Field observations vs snowpack tests: Which is best when?
Printable View
Field observations vs snowpack tests: Which is best when?
Ahh, yes, I remember this thread....an excellent thread and a great time of year to bring it back, as everybody is slowly gearing back up and getting ready to head back out into their various gnar.
This thread has the goods, if you look through the various posts...analytical snow pit datasets vs. slightly more casual probing, and general terrain and depth observations.
I think it depends how a person's head is set-up. In the laboratory, I'm very analytical...and know that aggregate minor mistakes just widens one's margin of error, so I do everything I can to absolutely minimize every error potential. But in the mountains, there are so many factors that you can't analyze every single one...so it becomes a factor of intuition combined with experience.
Some people are analytical and will always want to be digging pits to back up their observations out there....others will hardly dig a snow pit, even if they have the time and will go on their gut and some pole-probing. Is one better than the other? No...just different tactics to match different personalities. I'm sort of in the middle but tend more towards gut instinct and probing.
The KEY is whatever tack you take, NEVER rely on a single dataset....just like you wouldn't base all your movements on a few pole-probings in a specific area. Just like has been hammered in this thread and this forum, there are WAY too many variables in snowpack density...can be different 2 meters away, for a single test pit to give substantial OVER-RIDING validity. If it makes you feel better/safer to spend all day digging multiple pits in a grid pattern...then sure, your data will be more valid...but do you need to? Depends on the person.
Dig your snow pit or don't dig your pit...just make sure, whatever you do that you're comfortable with your decision. Your gut starts to tweak on you? Then you know you have to look around and re-evaluate.
Be safe out there, that's for sure. Just know that no matter how many different tests you use, the mountains tend to play by their own game...you're not the referee. Just a player. Knowing that you CAN get squashed by a big 350 lb linebacker if you don't stay on your toes and move lively...that's the pulse-quickening aspect of why we go out there, otherwise we'd all be staying home and watching the game on tv, me thinks.
--
strong post I doubt the great thread tag will ever be bestowed on the crap you start :rolleyes2but well we all start the threads we wantQuote:
Originally Posted by Alaskan Rover
scariest pit I ever dug
http://i5.photobucket.com/albums/y19...hbum/sf002.jpg
high danger day
collapsed as a partner dropped into pit to help
sympathetically released whole skiers right slope
knew shit was diceey that day but a few sucessful ski cuts and some sking of bed surfaces led to a great day of learning and a safe return to the trailhead witgh my crew
one of my partners asks
"what will you need to find in that pit to decide whether to ski that slope"
a lot
good question IMO
Fuck that was informative!!! Took me about an hour from start to finish but was cool to see opinions. If I am skiing my back yard I dont think I have ever dug a pit but just from observations and guys that have been rocking the same mountain for years. The mountain tells you what slides when, how often, under which loads. On heavy snow fall days we have average to steep lines that are very protected sub alpine that in the last 30 years no one has ever seen slide. But 30m west on a slightly different aspect its a totally different story.
If I see the winter develop season long I know whats there. If not ill dig but I cant say I do more than 4 a season. Its easy to get caught up in pit results. Identifying all the other signs is a much how to say "be totally aware of your surroundings". Get all nature boy and be part of the environment. Sounds hippy as shit but its the truth.
other side shot
glad the pit wasn't dug there
http://i5.photobucket.com/albums/y19...m/P1011327.jpg
Sometimes ya know what you're lookin for but the act of findin it is important
http://i5.photobucket.com/albums/y19...ski/tlp004.jpg
who wants to be dope on a rope lookin for shit in the pit today:wink:
http://i5.photobucket.com/albums/y19...ki/toph052.jpg
nice pics Dibbs- I dug very few pits last season, but most of the days I was bc were usually days after a storm since I ride lifts on pow days, and from paying attention to pole probing and skinning up/kicking some smaller cornices and stomping in the snow at switchbacks my group never really felt the urgency of digging a pit. we also didn't ski TOO much seriously dangerous terrain. It's neat seeing pics from high danger days and how that affected the pits you dug.
From Ed LaChapelle (1980)
Formally stated, the steps in avalanche forecasting are these:
1) Available data is collected about the place and time in question. Some of the data may be vague, (second hand reports, past weather trends), while others may be quite precise (snowfall records, avalanche records, weather maps).
2) A hypothesis about snow stability is formed on the basis of the initially available data. (A first estimate may see an unstable snow pattern, or the amount of snow required to overload a slope may be anticipated.)
3) The hypothesis is tested through observation and experiment. (Field checks are made for avalanche occurrence, mechanical tests are made for failure planes in new snow, or artificial release is attempted.)
4) On the basis of tests, the hypothesis is confirmed or revised. This test revision process may be repeated a number of times over time spans ranging anywhere from hours to months, if a sufficiently reliable picture about snow stability has not yet emerged.
5) Finally, the hypothesis is revised or confirmed to the point that it is seen to represent current reality of the snow cover. An evaluation or prediction is made. (Safe slopes are are selected for skiing, a degree of hazard is estimated, or on avalanche warning is issued.)
6) Actual avalanche occurrences (or non-occurrences) are monitored to check prediction accuracy.
