Avalanche basics

Most of us pay little attention to avalanches unless someone gets injured or killed in one. Be smart. Whether you go snowboarding, skiing, snowshoeing or climbing, learn all you can about avalanches so that you don't end up on the wrong side of a slide.

The sad fact is that most people caught in an avalanche trigger the slide themselves by traveling on or beneath unstable snow slopes. Typical avalanche victims are usually very skilled in their sport, but have little knowledge or skills for dealing with avalanches. REI’s 4–part series of articles provides some basic information you need to know.

The Potential Hazards

An avalanche can go as fast as 120 miles per hour, a force that can easily sweep you into trees and rocks, pull you over cliffs and tangle you in your ski or snowboard equipment. Nearly one–third of avalanche deaths are the result of trauma.

If you survive the trauma, you then have to deal with a mass of concrete–like snow encasing your body. What may start as powder snow heats up from the friction of sliding downhill, melts slightly and then refreezes solidly around you. The weight of this crud is often enough to force the air out of your lungs.

If you have been able to form a breathing space before the snow settles, you may have a decent chance of survival. That assumes all members of your party are wearing avalanche transceivers and know how to use them. From here, it's a race against time. Most people can't survive under snow much more than 30 minutes (the Black Diamond AvaLung can extend this time to about an hour), so becoming proficient with an avalanche transceiver, probe and shovel is a must for winter backcountry travel. About 70% of avalanche fatalities are due to suffocation.

The best defense, however, is to know how to read snow conditions and terrain and to avoid dangerous situations altogether.

Types of Avalanches

Loose–snow Avalanches

Loose–snow avalanches occur where there is little or no cohesion in the snowpack. They usually start at a point on or near the surface and gather more snow and momentum as they progress down the slope, often forming a triangular–shaped path. Snow clumps falling onto the slope from a cliff above can set off these slides, as can melting within the snowpack.

These avalanches can occur in dry or wet snow, in winter or summer. Winter loose–snow avalanches usually happen during or after snowstorms. In warmer months, wet loose–snow avalanches are usually the result of meltwater or rain. Either type can be dangerous.

Slab Avalanches

Even more dangerous are slab avalanches, in which a layer of snow breaks away from the layer beneath it and slides downhill. Most backcountry travelers are caught in these avalanches.

Slab avalanches are the result of many snowstorms or strong winds depositing layers of snow that change over time. Some layers settle and become stronger, while others grow weaker. Weak layers are frequently grainy or fluffy so that other layers can't bond to them.

An avalanche results when the top layer of snow, known as a slab, is not bonded well to the underlying layer and is disturbed by some kind of trigger, often a skier or climber. Rather than starting at a point on the surface, as loose–snow avalanches do, slab avalanches start out bigger and deeper, usually at a fracture line running across the top of the slide.

Contributing Factors to Avalanches

Terrain

Slope angle: Pay attention to slope angles when you're skiing or climbing, since avalanches occur most frequently on slopes between 30 and 45 degrees. A clinometer on a compass or one specifically made for snowpack study can be a useful tool when traveling in the backcountry.

Slope aspect: During winter, a south–facing slope is more stable than a north–facing one since it has sun exposure to melt and condense the snow. The tempting north–facing slopes that hold all the best powder are also more likely to have unstable layers of ’depth hoar,’ the dry, icy snow that does not stick to the adjacent layers. Since these slopes don't have the benefit of sun to warm and compact the snow over the winter, they tend to be less stable than south–facing slopes. By spring and early summer, however, a south–facing slope can mean more serious melting, resulting in dangerous wet–snow slides. The warmer weather in these seasons often allows the snow on north–facing slopes to consolidate, thus making them safer.

Terrain hazards: Snowpack is likely to be unstable on convex slopes, cliff bands, boulders or trees where the snowpack breaks, wind–loaded lee slopes or beneath cornices. It's best to avoid bowls, cirques and depressions where snow could settle after a slide. Steep, narrow couloirs (or gullies) tend to collect snow and present a trap to hikers or skiers caught in them, often without side escape routes if their sides are steep.

See Avalanches, Part 3: Reducing Risk for tips on what terrain is usually safest.

Weather

Precipitation: Snow is least stable during or immediately after a snowfall or rainstorm. Large amounts of snow falling in a short time are a likely sign that the snowpack could slide. A heavy snowfall of especially wet or dense snow over lighter powder snow can set up layers of instability. Rain tends to percolate down through the snowpack and warm the deeper snow. It also provides lubrication between layers, making them more likely to slide.

Wind: Wind is another indicator of snow instability. Often, high winds pick up surface snow on one slope and deposit it on the other side of the ridge (a so–called ’wind slab’) where it is prone to slides. Pay attention to the intensity and direction of the wind throughout the day.

Temperature: Temperature fluctuations cause a number of problems with snowpack stability. Temperature differences between the ground and new layers of snow, between different layers of snow, or even between the air and the top layer of snow, can cause changes in snow crystal formation. A common crystal type that is particularly dangerous due to its inability to bond with other snow crystals is know as ’hoar.’ Hoar snow, also called ’sugar snow’ because of its similarity to granulated sugar, can be found at any depth or at multiple depths in a deep snowpack. Another common temperature problem, especially in the spring, is rapid warming of a slope, which can lead to wet–snow avalanches.

Snowpack

Snowstorms pile up one after the other all winter long. Wind blows snow off of some slopes and on to others. Temperature changes cause snow crystals to metamorphose. If the snow’s consistency remains constant, the snowpack is homogenous and stable. It’s when the snowpack develops different layers of different snow types that it becomes unstable and hazardous. It is extremely important for the backcountry traveler (especially on slopes between 30 and 45 degrees) to take the time to analyze the layers of the snowpack and test their stability.

Human Factors

While terrain, weather and snowpack play a huge role in contributing to the likelihood of an avalanche, it is also important to recognize that ego, emotions and group–think can cloud your judgment and impair your decision–making ability. In fact, according to a recent survey of Canadian avalanche professionals, respondents identified ’human factors’ and ’choice of terrain’ as the leading causes of avalanche accidents.

Some common decision–making mistakes:

  • Familiar terrain: You may be more likely to take risks in terrain that you are familiar with. Conditions, however, can change minute–to–minute, so treat all terrain like it is the first time you have seen it.
  • Acceptance: Approval from the group can be a strong force. Pushing on because you do not want to go against the will of the group, even though things do not feel right, can lead to unnecessary risk.
  • Overcommitment: If you are overcommitted to your destination, you may block out your better judgment or ignore indications of danger by focusing only on achieving your goal. In mountaineering, this is known as ’summit fever.’
  • ’Expert halo’: This can occur when you perceive that someone else in the group has more experience than you, based on his or her having been in the area before or other backcountry education. While this may be true, it is better to ask than to just assume.
  • Existing tracks: Relying on an existing set of tracks in the snow can give a false sense of security. Just because someone else went through the area before you does not mean it is safe for you to travel through.
  • Powder fever: It is very tempting to ignore avalanche hazards when the snow is deep, fresh and untracked.
  • Social proof: It can be easy to succumb to the ’herding instinct’ and step onto a dangerous slope when other people have gone before you. Always assess the situation as if you were traveling alone. Speak up if things do not feel right.