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Avalanche Awareness
The content below is written by: Bruce Tremperhttp://www.avalanche.org/~uac/Common-questions.html#WHERE
Director of the Forest Service Utah Avalanche Center (Much of this material is reprinted from the Book "Staying Alive in Avalanche Terrain" by Bruce Tremper and published by Mountaineers Books, 2001.)
How fast do avalanches go?
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 avalanche is most dangerous?
Dry slab avalanches account for almost all avalanche fatalities. A slab avalanche is like a dinner plate sliding off the table. A cohesive plate of snow slides as a unit on top of weaker snow. The slab shatters like a pane of glass with the victim in the middle of the slab and usually there's no escape.
What causes slab avalanches to fracture?
Snow is a lot like people. It doesn't like rapid change. (Raise taxes slowly enough and no one notices.) Dry slab avalanches occur when the weak layer beneath the slab fractures, usually because too much additional weight has been added too quickly, which overloads the buried weak layer. Snow is very sensitive to the rate at which it is loaded or stressed. Two feet of snow added over two weeks is not a problem. Two feet of snow in two days is a much bigger problem. Two feet of snow in two hours is a huge problem. (Wind can easily deposit two feet of snow in two hours.) Then, finally the weight of a person can add a tremendous stress to a buried weak layer, not in two hours, but in two tenths of a second-a very rapid change. That is why in 90 percent of avalanche accidents, the avalanche is triggered by the victim (or someone in the victim's party). Wet slab avalanches occur for the opposite reason. Percolating water dissolves the bonds between the snow grains, which decrease the strength of the buried weak layer.
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). We call this "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. Rapid warming can also cause dry avalanches but this is much more rare.
Rain or melting of snow surface can also cause avalanches. For instance, rain on new snow almost instantly causes avalanches. 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. Stability analysis is a complicated process and it requires much study and experience to develop good stability analysis skills.
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.
How do people get caught in avalanches?
In 90 percent of avalanche incidents, the VICTIM or someone in the victim's party triggers the avalanche.
Where do most avalanche fatalities occur?
Only one tenth of one percent of avalanche fatalities occur at ski areas on open runs or on highways. This is because teams of avalanche professionals at ski areas and on highways routinely knock down the avalanches each morning with explosives before the public arrives. In other words, almost all fatalities occur in the "backcountry" where no avalanche control is done. When you step across that magic ski area boundary line, you are stepping back into the Stone Age. You must be your own avalanche expert. Most victims are recreationists and in 90 percent of the fatalities, the avalanche is triggered by the victim, or someone in the victim's party a job on the ski patrol doing avalanche control, and I've been studying controlling, and forecasting avalanches ever since. One of these days, I may even become an expert.
What do I do if I get caught in an avalanche?
You're first job is to GET OFF THE SLAB, which as you might imagine, is not very easy.
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. Snowmobile 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.
· Grab a tree. If you can't escape off the slab, try grabbing a tree. But you have to do it very quickly because avalanches quickly pick up speed. If you can't grab a tree quickly, then your best friend suddenly turns into your worst enemy. After about 4 seconds they can easily be traveling at 40 miles per hour, and you can imagine what a tree feels like at 40 mph. (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.
How do I judge the danger of avalanche terrain?
· Steepness. Almost all avalanches occur on slopes between 35 and 45 degrees. Slopes less than 30 degrees seldom produce avalanches and slopes steeper than about 50 degrees sluff so often that they tend not to build up into slabs. So it's the intermediate slope steepness that produces most of the avalanches. But the bad news is that exactly the kind of slopes we like to ski, snowboard or snowmobile usually produce most of the avalanches. A black diamond slope at a ski resort is usually around 35 degrees--prime steepness for producing avalanches.
· Anchors. Trees and rocks that stick up through the snowpack can help to hold the snowpack in place. But the anchors need to be fairly thick to be effective. For instance a thick, mature grove of evergreen trees anchor the slab quite effectively while a sparse grove of aspen trees have very little effect.
· Aspect with respect to wind. Recently wind-loaded, steep slopes are almost always very dangerous while recently wind-eroded slopes are usually fairly safe.
· Aspect with respect to sun. In the Northern Hemisphere as temperate latitudes, the direction a slope faces (aspect) is very important. For instance, north facing (shady) slopes usually produce more avalanches and more persistent avalanche hazard in mid winter. On the other hand, in the spring when wet avalanches occur from strong sun, south facing slopes produce more wet avalanches. At equatorial or Arctic latitudes, the aspect with respect to the sun has very little effect.
· Consequences. What will happen to you if the slope slides? It's very difficult to survive an avalanche if it strains you through thick trees or dumps you over a large cliff or deposits you into a crevasse or dumps you into a narrow gully (creating a very deep burial). On the other hand you have a fairly good chance of survival on a small avalanche path, without obstacles and a gentle run-out.