The essence is always to have in hand an opinion, no matter how vague or ill-informed at first, about the current state of snow stability. The opinion can be revised and improved as more clues become available. The error is to have had no opinion at all, to have started building no prior knowledge, before a decision has to be made
A fairly concise description of an incredibly complicated process. Snow pits? Definitely good for something but just one small piece of puzzle.
Here's how I look at digging and stability tests.
Day tours in areas where I'm familiar with the snowpack: I'll read the public bulletin and probably won't dig. I'll rely on field observations and poke around in the upper snowpack using a pole test or hand shear. If there's something ugly in the public bulletin I may investigate further.
Day tours in areas where I'm not familiar with the snowpack: Same as above but if the bulletin is reporting tricky conditions I'll dig once or twice to see what's going on.
Trips where I'll be in the area for more than a couple of days and familiar with the snowpack: Depends what's going on. If there's something to watch for or a significant change during the trip I'll dig to evaluate what's going on. If things are generally stable with little change I'll rely on field observations for the most part.
Trips where I'll be in the area for more than a couple of days and not familiar with the snowpack: I'll probably dig a decent profile at the beginning of the trip to get a baseline on what the snowpack looks like. For the first couple of days I'll dig several more times in various locations to get an idea of consistency in relation to what I saw in the original profile. After that I'll be comfortable with my read on the snowpack and will monitor any tricky layers or reevaluate if there's significant change.
Where digging really has it's place is when you're dealing with a persistent weak layer (PWL). It's handy to confirm distribution and consistency of the PWL:
If you dig a bunch of holes and the PWL shows up in all of them and produces similar results you know what you're dealing with is widespread and you need to choose terrain accordingly.
If you dig a bunch of holes and the PWL only shows up in specific areas (ie, a certain aspect and elevation) you can start to isolate the problem to those specific areas. This requires due diligence and keen observation skills in your investigation because you need to be sure you've been thorough in isolating the PWL to those specific areas. There's serious potential for dire consequences if you fuck this one up.
If you dig a bunch of holes and the PWL shows up randomly here and there with no consistency in it's distribution and/or test results you have a sketchy situation on your hands. You can stress out about what areas are safe and what aren't or you can just treat it like it's a widespread problem and choose terrain accordingly.
Nice pix. I take it you concluded the pit test pretty 'rapidly' after THAT. But at least there was a tree close by at the initiation point, if that slab had decided to go for a nice trip. Trees have saved my ass more than once!!Quote:
Originally Posted by skifishbum
Johnny Casino:
Thanks for the good vid of the professor discussing snowpack analysis. Some good points in there, for sure. That codger reminds so much of this professor at the Geophysical Institute in Fairbanks....actually he reminds me of about 15 people there!
But will someone PLEASE break the telescoping antenna off of their car and give it to this guy, so he can use the damn thing as a POINTER. Sometimes professors forget that they are NOT invisible.
Hey, Professor...get one of these: They work great!!! (as a lecture pointer, not a snow probe ;) )
http://buy.traxxas.com/images/products/2017.jpg
-
ah ha!!!
http://i5.photobucket.com/albums/y19...ki/toph052.jpg
now i remember you. my only memory is passing you while heading out to the g line. you had a few folks with you, i was solo. thanx for breaking trail part way out! you didn't seem very amused at my passing your group. oh well:)
twas a great run if i remeber correctly.
rog
Seems like weather observations, local slide activity, and local propensity for activity are pretty important.
I've usually decided to ski something before I dig. I go to UT from CO and the snow seems much more cohesive. Then go to the the pacific northwest maritime snow takes that up an exponent. Familiarity breeds contempt but maintaining a healthy respect will see you thru to another day.
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
Stupid CAC site has many dead links
Tent Ridge K-country in the Canadian Rockies. Your basic 30 degree unsupported planar slope. 3 skiers involved. Were digging a pit on that slope
http://www.clubtread.com/sforum/topi...TOPIC_ID=24657
Good thread.
That was an interest report.Quote:
Originally Posted by Hugh Conway
Hope "A" had a full recovery from the incident.
wildland fire tangent:Quote:
Originally Posted by Meadow Skipper
Isn't it a little like spinning a weather?...it's definitely not a bad thing to do, but it's usually not a surprise unless you just arrived somewhere; and it probably shouldn't be the single focus for a particular go/no-go decision. It's going to be part of a due diligence process in big organized structures like DOT and ski area management for legal purposes, and the specialists at the avy center are happy to get field obs to keep the forecasting sharp...but when you're out there on your own...meh...I'm probably not going to be calling obs in every hour from my 2 man single tree fire in the wilderness; same as when you're up on some piece of terrain on a regular basis and you pretty much know how things have set up.
dug one full and prob a good 5 haisties a day on our week in bc haities told me almost as much as diggin & shovel whompin
dug quite a few the last 3 in the wasatch don't base go no gos on em as much as general snow geekery
kno plenty of more savvy peeps than i who never fully quantify much as someone aptly pointed out in the other thread
reached the point of spatial varibility meh these days
as always group dynamics if your partners are into it you'll do more
Exactly. IMO there are a lot of parallels between wildland fire and travel in avy country.Quote:
Originally Posted by YetiMan
With a little adjustment, a number of these concepts are valid for avy country travel:
http://www.fs.fed.us/fire/safety/10_18/10_18.html