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Climb Safe

Climb Safe: Avalanche Safety

“There is no avalanche danger unless a human is there,” said Dick Jackson of Aspen Expeditions at the start of a Level 1 avalanche course.

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A typical mountain landscape showing slab (left) and point-release (right) avalanches. Prevailing winds have also created a monster cornice. In your summit-or-bust quest, how would you plot a safe course?

“There is no avalanche danger unless a human is there,” said Dick Jackson of Aspen Expeditions at the start of a Level 1 avalanche course. His words reminded me of Erwin Schroedinger’s famous thought experiment, where a cat is placed in a box containing radioactive material. The experiment proves that an event does not happen until you observe it (the cat is simultaneously alive and dead until the box is opened).

Avalanche awareness is just as confusing and unknowable a science as quantum physics, which explains why research indicates that climbers know so little about identifying dangerous terrain.

Last year, the American Institute for Avalanche Research and Education (AIARE) recorded 140 avalanche-related deaths worldwide (32 were in North America). Experts speculate that most of these deaths could have been avoided had the victims been more aware of the clues of instability, such as shooting cracks in the snow, recent avalanche activity on similar slopes, recent wind-loading, and hollow or “whoomphing” sounds. Also, recent research points to “human factors”—how behavior and personality affect decision-making—as a principal cause in most avalanche accidents. Climbers, who rank third behind snowmobilers and skiers in avalanche deaths, often rely on ignorant bravado and fuzzy logic to assess risks.

Avalanche awareness is just as confusing and unknowable a science as quantum physics, which explains why research indicates that climbers know so little about identifying dangerous terrain.

Though myriad books on avalanches exist, you must venture slopeward to gain the hands-on experience necessary to recognize and avoid  avalanche terrain. Amos Whiting, who like Dick Jackson is an IFMGA/AMGA guide, spends every day tromping through the world’s worst snowpack, in Colorado’s Elk Range, and is able to see the whole picture better than most of us. For example, on Green Mountain, our Elk Range classroom, I expressed concern that we were about to ski terrain that had failed relatively easily during a Rutschblock test, the standard test for snow stability. Whiting noted that we were skiing down a slope that we had already zigzagged up. If it was up to me, I would not have skied because of a “textbook” warning, yet Whiting called upon his experience to overrule the book.

No hard and fast formulas could ever describe complex backcountry reality, but certain rules exist to make snow travel safer. Primarily, avoid avalanche
terrain altogether; or recognize it, make logical risk assessments and choose safe paths. After all, as long as we’re educated and open to learning new things, we shouldn’t be afraid to venture into high and wild wintry worlds.


Dial a phone or log onto the Internet and you can access current weather and avalanche conditions for every major mountain range on earth. A huge amount of resources is poured into collecting this data. The Cyberspace Snow and Avalanche Center ( is a good place to start. It provides links to current avalanche conditions worldwide, and has accident reports (read them) and other educational resources, including quizzes.

An avalanche report will provide you with current weather and wind conditions, as well as a forecast. A snowpack analysis often follows (e.g. “Warm temperatures are helping stabilize recent snowstorms … ”). These variables are then distilled to a danger or instability rating using the standard scale: Low, Moderate, Considerable, High and Extreme. “Considerable” danger is the most common rating and means a condition where human-triggered avalanches are probable.

After collecting a weather forecast and danger rating, plan your route by mapping out the safest path. This process could include choosing a different
feature (following a ridge instead of a gully, or perhaps deviating to avoid traversing below a dangerous slope), changing departure time (when will the sun hit your route?), locating bailout options if conditions get hairy, etc. For example, if your route is east facing, and the report notes strong westerly winds, expect to find a wind-loaded, slide-prone layer of snow on your route. Select an alternative route or stay home.


Snow stays on a slope when the forces holding it together exceed the force of gravity pulling it down. When gravity wins, you have an avalanche. Often,
these forces are precariously balanced: It can take little more than the weight of one person to cause an entire mountain to slide. Understanding the dynamics of how snow slides is essential to avoiding terrain and features that could be trigger points.

“Loose-snow” and “slab” are two types of avalanches. A loose-snow (or point-release) avalanche originates from a single point and spreads out, fan-like, down the slope (the right-side avalanche in Figure 1). These avalanches usually occur in dry, loose snow, or wet, slushy snow, and on steep terrain (between 30 and 55 degrees). In general, loose-snow avalanches are relatively benign, though they can bowl over a climber in a steep gully. These avalanches often occur on bluebird winter days, when the sun weakens the snow at a particular point. For example, dark protruding features such as trees, rocks or cliff bands absorb the sun’s radiation and melt surrounding snow, weakening its bonds. By traveling next to a tree or beneath a cliff, a climber could start a loose-snow avalanche, taking the ground out from under his feet.

