopes, will form a wet, loose avalanche if triggered. This dense snow is extremely powerful and dangerous. However, if nighttime temperatures dip below freezing, the top few inches of the snowpack will freeze and the snow will be bombproof the next morning…until daytime temperatures soften the surface again. As the top of the snowpack melts, the entire snowpack becomes mushy and conditions can quickly move from being quite safe to quite dangerous. Once into a spring cycle the avalanche hazard plummets with freezing temperatures and rises with warm temperatures. This makes the hazard considerably more predictable in spring. In spring, southern slopes are usually safer early in the day and northern slopes will remain firm (and safe) longer into the day.

11) Aspect.  Because sun and wind work differently on slopes facing different directions, a slopes aspect directly affects its hazard on any given day. Southern slopes get more sun and are warmer so there is less faceting, less hoar frost accumulation, and more rounded crystals in the snowpack. These slopes also face the prevailing winds accompanying storms so they don’t accumulate as much snow. Northern slopes, meanwhile, are colder, have more faceting taking place in the snowpack, and will accumulate more snow during storms because they face away from the prevailing winds. Over the course of the winter, there will be more days when northern slopes present a higher hazard than the southern slopes. On any given day, however, you can only bank on the fact that each aspect will be different. There will be days when the southern slopes are safe and the northern slopes are not. There will be days when it’s the other way around. And this doesn’t even include east-facing and west-facing slopes into the mix. It gets complicated …which is, once again, why learning to master terrain is much more practical than learning to master snow. 

12) Slope shape and snow support. Does the shape of the slope increase or decrease the stress on the snow? Yup. Convex slopes stress the snow at the roll-over point and this is an obvious release zone to be avoided in sketchy conditions. Concave slopes support the snow better. A slope above a cliff band has less support than a slope that flattens out gradually into a meadow.

13) Dealing with dangerous slopes. If a tour necessitates crossing slopes that could slide, you’ve hopefully waited for safe conditions. Even conservative backcountry skiers, however, occasionally find themselves contending with slopes whose safety is suspect. At such times go out of your way to find the safest place to cross. For example, climbing higher and cutting the very top of a slide path keeps you above tons of snow that may get set in motion. Or dropping elevation and crossing a slope below the release zone, while presenting some danger, is safer than disturbing the release zone. If you must get up a steep slope you’re worried about, walking straight up the slope in a place delivering the least exposure creates fewer disturbances to the snow than cutting many switchbacks upward. If descending slopes you’re nervous about, have each skier stay in the tracks of the lead skier so that there’s less disturbance of the slope. And in all of these cases, keep spread out and ski one-at-a-time between pockets of safety. Only expose one person to the possibility of getting caught in a slide. 

14) Map study. Studying topographic maps at home and drawing a line on the map affording safe travel below, above, or beside dangerous slopes is key to navigating avalanche terrain. Maps give you the big-picture view of danger zones as well as the safe zones (valley floors, ridges, forests, benches).The navigation skills to follow the line you establish on a map is also critical. Recognizing the steepness of a slope from the spacing of the contour lines is another important skill in recognizing slopes you can ski as well as those you should avoid. Tip: A standard USGS 7.5-minute topographic map uses a 1 to 24,000 scale where each quarter-inch represents 500 horizontal feet. Given the standard contour interval (40 feet), here’s the approximate slope angle associated with the number of contour lines per quarter inch: 26 degrees (6 lines), 30 degrees (7 lines), 33 degrees (8 lines), 36 degrees (9 lines); 39 degrees (10 lines), 42 degrees (11 lines), 44 degrees (12 lines), 46 degrees (13 lines), 48 degrees (14 lines), 50 degrees (15 lines), 52 degrees (16 lines), 54 degrees (17 lines), 56 degrees (18 lines). The largest number of human triggered slab avalanches occur on slopes whose steepness is between 30 degrees and 45 degrees and these will show up on 7.5-minute topo maps as slopes with  7 to 13 contour lines per quarter inch.

