Long throughout the history of mountain backcountry travel there has been a constant threat that has sat waiting, just underneath the cold white surface. Snow avalanches have killed thousands of people traveling and living in the mountains, devastating both lives and property. Take for instance the tragedy that occurred on March 1, 1910 near Steven’s Pass when an avalanche tore two Great Northern trains from their tracks, killing 96 people. With potential catastrophes looming in the future, winter backcountry travel in the Cascades demands a very real respect. A part of paying toll to that respect is to instill knowledge about how and where avalanches can occur. This way you are prepared to take action to both prevent an avalanche, and respond when the unfortunate does happen.

Types of Avalanches:

In general, there are two basic types of snow avalanches that can occur on any given mountain slope. First there is the less destructive “loose snow avalanche” or “powder avalanche”, which in most cases in just a nuisance. Though still dangerous, the powder avalanche usually only involves a few inches of the upper layer of snow on a slope. These types of avalanches are less common, especially on very windy slopes, as the snow has little or no cohesion with firm ground. Make no mistake though; a powder avalanche can easily grow in size as it slides down a mountainside, which could potentially create extremely deadly forces.

The second type of avalanche is the most common and has the longer list of casualties next to its name. This type of avalanche is know as a “Slab avalanche”, which forms when several layers (or slabs) of snow form above a very loose base layer of snow crystals. The shear weight of the upper layers causes the weaker base layer to lose its grip, which in turn makes all the layers above let loose. There is a well-defined break point at the top of a slope where a slab avalanche occurs, revealing the layering that caused its release. It is this layering that has been the focal point of most avalanche research, and understanding it is key to being safe.

Snowpack Layers:

As snow falls on a mountainside over a period of time distinct layers form, creating strong and weak binding surfaces within a snowpack. Heavy snow binds easily on a slope creating a very strong layer, which can also act as a triggering mechanism. If this heavy layer is above a very fine powdery layer the stability of the slope should be questioned. As more snow accumulates above this powder layer, it will eventually give and act like ball bearings, helping the slope gather momentum. Trees and rocks can help to protect against this effect, but as the snow grows deeper their holding power diminishes. Old snow surfaces that have been sun baked and refrozen can cause instability as cold, light snow falls on top. This crusted surface creates a platform for the layers above to slide down on. These crusty layers form in areas around particular portions of a mountain, which makes the snow fall an equally important factor, as is the terrain it falls on.

Properties of a Slope:

Several properties of a slope must be taken into consideration while evaluating the hazard of a particular area that maybe unstable. Studies have shown that slopes between 25 and 65 degrees are the most prone to release their snowpack. A slope with a declination of between 30 and 45 degrees that recently got a substantial amount of snow is definitely a major risk. Another contributing factor is the actual profile of the slope itself. A wide open slope that curves outward forming a bulge should be an obvious danger sign of an avalanche prone slope. Whereas, an inward curving slope is less likely to avalanche due to tensions that build as gravity pulls down on it.

The direction the slope is facing also plays a very important role as well. The sun is mostly absent the northern facing slopes during most of the year, which in turn increases the time needed to stabilize a slope. A southern facing slope will usually get a lot of sun in the early spring, causing destabilization due to melt off. This continuous cycle of freezing and thawing also creates layers of ice within the snowpack. These ice layers can act as a platform for a slide to travel on. It is these extreme changes in weather and temperature that cause the layering and instability to form.

Weather Conditions:

Rapid changes in weather conditions will cause changes in the consistency and subsequently the layers of a slope’s snowpack. For this reason, you must be constantly aware of what the weather is doing and what it may do. The winds are a key force that you need to pay attention to, as just 15 mph sustained winds can redistribute a large amount of loose surface snow. This wind-blown snow can accumulate as much as 10-times more than what may already be coming out of the sky. Travel along the leeward (wind protected side) side may be more pleasant, but the added accumulations that wind can produce drastically increase the instability of a slope.

Also keep in mind that the temperature also plays a pivotal part in how much time it can take for a slope to become stable. Colder temps tend to maintain an already unstable snowpack, keeping powdery snow soft and light for longer. On the other hand, warmer weather allows the snow to “settle”, but can cause upper layers to melt and destabilize seemingly stable slopes.

Storms are the major instigator of changes in snowpack up in the high mountain elevations. With storms, usually come significant amounts of new snow of a variety of type. Depths can vary considerably with differing slope elevations and directions, even over short distances. The winds of a storm can form large drifts in precarious positions, where a careless footstep could trigger a large avalanche.

