 Poster: A snowHead
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...I know, what a nerd.
An instructor once told me in theory you should never go off piste during a dump or the immediate day after (general rule).
If that's so, presumably dry avalanches and weight of snow with no bonding? What causes slab avalanches? Because a slab, by definition, some of it is bonded to itself.
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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The instructor would have told you that so that he could get fresh snow before you.
I am not aware of any reason to not ski off piste during or just after a snowfall. Obviously route selection, slope selection and safety must be taken into account. When it is snowing there is the extra problem of poor visibility: Can you see where you are going? Can the rescue services see enough to find you if needed?
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Well, the person's real but it's just a made up name, see?
Well, the person's real but it's just a made up name, see?
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You need to Login to know who's really who.
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| Quote: |
I am not aware of any reason to not ski off piste during or just after a snowfall. |
maybe you should also go away and read that pdf 
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Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
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The Lewes avalanche in Sussex at 10.15am on 27 December 1836 (which killed 8 people and destroyed a terrace of 7 cottages, rendering 4 other cottages damaged beyond repair) resulted from a snowstorm which reached its climax two days earlier. But the fatal avalanche was preceded by three smaller avalanches which began while the storm was still in progress on the evening of 26 December.
I don't think there's any safe rule to follow. Check the prevailing avalanche rating number in the resort. As mentioned many times, even a level '3' is potentially very dangerous (and, as I understand it, most fatalitites occur at that rating, maybe because people misinterpret it as 'average')
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Non-expert disclaimer - but I think I can remember enough from the courses I've been on to muddle through this one...
A "slabs" is layer of cohesive snow (i.e. snow that sticks to itself) that forms and is poorly bonded to the layer of snow below it. A slab avalanche is caused when something happens to cause the bond between snow layers to fail - the cause can either be through loading (a person, more snow falling on top) or thermal.
One way in which slabs can be formed is when snow is picked up and transported by the wind. In picking up the snow and moving it, the snow crystals get bashed and buffeted into smaller, rounder crystals that compact more easily when they land. The rounder snow crystals, in turn, have problems bonding to the layer of crystals below, which have a different shape.
Wind transport also means that the leeward side of the slope will be more heavily loaded than the windward side. In extreme cases you see the windward side completely scrubbed and all the snow on the leeward side.
This is why you should be cautious on leeward slopes after snowfall. The higher the wind then, generally, the higher the resultant avalanche risk.
So, if it snows and is windy then you can get slabs. But you can also get slabs if it's just very windy (often for prolonged periods), as the wind will just transport the snow.
Conversely, heavy snow with near zero wind is supposedly relatively safe, but...
Even if there's no wind in resort it's difficult to know what's happening 1000-2000m above you...
And also...
Another way that slabs can form is when snow falls (without wind) onto surface hoar. Surface hoar forms when it's particularly cold after snowfall - the temperature gradient between the snowpack and the air promotes the metamorphosis (transformation) of snow crystals into longer, flatter "plates" that are difficult to bond to. You can recognise surface hoar as "sparkly" snow.
So, if it's snowed; been very cold sufficiently long for surface hoar to form and then snowed heavily again with low wind then you can still get a slab.
Surface hoar is always something to look for. Once buried, a layer of surface hoar can remain intact and cause problems later in the season once things start melting.
Slabs, however, can also form within the snowpack through snow metamorphosis due to the temperature gradients in the snow (you get different temperature gradients through various layers of snow). So, metamorphosis can cause the bond between two layers to break down if conditions are right.
At which point, you should start to get worried. And this is all why you need to pay attention not only to the general avalanche warning, but also read the bulletin that'll be posted on the lift office window. Do, also, remember that the avalanche level can change as you move around a resort - so you should try to clock the avalanche level in each sector - no just the one posted at the bottom.
Moving onto the "day after" point. The general point is that the longer that snow's been on the ground the more likely it is that the snowpack will have stabilised through metamorphosis. So the day after snowfall will generally be higher risk than two days after; which will generally be higher risk than three days after; etc.
But (and there's always a but with this stuff) - there's really no such thing as "general", only specific slopes.
IIRC statistically, most deaths occur on the first blue sky day after snowfall and at avalanche risk 3 in France and avalanche risk 2 in Switzerland.
