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Snow-geek questions

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Snow-geek questions

Postby Dave B » Mon Apr 30, 2012 4:02 pm

No doubt it's been an odd snow year with some sketchy observations still being made.

So, why all the facets and subsequent deep persistent layers this year? Is this strictly due to heavy early season snow followed by a long dry cold(ish) period? Was it the early season lack of snow or extreme temp. gradient that drove their formation (this year wasn't unusually cold right?)?

Also, in a wet slab, what is the physical boundary between the slab and the bed surface? I'd imagine it would differ from a winter snow pack with an identifiable weak layer between slab and bed surface, but is it a similar process with a wet slab in a transitional snowpack? Does the softening from solar radiation abruptly cease at some depth creating the boundary or is there a depth limit to the percolation of water?
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Re: Snow-geek questions

Postby TeamDino5280 » Mon Apr 30, 2012 6:57 pm

The reasoning for the PWL for this year is a combination of 3 things which you really already mentioned 1.) Consistent shallow snow pack 2.) Large temperature gradients and 3.) No large dumps to wipe the slate clean.

Facets, depth hoar, sugar snow whatever you want to call them are formed when there is a one degree Celsius or larger change in temperature for each 10cm of snow pack. Now Colorado has a continental snow pack which is the shallowest on average of all snow packs unlike maritime snowpack in the PNW which tends to be far deeper. Why is this important? Because shallow snow packs can lead to larger temperature gradient between the ground and the surface which leads to faceted snow which is why Colorado always has some form of PWL every year. This year in Colorado we received a semi decent amount of early snow and then it stopped, with only a dusting here and there or 8 inches here and there. This led to very long extended periods where the snow pack was exposed to large temperature gradients. Colorado never really experienced any really large dumps which would have done two things for us. One give the old faceted snow time to heal (if possible at that point) and more importantly a large dump of snow on the old facets could have caused a large natural avalanche cycle which could have wiped the slate clean of the old facets which is what happed around February in the Teton Pass area and the Palisades.
As for a Wet Slab the physical boundary is the same as any avalanche a week layer. One common layer is that as the water percolates through the snow pack is may have the opportunity to refreeze and cause a weak layer of icier snow to form below the surface. Once the next thaw cycle comes through and the upper snow pack begins to become saturated and the percolating water hits that newly formed week layer from previously melted water it can act as a lubricant to get things sliding. Other layers could be a dust layer or just temperature variations. I do not believe there is a limit as to how far the melt water will percolate through the snow I think the limiting factor would be the length of time of the melt freeze cycles. A long melt cycle would allow for the water to percolate further. Lastly I am not 100% on this so I apologize if I’m wrong; I think that the limit of direct solar radiation on the snow is 1 meter in depth.

Anyway that is my understanding of it all.
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Re: Snow-geek questions

Postby Dave B » Tue May 01, 2012 6:34 am

That was awesome, thanks for responding!
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Re: Snow-geek questions

Postby roguejackalope » Tue May 01, 2012 9:43 am

I seem to remember some pretty cold temperatures after some of the first snowfalls, as well, with a lot of clear, cold nights. Which really just adds to TeamDino's response, but that brought up another question... anyone have a resource to go back and look at actual temperature histories of various locations?
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Re: Snow-geek questions

Postby Dave B » Tue May 01, 2012 10:44 am

roguejackalope wrote:I seem to remember some pretty cold temperatures after some of the first snowfalls, as well, with a lot of clear, cold nights. Which really just adds to TeamDino's response, but that brought up another question... anyone have a resource to go back and look at actual temperature histories of various locations?


The Snotel site allows you to look at historic data, not sure how far back you can go but you can definitely look at the entire season.
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Re: Snow-geek questions

Postby roguejackalope » Tue May 01, 2012 11:21 am

davey_rocket wrote:
roguejackalope wrote:I seem to remember some pretty cold temperatures after some of the first snowfalls, as well, with a lot of clear, cold nights. Which really just adds to TeamDino's response, but that brought up another question... anyone have a resource to go back and look at actual temperature histories of various locations?


The Snotel site allows you to look at historic data, not sure how far back you can go but you can definitely look at the entire season.

Thanks, and I thought of that, but I was wondering if there was a more comprehensive resource for just temperature and weather history, not necessarily including snowpack data, that would open up more locations and other aspects of the weather history. The Snotel data usually serves but doesn't always give the whole picture. And I would imagine there are far more weather stations and sensors than there are Snotel sites, but maybe I'm wrong. I travel a lot for work and if I am gone for a week or two in the winter there is always a lot of catching up to do with snow conditions.

I just hope for better conditions next season. I took an Avy 1 course in December near Crested Butte, and the snowpack was a huge disappointment. They had even less snow in mid December than most other places in the state at the time. About 2 feet of snow on most shady aspects and it was almost entirely faceted. Our snow pit tests were... disappointing. I think the big culprit this year was the huge late October dump followed by almost nothing for about 2 months.
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