Avalanche Basics for Backcountry Travelers

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Winter and spring mean avalanches in the backcountry. Most of us pay little attention to these regular mountain phenomena unless someone gets injured or killed in one. Be smart. Whether you go snowboarding, skiing, snowshoeing or climbing, it's wise to 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 avalanches trigger the slides 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. Here is some basic information you need to know.

The Potential Hazards

A dry-snow 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 a third of avalanche deaths are the result of trauma. If you make it this far, you then have to deal with a mass of concrete like snow encasing your body. What starts 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. Seventy percent of avalanche fatalities are due to suffocation.

If you have been able to form a breathing space before the snow settles, you may have a decent chance of survival. That's assuming you and 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, so becoming proficient with an avalanche transceiver, probe and shovel is a must for winter backcountry travel. The best defense, however, is knowing how to read the snow conditions and terrain and avoiding dangerous situations altogether.

Types of Avalanches

Loose Snow

Loose-snow avalanches occur where there is little or no cohesion in the snowpack. They usually start at one 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. Loose-snow avalanches can occur in dry or wet snow, in winter or summer. The 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

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

Factors Contributing to Avalanches

Terrain

• Angle: It's worth your time to 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. The likelihood of a slide goes up with the slope angle, and is especially likely on convex slopes. Staying on open slopes below 30 degrees is a good idea.
• Aspect: During winter, a south-facing slope is more stable since it has sun exposure to melt and condense the snow. In spring and early summer, a south-facing slope can mean more serious melting, resulting in dangerous wet-snow slides. The attractive north-facing slopes that hold all the best powder are also most 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 the south-facing slopes. It might be enticing to venture onto those powdery, north-facing slopes, but you could easily set off a slide if you do.
• Terrain hazards: Terrain features where the snowpack is likely to be unstable include 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) over 45 degrees 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.

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 a good indicator of snow instability. Often, high winds pick up surface snow on one slope and deposit it on the other side of the ridge where it can start to slide as a wind slab. Pay attention to the intensity and direction of the wind throughout the day.
• Temperature: Rapid changes in temperature can cause changes in snow crystal formation and can lead to weak layers. Surface hoar, which is known to be a "weak link," forms during clear, still nights when the temperature drops. In spring, rapid warming of a slope can lead to wet-snow avalanches.

Snowpack

Snowstorms pile up one after the other all winter long. If the snow's consistency remains constant, the snowpack is homogeneous. It's when snowfall of different consistencies pile one on top of the other that the snowpack becomes hazardous. For instance, loose, cold snow on top of an old snow layer crusted over by the sun does not stick to the old snow. Long, feathery snow crystals known as surface hoar are one indicator of poor bonding between layers. Another type of snow that is found deeper in the snowpack is known as depth hoar. Its ball-bearing-type structure makes a poor bond with other snow layers. Many other snow crystal forms exist and tell something about snowpack stability. A snow crystal identification card can be helpful for study in the field.

Snow Tests

As you travel in possible avalanche terrain, make it a habit to test the snow throughout the day. A simple poke at the snow surface with your ski pole now and then can give you information that can prove useful (for instance, if the upper layers are changing as you progress up a slope). You can also perform more involved tests of the snowpack as follows:

• Snow pit: A snow pit is a way of observing layers in the snowpack to determine if there are weaknesses that could potentially give way. Dig this pit on an open slope after probing to see if there is any old avalanche debris, rocks or brush in the way. Make the sides of the pit smooth with your shovel. Use a glove to brush the surface of one wall to see if there are visible layering lines. Use a credit card or driver's license and, holding it lightly, slide it down the wall. Notice where the card catches on hard layers. Do the same starting at the bottom and sliding up. Next, do the finger test for soft layers, running your gloved hand first down and then up the wall. Note where the hard layers (possibly sun or wind crust) and the soft layers (grainy snow known as depth hoar) are located. If you don't detect any significant layers in the snow, you can continue on your trip. But if there are crusty or soft layers, you may want perform either or both of the following tests.
• Shovel shear: This test is used to locate a weak layer that allows the snow above it to shear off. Use your snow shovel to cut out a pit in the snow with the same aspect and angle as the area on which you want to travel. (Be careful not to do it on a slope on which you could start an actual avalanche, however!) In one wall of the pit, create a column of snow with 3 sides, about the width of the shovel blade. Leave the uphill side of the column attached to the snow pit wall. Then carefully place your shovel blade between the column and the back wall. Gently pull the shovel forward and see if any snow slides off the column. If one layer slides off, go down farther with your shovel to see if multiple weak layers exist. Cutting the back wall with a snow saw or a piece of string first may make the test easier to perform. How easily the snow slides with the shovel test is a somewhat subjective call. If it slides off the column without any help from the shovel, though, you can be sure the snow is unstable! Also, absence of sliding during the test does not mean that snow is not unstable anywhere else in the area. If this test doesn't give you results, you may want to try the following test.
• Rutschblock: This "sliding block" test shows how much force is needed to make the snow slide on a given slope. Usually performed by a person wearing skis, the Rutschblock test is a larger-scale version of the shovel shear test. The force comes from the skier above, though, instead of a shovel from the side. Choose a safe area on a slope similar to the one you will be traveling on. To make the block, use a shovel, snow saw and/or ski and cut a trench at least 3 feet deep. Make the block a ski length wide and about a ski pole length into the slope. Unlike the shovel shear test, the back wall is cut away to create a freestanding block. Use a piece of cord or a snow saw. Now, notice at which point the block breaks and a layer slides off. Is it when you:
  1. Test-dig out the snow block?
  2. Go to the uphill side of the block and step gently onto it?
  3. Flex your knees and let your weight sink into the block, but without jumping?
  4. Jump in the same spot with your skis on?
  5. Jump again?
  6. Either jump again in the same spot without skis on or leave them on and jump on the center of the block?
  If the block fails before step 5, the snow is showing signs of instability. If you have to jump in the middle of the block or take your skis off to make the snow slide, there's likely a low chance of avalanches on slopes with similar angle and aspect.

