I've heard friends say "the snow didn't have any energy," or "the new snow was humming with elastic energy." I've been wondering, how can snow have energy? Maybe when it's avalanching, but how about when it's just sitting there?
Cathy and Jeff Conaway described energy to me in terms of potential energy that accumulates in a column of snow before the snow settles or the weak layer disipates. Kinetic energy is released when the weak layer is triggered into an avalanche or it consolidates. Sounds like you need a Ph.D in geo-magitianism to know if the slope will avalanche or not.
Okay, what really is energy? I need a non-physics answer.
I consulted Tremper's Staying Alive in Avalanche Terrain. He writes "Researchers believe that shear quality does a good job of determining energy in the snowpack...High quality shears break on a clean, planar surface and pop out with 'energy' like they're spring-loaded. Canadians describe them as 'pops and drops,' meaning they pop out with energy or they collapse." Oh, that makes sense. That's why we emphasize shear quality in stability tests.
Energy is also used to describe avalanche release as one of the three slices of the snowpack stability wheel: 1) Strength from stability test results, such as the rutshblock or compression test, 2) Energy measured as the shear quality of the failure plane, and 3) Structure of the snow layering measured by yellow flags or lemons. While stability tests and shear quality show us how likely the weak layer is to fail at that spot (the pit location), yellow flags help determine if the failure will spread and cause an avalanche.
My question becomes: must we conduct a compression test and measure shear quality to determine snowpack energy? Not necessarily, I think. Some snowpack has energy that is obvious through red flags: whoomphing, shooting cracks and hollow sounds. You can also feel energy in the snowpack through your skiis, such as punchy conditions from new snow over depth hoar for or a skittery, buried crust.
Problematic energy is the type you can't observe through a keen backcountry snow sense. This is the energy you can't see, feel or hear. An example is that lingering depth hoar layer that shows up as a CT25 Q1 at 45 cm in the compression test. As Henry Munter from Chugach Powder Guides put it, "I guess I just don't see how anyone could, see, feel, intuit, or otherwise sense the energy stored in deep slab instabilities without getting some gloves and eyeballs into a pit..."
I'm now thinking there is 'observed energy' and 'measured energy.' What do you think? If you've read this far then you better send me an email!
Update on 2011-01-14
John Daley sent this to us after a Level 1 Avalanche Course at Hatcher Pass.
Eeva, Joe, and Kent,
Thanks for the great class. I learned a lot and have recommended it to a couple of friends.
We talked a couple of times about potential energy and engineering etc. I am not an expert but do have a few opinions based on my training and experience. Below is a bit of a lengthy opinion on the matter of potential energy and failure.
I believe it is fair to say that the snowpack in the mountains has potential energy, in some cases a HUGE amount of potential energy. This was bestowed upon it by the wind and storms that moved it up into the mountains. The physics definition is basically potential energy due to mass and position or as elastic energy. I think the vast majority of the potential energy of the snowpack is simply mass and elevation.
What I have a hard time with is the idea that a block of snow has a lot of potential energy in it solely because it has a spreading shear failure. This would insinuate that a similar block of snow in the same location would have less potential energy if it did not have a spreading shear failure. While there could be a small amount of elastic energy stored in one versus the other I think a better explanation would be that one block is more prone to failure than the other and that they both have similar potential energy.
If you study the engineering of materials there are all types of failures that can occur; tension, compression, shear, and various combinations. Failures can be gradual, sudden, ductile, brittle, catastrophic, or minor etc.
In my opinion snow has some mechanical properties similar to concrete, a very thoroughly studied engineering material. Concrete has compression strength in the 3,000 – 6,000 PSI range. However its tensile strength is typically 1/10 of that or in the 300 – 600 psi range. So concrete (like snow) is strong in compression and weak in tension. Of course concrete is typically reinforced with steel for this very reason and the steel is used for the tension strength of the material. The reason I bring this up is to point out that snow is more likely to fail in tension as opposed to compression. Many of the large slab avalanches shown in the class materials were initiated at the top of the slope or in the crown area were there were significant tension forces. Tension failures in many materials tend to be sudden and catastrophic.
Shear failures can also be sudden and catastrophic. In addition, shear can actually generate a form of diagonal tension. It gets a bit complex but shear forces in materials can create three dimensional stresses with compressive forces in some areas and shear forces in other areas. In materials like concrete, large shear often results in diagonal tensile cracks that can propagate rapidly.