Now, hopefully, someone will be along to correct all of this 
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Please explain more, Haggis_Trap, about why you think skiing off piste during or soon after a snowfall is bad. You only quoted part of my words, you omitted the part about "route selection, slope selection and safety". Are you saying the whole mountain is unsafe? If you are then you appear also to be implying that on piste skiing during and soon after a snowfall is bad. I choose my slopes based on snow conditions etc and I ski with the Alpine Experience mountain guides who I trust.
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You'll get to see more forums and be part of the best ski club on the net.
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| Haggis_Trap wrote: |
| http://www.freeskiing.nu/media/FREE_03.pdf |
Great find.
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| PhillipStanton wrote: |
Wind transport also means that the leeward side of the slope will be more heavily loaded than the windward side. In extreme cases you see the windward side completely scrubbed and all the snow on the leeward side.
This is why you should be cautious on leeward slopes after snowfall. The higher the wind then, generally, the higher the resultant avalanche risk.
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Yes, but I don't think it's extreme or rare. Most Scottish avalanches, I would think, are as you describe, with many cornice avalanches.
Again, looking at the Lewes avalanche, it resulted from a massive blizzard carried by a NE wind. So the tops of the South Downs were swept of snow, but Cliffe Hill bore a huge (about 5m high) wind-deposited cornice, curled over its top edge.
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snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
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Interesting...
re resort bulletins -most seem to be very general. The ones I gave read in France/Switzerland classify entire areas 100s of km wide as the same risk with a bit of info about altitude, wind and orientation of slopes. The rest seems to be up to the individual...
...and while I'm here: actual avalanches have a grading system separate from the 1-5 warning system. Any links on that? Google has come up blank for me.
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And love to help out and answer questions and of course, read each other's snow reports.
And love to help out and answer questions and of course, read each other's snow reports.
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It stays 2or 3 most of winter! 4 and 5 is definitely a Jagermeister day.
Any reports on that avalanche mentioned?
How is the grading done, eg what is a 2.5 avalanche?
Last edited by So if you're just off somewhere snowy come back and post a snow report of your own and we'll all love you very much  on Sat 29-11-08 14:41; edited 1 time in total
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You know it makes sense.
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| GordonFreeman wrote: |
Interesting...
re resort bulletins -most seem to be very general. The ones I gave read in France/Switzerland classify entire areas 100s of km wide as the same risk with a bit of info about altitude, wind and orientation of slopes. The rest seems to be up to the individual... |
You're right, they are. They're not going to say the south slope of xxx is high risk.
They do generally say something about snow conditions at different altitudes / aspects if it's relevant. They also provide information on observed wind speed and direction.
| GordonFreeman wrote: |
| ...and while I'm here: actual avalanches have a grading system separate from the 1-5 warning system. Any links on that? Google has come up blank for me. |
IIRC...
...and in the colloquial...
1 - not enough to bury someone
2 - big enough to bury someone
3 - big enough to bury a transit van
4 - big enough to bury a house
5 - big enough to bury multiple houses
I'll see if I can dig out my notes...
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Otherwise you'll just go on seeing the one name:
Otherwise you'll just go on seeing the one name:
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| GordonFreeman wrote: |
| It stays 2or 3 most of winter! 4 and 5 is definitely a Jagermeister day. |
Yes that is why more people are killed when it is posted considerable "3" than any other level.
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Poster: A snowHead
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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Thanks!
So, a 45 degree north facing slope gets a 25cm dump during day. Is this more dangerous right at the end of the dump (possible dry avalanche and something I was worried about on the Swiss wall last season during a whiteout) or 12 hours later when it has bonded more but not necessarily with the layer below?
I know freeze/thaw plays a part but just as an example...
People are eager to get out during a storm or the next day. It's usually the busiest time.
Also, there is a section in Freeskiing about not cutting the snow above people after you have skied down 1at a time. Seems counter intuitive as you almost always have to cut the snow above?
Last edited by Obviously A snowHead isn't a real person on Sat 29-11-08 15:11; edited 1 time in total
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Well, the person's real but it's just a made up name, see?
Well, the person's real but it's just a made up name, see?
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GordonFreeman, 35 degrees might be a better question.
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You need to Login to know who's really who.
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35 then
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Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
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| stanton wrote: |
| Adrian wrote: |
| I am not aware of any reason to not ski off piste during or just after a snowfall. Obviously route selection, slope selection and safety must be taken into account. |
Here is one reason. If the regional Avalanche bulletin has posted 3 or above your taking a considerable risk. |
I do not understand. Where in the original question did it mention avalanche risk? The original question included:
| GordonFreeman wrote: |
| An instructor once told me in theory you should never go off piste during a dump or the immediate day after (general rule). |
So I want to know why an instructor would say that. Clearly there are specific conditions when going off piste may be a bad idea. But the original question has "general rule". Why is it a general rule? What is the general problem that means do not go off piste?