Warning Signs

The first warning sign you should be aware of before you leave home is the avalanche report. If high avalanche danger is forecast for the mountains that day, plan your route in safe terrain or stay home altogether. Once you're in the field, use your powers of observation throughout your trip to stay alert to avalanche danger. The following are evidence of unstable snow and possible avalanches:

• You see an avalanche happen or you see evidence of previous slides.
• Cracks form at the top of the slope or around your feet or skis.
• The ground feels hollow underfoot.
• You hear a "whumping" sound as you walk, which indicates that the snow is settling and a slab might be released.
• You observe baseball-size snowballs rolling down a sunny slope — these indicate surface warming and a possible wet-snow slide.
• You see surface patterns on the snow made by the force of strong winds. This could indicate that snow has been transported and deposited in dangerous drifts that could release.

Route Selection

So where is a backcountry traveler to go? First, avoid areas that present any of the warning signs listed above. Instead, seek out ridges and hilltops, open valleys and gentle slopes without steep sections or chutes above them. Stick to windward slopes, which will not have cornices hanging above them or snow deposited there. Forested areas are generally a good choice, as long as they are thick enough and as long as slopes above don't avalanche onto them. Groves or sparsely wooded areas can slide, too.

If you absolutely have to cross a suspect slope, make sure that you go one at a time. Remove ski leashes and pole wrist loops and unbuckle your pack in case you need to remove them in a hurry. Zip up all clothing and put on your hat and gloves, just in case you're caught by the snow. Someone should always be watching the person in the "danger zone" and be ready to come to his or her rescue if an avalanche occurs.

The Human Element

A large part of staying safe in the backcountry depends on your group. (And it should be stated here that being part of a group is a far wiser way to travel than going solo. It's a no-brainer for avalanche rescue!) Your companions' experience and fitness level can affect the outcome of your trip, as can group size and dynamics. Before you head out, "take the pulse" of the group in terms of attitude, skill and preparation for what you're setting out to do. Is everyone willing and able to take on the same amount of risk? Are they all prepared to deal effectively with an avalanche accident? Is there anyone whose need for a summit or a perfect ski run may get the rest of the group in trouble? These are all things to iron out before you leave the trailhead.

The other important detail to take care of at camp or the parking lot is that of the avalanche transceiver (or "beacon"). Make sure everyone is wearing a working transceiver, knows how to use it and has it turned on to "send." Ensure that all units are compatible.

Avalanche Safety Gear

Avalanche gear is only as good as your knowledge and skill in using it. To improve your safety in avalanche territory, these 3 items are absolutely essential:

• Avalanche transceiver: This device must be carried by everyone in the group. The buried beacon sends radio signals that are picked up by searching beacons, leading them to the victim to be dug out. See our How to Choose an Avalanche Transceiver article for details.
• Snow shovel: An essential for testing snow and digging out victims.
• Probe: This is a collapsible aluminum pole, similar to a tent pole, made specifically for probing by itself or during the pinpoint search with an avalanche transceiver. Alternatively, some ski poles can be screwed together to form longer poles for probing.

The following items are also helpful:

• Snow saw: This is handy for cutting out blocks for snow shelters and igloos. It can also be used for snow stability tests.
• Snow crystal card: This plastic-coated card shows the various forms of snow crystals and their characteristics, including how well (or poorly) they bond together.
• Slope meter: This is used to determine the angle of a slope for avalanche probability.
• Black Diamond Avalung II: This innovative breathing apparatus pulls available oxygen from the surrounding snowpack so that you can breathe if you're buried in an avalanche.
• Recco reflectors: These passive transponders are embedded by some manufacturers into ski products such as jackets, pants, boots or helmets. They complement but do NOT replace the use of avalanche transceivers. Recco detector units are used by many search-and-rescue organizations.

Shop REI's selection of avalanche safety gear.

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