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发表于 2010-4-21 16:58:50
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ALEXYIM
這次備受關注,
1. 據報導是within bounds, 即是發生在滑雪塲範圍內.
2. 滑雪塲範圍內, 我想很少人會帶beacons (我也不會喇.) 所以要小心搜索.
有一點可以留意, 時間是在下午發生, 溫度上升可能是一個重要因素.
調
查正在進行中.
Hi Alex and all,
I always think of Springtime as the time when avalanches don't behave according to the forecast especially over the last few years when persistant weak layers have caused major slides both in and out of bounds.
These are great videos of avalanches which catch people by surprise and at this time of year that is exactly what happens when the freezing level goes up and snow becomes isothermal.
It is important not to determine your plans simply by the avalanche rating especially if the ratings are getting better.
This is the time when isolated remaining highly reactive pockets of cold slab snow which don’t bond well and are affected by the sun still exist on various slope aspects. They do not reflect the overall forecast rating and typically at this time of year you will not see much difference in the forecasts (Sad but true).
That is why it is important to look for notes in the forecast which have been carried over into the next forecast period despite improvement in the ratings and get recent observations (if possible) for the area you are going to travel in). These notes are there to let you know that some avalanches are not going to behave according to the forecast period and to be ready for them.
An example of this type of note can be taken from a previous email to Lulu regarding the singing pass trip.
This note remained in effect while the forecast improved and is there to tell you that a problem will continue to exist . You need to read the terrain and feel the differences in the snow consistency and density very carefully while you are out there.
Quote : “Loose new snow sits on wind affected surfaces in lee terrain. Isolated pockets of wind slab on solar exposed terrain are now covered by new snow.”
The next example of a note taken from today’s Whistler Report is one which could be carried over a few forecast periods despite the ratings improving (Note that today’s 3 day forecast calls for a “moderate” rating from below treeline to Alpine):
“Cornices and unsupported features are the most significant concern at all elevations. A mostly isothermal snowpack has been on the move for several days now. Snow that is located in steep terrain, especially below tree line, is falling from features and plowing deep trenches until it deposits dense debris when it reaches lower angle terrain”
That first video is a good example of how a highly reactive weak layer covered by a slab warming in the morning sun can catch you by surprise even if the Avalanche report ratings are improving. It demonstrates the need for properly looking at the slope visually for a few minutes in order to identify hidden build up areas and convexities and doing field tests (snow pits/shear tests etc).
Extremely important to note is how that gentle slope started pulling snow well back of the convex roll below (which is not seen in the picture).
It doesn’t matter how thick the slab layer was or how gentle the slope was. Once everything started to move, velocity built quickly and pulls everything along like the current in a fast moving stream (even in shallow terrain)
A snow pit does not take that long to do with a group and can help you manage risk in the field and the way you approach the route rather than just hoping the route you chose deals with the problem. This is especially important in areas which are rated as challenging and above including multiple convexities, ridges, cross loading, terrain traps and in cases where avalanche reports are changing fairly quickly.
Sometimes I’m not sure what to offer in terms of ideas to all of you on these subjects because I don’t speak Chinese and many of you may already have a lot experience and knowledge, so I only hope that this information is helpful. |
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