Considering avalanche risk. The value is some sort of "average" of the risks across the mountain, or the resort, or the region. Depending on which value you see. An avalanche risk of (say) three does not mean all the slopes on all the mountains have a risk of exactly three. Some slopes will be one and some slopes will be five. That is why I mentioned route selection, slope selection and safety. On some days the best route is down to the bar or back to bed.
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If you watch the reports that come in over each season you will find that very often avalanche fatalities/incidents occur on the first clear day after a storm.
So statistically, as a general rule, it is worth being very cautious immediately after a dump.
In terms of the snow, the new stuff is likely to have no cohesion to the old layer to begin with; this is likely to increase over time (ie get more stable) hence as a general rule skiing off piste during and immediately after a storm is bound to be more risky.
As to never going off piste during and immediately after a dump, well thats taking it a bit far, but I am always alot more cautious during these periods. Its always interesting (and worrying) to see where tracks appear & how quickly after each storm.
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The reason why this is a general rule is that something like 90% of all avalanches happen during or immediately after snowfall. It's not rocket science - the new snow either causes existing instabilities to avalanche because of the increased loading or it doesn't stick to the old snow and slides off itself.
Therefore, during a heavy snowfall or immediately after, it may be best to stick to gentler slopes for a day or so.
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You'll get to see more forums and be part of the best ski club on the net.
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GordonFreeman,
I haven't read any of the other contributions to this thread, however:-
There are a lot of factors which determine how likely a slope will avalanche.
1) Angle of the slope, some slopes never avalanche because they have very shallow gradient. Skiers usually like skiing slopes between 25 & 40 degrees which are the worst gradient for avalanches.
2) Early season the snow falls on the ground and doesn't bind to it and will avalanche easily.
3) Different snow layers build up when snow falls on different days. Sometimes the layers don't bind together and the top layer will slide over the lower layer.
4) Temperature difference between snow surface and ground. There will be a temperature fradient between surface and ground this will have an effect on the likelihood that there will be an avalanche.
5) Trees, avalanches rarely start in a forest because the trees heelp stabilise the snow, that's why men build avalanche barriers on steep slopes to stop avalanches occuring.
6) Wind will blow snow from one place to another, where it is blown to it builds up and can become unstable. Look at snow cornices which are created by wind and are likely to fall off.
7) Loading on the snow pack will cause an avalanche, the avalanche that kills you is normally triggered by yourself or someone in your party or that idiot skiing above you who shouldn't be skiing there while you're below them.
 Aspect, the direction the slope faces north or south etc has a bearing on how likely it is that an avalanche will occur.
9) How much snow has fallen and is it wet sticky snow or very dry powdery snow.
It is a very complex subject because there are so many variables.
If you want to know more there are books, DVDs and talks. Look up "HAT" Henrys Avalanche Talk on the internet there's lots of info there. If you ski in ValD he does free talks there thro most of the ski season, you can (at a price) ski with him and ask him questions.
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| stevomcd wrote: |
The reason why this is a general rule is that something like 90% of all avalanches happen during or immediately after snowfall. It's not rocket science - the new snow either causes existing instabilities to avalanche because of the increased loading or it doesn't stick to the old snow and slides off itself.
Therefore, during a heavy snowfall or immediately after, it may be best to stick to gentler slopes for a day or so. |
Eloquently to the point
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snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
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This is a good resource.
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And love to help out and answer questions and of course, read each other's snow reports.
And love to help out and answer questions and of course, read each other's snow reports.
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| Quote: |
| The reason why this is a general rule is that something like 90% of all avalanches happen during or immediately after snowfall. |
Exactly.
As said before - If you remember the simple statement
'90% of avalanches occur 24hrs after a snow fall, and are triggered by the victim'
Then your more avalanche aware than most people.
If you want to know more then spend 20 minutes reading the excellent pdf... 
You might even learn a couple of things (I did!)
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| Adrian wrote: |
| On some days the best route is down to the bar or back to bed. |
I hear you.
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You know it makes sense.
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Otherwise you'll just go on seeing the one name:
Otherwise you'll just go on seeing the one name:
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OOI, I can vaguely understand how snow bonds to each other, ie layers.