15) Group Dynamics. It’s important before leaving home that all members of a group to be on the same page about what the objectives of the day are. Each trip may have a main objective but there should be fallback options if conditions don’t warrant completing the main objective. These different objectives should be known and agreed upon by all. This reduces the likelihood of a group blindly following one course of action, or of the group splintering chaotically as different agendas are followed, or of one person (the strongest personality) hijacking the agenda of the day. The reports of avalanche accidents frequently reveal that group dynamics contributed to the poor decision-making that resulted in an accident. Keeping your group close enough together so that the head of the group knows what’s going on at the tail of the group is also very important for safe travel through avalanche terrain. Finally, make a conscious effort to migrate toward skiers with similar attitudes about risk. Whether you’re aggressive or cautious, find others who approach the sport the same way so that the group forms harmonious decisions rather than decisions that fracture the group into subgroups or force some group members onto terrain they don’t want to ski. This article, by local skier Rob Mullins, gives perspective to the human factors. 

16 Keep learning. These rules are just the starting point. Learn more by getting out with experienced skiers willing to teach you, taking avalanche courses, and reading more. We've got lots of additional articles at this website -- type 'avalanche' into our 'Quick Search' box and you'll find lots of good resources. Good reference books on the topic include: The ABC of Avalanche Safety (Mountaineers Books); Staying Alive in Avalanche Terrain (Tremper, Mountaineers Books). Backcountry Avalanche Awareness (Jamieson). Backcountry Skiing: Skills for Ski Touring and Ski Mountaineering (Mountaineers Books) is also extremely useful for avalanche basics and for its practical tips for staying out of trouble by managing the terrain effectively. Also, click through the screens, scenarios, and quizzes at the Avalanche Weather Forecast website as well -- it's an excellent resource.

17) Videos. Some of these avalanche videos will also increase your knowledge...and your apprehension of avalanches. Keep things in perspective as you view this stuff -- too much exposure to the grim side of avalanches may induce avalanche paralysis, but respect for (and fear of) avalanches doesn't mean you need to become a couch potato.

  • Shovel compression test video: Tom Janisch described this technique in our session but this 5-minute video shows how it’s done in the field. The video is good but interpretation of the results could be better.
  • Rutschblock test video: This is another test Tom described that evaluates snow stability in the field. This 3.5 minute video does a good job of showing you how to perform and rate the test, but could do a better job of interpreting the results.
  • A Dozen More Turns: This 31-minute video tells a sad but important story in the effort to help winter backcountry enthusiasts re-calibrate their decision-making processes. The video intertwines some educational information to further the viewer’s knowledge of avalanches. More importantly the story is a visceral one that will help most people dial back the urge to overextend their knowledge or their luck.
  • Xavier Delerue Avalanche. A good 3-minute video with amazing footage of a massive slide engulfing a pro-snowboarder and his support team that ignored warning the signs. A happy ending through a combination of skill, preparedness, and luck. The boarder caught in this slide used an ABS (airbag system) backpack.
  • ABS Backpacks. This 5-minute video covers the avalanche air bag system (ABS) that originated in Europe and is more widely used there. The system uses pressurized nitrogen to nearly instantly inflate airbags that balloon out of the pack and that keep victims afloat in an avalanche where they can often rescue themselves or can be quickly found. Skip the first minute of the video and ignore some of its hubris. More about this system.
  • Stay Alive. A ten-minute video focusing on the backcountry Bridger Bowl, Montana that has good information in the middle. The first 2.5 minutes and the last 1.5 minutes can be skipped. The footage of a slope breaking apart where others have skied should be seen. Wow.
  • Keep your guard up video: A several-minute video with general advice about never being complacent in avalanche terrain. Some good avalanche footage
  • Helmet Cam Video. This 8-minute video as a skier gets engulfed in the slide he caused and then waits helplessly as he whimpers while trying to breathe is very sobering. It gives you a taste of place you never want to visit.