How Avalanches Work

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Introduction to How Avalanches Work

Q: What is a snowpack?

A: In areas that get a lot of snow, the snow on the ground forms a snowpack. The layers within the snowpack have different qualities due to the shapes of the crystals in the layer.

Avalanches can be surprising, sublimely beautiful and deadly. They can sweep trains off their tracks, crush buildings, uproot trees and bury people. Some avalanches have even covered entire houses with people still inside them. Even though movies and news reports say that they "strike without warning," most deadly avalanches start when victims trigger them.

So what do you do if you're caught in an avalanche? How can you stay alive, and what does it take to rescue people who've been buried in the snow? In this article, you'll learn how avalanches form and what triggers them. You'll also learn how to survive and how to rescue others.

Avalanches are a serious danger for winter sport enthusiasts. Read all about ice climbing safety and watch video from Discovery’s Fearless Planet.

The Properties of Snow

To understand how avalanches form, you need to understand the properties of snow crystals. Depending on the temperature, humidity and other atmospheric conditions, snow crystals can have a variety of shapes, but all are generally hexagonal or six-pointed.

Photo courtesy Beltsville USDA Agricultural Research Center
A six-pointed snow crystal, viewed through a scanning electron microscope.

In areas that get a lot of snow, the snow on the ground forms a snowpack. The layers within the snowpack have different qualities due to the shapes of the crystals in the layer. For example, six-pointed crystals can interlock more easily than needle-shaped crystals, so they create a steadier layer. On the other hand, when super-cooled water comes into contact with snow crystals in the air, it creates rime. Heavy rime deposits can cause pellet-like snow called graupel, which creates a very unstable layer.

Snowpack layers also have different qualities because of changes that take place once the snow is on the ground. Changes in the weather lead to changes on the snowpack's surface.

If the top of the snowpack melts and re-freezes, it can form a layer of slick ice.
If air just above the snowpack reaches the dew point, the snowpack can develop hoar, which is a light, feathery crystal that does not bond well to snow.
If the top of the snowpack freezes and thaws repeatedly, it can develop clusters of frozen particles with space in between, which creates an unstable surface for the next layer of snow.

Photo courtesy Beltsville USDA Agricultural Research Center
A hexagonal snow crystal with rime along its edge.

Changes within the snow pack take place due to the temperature gradient -- the difference in temperature between the upper and lower layers. The snow near the bottom is relatively warm (close to 0° Celsius/32° Fahrenheit) because of residual heat from the ground. The temperature in the upper layers depends on the temperature of the air. Snowflakes within the snowpack undergo different types of metamorphosis depending on the size of the temperature gradient.

In snowpacks with a high temperature gradient -- a large difference in temperature -- crystals tend to develop facets. The flat surface of a facet cannot bond well to other surfaces. Heavily faceted crystals located deep in the snowpack are called depth hoar and create dangerous instability.

Photo courtesy Beltsville USDA Agricultural Research Center
Depth hoar crystal. These crystals cannot bond well to one another and create instability in the snowpack.

On the other hand, low temperature gradients and consistent sub-freezing temperatures cause rounding, which allows crystals to compress more tightly. The exchange of water vapor during rounding also creates bridges between crystals and parts of crystals, creating a firm, stable snowpack.

Photo courtesy Beltsville USDA Agricultural Research Center
A rounded snow crystal that has spent
several days in the snowpack.

Regardless of whether they are the result of temperature gradients, atmospheric conditions during snowfall or melting and refreezing, strong and weak layers of snow make avalanches possible. Next, we'll look at how avalanches form and what can trigger them.

Avalanches in Other Materials In the right conditions, avalanches can form in materials other than snow, including volcanic debris, mud and Martian sand. Usually, the volume or weight of the material overcomes the friction holding it in place. Rain or melting snow can also reduce the amount of friction, leading to mudslides and rockslides.

Avalanche Formation

The Typical
Avalanche Victim
According to the Colorado Avalanche Information Center, most avalanche victims are men in their twenties. The majority are also experienced in
back country sports.

