 Poster: A snowHead
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| rob@rar wrote: |
Yes, I appreciate that. I'm just trying to get a feel for what people think is "low angle terrain". If the steepest pitch of the Face du Bellevarde is 28º it's only beginning to get into the steepness where slab avalanches normally can occur. Or am I mis-understanding things? |
I would have to find the post on sH where I gave the exact figure I measured on la Face - but don't forget that a lot of the off piste surrounding the run is steeper. If you ski down through the avalanche protection it is more like 35 degrees and very likely to slide, ditto for the off piste off to skiers right (banane etc?).
To extend on what you said above what is important on la Face when the risk is 3..5 as on any route, are the slopes above you as well as the one you are on.
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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If you place one pole vertical and the other horizontal, each with a tip touching the snow, and the handles together, then the snow angle is 45 deg. It assumes you can approximate the vertical and horizontal accurately. I have graduated my poles to also show 40deg, 35, 30 etc.as I slide the horizontal pole downwards. I am sure I have recorded 35deg in a couloir in la Grave and the same on Orientation in Val . These were on the steep bits, and only a short length of reading.
philingle, has some strong credentials so I will not pontificate but I will test out my angles whilst away this season. I feel some of his angles are more likely to be averaged over longer parts of the slope or even overall. It's the brown stain bits we are really wanting to notch up. Might even buy one of those gadgets Achilles mentioned early in the thread.
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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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| Quote: |
| It is often a misconception that they act as 'pins' to stabalise the snow. This is wrong, the air gap that forms around the rock allows cold air to get closer to the ground, so increasing the temperature gradient and the creating depth hoar, so they are weak points. |
If you have a rock in the ground it has three effects:
i. it conducts heat into the snowpack leading to a strong TG
ii. the snowpack surrounding the rock is thinner leading to a strong TG - this is why you often get goblet where there are rock outcrops and this is why you will get air pockets - I've never heard the cold air theory, maybe you have a reference?
iii. it is a stress concentrator and can aid triggering (thinner snowpacks are weaker snowpacks).
That said, if you had a choice between a large field of slabby looking snow and scrambling up some rocks I would probably favour the rocks but I would be very wary of the snow immediately below the rocks. It comes down to a judgment call on the actual conditions.
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You need to Login to know who's really who.
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| davidof wrote: |
| rob@rar wrote: |
Yes, I appreciate that. I'm just trying to get a feel for what people think is "low angle terrain". If the steepest pitch of the Face du Bellevarde is 28º it's only beginning to get into the steepness where slab avalanches normally can occur. Or am I mis-understanding things? |
I would have to find the post on sH where I gave the exact figure I measured on la Face - but don't forget that a lot of the off piste surrounding the run is steeper. If you ski down through the avalanche protection it is more like 35 degrees and very likely to slide, ditto for the off piste off to skiers right (banane etc?).
To extend on what you said above what is important on la Face when the risk is 3..5 as on any route, are the slopes above you as well as the one you are on. |
Yes, I appreciate that. I'm not going to blindly ski any slope that feels "black-like" regardless of what is above or near it. Just trying to get a feel for these things so that I'm appropriately cautious, not over-cautious.
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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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| rob@rar wrote: |
Yes, I appreciate that. |
I realize that is common sense for a lot of people I was just rounding out the story you'd started (although I see Phil posted a similar thing).
The most dangerous thing about back country skiing is we get too much positive feedback - most trips pass without incident and one can begin to believe that one's knowledge rather than a dose of luck, had something to do with that.
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You'll need to Register first of course.
You'll need to Register first of course.
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| davidof wrote: |
| I realize that is common sense for a lot of people I was just rounding out the story you'd started (although I see Phil posted a similar thing). |
And I'm grateful to both of you 
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Cold air is what causes a strong Temperature Gradient (TG).
TG is only applicable on a layer by layer basis in the snow. and it is the difference in the temperature between the top of the layer and the bottom of the layer divided by the thickness of the layer.
