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舊 2008-11-30, 12:02   #1
Mike
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註冊日期: 2008-05-19
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Search is continuing for a snowboarder who was caught in an avalanche whilst boarding off piste in Les Diablerets last Wednesday (26th Nov 2008), the snowboarder was overtaken by a 60 metre wide avalanche on the glacier du Dar, whilst boarding off piste with another boarder.

Roddy Clarke, HAT presenter, beeper trainer and snowboard instructor commented on the risks of heavy early season snowfalls, "Early season snow can be unpredictable and it is sometimes hard to read or predict stability. This is due to a lack of knowledge of what the base layers are like. New snow sitting on grass or smooth rock, coupled with a lack of time for settling to occur, can lead to quite dangerous circumstances" he said. "Remember at this time of the season, this is a virgin snowpack and will not have had the stabilising effect of skier compaction even on the classic itineraries!"

So what is the cause of Avalanche:
The word avalanche comes from Savoyard lavantse, itself coming from lower Latin labina, landslide.

An avalanche occurs when a mass of snow falls down a mountainside. That is because new snow (which is not wet) accumulates on a more heavy snow layer. Since the new snow layer is not compact, it could slide down toward the base of the mountain.

An avalanche or snowslide is the rapid movement of snow down a mountainside. Most avalanches occur on slopes of 25 to 50 degrees – the same slopes favoured by many skiers, snowboarders and snowmobilers.

The most hazardous type is called a dry slab avalanche. When a weak layer in the snow cover can no longer support new layers of snow above, the crystalline structure collapses, sending the top layers down the slope. Basically, a slab avalanche is like a dinner plate sliding off a table with the victim caught in the middle of the slab. On average, dry slab avalanches travel from about 100 to 130 kilometres per hour – too fast to outrun.

The risks of an avalanche increase during major snowstorms and periods of thaw. Temper said wind can gather up snow and deposit it 10 times faster than snow falling from storms. The makes wind the most common weather-related cause of avalanches.

What causes an avalanche to release? The most simple answer is that there is too much stress on the slope for the snowpack to support it. This can occur naturally as the snowpack slowly accumulates snow and eventually releases on its own. For the backcountry skier/snowboarder however, the slope can release when the weight of a skier/snowboarder cutting across a slope provides just the amount of stress to release a slab. Most accidents are caused by skiers/snowboarders releasing the slopes under which they become buried. Avalanches could also be triggered by loud noise (yells, firearm shots, etc.).

If you walk in a trail after a snow fall, do not walk in the middle of it, but rather on the side. Avalanches always fall down in the middle of the path; thus, if you are on the side, the risks of being hit by the avalanche are lower.

Last word - listen and obey the Avalanche Warning issued. :)
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舊 2008-11-30, 12:13   #2
Mike
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AVALANCHE WARNING SYSTEM (adopted in the Alps)

The risk of avalanche is assessed on a five-level scale by specialists in nivology (the science of snow) and mountain climatology. Flags are used on the ski slopes, and especially on their limits with the off-piste slopes. It is more a precaution and information measure than an operational action.

Explanations on the different hazard levels:
In the Avalanche Hazard Scale, the avalanche hazard increases progressively from one level to the next, whereby the stability of the snow cover decreases at a constant level and the hazard zones spread out over the land in terms of number. The additional stress which is required to initiate triggering decreases at the higher levels.

Level 1: Low hazard
The snowpack as a whole is firmly secure. Self-triggering (spontaneous slides) will hardly occur other than smaller slides on steep slopes. For artificial triggering, even on extremely steep slopes, high levels of additional stress on the snow cover are required (e.g. through detonations).
The conditions outside secured zones may generally considered to be safe. The hazardous zones are few, limited to extremely steep slopes, and easy to locate.

Level 2: Moderate hazard
The snowpack is only moderately bonded on some steep slopes which are described in the avalanche bulletin in general terms by altitude, exposure or nature of terrain. If they choose their route carefully, skiers thus have a predominance of favorable conditions. Nevertheless, the possibility of triggering an avalanche should not be ignored, especially in the case of additional stress, e.g. a group of skiers ascending or descending close together. Moreover, on steep slopes with unfavorable snow cover conditions, the possibility of an avalanche being triggered by an individual should not be excluded. Traffic routes and populated areas are hardly at risk at all from spontaneous avalanches, as these will, at most, arise in isolated instances. In principle, securing measures in the area of controlled ski descents are not necessary either.

A plain yellow flag means that the risk of avalanche is low (levels 1-2).

Level 3: Considerable hazard
The snowpack is only moderately or poorly bonded on many steep slopes. Above all, on the steep slopes having the exposure conditions and altitude indicated, even a small additional stress, for instance caused by a single skier, could trigger an avalanche.
The danger of spontaneous avalanches may vary a great deal; with a weak snow cover structure and not much snow depth, only occasional instances of medium-sized avalanches need be expected. However, if the level is announced where there is new snow, or in conjunction with the warming effects of the day, depending on the influence of the weather, isolated instances of large avalanches must be anticipated. This then determines detonating activity (above all when there is new snowfall) or closing of areas at certain times (above all, when the temperature rises) for exposed parts of traffic routes, and above all in the area of ski runs which have to be secured. Ski tours and descents outside secured zones require experience and the ability to assess avalanches. Steep slopes at the altitude and with the exposure conditions indicated should be avoided where possible.

Level 4: High hazard
The snowpack is poorly bonded on most steep slopes. Even where there is only minor additional stress, triggering is probable. Depending on the snow cover structure and quantities of new snow, a large number of spontaneous medium-sized avalanches, and an increased number of large avalanches, must be expected.
Parts of traffic routes and populated areas in the locality affected by such avalanches are likely to be endangered. Increased detonation and closures are called for in terms of safety measures.
Conditions outside secured zones are unfavorable.

A black and yellow chequered flag means that the risk of avalanche is high (levels 3-4).

Level 5: Very high hazard
The snowpack is generally poorly bonded and thereby largely unstable. Numerous large spontaneous avalanches may be expected, which requires comprehensive safety measures (closures, in certain circumstances also in the form of evacuations, etc.).
Ski tours should not be recommended and are mostly not even possible.

A plain black flag means that the risk of avalanche is very high (level 5).
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