Question re snowpack temp gradient in super cold temps

So I'm re-reading Tremper's text, second edition. (Ordered the 3rd, but haven't gotten it yet.)

On page 83-85, he discusses how shady N to E facing slopes are generally more dangerous than sunny S-W. I get it: colder air = larger temp gradient (assuming ground temps around freezing) = more faceting.

But then he says,

"During arctic outbreaks with temperatures less than -20C [-4F], faceted snow grows better on sunny slopes because shady ones are too cold. [...] Sometimes it's so cold on N-facing slopes, the avalanches are in a deep freeze and nothing much happens."

I don't understand this phenomenon of it being too cold to grow facets. Can anyone explain? Yes, I understand it can be too dry to grow surface hoar, but Tremper's nomenclature is facets are "temperature gradient layers" not surface hoar.

Without being intimately familiar with the Tremper book I would venture the guess that he is probably talking about NSF (near surface faceting).

Those cold but sunny slopes get increased solar gain throughout the day and then give that heat back up when the sun goes down leading to a weakened layer on the surface to about 5 CM below. If that regime continues for a long period you can develop excellent recrystallized snow and skiing and a rotten layer when it does snow.

Not sure about his summation that totally cold and shaded slopes have little change in grain type and size.

yeah, he discusses NSF and the 3 modalities by which it forms later in the book. Pretty sure he calls what you're talking about diurnal recrystallization. But I'd think the colder it gets, the worse the NSF would be. Even if it doesn't get *worse* with colder temps, I don't see why it would get better, and I don't see why it would just stop happening at -20C/-4F.

And regardless of NSF, if it's getting super cold, you'd have a bigger overall temp gradient to possibly grow depth hoar, right? (assuming constant snowpack depth)

Edit: Thanks for the quick response, though!

Not really, the same 1 Deg C p/10 CM seems to be the gold standard although a shallow snowpack coupled with a big temp gradient should rot things faster although it may not increase the size of the grains as dramatically.


And -20C while damn cold, converts to -4F so considering the temps at night we have been experiencing this past week (-15F) I am not sure where he gets that conclusion.

Tremper is a smart guy and definitely knows his regimes so there is that. I found his book on Google but it doesn't contain those pages.

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Originally Posted by Not bunion

And -20C while damn cold, converts to -4F so considering the temps at night we have been experiencing this past week (-15F) I am not sure where he gets that conclusion.

Tremper is a smart guy and definitely knows his regimes so there is that.

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Yeah, I'm at the same place -- definitely a smart guy but I just don't see how -4F = "the avalanches are in a deep freeze and nothing much happens" on shaded aspects.

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Originally Posted by auvgeek

Yeah, I'm at the same place -- definitely a smart guy but I just don't see how -4F = "the avalanches are in a deep freeze and nothing much happens" on shaded aspects.

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I don't get it either. I will say something that I've noticed happens sometimes in CO is that north aspects (especially slopes that are protected from the wind) can be so cold for so long and never see the sun that the entire snowpack just facets away to the point where there is no slab anymore. When there's no slab, there's no danger of a slab avalanche. But I'm not sure that's what Tremper is talking about here.

Dead on right with that particular phenomena, too weak to propagate a fracture.

I may be reading this wrong. Is he saying it’s too cold to avalanche, or too cold for the facets to grow. My understanding is that metamorphism occurs faster at warmer temperatures. Of course, warmer temps lessen the gradient. So there is a sweet spot of being warm enough for for the rapid vapor transfer and cold enough to create a large gradient. When it’s really cold that sweet spot is on the solar aspects instead of the shady aspects where we usually think of faceting occurring.

I would suspect the slowing of the faceting process in temps colder than minus 20C on non solar slopes/aspects has something to do with the slowing of the molecular vapor transmission required for facets/depth hoar to grow? Also, wonder how factors such as shallow snowpack vs. fat and different mf crusts within said snowpacks alter the vapor transmission rate...etc...

Prolly not much going on as temps approach zero Kelvin when atoms stop moving. Knee jerk response trying to recollect knowledge gleaned from a few decades ago...will have to research to confirm or refute.