So based on the above I think it would be fair to say the snow pack can fail in a number of ways. Tension and shear failures in particular have the potential to be sudden and catastrophic. The extended column test can show if the snowpack in that area has potential for a sudden failure.
Below is a link to a physics page that talks about potential energy:
http://www.physicsclassroom.com/Class/energy/u5l1b.cfm
I realize that all this is a bunch of nerdy techno engineering speak. This was the ONLY minor point that I had trouble with in the class. I am not an expert on this and am just expressing my opinion. In reality I place a higher value on practical experience and knowledge than on theory. You guys were certainly great about that.
Cheers,
John
Update on 2010-02-26
Joe Lujan from Anchorage sent me this email:
Hey Joe,
I think you make a good point in your spiel about observed and measured energy in the snow. Obviously, stability feels more intuitive to us if we have some way to quantify it. CT25 is safer than CT7, for example. The higher the number, the safer. But I don't think many people consider shear quality or depth. Like you said, CT25 may not be so good if it pops like a mouse trap 45 cm down. Now we may be able to observe this "energy" as we hike, as long as it is a fairly shallow energy. Yeah, I can see shooting cracks in front of my skis, but if those cracks are failing three feet down as I skin, I should get the hell out of there. I am likely observing cracks shooting in the surface snow. This is a common occurrence, but knowing it is shallow, many people push on. So, we have our observed energy in the snow. This is the whoomphing, the shooting cracks. Whatever we notice on the way up. We have the measured energy. The CT scores combined with the shear quality. This is what we "measure" in our stability tests. But I think the unnerving fact is what we don't observe or measure. We won't likely see shooting cracks if the failure is three or four feet deep. Nor will we measure the same weakness in a compression or shear test in every pit we dig. So what do we do? If we tell people to question their pit results, even after noticing no signs of instability on the way up, they will begin to wonder why they dig pits in the first place. But if we tell people to trust the numbers and only the numbers, shy of digging twelve pits in different locations in a single day, I think some bad things will happen. I think the kind of "problematic" instability or energy we are talking about is neither observed nor measured in many cases. It lies somewhere in between. And hopefully when one deals with this kind of "dragon," they either realize that they are not ready for this kind of situation and turn around, or they accept the level of risk. There are many times that you simply can't be sure the snow will not slide, but if you have the experience, knowledge, skiing chops, and confidence to accept that fact, hopefully more often than not, lady luck will shine upon you. So, I guess I'm thinking that there is observed energy, measured energy, but also something else. What exactly that is, I'm not sure. Maybe it's localized energy which is so localized it is hard to observe or measure. We've seen different pit results in different locations. We've felt wierd collapses and density inversions for fifty steps that suddenly disappear just as we begin to grow concerned. We hope these feelings and observations don't slip through the cracks, but again I think it comes down to the inherent risk. We choose to accept our judgment, and accept the fact that maybe we can't sense all energy in the snowpack. But we can find enough facts to make a safe decision that gets us home safely so we can drink beers and reminisce about our day. Let me know if this makes sense. It's late.
Cheers,
Joe
Update on 2010-02-26
Ryan Hokanson sent me this email on Feb 24:
I think that all snow (or other objects) capable of falling has potential energy, and when it falls that potential energy is converted to kinetic energy. When a snow slope is pasted up high on a hill, not avalanching yet, it has high potential energy and no kinetic energy. As it falls (avalanches) the potential energy is converted to kinetic energy, which is used to snap trees in half and demolish houses.
I think when people talk about 'energy' in a snowpack, they're really talking about how easy it would be to trigger this conversion from potential energy to kinetic. The ease of doing this is influenced by slope angle, tensile strength of the snowpack, sliding surfaces, natural anchors, etc. I think if you look at it from a purely physics viewpoint, any 100 gram falling X distance has the same energy as any other 100 gram object falling X distance. Thus, ALL snow has the same potential energy as an identical MASS of snow capable of falling the same distance. It's really stability they're talking about, ie, how easily the energy is released.
Now post some euro ski photos!!!!!!!!
