 Poster: A snowHead
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so I've heard a few people say this in resort...that "December was too warm and the avalanche risk is higher as the new snow has not bonded".
I thought in general some warmth for freeze thaw was supposed to be good for stabilising the snowpack so what's the issue this season?
or is it just that a lot of snow fell on the old snowpack and hasn't bonded because it HASN'T been warm since?
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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It takes serious warmth to permeate through metres of snow to bond old dodgy layers and destroy hoar crystals. Just doesn't really happen, especially on shaded slopes.
I was in Öst Tirol and Süd Tirol yesterday, and the amount of (old) large slides right next to the valley road was incredible. Serious debris piles and crowns, often on not very steep slopes.
Last edited by Obviously A snowHead isn't a real person on Wed 12-02-14 20:16; 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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Whereas in North Tirol the snowpack is very thin and I really don't trust it either. Plus there's been a metric poo-poo-tonne of wind.
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| clarky999 wrote: |
It takes serious warmth to permeate through metres of snow to bond old dodgy layers and destroy hoar crystals. Just doesn't really happen, especially on shaded slopes.
I was in Öst Tirol and Süd Tirol yesterday, and the amount of (old) large slides right next to the valley road was incredible. Serious debris piles and crowns, often on not very steep slopes. |
but surely it's similar every year? gets colder in January onwards.
The layers have to bond somehow or is it that the snowfall is more gradual (normally) and so has time to bond.
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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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It's quite complicated, but depends where you are talking about. The statement you quoted is probably a bit simplistic.
In Austria we had a good amount of early season snow, then from December about 6 weeks of warm temps and no snow - which is v unusual. With a thin snowpack weak crystals develop much more due to the stronger temperature gradient, which will probably last all season, whatever happens with snow from now on. If enough snow falls to bury them deep within the snowpack then human triggering becomes less of an issue, but they are still there and triggering would be possible.
A season or two ago we had the opposite. Really warm and dry November, with snow coming in mid/late December, falling onto unusually warm ground. There was an increased and persistent risk of full depth slides, and there were way more glide cracks on all aspects and altitudes than normal.
There are plenty of reasons for current increased avalanche danger though. Large and quick additional loading from new snow (like the recent 'super storms'), especially when it's fairly warm. Generally warm temperatures. Lots of wind moving snow about.
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| clarky999 wrote: |
I
A season or two ago we had the opposite. Really warm and dry November, with snow coming in mid/late December, falling onto unusually warm ground. There was an increased and persistent risk of full depth slides, and there were way more glide cracks on all aspects and altitudes than normal.
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I've never quite got this part. Surely snow has to bond to the ground/grass/rock at some point?
How does it bond with good strength?
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The issue wasn't that it was too warm, the issue was that there had been no new snow, and it was, in fact, too cold. There was a strong temperature gradient throughout the pack which had formed late Nov, early Dec, which meant the snow turned into sugary, faceted crystals. While this is nice to ski on, it's a horribly weak layer to dump a load of fresh snow on top of and will remain unstable for a long time thereafter.
Below 1800-2000m, it wasn't so bad as the start of the storm which dumped down in mid Dec fell as rain on the lower slopes, and the water permeated this faceted snow pack and help consolidate it. Higher, it just fell as snow, so it never got the chance to consolidate.
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The ground is warming up period regardless of what season...
Rock erosion and landslides are on the increase off season
Avalanches can occur at anytime no matter what risk warning is given.
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| feef wrote: |
The issue wasn't that it was too warm, the issue was that there had been no new snow, and it was, in fact, too cold. There was a strong temperature gradient throughout the pack which had formed late Nov, early Dec, which meant the snow turned into sugary, faceted crystals. While this is nice to ski on, it's a horribly weak layer to dump a load of fresh snow on top of and will remain unstable for a long time thereafter.
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GordonFreeman, Weathercam posted a very useful video from his avalanche course at La Grave which explains and shows you some of this here http://snowheads.com/ski-forum/viewtopic.php?p=2446156 .
And when that gets a nice heavy dump of snow on top as has happened recently you can imagine how "stable" it all becomes 
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| feef wrote: |
The issue wasn't that it was too warm, the issue was that there had been no new snow, and it was, in fact, too cold. There was a strong temperature gradient throughout the pack which had formed late Nov, early Dec, which meant the snow turned into sugary, faceted crystals. While this is nice to ski on, it's a horribly weak layer to dump a load of fresh snow on top of and will remain unstable for a long time thereafter.