There's a local avy tech with a master's degree in snow science stuff, i'll pick her brain and update.

We have cold temps with a lot of wind affected alpine surfaces up in northwest b.c. at the moment. Lots of windboard, strattagucci, breakable crust interspersed with some smooth creamy facets pitches. Temps are cold enough to maintain the crusts...sometimes after extended droughts with clear sky, the surface crusts start to facet out and ski quality improves somewhat....not this time...yet.

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Originally Posted by swissiphic

I would suspect the slowing of the faceting process in temps colder than minus 20C on non solar slopes/aspects has something to do with the slowing of the molecular vapor transmission required for facets/depth hoar to grow? Also, wonder how factors such as shallow snowpack vs. fat and different mf crusts within said snowpacks alter the vapor transmission rate...etc...

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But the ground is still at (approx) 0C/32F, so isn't there still going to be a high temp gradient in snow near the ground? So even if most of the snowpack is super cold and there's not much of a temp gradient at the air surface, then it seems like you'd have kind of the opposite of a NSF situation going on -- the ground is warm and the first few cms of snow are undergoing a high TG. I don't see how this snow right at the ground would be cold enough to not have any vapor transfer, unless like adrenalated said, it's all faceted.

But I'm just starting to wrap my head around the fact that the temp gradient doesn't matter in and of itself, except for the fact that vapor pressure decreases nonlinearly with snow/ice temperature and facets form when water vapor diffuses rapidly. So these comments about being too cold for rapid vapor transfer are interesting, but I don't really understand it well enough yet to have decent intuition about how it changes when it gets really cold (let alone how an insulating layer in the snowpack would affect things).

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There's a local avy tech with a master's degree in snow science stuff, i'll pick her brain and update.

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That'd be sweet!

I'm not an expert, just trying to learn here.

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Originally Posted by auvgeek

But the ground is still at (approx) 0C/32F, so isn't there still going to be a high temp gradient in snow near the ground? So even if most of the snowpack is super cold and there's not much of a temp gradient at the air surface, then it seems like you'd have kind of the opposite of a NSF situation going on -- the ground is warm and the first few cms of snow are undergoing a high TG. I don't see how this snow right at the ground would be cold enough to not have any vapor transfer, unless like adrenalated said, it's all faceted.

But I'm just starting to wrap my head around the fact that the temp gradient doesn't matter in and of itself, except for the fact that vapor pressure decreases nonlinearly with snow/ice temperature and facets form when water vapor diffuses rapidly. So these comments about being too cold for rapid vapor transfer are interesting, but I don't really understand it well enough yet to have decent intuition about how it changes when it gets really cold (let alone how an insulating layer in the snowpack would affect things).

That'd be sweet!

I'm not an expert, just trying to learn here.

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Again, just guessing a bit but even if ground is zero and the t.g. is strong, i'd speculate there's a need for adequate pore space for crystal growth...if it's a deep, well settled snowpack comprised of tightly compacted small rounded grains, the weight/pressure of overlying snow may well prevent the crystal growth from happening?

In a shallower snowpack...dunno but seems like you see biggest depth hoar crystal growth in shallow snowpacks so maybe that's where crystal growth maintains progress? Some of the biggest depth hoar crystals I've observed was in a 70 cm snowpack up north in the Yukon where it's brutally cold...

Still patiently awaiting an answer from my avy tech bud.

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Originally Posted by swissiphic

I would suspect the slowing of the faceting process in temps colder than minus 20C on non solar slopes/aspects has something to do with the slowing of the molecular vapor transmission required for facets/depth hoar to grow?.

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This.

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Originally Posted by covert

This.

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I think it is probably partially this - the effective diffusivity (i.e. how easily water vapor can move through the pore space) of water vapor decreases with temperature. The other side of it is that the saturation vapor pressure decreases significantly as it gets colder (i.e. the air can hold much less water vapor). You'd have to monkey with some equations to sort out the trade offs, but the gist is that you have less water available and it moves slower.

Yeah, but isn't that quote in reference to avalanches releasing rather than weak layer formation? If so (I don't have my copy here at home), I would guess that is more of a slab mechanics issue wrt cold air. Good points on effective diffusivity -- always forget about that.