Snowpits

davidof · Posted: Thu Jan 13, 2005 7:50 pm Post subject: Snowpits

I don’t know if this is of interest but I’d like to share the results of a snow profile I made today. I don't dig many pits so excuse any gross errors I have made. The pit was dug on a sheltered n-e facing slope (330 degrees) at 2pm in the Belledonne mountains in the French Alps. There had been about 5cm of snow in the morning. The ski was clear but their was cloud to around 1750 meters altitude.

Recent avalanche activity included some sluffs originating off the steep slopes which then took the surface layer with them. I knew there was a crust that had formed, probably on the 26th of December. There was about 30cm of fresh snow on the 28th of December. I also found a crust buried much deeper 63cm which dates from earlier in the year. I’ve marked both as freeze-thaw crusts although some locals think the 26/12 crust was due to fog formed by a temperature inversion.

http://piste-hors.chez.tiscali.fr/images/snowpits/13-jan-2005/profile.jpg

Shear tests showed a sliding plane, not at the first crust but at the 30cm point. When I look more closely this appeared to be the interface between two layers of similar crystals but with a slightly different density. I wonder why the snow isn’t better bonded at this interface? I assume the snow is from two different events.

I’ve included photos of the crystals in 3 layers. Anyone care to say what the crystal types are and from what layer they come?

http://piste-hors.chez.tiscali.fr/images/snowpits/13-jan-2005/snow-1.jpg

http://piste-hors.chez.tiscali.fr/images/snowpits/13-jan-2005/snow-2.jpg

http://piste-hors.chez.tiscali.fr/images/snowpits/13-jan-2005/snow-3.jpg

Gary Brill · Joined: 07 Dec 2004 Posts: 1717

#2 appears to be angular facets that are large. Beneath a crust?

#3 appears to be small grained and bonded, but there are some facets. either rounds with good bonds or more likely part of a sun crust.

davidof · Posted: Fri Jan 21, 2005 10:18 pm Post subject:

Snowman · Joined: 06 Dec 2004 Posts: 791 Location: Ask Heisenberg

One of the problems is that people get hung up on the textbook photos and definitions of faceted crystals (kinetic forms) and rounded crystals (equilibrium forms).

The reality is that both processes occur, sometimes simultaneously and sometimes sequentially.

So its typical to find facetted crystals which are beginning to round and bond. Or rounded crystals which underwent some facetting.

Understanding which processes occurred when is greatly assisted by some knowledge of the weather history of the area.

Jim
Avalanche Center
AlpenPro

Valdez Telehead · Joined: 06 Dec 2004 Posts: 1568

I looked at your profile. You indicated you got a sheer at 30cm, but your profile does not identify that weak layer. Did you miss it identifying it on your profile?If so it was very thin. There has to be something their measurable. How did you identify each layer for your profile..drag a card, tap the pit wall????

Note how the temp grad lessens dramatically below the crust later. Those two layers appear well bonded with similar densities and temp.

????

Nice work up BTW.