Avalanches have three ingredients -- snow, a sloped surface and a trigger. A weak layer within the snowpack, caused by ice, surface or depth hoar, faceted crystals or graupel also contributes to the process. If the weak layer is near the surface, it causes a sluff -- a cascade of loose, powdery snow in an inverted "V" shape down the slide of the mountain. Sluffs are like sand rolling down a dune, and they usually cause minimal damage to people and property.

If the weak layer is deeper in the snowpack, it can cause a slab avalanche, which is far more dangerous. In a slab avalanche, a strong, cohesive layer of the snowpack slides down over a bed layer of snow, like thawing snow sliding down a car's windshield. Sometimes, the entire snowpack breaks free from the mountain and slides over the ground.

The strength of a slab avalanche depends on the properties of the slab and the depth of the weak layer, also called the failure layer. Hard, cohesive slabs create very large chunks of solid slow, while softer slabs create smaller blocks. Slabs of wet snow cause generally slower avalanches than dry slabs, but they typically hit obstacles with more force.

Avalanches usually start on mountain slopes that are at a 25 to 60 degree angle to the ground. Slopes less than 25° generally aren't steep enough to produce avalanches, and slopes steeper than 60 degrees usually sluff their snow constantly, giving slabs little chance to develop. Most avalanches begin on 35 to 45 degree slopes.

Photo courtesy MorgueFile
Debris from a slab avalanche

Most slab avalanches take place on leeward, rather than windward, slopes. They can have a natural trigger, like a sudden change in the weather, a falling tree or a collapsing cornice -- an icy overhang of wind-driven snow near the ridge. In spite of what movies and cartoons depict, the trigger is almost never a loud noise. In most fatal avalanches, people create the trigger. Once they begin, they have three segments:

A starting zone, often above the tree line and near the ridge, where the slab breaks away from the rest of the snow.
A track, or the course the avalanche follows down the mountain. You can often see avalanche tracks even in the summer because of missing trees.
A runout, where the sliding snow and debris eventually comes to a stop.

When the snow stops, it compacts and sets up like concrete. This is what makes avalanches so dangerous to skiers, hikers and snowmobilers -- they generally cannot dig themselves out and must wait for rescue.

Unexpected Avalanches You don't have to be in the wilderness or pursuing an "extreme" sport to encounter an avalanche. For example, on March 31, 1982, an avalanche struck the Alpine Meadows ski area in Tahoe City, CA. Of the seven people who died, three were in a building that housed the ski patrol and avalanche control headquarters, and four were in the parking lot.

Avalanche Prevention and Control

Avalanche fatalities are most common in the winter months, but since early-season snowfalls and spring thaws are also dangerous, they can occur in every month of the year. In addition to the threat to human life, avalanches can cause tremendous damage to buildings and property. They can also close roads, cover train tracks and disrupt local economies. So, ski patrols and other organizations usually take steps to prevent major avalanches.

Photo courtesy National Park Service
A bulldozer clears avalanche debris from a road
in Glacier National Park.

One technique is to deliberately trigger small, controlled avalanches when no one is on the slope. Staff and researchers first study the snowpack either by digging pits and analyzing each layer or by using radar technology. They then start an avalanche with explosives or artillery fire. On small test slopes, they may also perform ski checking by deliberately skiing along fracture lines high on the slope. People performing ski checking always work with at least one partner, who remains in a safe location in the event that the skier gets caught in the avalanche.

Photo courtesy National Park Service
A rescue volunteer in Olympic National Park
investigates an avalanche surface.

Other techniques involve preventing the conditions that lead to avalanches or interrupting the flow of snow. In some locations, fences, posts, nets, anchors and windbreaks change the way snow collects, reduce the size of the slab or provide physical obstacles in the event of an avalanche. Authorities in parts of the United States and Canada have also reforested areas that underwent heavy logging (clear-cutting in avalanche-prone areas is illegal in most of Europe).

But avalanches can happen in spite of all preventive measures, especially in the first 24 hours after fast, heavy snowfall. Next, we'll look at how people can avoid triggering avalanches.

