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Avalanche Awareness & Avoidance
| Introduction to Avalanches |
An avalanche is a fascinating and naturally occuring phenomenon that is caused by unstable snow conditions. Since it is practically impossible to predict where and when the next avalanche will occur, significant amounts of information such as weather forecasts and avalanche forecasts are provided for winter sports enthusiasts to aid their understanding of avalanche dangers and potentially prevent one from occuring.
The following avalanche information is not to be accepted as a set of instructions, it is designed merely to provide guidance for those who wish to go off piste, and facilitate informed decision-making. It is also worth noting that avalanches are not the only dangers of off-piste: natural obstacles such as crevasses, buried rocks or cliffs are also potential hazards. It is highly recommended that you take a mountain guide whilst venturing into unknown territory. |
| Awareness & Avoidance of Avalanches |
Being caught in an avalanche is perhaps the greatest fear of all those who undertake off-piste skiing or boarding and ski-touring. The boom in the snowsports industry, high lift-stations, and fat skis and snowboards have given greater access to the mountain for many who do not have the necessary experience to deal with the conditions and to know the warning signs. Snow conditions can vary from week to week, and even hour to hour as weather conditions alter the structure of the snow. Knowledge of the snow and understanding of the terrain are necessary to avoid in being caught in an avalanche, however the best form of avalanche safety is really avalanche avoidance!
An avalanche consists of a fast moving mass of snow, ice and other debris collected en route. As snow falls it forms layers which bond together with already laid snow and the ground. When these frictional bonds between a surface layer, the ground and surrounding snow are weak or poorly formed, it can take just the weight of a single skier to destabilise the entire mountain surface.
The risk of avalanches on the patrolled runs is minimal as ski resorts go to great lengths to protect their pistes - incidents do happen but are thankfully infrequent. However, avalanches and accidents can occur just off the pisted areas. The important point is to be prepared before you venture away from the pistes. An avalanche doesn't need to be of blockbuster movie proportions to be hazardous to your, or others', life.
Who gets caught in avalanches?
In general, its more likely to be a backcountry enthusiast or climber that gets caught in an avalanche as they are generally crossing hazardous terrain which would not have been controlled by the local 'security des pistes'. A common perception for those that get caught and injured in an avalanche is that they were obviously at fault and shouldn't have been there in the first place. Whilst this can be true for some avalanche victims it is important to stress that an avalanche is not always the fault of the backcountry skier or climber. Often unstable snow conditions, warm weather and melt water can affect the snowpack, creating a weak snow layer that will easily trigger a slide. The winter of 2005-06 was the one of the deadliest to occur for a long time. That year up to 55 people were killed in the French Alps during the course of the ski season that ran from December through to April. Not all of these were as a result of
Some avalanche quick facts:
- In 90% of avalanche incidents involving skiers and snowboarders, the victim or someone in the victim's party triggers the avalanche
- The common avalanches to occur are slab (which account for almost all avalanche fatalities. A cohesive plate of snow slides as a unit on top of weaker snow) or flow (powder) avalanches
- Experience indicates that about 90% of avalanche victims will be recovered alive if they are pulled out within the first 15 minutes. After approximately 45 minutes, the chances of survival drop dramatically to between 30-40%. After 2 hours, the chances of being pulled alive are neglible
- Approximately 50% of victims, partially buried survive if their airways are clear. Of the 50%, only about 5% will suffer from deadly injuries due to the fall. Of the other 50% of victims who die, death will usually be attributed to a late rescue as carbon dioxide builds up in the snow around their mouths and they suffocate
- The effects being caught in an avalanche potentially include having clothing, rucksacks, and other equipment ripped off the victim. Wearing a transceiver anywhere else but properly secured around your body can increase the chances of it being separated from you and therefore rendering it virtually useless
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last updated 8-Jan-2008 |
| What causes slab avalanches to fracture? |
Even though the answers to these last few questions are 'common knowledge' among professionals, Bruce Tremper provides the clearest explanations we know of. And so we often quote and paraphrase him:
"Snow is a lot like people. It doesn't like rapid change."
When people go through a lot of changes in a short amount of time, they can become very stressed and break up. Snow is the same - it needs time to adapt to change. Different types of people and different types of snowpacks have different tolerances to different types of change and stress. Dry slab avalanches occur when a relatively weak layer beneath the slab fractures, usually because too much additional weight has been added too quickly.
Snow is very sensitive to the rate at which it is loaded or stressed: a metre of snow added over two weeks is not a problem. A metre of snow in two days is a big problem. A metre of snow in 3 hours is a huge problem (wind can easily deposit a metre of snow in three hours). Finally, the weight of a person can add tremendous stress to a buried weak layer, not in 3 hours, but in two tenths of a second - an instantaneous change. That is why in 90% of all avalanche accidents, the avalanche trigger is the victim (or someone in the victim's party) and 90% of the time it is a slab avalanche that is involved. Wet slab avalanches occur for other reasons: percolating water dissolves the bonds between the snow grains, which decreases the strength of the ice 'skeleton' and adds lots of stress to a buried weak layer - if there is one. |
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last updated 8-Jan-2008 |
| Is the slope safe after an avalanche has gone? |
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Usually it's safe. But there are a number of things to watch out for: multiple avalanche starting zones ('hang fire') looming above, dense snowfall, hard wind. If there is rapid melting going on, then there's a good chance of another avalanche coming down. It's a hard call, especially if you are going to rescue someone. |
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last updated 8-Jan-2008 |
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