Wet snow can also wreak havoc on an already weak and loaded slope by adding weight to any already burdened weak layers. As well, rainfall can very rapidly overload these buried, unstable layers, sometimes even releasing an avalanche almost instantaneously with the onslaught of rain. Rain may also work its way down through layers of the snowpack until it reaches a barrier, such as a layer of ice. It will then act as a lubricant, which has the potential to produce a large wet-slab avalanche. Possibly even causing multiple slides on the same slope as the rain percolates progressively deeper into lower snow layers.

Things to Watch For:

So, by now it should be rather obvious that it is important to keep an eye out for changes in weather conditions. But, what are some other things to watch for? Well to begin with, old slide paths can point out an area that has an apparent history of avalanching. Also, debris located at the base of a slope indicates dangerous conditions exist – especially on other slopes with similar aspects. Look for broken tree limbs, or torn off trunks, and large blocks of snow in piles along slopes that are suspect. Most importantly though, be sure to stay updated on local weather and avalanche forecasts so that you are not caught by surprise.

Knowing what routes to take in order to avoid avalanches is a large part of being able to safely travel through areas that are prone to slide. The safest routes are normally on ridge tops just slightly on the windward side to avoid punching through any cornices. If travel along the ridge top were not possible, then the next safest route would be at the valley floor, far away from the base of the slope. An avalanche could easily sweep you into a crevasse or off a cliff below you, should you choose to travel too closely to the base of a suspect slope. With all this knowledge though, sometimes crossing a slope that has shown several signs of sliding cannot be prevented.

Crossing Safely:

If crossing an area that has any possibility of sliding cannot be avoided, listen to the sound your boots make while walking into the area. If the snow sounds hollow, particularly on a leeward slope, you can be sure conditions are right for an avalanche. Cross the potentially lethal slope by staying high and near the top, and cross one person at a time. This way in the event that a slide occurs someone can hopefully observe where you land. Ascend and descend in a straight line, as traversing or cutting switchbacks up a slope can trigger an avalanche. Remove ski pole straps, ski safety straps, and loosen all equipment so that they can be quickly discarded. Take advantage of any areas that offer cover, such as dense timber, ridges, or rocky outcrops, and use them as an “island of safety”. Most importantly, travel fast; the more time you spend exposed on an open, avalanche prone slope, the less time you have to live.

Avalanche Hazard Scale:

• Low Avalanche Hazard: mostly stable snow exists, and avalanches are unlikely except in isolated pockets on steep, snow-covered, open slopes and gullies. Backcountry travel is generally safe.
• Moderate Avalanche Hazard: areas of unstable snow exist, and avalanches are possible on steep snow-covered, open slopes and gullies. Backcountry travelers should use caution.
• High Avalanche Hazard: mostly unstable snow exists, and avalanches are likely on steep snow-covered slopes and gullies. Backcountry travel is not advised.
• Extreme Avalanche Hazard: Widespread areas of unstable snow exist, and avalanches are certain on steep, snow-covered, open slopes and gullies. Large and destructive avalanches are possible. Backcountry travel should be avoided.

Getting Caught:

Actually being caught in an avalanche is an experience many people do not survive, so prevention should be the foremost thing on your mind. In the case that you have followed all the guidelines, but still are caught in a slide, hopefully someone is watching as it overtakes you. Discard all of your equipment immediately and making swimming motions to try and keep yourself on top of the slide. As it begins to slow, get your hands in front of your face and try to make an air space in the snow. If you know you are near the surface try to reach a hand or foot up out of the snow. This will increase you chances of being found once the slide comes to a stop.

If it happens that you are the one watching a victim being swept away, mark the location where you last saw them. You should begin searching down slope from this point, using a probe if there are no obvious signs of the victim on the surface. Wearing an avalanche beacon can considerably increase you chances of being found if caught, but will only help if the others in you party use them as well. You are now the victim’s best chance for survival, so do not abandon the victim to get help as time is of the utmost importance. After just 30 minutes of being trapped in an avalanche the victim will only have a 50 percent chance of surviving. So, be prepared and make sure that you do not let you guard down, especially on suspect slopes.

Conclusion:

With the growing numbers of people today picking up winter outdoor activities, more accidents involving avalanches are occurring in the backwoods. In most cases, poor judgment was the foremost contributing factor in the cause of the accident. New and continuing research has helped define the causes of avalanches so that understanding how to avoid them is now commonplace. Constantly and consistently applying this understanding is the only weapon we have in the battle against one of Mother Nature’s most feared events. By just understanding a few basic principals, and by exercising caution along with good judgment, it is possible to be safe while playing in the outdoors during winter.