Avy Hazard and Decision Making (from Cement Basin post)

That's right - never, ever go first on a powder day. You have to stay in bed and wait for photos so you can see whether things look stable!

Having not been familiar with the "history" of this season's snowpack/weather (only what I gather from Crystal snow report..) up there, I would have in mind the possibility of depth hoar to some degree (or none?) on N. facing slopes-- but I suppose since the sun angle is so low this time of year, this could be a mute point?

Curious what others thinks...

So if it was below freezing when it started snowing, and there was wind blowing snow onto the slope and consolidating it, what evidence was there that the bond to the crust was good?  I'd like to understand why you guys think the avy danger was moderate/low when I can't figure out how its possible to tell how well the bonding was here....  I understand why a few tracks down the slope was re-assuring, but other than that, was there anything else?

The bond was good for several reasons which I'll restate once again:

* Temps dropped from above freezing at all elevations, to below freezing near the onset of precip (within a few hours before the onset even at 6800 ft). The air temps don't need to be above freezing when the snowfall starts to ensure a good bond -- it is the snow surface which must be near 32 °F for this, not frozen solidly and well below 32 °F. That situation was ensured by the warm temps overnight. Even at 6800 ft, the snow surface temp could not have dropped below 32 °F on this day, there were just not enough hours of cooling prior to the snow starting.

* Even more important, the surface crust was very lumpy and bumpy, with no smooth icy rain crust anywhere, and only somewhat smoother on isolated steep south-facing aspects which had a sun crust. All other areas had no sun crust (since it can not develop on any other areas this time of year), only a rough crust formed due to settlement and sublimation and wind erosion during 3+ weeks of drought. This type of rough crust was obvious at every location and elevation that I skied this December (Rainier, St Helens, Crystal, Snoqualmie), except some protected flat areas which developed surface hoar (which is not an avy concern when it occurs only on flat areas). All crusts are not the same. This rough old crust ensured a better bond and poorer sliding bed surface.

There was lots of "anything else" too which made the avy hazard look low-moderate. Essentially all observed signs showed a lack of instability, except on the isolated S and SW aspects up high which involved too little snow to be hazardous (thin slabs, no propagation). No red flags. No hard windslabs on loaded lee aspects, only cohesionless snow. Etc, etc.

I'm also not convinced how a wearing a pack would help flotation in an avalanche. I see how it would protect the spine, but the density of the pack is probably going to be denser than the snow in motion - while this would slightly reduce the skiers average density, it makes them heavier and bulkier, making it harder to stay on top of the snow. I guess i'd have to see statistics similar to the airbag ones to believe this.

This is simple physics, which is probably the reason that it is so poorly understood (blame the terrible state of science education in the US) and perhaps why the conventional wisdom for so long was to ditch the pack in avalanches. For flotation in an avalanche, it does not matter if the pack is more or less dense than the snow in motion, or how heavy the pack is -- all that matters is that it is less dense than what it is attached to (your body). So the total density of pack+human will always be less than the density of the human body alone, which is the main thing that matters for better flotation in a fluid (such as the flowing snow of an avalanche).

There is also a secondary effect, which is that larger objects tend to sort themselves to the surface of a shaken mixture of solid particles of similar density due to packing (like shaking a box of cereal, the large unbroken pieces end up on top, smaller bits go down). So anything which makes you bigger and bulkier (like a backpack) will help slightly in this manner too, although this effect is less important than pure flotation (since an avalanche behaves more like a flowing fluid, and less like an agitated mixture of solid particles).

