Printable View
I do lots of stopping and sniffing the air and bat sensing the subsurface layers. I also usually try to ski as if I don't have a beacon but at the same time pretend that the snow is 100% unstable. By that I mean always using terrain features as waypoints and always have exits planned, turn for turn. I have probably dug 5 full pits over 15 years of active bc skiing. I like steeper, unstable snow vs medium angle mid-stable snow.
Good thread. Column test fracture characteristics correlated with skier triggerering:
http://vimeo.com/30996756
For academic work, I have to dig pits once or twice a week so I'll admit, I get a feeling for whats going on through that.
For my free skiing, I hardly ever dig pits. I simply feel that spatial variability is so great, especially in more complex terrain (can't wait for the talk on this saturday!) that unless you can dig multiple pits on the slope you're about to ski you aren't learning much. If There is a specific layer I am concerned about I will dig to check it out.
In my humble, non-expert opinion, your time is better spent looking at past weather and following it through the entire season. Also, in the early season, walk around your favorite zones and look for triggers. Its always good to know a general locaiton of the big ones.
In short, pits are awesome and have great uses, but don't ever make the decision to ski a slope based simply on a pit showing no danger.
ok so I just came across this and its a great thread, lots of good info and I feel that I more or less have been operating in accordance with the general consensus of the thread....Had one question that is more just a point of curiousity that was raised when I was unable to answer a buddies question. with the different failure rates in a compression test of: 1: before tap, 2: 10 finger taps, 3: 10 elbow taps, 4: 10 arm taps and 5 being no failure....do those more or less correspond with extreme, high, considerable, moderate and low avy danger? or am I way off?? Like I said I dont want it to come off like I'm assuming that one result is what dictates my impression of the conditions for the day. I just read the classifications out of the "avalanche handbook" and was curious as to what people think
No not really.
The number of taps gives you a rough index of the dynamic force required to cause a fracture in the test column. But as most people here already know, it's not really very precise and it's difficult to apply the information elsewhere. That's why I refer to it as an index of the force required to cause a fracture.
Shear quality is much more important because fractures at/around interfaces are likely during skiing. The question is: will the fracture propogate? Shear quality, especially from rustchblock test, gives you useful information about propogation propensity, according to The Avalanche Handbook, shear quality can constitute Class I information ( data that reveal direct information about instability ).
Generally, the results of snowpack tests are Class II data, which means there is uncertainty about exactly what the test reveals about instability.
Quote:
Originally Posted by point it
Hey Cookie... you in town? Coming out with us to get some fresh as soon as its ready like last year?? That was a great day!
Found a good link that might be of interest to some folks...
Here's more info on what Cookie's talking about...
http://members.shaw.ca/coldsmoke/oth..._character.jpg
Yes, I'm in town. Limiting my travel a great way to avoid triggering avalanches at home.
I'll be ready for the fresh stuff as soon as the rocks are covered. Keep me posted?
ok makes sense, thanksQuote:
Originally Posted by CookieMonster
That's an excellent link Johnny Casino.
Example 1
I've prepared a false colour map of the relative strength of the interface between new snow and a buried crust for an area of snow about the size of a rutschblock.
Brighter is weaker, and it's pretty obvious that the characteristics of the weakness are not suitable for fracture propogation, at least not across the whole block. ( Research by McClung discusses the characteristic size and strength characteristics of macroscopic weaknesses required for avalanche formation. *1, *2 )
Figure 1.1. This image is not scientific and is being used for illustrative purposes only!
http://i.imgur.com/FgGHU.png
Example 2
This example contains a false colour map of the relative strength of the interface between new snow and buried surface hoar.
Again, brighter is weaker, but this time the characteristics of the weakness are suitable for fracture propogation across the whole block. Fracture character provides direct information what happens when force is applied to the weakness, especially its propensity to propogate, and possibly something about the manner in which it will propogate ( which is why the terms sudden, planar, pop, and drop are used ). In that sense, fracture character tells you whether or not the weaknesses look the image below, or the image above.