1/ surface hoar - once formed does that prevent any bonding between the next new layer?
2/ how does the snow bond with the ground early in the season?
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Poster: A snowHead
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marmot, who is speculating? There is a lot of good advice over the last few posts.
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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| marmot wrote: |
| read this book (at the very least) before even thinking of venturing off piste |
I've done four evening talks; a six day long professional course; read probably six or seven books and done lots of transciever search practice.
All I know is that I (honestly) know diddly squat.
I don't know about other people, but I find this stuff really difficult to internalise because of all the variables. It's a complete bug to apply.
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Well, the person's real but it's just a made up name, see?
Well, the person's real but it's just a made up name, see?
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| GordonFreeman wrote: |
OOI, I can vaguely understand how snow bonds to each other, ie layers.
1/ surface hoar - once formed does that prevent any bonding between the next new layer?
2/ how does the snow bond with the ground early in the season? |
Surface hoar, once covered, can persist. But it can also break down. It depends on the temperature gradient within the snowpack.
It's no so much that the next layer can't bond - it's just that the layer boundary is relatively weaker.
In good years snow melt-freezes into the top layer of the ground and is strongly bonded. In bad years the snow never really bonds to the ground properly.
davidof's site Pistehors has some really good stuff, including assessments of avalanche accidents. It's interesting reading the one for 2005-2006:
| pistehors wrote: |
| Contrary to much press speculation the reason for these deaths was not a sudden change in behaviour of off-piste skiers and snowboarders but due to extremely unstable snow conditions. The cold autumn and early winter, with thin snow cover and many clear sunny days were ideal for the formation of a weak layer of depth hoar and facetted crystals... |
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You need to Login to know who's really who.
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PhillipStanton, years ago, I saw a telly programme about avalanches, in which some people at the Davos avalanche centre were interviewed. What they made abundantly clear was that, relatively of course, they also know diddly squat! That was very sobering and put me off skiing off-piste at all, for years. (Nowadays, being wrinkly and having no dependents, I'm less bothered about the danger.)
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Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
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| Hurtle wrote: |
| ...being wrinkly and having no dependents, I'm less bothered about the danger... |
Now there's a quote that could get a girl into trouble 
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You'll need to Register first of course.
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PhillipStanton, wash your mind out with soap!
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Just another skiing example, our group were having a great ski day the snow just kept coming down and we were after the fresh, around noon temps warmed and we got groppel (nice little round balls) we all knew that was a warning but we kept going, off-piste skiing was really good. It cooled back down went back to snow flakes instead of groppel we all knew it was time to play it safe but it was a really fun day, crossing a open bowl the snow started cracking and whumping I was leading and almost to the next subridge I looked back and the group was backing up so only one skier was crossing the open bowl at a time.
Now we had seen what came down earlier we had warning noises and should have angled back inbounds heck no did I mention it had been a great day so we skied the final line it was fantastic.
Next day driving up the last canyon we had skied last had slid and broken up to 12" trees in the slide.
Skiing the storm had been unwise and not giving the snow a chance to settle and bond could have cost us lives, normally I'm smart enough to turn around and skin back inbounds on these days, group dynamics and good turns can drown out wisdom.
Another thing about surface hoar is the big hoar flakes can collapse under weight of more snow layers which can cause a slide.
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The majority of fatal avalanches are caused by windslab (the kind of slab someone mentioned before that is made up of flakes with all the extremities broken off by swirling in high wind and then deposited on lee-slopes). This type of snow creates slabs which are self-cohesive but do not bond well with other layers. It "creeps" differently to other snow over time - often ending up as a crust with air cavities between it and the layer below. If you are skiing along and the surface is giving way with a little "wump" sound, often breaking into rafts - that is probably wind slab. Beware!
Prevailing winds are from the South and West in the Alps but can sometimes be from other directions - and may also be funnelled by the mountains. So most wind slab is formed by high winds on North and East facing slopes. Wind slab is formed in the lee of ridges in particular, where the wind suddenly drops. If you stick your pole into it and twist, wind crust often squeaks. Watch out in particular when there have been recent high winds. Wind crust is smooth - if you see wind-caused wrinkles in the snow it will not be wind-crust and will usually be safe.
If in doubt ski down along the tops of ridges rather than in the gulleys and bowls between.
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You'll get to see more forums and be part of the best ski club on the net.
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FAQ