Slab avalanches are the big, climber-eating ones (the left-side avalanche in the cover image). They occur when a strong, cohesive area of snow slides as a unit because the bond between this layer and the snow beneath it fails, usually on a slope between 30 and 45 degrees. Slab avalanches occur in all weather conditions, can involve a small area or an entire face and are more difficult to predict than loose-snow avalanches.


Four variables determine avalanche danger:

1. Terrain. (The slope must be steep enough to slide.)

2. Snowpack. (Must be unstable enough to slide.)

3. Weather. (Increases or decreases stability.)

4. You, the climber. (Where and how you travel affects the probability of a slide.)

<strong>Figure 2.</strong> Using ice axes to measure slope angle. when both spikes touch the snow as shown, the slope is 45 degrees.
Figure 1. Using ice axes to measure slope angle. when both spikes touch the snow as shown, the slope is 45 degrees.

Terrain is your friend because it remains constant, unlike snowpack and weather, which are always in flux. Choosing safe terrain is key to the decision-making process. First ask yourself, “Am I on, below or above terrain that is steep enough to slide?” If the answer is yes, begin assessing risk by choosing the safest time of day (or year) to be there, and determine how long it will take for you to travel through dangerous terrain.

Slope angle is the most important component of terrain. Slab slides can occur on any incline between 25 and 55 degrees, but most frequently run at 38 degrees. With a trained eye, you can usually guess the angle within a few degrees. To measure angle, place two ice axes or poles of equal length at a right angle to one another, making sure one axe is completely vertical. Now touch the spikes to the snow slope. If both ends just touch, then the slope is
45 degrees (Figure 2). Also, remember that while you could be walking on flat terrain, you might be below a slope that could slide—avalanches have been known to run across flat ground for hundreds or thousands of feet. Analyze maps and route beta beforehand in case you won’t be able to see
what’s lurking above you while on the route.

Always aim for safe terrain: high ground such as ridges, slopes less than 20 degrees or greater than 50 to 60 degrees (which sluff often, but are too steep to form a slab) and paths through large, dense trees. Avoid wind-loaded (leeward) slopes and terrain traps, i.e., features such as cliffs that naturally increase the danger if you are swept down.

“Classic” avalanche paths are fairly easy to observe. They include wide-open slopes cutting through forests, couloirs and gullies. Usually, however, avalanche paths are less defined. For clues, look for “flagged” trees (where the branches are missing on the uphill side) and old avalanche debris (clumpy snow at the base of a steep slope).


Convex slopes are normally found where a ridge rolls over into a steep face and, by nature, stresses the snow’s bonds (imagine snapping a stick over your knee). One wrong step on the right weakness could shoot a fracture line across the entire slope. A gut-dropping WHOOOMPH! signals a release of air as the snow’s cohesive bonds break. If you’re still standing, it’s time to get to safe ground.

Concave slopes are often found at the bottom of a steep slope and, like convex slopes, are under high stress because they support the weight of the slab above. Avoid them.


Gullies and couloirs are drainages, but are attractive in that they usually offer the easiest routes up. It’s hard to know what conditions to expect in
a gully, and once you enter its narrow confines, it is usually too late to do anything. Traveling up on the side of the gully tends to be safer than tromping back and forth across it or heading straight up the center. Wait until you are certain conditions are stable before climbing an ice-filled gully below a snowfield or cornice.

Cornices are wind-sculpted snow formations that point in the direction the wind predominantly blows. A cornice above you indicates that you are on a leeward, wind-loaded slope, which is dangerous simply because of increased snow deposit. Blue skies do not mean that it hasn’t been “snowing,” as wind could have dumped a ton of fresh powder. Cornices are easy to spot from a distance, and choosing to travel on the windward sides is safer. Cornices also tend to break off, which could trigger an avalanche. Traveling on top of a corniced ridge, while safer from avalanches, presents its own dangers. Rope up in such places, follow in each other’s steps and be prepared if one of you breaks through the cornice.


Like your laundry, the snowpack accumulates over the course of a season: Layer upon layer of snow forms after every new weather and wind incident. One rule of thumb is that the snowpack doesn’t like rapid change. Rapid loading of snow (in Colorado, defined as one inch or more per hour for at least 10 hours, but it’s different everywhere) and rapid melting contribute to dangerous snow instability.

Watch for a strong layer on top of a weak layer. Slab avalanches tend to run when a strong, cohesive layer (which can be hard or soft) sits on top of a layer of weak, often sugary snow. Picture a cement block resting on top of wine glasses: The glasses can support the cement block at an angle of 20
degrees, but as the angle increases to 45 degrees, the glasses fail and the block slides. If you have to slog through weak snow, i.e., snow that can’t
support your weight, there may be no slab-avalanche danger because the snow is uniform—it’s all weak, so no slab exists. On the other hand, homogeneously weak snow tends to form loose-snow avalanches. Avoid loose-snow trigger points during these conditions.

