depth hoar

[euphoria]

Hi. Can someone shed some more light on depth hoar for me? I have read about it all over but dont really understand it. I have also taken an avy 1 class and I would have brought it up then, but it didnt dawn on me then. Why is depth hoar not much of an issue in deeper snowpack? If its present in shallow snowpack, and doesnt disapear, then wouldnt it be assumed that it is still there in deeper snowpack? I dont quite understand how deeper snowpack can actually make it more stable.

[bucksaw]

Depth Hoar: Faceted snow near the ground

As the snow pack becomes deeper the likely hood of a little trigger (you) causing a release far down in the snow pack is less then when the snow pack is shallow.

I would not assume this "safer" it changes the likely trigger.

[RR]

The trigger...it's all about what it takes to make it release. Don't forget that cohesion of any layer is import to the force. If a strong layer should be cut by meltwater later in the season it's cohesion is reduced...

Is that cut enough to bring the DH into play or not? Always good the think about it.

[yuckster]

Hi. Can someone shed some more light on depth hoar for me? I have read about it all over but dont really understand it. I have also taken an avy 1 class and I would have brought it up then, but it didnt dawn on me then. Why is depth hoar not much of an issue in deeper snowpack? If its present in shallow snowpack, and doesnt disapear, then wouldnt it be assumed that it is still there in deeper snowpack? I dont quite understand how deeper snowpack can actually make it more stable.

Never safe, only "safer." In the late winter, spring--or even in the late summer after it's all but forgotten by us humans--it's common for the deep buried layers to wake up and avalanche to the ground layer. Timing when they wake up is the tricky part.

[mainwaring]

http://www.avalanche.org/~uac/encyclopedia/depth_hoar.htm

the link above is a good resource and i've copied and pasted a few sections in answer to your questions (in quotations).

Why is depth hoar not much of an issue in deeper snowpack?

"Remember: thin snowpack means a weak snowpack. Thicker snowpacks insulate the cold air from the warm ground, have a small temperature gradient and thus a stronger snowpack. For this reason, you usually don't find bad depth hoar under the thick layers of wind loaded snow near the ridgetops. It's usually much weaker at mid-slope and especially near the bottom of basins, where thin snowpacks combine with cold air pooling, and around rock outcroppings."

so, it is an issue, especially in continental snowpacks. more so when it is deeply buried as it is more difficult for the trained and untrained observer to predict when it may become an issue, and by definition a deep layer means more snow to propagate and larger avalanches. deep weak layers can persist for many months and can lie in wait to finally propagate.


If its present in shallow snowpack, and doesnt disapear, then wouldnt it be assumed that it is still there in deeper snowpack?

"Large-grained depth hoar persists longer than any other kind of weak-layer. And as long as it does, you just tiptoe around and accumulate gray hairs. Usually the larger the grain size, the more persistent the instability. The time-honored adage among experienced avalanche professionals is: "Never trust a depth hoar snowpack." In other words, it's always guilty until proven innocent."

therefore, it shouldn't be "assumed". professionals will study interface layers (including surface layers) to evaluate bonding at each one. surface hoar or ground hoar from early season will tend to bond with the snowpack throughout the season as the snow crystals metamorphose from facets to rounds (sintering). this process can actually make the "deeper snowpack" you refer to more stable.

[icelanticskier]

the deeper you bury the whore, the better off you'll be. whore-ay!

rog

[euphoria]

Thanks everyone. So from what I am reading here (and from http://www.avalanche.org/~uac/encyclopedia/depth_hoar.htm thank you mainwaring i actually have read that one before but needed a diferent view of it), deeper snowpack can actually cause the facets to change back and have better bonding compared to shallow snowpack. However, its not always the case and even in deep snowpacks, depth hoar can stil be present, which means a larger avalanche can be triggered. Is that correct?

[RR]

Thanks everyone. So from what I am reading here (and from http://www.avalanche.org/~uac/encyclopedia/depth_hoar.htm thank you mainwaring i actually have read that one before but needed a diferent view of it), deeper snowpack can actually cause the facets to change back and have better bonding compared to shallow snowpack. However, its not always the case and even in deep snowpacks, depth hoar can stil be present, which means a larger avalanche can be triggered. Is that correct?imo, that's true.

Not just bigger, but more destructive too.

[yuckster]

these days the talk is about "persistent weak layers" which is a superset of "depth hoar" and includes things like a rain crust buried 3 months ago that finally gets overloaded with new snow that it can't support...