In any snow pack over around 10cm deep the ground is at 0 degrees C, then the temp gets colder as you get further away from the ground. if you have plenty of layers of snow between the ground and the cold air then the TG in each layer will be low. This is good, it tends to the creation of 'grins fins' or fine grains, symbolised by a dot on snow charts. which form stable snow.
a medium gradient in any layer (between 5 and 20 degrees C per meter) will lead to the creation of 'faces plane' or plates, symbolised by a square. these can create a weak layer, but here the process is reversible if the TG drops again.
a strong TG is when there is more than 20 degrees per meter, this will lead to the creation of 'goblets' or depth hoar (or surface hoar) symbolised by a V shape. This process is irreversible, and they are there for the season. and are a very weak layer.
If there is an air gap down the side of the rock, then there is less snow to insulate the lower layers and hence a greater TG will be generated in these lower layers.
similarly if there is a small amount of snow and cold weather the TG will be high. So a thin layer of snow followed by a long cold period at the begining of the winter is a disaster, you get a high TG and irreversible creation of goblets that will be a weak layer near the base all winter.
A large dump of snow at the begining of the season, like this year, is much better, because the TG will be low in that base layer, whatever you throw at it later. That doesnt stop you having another small snowfall and cold spell and creating a weak layer higher up mind.
Warm air is all good, it causes a low temp gradient and hence good sintering of fine grains.
refs
http://www.crdp.ac-grenoble.fr/neige/docs/traduc/snow_en.pdf - very good article
http://www.anpnc.com/recueil/chapitre%203-p24-36.htm - good diagrams
http://www.anena.org/savoir/nivologie/nivologie_connaissance_de_base_suite.html - a bit simplified
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| davidof wrote: |
| too much positive feedback - most trips pass without incident |
indeed, 95 percent of snow that falls is stable and will never avalanche. So someone going out at random with no idea what they are doing at all will come home witout incident 19 times out of 20
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You'll get to see more forums and be part of the best ski club on the net.
You'll get to see more forums and be part of the best ski club on the net.
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philingle, nice links there, cheers.
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Good stuff on this thread. I agree that sustained 30 degrees is unusually steep for a piste, black or not. I'd have thought that the pitch of black runs was typically in the 20s excluding a flatter run-out at the bottom.
Personally, I've always found that 40 degs feels flippin steep and I don't think I've skied anything steeper than 45.
What sort of angle do you need to get slough going? Don't think I've ever managed that on-piste (obviously un-pisted!).
J
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snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
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| philingle wrote: |
If there is an air gap down the side of the rock, then there is less snow to insulate the lower layers and hence a greater TG will be generated in these lower layers. |
Yes, I don't really agree with your explanation.
If there is cold air above the snow and cold air below the snow you don't have a TG.
What causes depth hoar around rocks is the face that you have cold air above the snow and warm rock below the snow and frequently a thinner snow-pack over the rock outcrop. The transformation of the grains into facets (gobelet) and the geothermic energy melts the lower layers of slow leaving the hollows. You end up with more depth hoar surrounding rocks and a poorly anchored snow-pack.
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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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| jedster wrote: |
| What sort of angle do you need to get slough going? Don't think I've ever managed that on-piste (obviously un-pisted!).J |
As I mentioned earlier, the snow on Glencoe's 'Spring Run' frequently sluffs a few feet but not when as well skied as in this picture. It's about 25 degrees gradient average but has a couple of steeper bits.
Spring Run a 'red' at about 25 degrees sluffs in fresh or heavy spring snow.
Fly Paper a 'black' at 40 degrees avalanches in fresh snow.
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When I was in an avalanche I reckon we were on a slope of less than 30º, but the last of our party skied a slightly steeper bit further up which we had missed and the slab he released collected much of the less steep snow below (and us) and the whole thing finally ran out of steam after 300yds as the slope got less and less steep further down
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You know it makes sense.
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Many Recta compasses also include a clinometer.
Last edited by You know it makes sense. on Thu 2-02-12 13:00; edited 1 time in total
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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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Poster: A snowHead
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I was just interested in what a 30 degree slope would compare with on piste, so basically if off piste and you look at a slope and think .... mmmm ...... maybe a red or an easy black if it was a piste, then it is probable that the slope is 25 degress or less, and below of the 30 degrees avalanche 'rule of thumb' which is nice to know.
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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Interesting diagram, I must admit I've never seen snow that falls in rectangular blocks like that .
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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.
You need to Login to know who's really who.
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