In an attempt at science, my friends Adam Roy and Stephan Drake lived in a hut on the back side of Aspen Mountain a few winters ago, where they religiously boot-packed an entire backcountry bowl for the season’s duration, reasoning that they could eliminate underlying weak layers. One ripping spring day, Adam and Stephan were about to drop into their laboratory when the whole bowl slid, proving once again that mountains do whatever they want.

By digging a pit about three feet deep, you can analyze the different layers of snow. Cutting a credit card or compass down through the snowpack and feeling for changes in resistance indicates different layers. Analyze them: Is there a layer of strong slab on top of weak snow? How many layers can you feel? Do they seem bonded? Poking the different layers of the pit with your fingers is another method of detecting strong and weak layers: How easily can you plunge one, two or four fingers? Climbers don’t normally carry shovels, or climb with skis, but an experienced mountaineer can plunge a pole through the snow and feel the different layers: This test can be performed hundreds of times during a climb.

Snow that is close to the ground (which is relatively warm) tends to undergo a process called faceting, which produces large, angular, loose  “sugary” crystals, indicating a weak layer. This unstable snowpack is Colorado’s status quo.


Precipitation, wind, temperature and sun play a direct role in determining snow stability.

Rain causes rapid instability both by weakening the snow’s cohesiveness and lubricating its layers. Extended periods of heavy and dense snow will also raise the avalanche danger.

Wind raises avalanche danger on leeward slopes when it is strong and constant enough to move snow. Windward slopes tend to provide safer terrain since they contain less snow. Windblown snow creates a “wind crust,” which tends to be dense, cohesive and easy to travel on. In general, wind crust is safer, while snow that is deposited by wind—a “wind slab”—is dangerous. A wind crust is rippled and scoured, while a wind slab tends to be smooth, hard and crusty. When you break through a firm layer of snow evenly, you are probably on a wind slab. When the surface breaks unevenly, you are likely on wind crust.

Cold snow is stable snow, but when the temperature is at or above freezing, with some meltwater running, the snow changes and deforms rapidly. Beware of any rapid changes to the temperature gradient within the snowpack.


Most avalanche research is currently focused on the role of human behavior in accidents. One disheartening study concludes that humans are terrible at
assessing risks. The “blue-sky syndrome,” for example, incites a belief that everything is fine, though danger could be high. When weather, darkness
or fatigue suddenly appear, hasty decision-making becomes a factor consider building a snow cave in a safe area to wait for more stable conditions instead of rushing toward disaster. One study indicated peer pressure is another human factor; when there are a few males and only one female, the group tends to suggest taking higher risks.

Though we didn’t know it at the time, we were traveling on the convex part of a heavily deposited wind slab after a big storm.

One of the biggest human factors is ignorance. Around Easter a few years ago, three friends and myself tried to climb the Minarets, a New Zealand peak. We attempted the ridge that was our route some four times, failing because we were too slow, too inexperienced, too lost and too tired. The route required that we drop off the rock ridge onto a massive snow slope, and then traverse around crevasses toward the summit. We laughed at the ineptitude to which the knee-deep snow on this slope reduced us. That night it stormed, covering our tracks. The next day, we were surprised to find the snow as hard as rock, which allowed us to race almost to the summit, having to turn back because we were still too slow, too inexperienced and too tired.

Though we didn’t know it at the time, we were traveling on the convex part of a heavily deposited wind slab after a big storm. The sun was hot that day,
and on the retreat, we sank into melted-out pockets of waist-deep stuff. It was probably as dangerous as it gets, though somehow, we four idiots made
it back safely.

Your and your partners’ attitudes toward achieving goals and taking risks are the strongest contributors to avalanche accidents. Before entering avalanche terrain, you should have an idea of how experienced the team is and how prepared you are to turn back or deal with a disaster. Erring on the conservative side, learning and being humble give you a good shot at having a long, fun career up high.


Websites is a great website with easy links to every forecast center in the country. You can find statistics, up-to-date forecasts and accident reports and education resources.

The Cyber Space Avalanche Center ( is another great resource with cool quizzes to test yourself and an online store with educational books and videos.


The American Avalanche Association is a prestigious organization that puts
out The Avalanche Review.

American Mountain Guides Association will hook you up with courses from certified guides in your area.


Avalanche Safety For Skiers and Climbers, by Tony Daffern, and The ABCs of Avalanche Safety, by Sue Ferguson and Edward LaChapelle, are two comprehensive easy reads that you can get from The Mountaineers Books.