Avalanche Awareness
by John Moynier
(new 12/28/98)

[Site Map] [Bookstore] [Home Page] [Rappel to the Bottom]

This article, by John Moynier, discusses the basic principles of avalanche hazard evaluation--terrain, weather, snowpack and human factors.

John Moynier is an American Mountain Guides Association (AMGA)-certified Ski Mountaineering guide living Bishop, CA. He provides a daily avalanche forecast for the eastern Sierra at www.csac.org/Bulletins/ and teaches avalanche awareness programs for the Bardini Foundation. John is the author of Sierra Classics, Backcountry Skiing in the High Sierra and Avalanche Aware. These excellent titles by John Moynier are included in the Backcountry Bookstore.

Although we don’t always like to admit it, much of what we consider backcountry skiing and snowboarding is basically a dangerous game of "Avalanche Roulette". Every time we head into the snowy hills, we roll the dice and place ourselves at risk. As a result, everyone skiing in the backcountry needs to recognize the risks. Although some areas may be consistently more hazardous than others, there will likely be serious avalanche hazard at some point wherever steep slopes and snow are found together. This doesn't mean that there will always be risk, but in order to become confident in our ability to determine hazard we need to educate ourselves, practice our techniques and use the proper equipment.

In order to travel responsibly, you need to learn all you can about avalanche hazard by reading and taking field avalanche courses which focus on hazard evaluation and self- rescue techniques. It is the responsibility of each member of a group to evaluate their own risk when traveling through this terrain and communicate their concerns to the rest of the group. Avalanche awareness is developed through education, as well as practical experience and observation. Without a well-developed sense of hazard awareness, you are playing "avalanche roulette" every time you head into the winter backcountry. Sooner or later the odds are going to stack up against you.

[Climb to the Top] [Rappel to the Bottom]

Hazard Evaluation

Most backcountry skiers and snowboarders will eventually find themselves exposed to avalanche hazard at some time. That is because, by definition, prime backcountry ski terrain is often considered prime avalanche terrain, and whether we know it or not, we frequently bet our lives against the snow-loaded house every time we venture into the world of the "steep and deep". Skiing or snowboarding in the backcountry without adequately assessing the potential hazards is akin to crossing a busy street without looking for traffic. It’s much better to make a conscious and informed decision to accept risk, rather than to foolishly court danger wearing a blindfold. Learn to recognize avalanche terrain and gather all the clues you can about the weather and the snowpack so you can factor these into your assessment of risk.

Avalanche hazard evaluation boils down to four components: terrain, weather, snowpack and the human judgment. Away from the controlled slopes of the ski area, we use an assessment of the first three factors to guide our own actions. Avalanches are a natural event, avalanche hazard only occurs when we place ourselves in harm’s way. Unfortunately, that’s what many of us do, time and time again. Really, the only thing we can do about terrain, weather or the snowpack is to decide when and where we’re going. That’s where the "informed decision" process becomes so important. What we do with our evaluation can make the difference between a fun day out with friends and an unnecessary tragedy.

[Climb to the Top] [Rappel to the Bottom]

Terrain Considerations

Slope--When planning a trip or picking a route, remember that slope angles of 30 to 50 degrees (advanced to expert alpine runs at a ski area) are the most likely to slide, especially on convex slopes that are under more internal tension. Avalanche statistics show that slopes of 38 degrees are prime avalanche terrain. Note that even low-angled slopes can be threatened by steeper slopes above. Steeper slopes generally slough off new snow before they build up significant accumulations, but really wet snow can stick to very steep slopes, especially when deposited by strong winds.

Aspect--Slope aspect is a prime factor in determining avalanche hazard. Sunny aspects are most likely to slide right after a storm as a result of rapid warming, but are also the quickest to stabilize due to the warmer temperatures. Shady north slopes are less likely to be affected by the sun, but are also more likely to harbor weak layers of poorly bonded crystals over long periods of cold, dry weather, especially early in the season. Orientation to wind is even more critical. Windward slopes are likely to be scoured during storms, becoming more stable and sporting a noticeable crust layer. On leeward slopes the wind often deposits thick, cohesive layers of snow known as "slabs", which pose the single greatest avalanche concern for unwary backcountry travelers.