Below 1800-2000m, it wasn't so bad as the start of the storm which dumped down in mid Dec fell as rain on the lower slopes, and the water permeated this faceted snow pack and help consolidate it. Higher, it just fell as snow, so it never got the chance to consolidate. |
There are always parts of the mountain that remain cold so aren't crystals always going to be in the snow pack at some stage?
I guess this is partly why North East slopes are more avalanche prone as they are colder and not in the sun so much?
| clarky999 wrote: |
It's quite complicated, but depends where you are talking about. The statement you quoted is probably a bit simplistic.
In Austria we had a good amount of early season snow, then from December about 6 weeks of warm temps and no snow - which is v unusual. With a thin snowpack weak crystals develop much more due to the stronger temperature gradient, which will probably last all season, whatever happens with snow from now on. If enough snow falls to bury them deep within the snowpack then human triggering becomes less of an issue, but they are still there and triggering would be possible.
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Why less of an issue? Isn't more weight on top of a thick snowpack more likely to slide?
Is the gradient warm on the ground, cold on the top?
Last edited by Ski the Net with snowHeads  on Mon 17-02-14 19:50; edited 1 time in total
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snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
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| GordonFreeman wrote: |
| feef wrote: |
The issue wasn't that it was too warm, the issue was that there had been no new snow, and it was, in fact, too cold. There was a strong temperature gradient throughout the pack which had formed late Nov, early Dec, which meant the snow turned into sugary, faceted crystals. While this is nice to ski on, it's a horribly weak layer to dump a load of fresh snow on top of and will remain unstable for a long time thereafter.
Below 1800-2000m, it wasn't so bad as the start of the storm which dumped down in mid Dec fell as rain on the lower slopes, and the water permeated this faceted snow pack and help consolidate it. Higher, it just fell as snow, so it never got the chance to consolidate. |
There are always parts of the mountain that remain cold so aren't crystals always going to be in the snow pack at some stage?
I guess this is partly why North East slopes are more avalanche prone as they are colder and not in the sun so much?
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N facing slopes are certainly subject to more surface hoar formation as sun can "burn off" light surface hoar on Southern aspects. Not sure if N facing slopes get more depth hoar formation, but I guess it is likely.
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| clarky999 wrote: |
It's quite complicated, but depends where you are talking about. The statement you quoted is probably a bit simplistic.
In Austria we had a good amount of early season snow, then from December about 6 weeks of warm temps and no snow - which is v unusual. With a thin snowpack weak crystals develop much more due to the stronger temperature gradient, which will probably last all season, whatever happens with snow from now on. If enough snow falls to bury them deep within the snowpack then human triggering becomes less of an issue, but they are still there and triggering would be possible.
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Why less of an issue? Isn't more weight on top of a thick snowpack more likely to slide?
Is the gradient warm on the ground, cold on the top? |
More weight is indeed a risk right after a storm. However, a well bonded layer can "bridge" over patches of weakness so can be safe to ski over even if there are buried instabilities. Depending on density a single skier only transmits enough force to fracture a weak layer to a depth of a about a metre or so. I.e. if a weak layer is buried more than a metre deep a single skier is unlikely to trigger it. If a skier falls then more force may be transmited, if there is a group skiing close together then more force can get transmitted, or, if a skier hits a thinner layer (e.g. caused due to wind transport) then again, trouble...
Yep, temperature gradient is the difference between ground temp, which is almost always at freezing point and the surface temp, almost always below freezing except late in the season. The steeper the gradient the faster the moisture is transported and the faster the transformation of buried layer into facets. More maths than you bargained for here: http://www.avalanche.org/moonstone/snowpack/the%20formation%20rate%20of%20depth%20hoar.htm. The rough rule of thumb is anything more than 1 degree C per 10cm of snow pack is bad news. So, if it gets down to -20 at night you want a 2m snow pack. Roughly.
When the surface gets to zero you have an isothermal snow pack, in this case you are going to be more worried about wet slides. More to the point if this is happening then it is probably mid afternoon in late spring so you need to wonder why you are not on the terrace drinking cold beer 
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