Virtual Avalanche Control Even if you've never wanted to be a park ranger or a ski patroller, launching artillery to start an avalanche might sound like fun to you. You can play an avalanche control game at ParkWardens.com.

Avoiding an Avalanche

Avalanches are most common on smooth, steep slopes, without a lot of obstacles or tree cover. Unfortunately, these are the sorts of areas backcountry enthusiasts like to use for skiing, hiking and snowboarding. Also, some popular activities, like high marking (driving a snowmobile as high as possible up a steep incline and making an arc back down the slope) are exactly the kind of actions that will likely start an avalanche. This makes avalanches in areas where people are likely to be inevitable.

Photo courtesy NOAA
A clear fracture in a snow slab

Preventing an avalanche is far easier than surviving one. The most important step is to be aware of your surroundings and to monitor weather conditions closely. Most countries use a warning system to tell people how likely avalanches are. Lots of parks and resorts have hotlines and web pages with up-to-date information about the area avalanche forecast. In addition to keeping up with this information, you should:

Photo courtesy Image After
Most areas provide avalanche forecasts, but survival in the backcountry still requires constant vigilance.

Take an approved avalanche safety course before going into the backcountry.
Take a partner with you.
Carry a shovel, a rescue beacon and an avalanche probe. Make sure your rescue beacon is under your outer layer of clothing, has fresh batteries and is set to "transmit."
Measure the angle of the slope. Most outdoor supply stores sell small, inexpensive inclinometers for this purpose.
Look out for shady areas and places where snow collects.
Be alert for fracture lines, hollow sounds, and "whumphing" noises, which can all signal an impending avalanche.
Dig a quick pit -- a deep pit with smooth sides in which all the layers are visible -- to examine the snowpack.
Test the stability of the snowpack. The U.S. Forest Service has tutorials on three basic tests -- shovel shear, compression and rutschblock. Another common test is called the stuffblock test. You can also test use a belay system and kick or cut down cornices to test whether the terrain below is stable.
Avoid obvious avalanche tracks and areas with previous avalanche activity.
Travel above avalanche-prone areas instead of through the center. If you must cross a hazardous slope, do so one at a time to minimize risks.
Never travel above your partner.
Do not assume that existing tracks from other people mean that an area is safe.

But even if you follow all of these steps, you could still be caught in a naturally-occurring avalanche or one started by someone else. Next, you'll learn the steps you can take to survive an avalanche.

Filming an Avalanche You may have seen dramatic film footage of an avalanche coming straight for the camera. Usually, filmmakers capture this footage by placing the camera and an avalanche transceiver in a heavy, padded crash box. They typically use timers to start and stop the camera so they don't waste film, and they trigger the actual avalanche with explosives. You can learn more about filming avalanches at NOVA Online.

Surviving an Avalanche: If You're the Victim

Avalanche fatalities usually have one of three causes -- physical trauma, suffocation and hypothermia. Survival techniques involve getting out of the avalanche before it stops, preventing total burial and reducing the amount of time that the victim is buried.

Photo courtesy National Park Service
Most victims are buried in the debris in the avalanche runout.

Virtually all avalanche safety courses will advise you to try to get away from the avalanche as soon as you realize what is happening. The Forest Service National Avalanche Center advises skiers to ski off the slab and snowmobile drivers to try to outpace the avalanche. Call for help one time so your party knows you are in danger. Then, close your mouth so snow doesn't block your airway. If you can't get away from the avalanche:

Abandon ski equipment. It can drag you down and provide more torque on your extremities, leading to broken bones. If your pack is light and has emergency equipment inside, keep it with you.

If you are thrown from your snowmobile, try to get away from the machine.

Use swimming motions to fight your way to the surface of the avalanche.

Try to grab nearby trees to get away from the snow.

As the snow slows, cup your hand or arm over your mouth so you will have an air pocket. Thrust any part of your body through the snow as it comes to a stop so rescuers can see you.