By the way, at a specific gravity of about 0.3, the loaded pack is actually comparable to avalanche snow in motion, and less than most avalanche debris deposits. See this chart from The Avalanche Handbook, 3rd Ed., p. 134 (units divided by 1000 to convert from kg/m3 to specific gravity):

[tt]
Small dry avalanche deposit: 0.2
Medium-large dry avalanche deposit: 0.3-0.4
High-speed dry avalanche deposit: 0.5
Wet avalanche deposit: 0.5-0.6

Dry flowing avalanche: 0.1-0.2
Wet flowing avalanche: 0.15-0.3

For comparison:

Water: 1.0
Human body: 1.01
Loaded backpack: 0.33, for typical 30L pack weighing 10 kg or 60L pack weighing 20 kg
Human + 30L daypack: 0.8, assuming 70 kg (154 lb) human + 30L pack weighing 10 kg (22 lb)
Human + 60L overnight pack: 0.7, assuming 70 kg (154 lb) human + 60L pack weighing 20 kg (44 lb)
Human + 30L airbag pack: 0.33, assuming 70 kg (154 lb) human + 30L pack weighing 12 kg (26 lb) with 150L airbag deployed
[/tt]
These numbers are obviously rough approximations, not exact values!

So the daypack reduces the density of the human+pack combo by 20% relative to the human alone, the overnight pack by 30%, and the airbag pack most of all. All of them will float better in flowing avy debris than a human body alone.

I have never seen any avalanche statistics which have info on whether the victims were wearing a pack (and most importantly, including pack volume and weight), this data appears not to be collected. However, I think that (thankfully) there are just not enough avalanche burials to ever generate statistics which would demonstrate this effect, even if the data were properly recorded. It would require thousands of avalanche burials, of victims without and with packs of various sizes, all with burial depth info (or whether they stayed on surface), in order to get a statistically significant result. The lack of stats does not mean that a physically-certain result is false -- the flotation would be measurable if the data existed.

Thanks for the feedback on the crust, interesting stuff.

As for how density effects a skier in an avalanche, this is most definitely not simple physics, as I don't think some of the assumptions are valid:

* i'm not sure the 'granular convection' referred to is valid.
    -i'm not sure this phenomenon exists where there is lots of turbulence
    -i'm not sure what the effect is when there is only one large object - when one thinks about a cereal box, small particles filter down through the cracks between large particles.  But when there is only one large particle and no walls to the 'container', I would expect this effect not to exist, or at least not be as pronounced.

*I would assume the density of dry moving snow, of 0.1-0.2 here, as once the snow is 'set up', the density doesn't matter as your stuck...  In this low density, turbulent environment, I would expect bulk to have a bigger effect than buoyancy.  Its like having two rocks of similar mass where one is larger than the other bouncing around the bottom of the river.  I would argue the bigger rock is going to get more beat up.  When you go to an airbag however, there is a huge reduction in density, only 1.5-2x that of moving snow as opposed to 4x-8x with pack & 5x-10x without.

*Further to the last point - if a person is on their back, stroking with your arms with snow pouring over you, the frontal area top down is ~2 sq ft.  With a backpack, its easily twice that, doubling the force pushing the body around.

As an aside, when the avy airbag report came out last year, I did a quick analysis, and it seems their effect is so pronounced, it is statistically significant even with a tiny sample:

docs.google.com/spreadsheet/ccc?key=0ApQ...thkey=CJj5u-8B#gid=0

The granular convection is very likely a minor effect in avalanches, probably better to have avoided mentioning it. Most important is just plain old buoyancy, which is the "simple physics" I was referring to. So the skier who sets off the avy starts out on the surface of the suddenly fracturing and fluidizing slab. Then his tendency to sink into the fluid (versus stay on the surface) depends primarily on his density relative to that of the flowing snow. If the pack reduces his density, then he is more likely to float than he would be without the pack. That's the simple effect that I think is physically certain, and not likely to be reversed based on any assumptions.

I agree that the airbags are such a huge effect that it requires only a few incidents to be significant. But not everyone has (or can afford or wants to carry) an airbag pack, but almost everyone is wearing a backpack while bc skiing. So the choice on whether to ditch the pack in an avy (or whether to undo the belt/strap when crossing an avy path, vs. securing the pack tighter) is a critical one.

I am incredibly surprised by the arm-chairing in this thread.

Conditions were obviously stable. The evidence shows this: from telemetry to field obs to the skiing and the photos. How much explanation is required when the essential difference is that some people are more uncertain than others?