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 Post subject: Science of facet formation
PostPosted: Fri Nov 02, 2007 11:41 am 
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Posts: 549
Location: Stowe, VT
So, when I took my Avy class last year, our instructor didn't know the scientific reason for facet formation (actually, he knew water vapor was travelling away from the surface, but not why). Not really important, but I'm an engineer, I always want to know why. Anyway, here is my theory, please tell me if you think it's wrong or right, so when I tell people about facet formation I don't make a complete d!ck out of myself (well, okay that might still happen, but not cause I have bad info).

Since the earth is always being heated (generally accepted in avalanche science to never fall below 0 degrees C) there is a fixed vapor pressure in the air spaces lurking within the dirt. Since the snow and atmosphere are below this 0 degrees C, their vapor pressure is lower. My belief is that at a boundary like this, the two parameters will try to equalize. Take this pressure difference and divide it by the width of the boundary (depth of snowpack, cause thats what divides the air from the earth) and you get a vapor pressure gradient. This gradient causes a flow of water vapor, and this flow is what attaches to the ice crystals and creates the facets.

It just so happens, in avalanche forecasting, we measure this vapor pressure gradient using the handy stand-in of temperature gradient in the snowpack. Since vapor pressure is dependent on temperature, knowing the gradient tells us the flow, and it turns out that 1 degree C per 10cm of snowpack is approximately equivalent to a "bad" vapor pressure and resulting amount of vapor flow.


Wow, anybody still reading? Anybody care? This is the kind of stuff I think about when I'm bored. :roll:

Shep


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 Post subject: Science of facet formation
PostPosted: Sat Nov 03, 2007 8:44 am 
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I think you have it almost right. I haven't heard or read of any significant vapor transport from the dirt to the snow. For example, you get facet growth above rock slab which, of course, have no spaces lurking within them. I think it is solely the heat of the ground being at or near 0C (or at least warmer then the snow above it). I think you've got the vapor pressure gradient thing correct.

ronco (and yes I'm an engineer too)


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PostPosted: Sat Nov 03, 2007 11:38 pm 
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sounds about right. There is always vapour movement within the snowpack to a certain extent, but the gradient is what determines if a facet or sintered-round will form(weak or strong).

here is some science and numbers for ya.

http://www.schulich.ucalgary.ca/Civil/A ... Crusts.pdf

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 Post subject:
PostPosted: Mon Nov 05, 2007 2:12 pm 
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Location: Stowe, VT
Awesome link Camgina! It's cool to know that I used the terms correctly
(even if I got the details wrong :) ). And a good point, ronco, that the earth does not need to be a moisture source since the snow will be it's own moisture source.

Thanks!
Shep


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 Post subject:
PostPosted: Sat Dec 01, 2007 8:34 pm 
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Joined: Tue Sep 25, 2007 8:17 pm
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Location: bc
I've always wondered, when there is a clear night, and the surface snow leaks its energy in the form of infrared which is free to pass into space to , and the snow surface becomes very cold (ie, the air temp up 2 m high is -5C, while the snow surface temp in the first cm is -30C, and the air just above it is pretty cold too)
if there is moist air, and low speed wind, surface hoar grows due to the super cold surface of the snow
It seems that surface hoar grows faster if the air is more moist, and if the wind is slightly higher (although not too high to break them apart)

It seems that the lee side of open rivers grow a lot more surface hoar, and I've even seen surface hoar growing on rocks and trees

so how much of the water molecules that are deposited to form surface hoar come from the air, and how much of it comes out of the snowpack and deposits when it hits the super steep temp gradient in the last few cm's before the air

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 Post subject:
PostPosted: Wed Dec 05, 2007 12:02 am 
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it depends on conditions. i dont think there is a quantitate answer that can be given. surface hoar forms from sublimation in the snowpack as well as outside the snowpack(dew). it all depends on conditions.

as for the creek example - more moisture = more crystal growth (conditions dependent). a small amount of wind is necessary for surface hoar growth.

you will almost always find SH on creek beds. Why - there is more moisture & cold air follows the path of least resistance making creek beds the cold places, and often sheltered

as for lee and windward sides. it could be more a factor of radiation exposure than wind orientation.

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 Post subject:
PostPosted: Fri Dec 07, 2007 8:36 pm 
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Location: bc
hmm yes now I realize a bit more, a stream has a slight grade to it, and an inversion produces light katabic very moist winds, which will follow the stream... I have even once seen, when we made a fire, the smoke from the fire followed PERFECTLY the shape and direction of a stream on a clear night (sharp bends and meandering shape etc.), except about 10 feet above the stream.

So I guess the surface hoar ice molecules can be drawn from vapor from 3 places: the ground and the snowpack, and it could be deposited directly from the humid air right above as that humid air hits the super cold surface of the snow.
I was thinking that the downwind side of a stream (if the wind is going across the stream) has much larger surface hoar because the water from the stream is being evaporated and then deposited when it hits the super strong "cold wall" of a temperature gradient just above the white Infrared dumping snowpack.

Anyone ever seen "diamond dust"? When it's "snowing" on a perfectly clear night? It's neat!

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 Post subject:
PostPosted: Wed Dec 12, 2007 8:41 pm 
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Location: N. VT & Central Wasatch
Thought you might like this too Shep...

http://www.eos.ubc.ca/research/themes/geophysical.html

...and it's relation to snow avalanches. Always thought geo fluid dynamics w/ a concentration on avys would be a cool doctorate thesis.


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 Post subject:
PostPosted: Thu Dec 13, 2007 8:51 am 
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Location: Stowe, VT
Boy, maybe I should go for the Piled High and Deeper after all... The field work would be exciting. :)

Shep


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 Post subject: V
PostPosted: Fri Mar 07, 2008 11:42 am 
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Posts: 255
Location: powder central, bc, canuckistan
hate ta be that guy but i thot i mite correct some inaccuracy re surface hoar....
which is also called hoar frost, indicating how it is a form of precip, well ooops more correctly sublimation not precipitation. as was stated the cooling of the snow surface and the warmer moister air are key. this creates a temperature gradient and thus a vapour pressure gradient at the snow surface thats backwards / upside down, so to speak. the steep temp gradient in the upper snowpack will promote faceting in that snow, and the snowpack is a source of water vapour to the air yes but the frost is a deposit from the air to the snow.
on a tangent, yesterday in cougar brook we noticed that there had been a cycle of avalanching on both sides of the drainage that had run on V and was confined to about 30 metres above the big flats in the upper brook. huge slopes above (some of which had otherwise cut out on loaded easterly start zones) but just the lil bits at the bottom had collapsed!

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 Post subject:
PostPosted: Wed Mar 12, 2008 9:50 am 
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Location: Boulder, CO
stomppow wrote:
, and I've even seen surface hoar growing on rocks and trees


Another point I noticed, if the snow was more globular/spiny looking instead of feathery xl's, you may have been seeing rime. You get rime on the windwad side from moisture deposited by high winds.

http://www.avalanche.org/~uac/encyclopedia/rime.htm


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 Post subject:
PostPosted: Fri Mar 14, 2008 11:47 pm 
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Posts: 476
Location: Meyers, CA
or splitboards...

Image

Image

Nice thread.

Also, I've been enjoying the Gallatin youtube vids....here is one examining Near Surface Faceting.

http://www.youtube.com/watch?v=u7z9iiFf ... C6&index=2


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