Wait for rescue. Stay calm. Conserve oxygen. Do not try to call for help unless you hear rescuers above you.

Most sources say that a person who is completely buried can live for about 18 minutes. Even though snow is porous and contains a lot of trapped oxygen, victims breathe their exhaled air, causing carbon dioxide poisoning. Warm breath also melts the snow, and it can re-freeze as a solid, non-porous layer of ice that oxygen cannot easily penetrate. With sizeable air pocket that is open to the outside, you can avoid suffocation, but you still run the risk of hypothermia and shock.

Next, we'll look at the steps to take if you witness an avalanche.

Avalanche Survival Gear

Photo courtesy HowStuffWorks Shopper
The AvaLung

In addition to probes, transceivers and shovels, some people use high-tech safety gear to improve their chances in an avalanche. One device, called the AvaLung, lowers the likelihood of suffocation. It pulls air from the snowpack through a mouthpiece. When you exhale, it diverts the carbon dioxide-rich air to the snow behind you so you don't re-inhale it.

In any system of moving particles, large particles tend to stay on top while smaller particles filter to the bottom. Inflatable airbags and vests increase a person's size relative to the particles of snow and debris around them, so they tend to stay closer to the top of the flowing avalanche. The airbags also provide additional buoyancy and may protect against physical trauma from the moving debris. Two airbag systems currently available are the ABS avalanche airbag and the Avagear avalanche life jacket.

Surviving an Avalanche: If You're a Witness

Search & Rescue Robots Most people are familiar with the idea of St. Bernards or other dogs taking part in rescue and recovery efforts. Robots might also take part in search and rescue missions. Sandia National Laboratories has experimented with robots and algorithms used in national defense and biological weapon detection as a method for seeking avalanche victims.

If you see someone swept away in an avalanche, you are their best hope for survival. Some ski areas do have search and rescue teams as well as rescue dogs, but going for help can take longer than the victim can survive. According to accident statistics, 92 percent of victims survive if their party digs them out within 15 minutes. Only about 25 percent survive after 45 minutes [ref]. Large groups can send someone for help, but small groups and individual survivors should not leave the scene unless help is just a few minutes away. Some experts say that lone survivors should conduct the search themselves. Others say that if the only survivor has no visual clue about the victim's location and if the victim or survivor was not wearing a beacon, the survivor should go for help.

If you see an avalanche overtake someone, you should:

Watch him closely without looking away.

Keep your eyes fixed on the last point at which you saw him if he becomes completely buried.

Wait a minute or so after the avalanche stops for the snow to settle. Evaluate the risk of another avalanche. If one seems likely, have someone keep watch while you search for the victim.

Conduct an efficient search.

Photo courtesy HowStuffWorks Shopper

Everyone who goes into the backcountry should have an avalanche rescue beacon, an avalanche probe and a shovel. These three items are central to any search and rescue operation.

An avalanche beacon is a small transceiver. When an avalanche occurs, survivors switch their beacon from "transmit" to "receive" and use the signal to locate the victim. New beacons display the direction of the signal as well as its strength, but educators recommend that people practice locating buried beacons before going into avalanche territory.

Avalanche probes are 10'-12' poles that collapse into segments that are about 2' long. They allow you to find solid objects buried in the snow. Some ski poles have removable grips and baskets and can also function as probes.

Shovels are a basic necessity for digging out buried survivors. Avalanche snow is very solid, and digging with a shovel takes far less time than digging with your hands.

You can conduct your search using a variety of methods, as long as it covers all of the terrain in which the victim can be buried and works within the beacon's effective range. If you are with a large group, you can stand shoulder to shoulder and work your way down the slope, beginning at the point where you last saw the victim. Smaller parties and lone survivors can zigzag down the slope.

The closer you get to the victim, the stronger the signal will be. When you find the victim, dig him out quickly. Clear his airway and perform CPR or artificial respiration if necessary. Treat for hypothermia and shock, and get help as quickly as possible.

For lots more information about snow and avalanches, check out the links on the next page.

Lots More Information

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How Avalanches Work