Cornices--The wind can also form extremely large and potentially dangerous cornices on ridge tops. These features can be very unstable and may threaten more moderate slopes below. When skiing along a ridge, always stay further back from the edge than appears necessary as cornices may bridge undiscernible and inobvious gaps in the crest. The soft, pillowy area just below a cornice is often a slab, formed by the eddying effect of the wind. When a cornice collapses, it can trigger an avalanche of the unstable slab below.

Routefinding--Once in the backcountry, it's important to pick a route that avoids primary avalanche terrain if at all possible, as conditions may change from day to day or even hour to hour. When in doubt, pick a route that follows a ridge top or wide valley bottom if possible. Look for signs of previous avalanche activity such as vegetation cues, old fracture lines and debris piles and avoid them. Stay clear of likely slide paths and run out zones if at all possible. Densely timbered slopes are generally safer than open slopes, but they don't necessarily mean safety. If you can comfortably ski through the trees, an avalanche can definitely slide through the area.

[Climb to the Top] [Rappel to the Bottom]

Weather Considerations

Local weather--Once you learn to recognize avalanche terrain, the next step is to recognize whether it will stay put while you’re there. Any significant changes in the local weather should be noted and evaluated, as these can have a dramatic impact on the stability of a slope. Events of special concern include rain or warm winds, as well as dramatic increases in temperature or precipitation intensity. Temperatures well above or below freezing can also lead to decreased stability of the snowpack, as can any sudden change in temperature, as these factors affect the relative density of surface layers, as well as the bonding of the crystals within a given layer of the snowpack.

Precipitation--It's important to note that weather events are key to developing avalanche hazard. Most slides occur either during or immediately after a storm or wind-loading event. New deposition of an inch an hour or 12" in 24 hours should be warning signs. Double this and you’ve got serious problems in steep terrain. Note that even moderate winds can increase the hazard by transporting snow from one area to another, often depositing dangerous slabs on lee slopes in a short period of time.

Wind--Wind is a very effective transporter of snow, as well as warming agent. Even at wind speeds of ten miles per hour, snow crystals can be redistributed. Higher wind speeds can cause dramatic erosion of snow on windward slopes and deposition on leeward slopes. Local down-valley or up-slope winds can also load snow in unexpected places. Winds with high relative humidity can cause riming of snow crystals, resulting in slick sliding surfaces or deposits of a very cohesive layer of snow.

[Climb to the Top] [Rappel to the Bottom]

Snowpack Considerations

Physical evidence--The most important clue to snowpack stability is signs of recent avalanches on slopes of similar aspect and slope angle. Fresh fractures, blocks of debris and large sunballs are very important clues that snow conditions are unstable and must not be ignored. Widespread settling of the snow from tree branches or exposed cliffs should also be taken as a sign of rapid settlement and potential instability. Cracks forming in the snow, "whoomphing" sounds or hollow feeling snow underfoot can indicate potentially dangerous slab conditions overlying weaker layers.

Layers--The snowpack itself is also of primary interest. Layers of snow within the pack can act cohesively as slabs. Poor bonding between layers is a common cause of avalanches, as the overlying layers cut loose in shear failure on the contact surface. Density incongruities should also be noted. Denser layers overlying less dense layers are always a concern. Other potentially weak areas include buried ice crusts or thin weak layers of poorly bonded snow like buried surface hoar or old faceted crystals.

Faceted Crystals--The processes that lead to increased or decreased stability are also important. A deep snowpack and moderate temperatures will lead to a small temperature gradient (TG) through the pack. The physics of this leads to greater snowpack stability through a process that results in a net decrease in crystal size and a net increase in the bonding strength between the crystals. The crystals develop obvious facets or angular characteristics that offer poor bonds with other crystals. Shallow snowpacks and/or very cold temperatures can lead to significant temperature differences between the ground and the surface. "TG" conditions tend to "rot" the snow from the ground up towards the surface by destroying the bonds between the crystals as the crystals themselves grow. This can also happen higher in a snowpack, too, especially on cool clear nights.

