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Posted by: Linda Chalat
January 22, 2007

AVALANCHES CAN ONLY OCCUR ON SLOPES STEEPER THAN 25 degrees

Most slab avalanches occur on slopes with starting zone angles between about 30 and 45 degrees, but slabs occasionally occur on slopes less than 30 degrees. Slopes steeper than about 50 to 60 degrees tend to sluff snow constantly, and slopes about 25 degrees or less are generally not steep enough. Skiing and snowboarding are most attractive on slopes in the 30-40 degree range, the same slopes most prone to avalanches

And since all of the snow is connected, you can be traveling on a flat slope or snow covered road and if the snowpack is unstable enough, even though you are not on a steep slope, you can trigger a slide on a steeper slope above you. As demonstrated by the massive slide which roared down Stanley Mountain on January 6th and pushed two cars off U.S 40 near Berthoud Pass - injuring at least eight people and temporarily closing the main route to the Winter Park ski resort. It is critical to know what is above you as you travel.

The better you are at determining slope angle and recognizing hazardous terrain, the better choices you can make when traveling in avalanche country. Nearly all avalanches that involve people are triggered by the victims themselves or a member of their party.

AVALANCHES CAN ONLY OCCUR WHEN THE SNOWPACK IS UNSTABLE

The snowpack accumulates layer by layer throughout the winter with each new precipitation, temperature and wind event. There are both strong and weak layers within a snowpack. Strong layers tend to be denser layers comprised of small round snow grains that are packed closely together and are well bonded to each other or cohesive. Weak layers tend to be less dense layers that are comprised of poorly bonded or cohesionless grains, often cold-temperature snowfalls. These layers often appear loose or "sugary". Because weak layers prevent strong layers from bonding with one another, it is important for the backcountry traveler to know the relationship of these layers.

Dry slab avalanches typically travel 60-80 miles per hour. They reach these speeds within about 5 seconds after they fracture. Wet avalanches usually travel much slower, around 20 miles per hour.

WHAT KIND OF WEATHER PRODUCES AVALANCHES?

Wind is the most common cause of avalanches. Wind can deposit snow 10 times faster than snow falling from storms. Wind erodes snow from the upwind side of obstacles and deposits snow on the downwind (lee sides), a phenomenon called "wind loading". The added weight from snowstorms also causes avalanches. If the weight of new snow is added faster than the buried weak-layer can adjust to its load, then it fractures and forms an avalanche. Strong sun or warm temperatures can also cause melting of the snow and creates wet avalanches. Wet avalanches occur because of a decrease in strength of the buried weak layer because water dissolves the bonds between the snow grains. But wind, snow or rapid warming do not always produce avalanches. It depends on the condition of the pre-existing snow and the conditions during the storm. With very stable snow pre-existing snow, even heavy, new snow with wind can bond well and be perfectly safe in the right conditions.

Interestingly, noise does NOT trigger avalanches. It's just one of those myths that refuses to die. Noise is simply not enough force unless on the magnitude of an extremely loud noise such as an explosive going off at close range. Even sonic booms or low flying helicopters trigger avalanches only in extremely unstable conditions in which natural avalanches would likely occur.

Avalanche victims are almost exclusively backcountry recreationists --snowmobilers, climbers, snowboarders, snowshoers, skiers and hikers. Snowmobilers lead the list with twice the number of fatalities as any other activity.

WHAT TO DO IF CAUGHT IN AN AVALANCHE?

Escape the slab!
Skiers and boarders technique:
If you're descending on skis or snowboard, try heading straight down hill to build up some speed, then angle off to the side off the moving slab. If you're close enough to the crown, you can try running uphill to get off the slab, or running off to the side. If you're ascending when the avalanche breaks, there's really not much you can do.

Snowmobilers technique:
If you're on a snowmobile you have the advantage of power. Grab some throttle and use your power and momentum to your advantage. If you're headed uphill, continue uphill. If you're headed across the slope, continue to the side to safe snow. If you're headed downhill, you're only hope is to try and outrun the avalanche. Remember that large avalanches travel 60-80 mph and they are difficult to outrun. Also remember that a disproportionate number of avalanche fatalities occur when one snowmobiler gets stuck on a slope and another person rides up to help them. Never go up to help a stuck buddy unless there are several other people in a safe place who can dig you out. This, of course, requires that everyone is wearing beacons and shovels and has practiced regularly with them.

If you cannot escape the slide:

Grab a tree. If you can't escape off the slab, try grabbing a tree. But do it very quickly because avalanches quickly pick up speed. After about 4 seconds you will be traveling at 40 miles per hour, not surprisingly, a quarter of avalanche victims die from trauma from hitting trees and rocks on the way down.

Swim. If you can't escape off the slab or grab a tree, then you need to swim hard. A human body is about three times denser than avalanche debris and it tends to sink unless it's swimming hard.

Clear an air space in front of your mouth. As the avalanche finally slows down and just before it comes to rest, try and clear an air space in front of your mouth. This helps delay the buildup of carbon dioxide in the snow around your mouth, which allows you to live longer under the snow.

Push a hand upward. Visual clues allow your friends to find you faster. You may not know which way is up, but take your best guess.

After the avalanche comes to a stop, the debris will instantly set up like concrete. So any actions you take must occur BEFORE it comes to a stop. Unless you are very near the surface or have a hand sticking up out of the snow, it's almost impossible to dig yourself out of an avalanche.

For more information, visit the U.S. Forest Service National Avlanche Center.

Do You Know Your Snow?

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