[icelanticskier]

while living in utah and doing 95% of my ski days in the bc, we used to get accustomed to skiing constantly deepening surface hoar when the weather would be high n dry for sometimes multiple weeks on end. cold clear nights and sunny cold days. after a couple/few weeks of this pattern we'd be skiing the most rippable fabulous mid-boot top deep hoar. great skiing actuallyn as most folks had given up weeks earlier waiting for it to snow again. well, when it does snow again...watch out! lots of weight on heaping piles of crispy fragile potatoe chips is recipe for disaster.

danger hoar is often much closer to the surface than many would think. leave yer shovels in the pack and just swipe yer mitt as you travel from time to time.

rog

[yuckster]

while living in utah and doing 95% of my ski days in the bc, we used to get accustomed to skiing constantly deepening surface hoar when the weather would be high n dry for sometimes multiple weeks on end.

Before and after our Chugach trip last March, I was keeping tabs on the Turnagain Pass avy forecast. They had a dry/cold/clear spell in January/February that got so cold and calm and lasted for so long that the surface facets grew super deep and started failing on themselves right before they finally got another storm which buried them! :eek:

[mainwaring]

deeper snowpack can actually cause the facets to change back and have better bonding compared to shallow snowpack.

a deeper snowpack creates an environment of greater insulation from external temps which fosters temperature consistency (in parlance, consistent temperature gradients). in such an environment, a surface hoar layer that has been buried by newer layers will tend to metamorphose into rounds from facets, a process called "sintering".

http://www.avalanche.org/~moonstone/snowpack/the%20formation%20rate%20of%20depth%20hoar.htm

ed lachappelle knows his stuff (redacted and quoted from above):

"Once snow has been deposited from the atmosphere, its subsequent evolution is strongly influenced by the internal diffusion of water vapor. In the absence of strong temperature gradients, vapor diffusion from one part of a snow crystal to another along pressure gradients established by differences in surface curvature results in the reduction of complex crystals to isometric snow grains. This process is termed destructive metamorphism. on the other hand, a strong temperature gradient in the snow cover establishes a gross vapor gradient which overrides those due to surface tension differences. This causes snow crystals to sublime and redeposit around new centers of crystallization. The latter process is termed constructive metamorphism, and leads to mechanically weak snow layers consisting of cup shaped crystals, or depth hoar.

Precipitated snow thus represents an unstable form of ice crystals under the temperature regime normal to a temperate snow cover. It has been suggested (Bader, (1)) that depth hoar is an equilibrium form of crystalline ice, and the observed stability of these crystals, once formed, appears to bear this out. Mature depth hoar structure persists for extended periods of time with very little physical change as long as snow temperature remains below freezing and superimposed compressive loads are light."

However, its not always the case and even in deep snowpacks, depth hoar can stil be present, which means a larger avalanche can be triggered. Is that correct?

if sintering does not take place (see constructive rather than destructive metamorphosis above, for example) depth hoar will be persistent. as the original website notes, with differing layers on top of depth hoar, with varying degrees of severity:

"A hard wind slab on top of depth hoar is double trouble. It's like laying a pane of glass on top of a stack of champagne glasses. It bridges a person's weight out over a larger area allowing them to walk on eggshells without breaking them until they either give it a hard thump, reach a place where the slab is thinner, or where the depth hoar is weaker, and then the whole slope shatters catastrophically. Fractures involving hard slabs commonly form above the victim, leaving very little chance for escape. Wind slabs on depth hoar exist throughout most of the season in continental climates, and when you add large populations to the equation it also means large numbers of fatalities. It's easy to see why Colorado leads the nation in avalanche fatalities."

does any of that make sense?

i would offer that, its been more than 15 yrs. since i really spent any considerable time with my face in a snowpack that was anything more complex than east coast cohesion / compression snowpacks...i.e. mid or continental snowpacks (where depth hoar is more persistent). more of my continuing knowledge these days is academic rather than field, so some of my answers may not be the most recent info. i would bounce your questions off your L1 instructor as a good source. email them, i'm sure they'd respond.

[petebanta]

Some excelent info on somethiung you do not see as much in the east. Google pic search "Depth Hoar" for some good pics that illustrate the crystal-y layer that can grow on the surface on cold mtn nights (like rime, but more delicate, and you can imagine that is not a good layer to build on)...

[petebanta]

Just noticed I am a "Little Headwall Master" now... :happyhiker:

[Dan9]

Last winter there were some pretty nasty facets at the bottom of the snowpack on Mont Royal, on the east side, sitting on top of a layer of ice right on the ground. I got to 'experiment' with that scenario on a shallow snowpack and it is SCARY. Like a bunch of little greased up ball bearings sitting on one giant banana peel.