Figure 2.1. This image is not scientific and is being used for illustrative purposes only!
http://i.imgur.com/VkS5i.png
I could be wrong ( I certainly DO NOT speak for Dave McClung ), but I believe McClung characterises 'fracture character' as Class I information that reveals direct information about instability because 'fracture character' tells you whether or not the characteristics of macroscopic weaknesses ( their size, strength, reaction to stress ) are suitable for avalanche formation. In the case of a rutschblock, sudden planar fracture characteristics means that the conditions required for avalanche formation are present. *3
The number of taps tells you that you can cause a fracture, but doesn't reveal whether or not the characteristics of the weaknesses are suitable for avalanche formation - unless the test score is very low. With fracture character, it's fairly safe to say that sudden planar results mean conditions are suitable for avalanche formation regardless of the number of taps.
Take this with several grains of salt. I've written a blog post that discusses mixed mode crack propogation / shear fracture in a ( fairly? ) approachable manner. It wraps up various failures into the concept of delamination, which is an easier way for most people to think about fracture mechanics and avalanche formation.
http://avalanchesafety.blogspot.com/...intaining.html
References
*1 http://www.geog.ubc.ca/avalanche/pub...ectLaw_JGR.pdf
*2 ( Work by Schweizer, Simenhois, Birkeland et al. )
*3 McClung discusses failure characteristics in a rutschblock as being highly representative ( size, strength, etc. ) of the initial failures required for skier-triggered avalanches. I couldn't find the source.
I always go into a pit with a question like "how good is the new snow bonding to the old surface?" or "is that layer that they were talking about on the avy report a week ago is still reactive?"
Good rule to remember:
A PIT NEVER TURNS A "NO" INTO A "YES", BUT IT CAN TURN A "YES" INTO A "NO".
Interesting stuff CookieMonster.
Let me know if my interpretation is wrong.
The shaded regions are less rigid under stress (more elastic, so they can absorb energy without breaking and can help propogate forces through the snowpack).
So the areas where the snowpack goes from elastic to inelastic rapidly are where you will get your fracture plain.
Then temperature gradients at crust layers are going to cause facetting / high inelastic-elastic gradient to from at the crust, and that's again where the fracture plain is.
I was wondering though. Could you put a z axis on that figure (or a similar one) and show the effects of a crust layer and facets?
Don't know how I missed your comments.
I'm not sure if your interpretation is accurate. The sample diagrams in my post are included as a non-scientific illustration of how poor bonds at interfaces influence shear quality.
Sent from my Paranoid Android TGR Forums
No worries.. I sort of figured that out after my post.. Classic.Quote:
Originally Posted by CookieMonster
bumpin a good thread for the jongs who can't search very well
Another bump
Whether pit tests are useful depends on the avalanche "problem"/"concern" (or whatever the next new term for this is) and what you're doing with it (just out for a tour, trying to forecast?). Here's a nice guide from AIARE
http://www.cbavalanchecenter.org/use...IARE_A_O_R.pdf
My own understanding is that pit tests are useful for persistent layers, somewhat useful for instabilities within storm snow (though less formal tests may tell you just as much), not useful for other concerns.
Did an extended column test last weekend and reminded how interesting snow layers can be. There wasn't avi danger and I had already done a few laps but the pit confirmed what I was seeing and hearing. Just one piece of the puzzle. Dug it while taking a break on the up.
I'm impressed that in three pages I'm the first one to mention saw test. Seems underutilized.
good info. im gonna read up on some of these. i liked the "it never turns a no into a yes, but the opposite is true" quote.
http://www.snowpit.com/articles/traps%20reprint.pdf
May help add something to the original discussion. Sounds a bit like 3.2
Quote:
Originally Posted by komo
This is a great thread. Wish I had read it BEFORE this last weekend.
I don't dig pits very often, mainly it's a quick pit to see what layers there are and then a compression test to see what the likelihood of fracturing is.
The avalanche forecast covers a large area and layers/instabilities in new snow is usually what we're concerned with around here and as was proven to me this last weekend, those instabilities can be highly localized.
To me, the idea of digging the pit is to look for something that says "don't ski this" even though other signs point to "go." Unfortunately that can lead to a false sense of security.
since the topic has come up on the usefulness of pits. I was wondering what percentage of avalanches were pits dug prior to the event. How about the high profile ones?
that'd be an interesting number. I remember this one:Quote:
Originally Posted by BFD
http://utahavalanchecenter.org/avala...eshoe-mountain
and one in Canada - Rockies maybe? where the pair triggered it digging or going to dig a pit