Other Crystal Processes--Alternating very warm days and cold nights can lead to a very strong snowpack, bonded by the melt-freeze or "MF" process. As the snowpack cools at night, free water in the snowpack re-freezes to glue the crystals together. All of the above processes might occur in the same snowpack. You can check on the snow conditions below the surface by digging a pit in the snow or probing with your ski pole. If you are still unsure about a slope, you should perform a shovel shear or Rutschblock ski test to determine stability.

[Climb to the Top] [Rappel to the Bottom]

Human Judgment--Safe Travel Considerations

Decisions--Judgment, knowledge and experience become your guide when traveling in the backcountry. You should form an opinion about stability before you even leave the trailhead and be prepared to change your opinion as conditions change. Gather all the information you can before you go by calling the National Weather Service or accessing their website on the Internet. Get a local avalanche forecast if one is available or call someone in the area to see if they can give you any local observations. You can also check out the local forecast on the Internet at www.csac.org/Bulletins/.

Observation--Keep your eyes and mind open as you travel, nature generally provides obvious clues to stability if you know how and where to look. Listen to the snow, feel it with your poles and skis, and constantly evaluate each step or turn.

Preparation--Be prepared to augment or even completely overhaul your assessment along the way. Make sure everyone in your group carries a beacon, shovel and probes and knows how to use them. Finally, don’t be too stubborn to leave the slope for another day. If you constantly play roulette with Mother Nature some day you’re going to lose.

Communication--Always make sure everyone understands and approves of the groups' objectives and that alternate routes have been taken into account. Listen to your intuition. If your gut tells you things are amiss the time to act is now. Don't let the idea that "I don't want to spoil it for the group" lead to tragedy. Stand up for your opinion and don't worry about being a "party pooper". It is much better to err on the safe side and come back another day than to have to dig a buddy out of pile of debris. Communicate your plans constantly, and make sure everyone understands.

Equipment--Avalanche transceivers or "beacons" are the best insurance policy you have if you are completely buried in a slide. If your partners follow good protocol and have practiced search techniques, then they should be able to find you within a short period of time. That doesn’t mean that you will still be alive, however, but at least you have a much better chance of surviving than without a beacon. In order to dig through heavy avalanche debris, each person will also need a sturdy shovel. A set of probe poles will also make their efforts more efficient as they can pinpoint your location without wasting effort digging a foot or two to the side. Remember time is critical.

Safe travel--If you must travel in a hazardous area, limit your exposure. Cross one at a time with beacons on transmit and all eyes on the person crossing. Take advantage of safer areas like dense timber, rock outcrops, ridges and wide valley bottoms. If you must ascend or descend a dangerous slope, stay close to the edge and chose as vertical a line as possible. Kicking steps straight up or down is much safer than cutting the slope with traverses, turns or sitzmarks. Don't assume a slope is safe just because someone else has skied it and remember that traveling on a lower-angled slope can trigger steeper slopes above or sympathetically release a slope some distance away when things are really hazardous.

Caution--Finally, the best overall strategy is to use caution and be prepared. As noted above, make sure everyone has a functioning avalanche transceiver and knows how to use it. These should be turned onto transmit the start of the day, checked, and left on until safe in camp. Each person should also have a sturdy shovel and avalanche probe poles. The ski area maxim: "Be aware, ski with care" is the absolute law of the backcountry. Travel one at a time in suspect areas, and always pick the safest route possible unless you are absolutely sure of a slope and snowpack stability. Backcountry skiing can be a very safe and rewarding experience if everyone accepts these responsibilities.

 Backcountry_Resource_Center--Paul Richins, Jr.

[Site Map] [Bookstore] [Climb to the Top] [Home Page]