How do I judge snow stability?
Look for obvious clues:
· Call the avalanche report. The many mountainous areas in North America have an avalanche center that issues regular avalanche advisories. This gives you an easy, overall view of snow stability for your area. The best sign of avalanches are other avalanches. You can't get much more obvious than that. But it's surprising how often people miss this clue.
· Collapsing snow. When you hear the snowpack collapse catastrophically with a giant "whoomph", that's the sound of the snowpack screaming in your ear that it's extremely unstable. Stay off of steep slopes and stay out from underneath steep slopes.
· Cracking snow. Recent wind loading, especially, creates cracking snow. The longer the crack, the more dangerous. Stay off of steep slopes.
· Avalanche weather. Just like people, avalanches do not like RAPID changes.
Recent rapid loading of new or windblown snow
Recent rapid warming
Recent rapid melting
Rain on new snow
Active Tests:
· Use test slopes. Find a small, safe, steep slope and go jump on it to see how it responds. You can do this on a snowmobile, snowboard, on skis or on foot.
· Cornice test. Find a refrigerator-sized cornice and tumble it down the slope. Hint: ALWAYS wear a belay rope and use a snow saw or thin avalanche cord to cut the cornice.
· Snow Pits. Take a reputable multi-day avalanche class to learn how to dig in the snow and do stress tests on the snowpack. These can range from simply pushing your ski pole into the snow or digging down with your hand to full-blown pits using a shovel. These require lots of experience to interpret them effectively.
· Integrate the information. Never base your stability evaluation on just one test or observation. That's like deciding to get married after the first date. Bad mistake. Stability analysis means integrating many different pieces of information together.
Safe Travel Techniques
· One at a time. There always needs to be someone left in a safe spot to do the rescue. Never put everyone on the slope at once. With large groups, split them in half and stay in visual and voice contact.
· Have an escape route planned. Always think avalanche. What will you do if you trigger an avalanche? Have a plan first.
· Use slope cuts. Keep your speed up and cut across the starting zone, so that if you do trigger an avalanche, your momentum can carry you off the moving slab into safer terrain. You can do this on skis, snowboards or on snowmobiles.
· Watch out for cornices. They always break farther back than you think. Always give them a wide berth. NEVER, NEVER walk out to the edge of a drop-off without first checking it out. Many people have needlessly died this way.
· What are the alternatives? Use terrain to your advantage. Follow ridges, thick trees and slopes with safer consequences. You can almost always go back the way you came. The route got you there, it will most likely get you back as well.
· If there's no other choice, go underground. You can almost always weather out a bad storm or bad avalanche conditions by digging a snow cave in a protected area. You may be uncomfortable but you will be alive.
If you see your friend get caught in an avalanche... · Watch them closely. Mentally fix the last seen area and closely watch to see where they end up. This will greatly reduce the search times if you have a good idea where to begin the search.
· Should you go for help? NO! First, they may not need help and you would needlessly endanger the lives of rescuers. Second, they only have a precious few minutes to breathe under the snow, so every minute counts. If you go for help they most likely will not be alive when you return with a rescue team. Spend about a half hour or an hour searching before you go for help.
· Is it safe to go in? Yes, usually it's safe. But if your friend is buried in a place with multiple avalanche starting zones looming above and it's snowing hard or blowing hard or there's rapid melting, then there's also a good chance of another avalanche coming down on top of the search area. It's a hard call. If you think it's too dangerous then it probably is. If it's too dangerous then you should go for help. It's a job for professionals.
· Find a safe route to the avalanche debris. Often you can descend down the avalanche path or come up from the bottom onto the debris.
· Do a beacon search. If the victim is wearing a beacon, turn yours to receive and make SURE everyone in your party is turned to receive. Go fast and cover a lot of ground. Look carefully for clues, hands sticking out of the snow, snowmobiles, skies, gloves. In most snowmobile burials, the victim is usually just uphill of their snowmobile.
· If there's no beacon--probe. If the victim does not have a beacon then it's a needle-in-a-haystack situation. You have no choice but to look for visual clues and probe. Move quickly. Use a ski pole, collapsible probe or tree branch to randomly probe. Concentrate on debris piled above trees or on benches or any other area with debris accumulation. Probing is difficult, tiring and time consuming, so don't get discouraged. If you don't have any success in the first hour or so, then you need to think about going for help.
· With multiple burials, go for the shallow burials first. Get them breathing but don't take the time to get them completely dug out, just keep moving and find the next victim and get them breathing, and so on. Get as many people breathing as possible before returning to treat the injured.
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