Hope you're having fun...R
Update on 2010-02-26
Mike Bromberg from Crested Butte sent me this email on Feb 26:
Anyways there are some very interesting points in your post with regards to "observable energy"- remember that shear quality is only one piece of the ever expanding jigsaw puzzle and that fracture character (the pops and drops) and ultimately the propagation potential is more important when determining "energy" in the snowpack. Many nowadays, especially Canadians, will argue that fracture character trumps all other observations with regards to snowpack test observations, hence we now code the compression test without the # of taps because the tap strength is highly subjective (CTM @ 45cm instead of CTM14 @ 45). http://www.ucalgary.ca/asarc/files/asarc/FractureCharacterIssw02.pdf is a good paper. Most agree that the Q scale (even in it's revised form) is not exhaustive enough of an observation tool, thus the fracture character is often preferred.
Also, what about the actual mechanics of a natural avalanche? It all adds up when we talk about stress exceeding strength and failure occurs in terms of an artificial trigger, but little is known about fracture mechanics in a natural avalanche. This paper below is super interesting on that topic. Yes this is somewhat off topic, but interesting. http://www.wsl.ch/personal_homepages/schweizj/publications/Schweizer_Review_slab_release_ISSW1998.pdf
I guess to answer your question the same way you did: No, you don't necessarily need to dig a profile or conduct a compression test to determine if there is energy within the snowpack, however many would suggest that any of the large column tests ECT, PST, and RB would be more beneficial in forecasting propagation propensity and therefore "energy" than a CT anyways.
Given your test results you then have some frame of reference for how that particular snowpack structure reacts during tests, other than just how it feels under foot - an informed guess at best.
Good stuff,
I hope your getting wicked d-sendy out there! where are the trip reports?
Mike
Update on 2010-05-17
Here is Henry Munter's original note to me:
Yo. Lemons! I think that structure really bridges the gap between strength and energy, just like that schematic would suggest. I think that if you know that there is a 4-5 lemon setup, let's say a bed surface of pencil rounds, a weak layer of 3mm surface hoar, and then a slab of finger rounds that turns into perfect powder less than a meter away at the surface. This snowpack is a rock prime for trundling, a skateboard at the top of a ramp, or a drunk on ice skates. You don't actually have to observe the motion (as in a shear quality assessment) to know that there is a lot of energy stored there.
The thing with the lemons and the checklist profiles is that they all come from data sets specific to certain areas and snow climates, and a few of the parameters are a little bit artificial. In Alaska we deal with a bunch of different snowpacks on a regular basis, within the same ranges, within the same seasons. Still, certain snow structures have energetic relationships, where crust and facets might be a skateboard and concrete, and crust and pencil rounds might be a skateboard and gravel.
I still worry about the ways that we measure strength though. That simple schematic says that a RB score of 2, or a CT of 8, is not strong, where an RB score of 5 and a CT score of 22 is a lot stronger. That's true, but those scores relate directly to the depth of the weak layer. If you've got a weak layer 140 cms deep and you get it to break at 17 on a ct, that's moderate strength by the scale, but that's awfully easy for force that's been absorbed by 140 cms of snow...
I've been thinking about what you were saying about your friend who uses his sixth sense to determine snowpack energy. It's interesting to me because Clark always does a similar thing, referring to the "tension" in the pack. He digs pits all the time, so he might be more calibrated, but I still don't know what he actually "measures", other than the feeling under his feet. I suppose in a maritime pack we're really just looking for new snow instability and windslab, the latter of which does have a "tension" that you can feel when you're skiing on or around it. You can also see windslab, most of the time at least. I guess I just don't see how anyone could, see, feel, intuit, or otherwise sense the energy stored in deep slab instabilities without getting some gloves and eyeballs into a pit though....
Update on 2010-05-17
Here is Jeff Conaway's note:
Yeah that is a hard question. A term thrown around quite a bit. I guess I think of it as potential energy. When you left a ball up 2 ft it has overcome the gravitational force applied to it and has that potential energy. When you drop it that potential is converted to kinetic. As snow accumulates potential energy is stored in the snow pack before consolidation (just thinking about snow in a column, not in reference to the whole mountain). The gravitational force is continually compressing the snow pack as well as any mass applied above. Of course there are layers that don't bond and sit there with energy in them for some time. I use that energy term to wind loaded snow that seems out of equilibrium and to a snowpack that has a layer that can collapse. So that is my quasi science geek/ back country dirtbag skier answer.