Slab Avalanches & Time of Day

[Robbie Crouse]

Hi all,

In my continuing avalanche education, one of the things that is talked about is the time of day wet slide avalanches occur in the spring. Obviously when the sun hits any overnight frozen snow, it weakens and thus increases the possibility of an avalanche (dependent on slope aspect). But in general, the main idea is start as early as possible.

My question is whether there is any pattern at all in the time of day for deeper, persistent slab avalanches. Would it be the reverse, since extra warmth in the colder, drier months would actually decrease the temperature gradient in the snowpack? (If so, it would almost make sense to start a bit later once the sun was already warming up the snow). I was just curious if those more knowledgeable than I had an answer.

[SchralpTheGnar]

During the winter one thing that can cause increase in danger is rapid temperature change, but the general warming in a normal day isn’t gonna much if anything to change the underlying layers because the deep persistent layers won’t change temperature

[spoony]

I agree with Schralp. Winter slides are generally not as heat-dependent as spring/wet slides, but one thing to look for with mid-day warming in the winter is areas of very thin snowpack and areas near exposed rocks. Thin snowpack might get heated and slide more easily than a deeper snowpack, which is more insulated. Exposed rocks can generate melting in snow around them, which can trigger movement. Either of these movements may propagate into a weak layer and cause a slab slide.

[Dave B]

My question is whether there is any pattern at all in the time of day for deeper, persistent slab avalanches. Would it be the reverse, since extra warmth in the colder, drier months would actually decrease the temperature gradient in the snowpack? (If so, it would almost make sense to start a bit later once the sun was already warming up the snow). I was just curious if those more knowledgeable than I had an answer.

In winter, temperature gradients drive the formation of facets and weak persistent layers on the time-scale of days, not hours, and rounding (strengthening) of the angular/faceted grains can take ~10x longer than it takes for the facets to form in the first place. Facet -> rounded metamorphism happens so slowly that a typical late spring snowpack in CO is still a sh*t load of facets and DPL bridged by strong freeze-thaw layers that are safe(r) to travel on.

In other words, time of day will have very-little impact on the likelihood of DPL being triggered. The exception would be, as gnar mentioned, huge temp swings during a 24h period can cause shallower/surface slides, and those can step down to persistent layers and initiate bigger DPL slides. If it's an unusually warm day after new snow, get off the snow before things start warming up.

[Robbie Crouse]

This is all really interesting and helpful. Thanks. It also triggered (!) two more questions. First, Dave, is "bridging" the term used for the changing of facets into more rounded, denser and therefore more cohesive/bonded snow? As I understand it, that only happens when the snowpack is large enough that it insulates the underlying snow and keeps the temperature gradient low. Is that right? Second, would early season (late fall / early winter) snow parallel somewhat to very late season (late spring/ early summer) in its freeze thaw cycles? From observations, it doesn't seem like wet slides really occur in October or November, only in May/June.

I really appreciate the input.

[Dave B]

This is all really interesting and helpful. Thanks. It also triggered (!) two more questions. First, Dave, is "bridging" the term used for the changing of facets into more rounded, denser and therefore more cohesive/bonded snow? As I understand it, that only happens when the snowpack is large enough that it insulates the underlying snow and keeps the temperature gradient low. Is that right? Second, would early season (late fall / early winter) snow parallel somewhat to very late season (late spring/ early summer) in its freeze thaw cycles? From observations, it doesn't seem like wet slides really occur in October or November, only in May/June.

I really appreciate the input.

Bridging is more of a physical/structural process than thermal. As the snowpack starts to go through freeze-thaw cycles, upper layers consolidate as melt water infiltrates deeper into the pack. At night, at least on spring nights with a hard freeze, the top layers of the snowpack freeze solid and provide a structural support and spread the weight of skiers/climbers over a much larger area allowing allowing for safer travel without triggering weaknesses in deeper layers. Once that top layer starts softening too much in spring, that support starts to diminish and the risk of wet slides (and wet slides that can break down to DPLs) increases.

And as far as fall/early winter snow packs, no the freeze-thaw is quite different from spring. Day length and incoming solar radiation dominates the snowpack energy balance and thus metamorphism processes. In late fall and early winter days are very short so there may be some minor surface melting and refreezing but orders of magnitude lower than during spring melt when the sun is up for ~15 hours versus ~9 hours in December.

[Jorts]

First, Dave, is "bridging" the term used for the changing of facets into more rounded, denser and therefore more cohesive/bonded snow? As I understand it, that only happens when the snowpack is large enough that it insulates the underlying snow and keeps the temperature gradient low. Is that right?

Dave beat me to it, but just throwing it out there anyway... Bridging/capping is when the weak layers underneath are no longer effected by the overlying slab. Basically the massive supportable slab on top bridges the weak layers so your weight on top is not going to make a difference impacting them. For example, if you have a foot of of basal facets with a 6 ft slab on top consistent across the entirety of the slope, you generally don't have to worry about effecting that underlying weak layer and causing a collapse/propagation. There's a big gray area between bridging and deep persistent slabs though. We have a lot of spatial variability in our thin snowpack. Just because you dig high on a slope where there's a lot of wind deposition and your have an ECTX doesn't mean that's representative of lower on the slope. Think of some of those giant deep crowns you've seen.

Second, would early season (late fall / early winter) snow parallel somewhat to very late season (late spring/ early summer) in its freeze thaw cycles?

No. It's all about temperature gradient with regards to snow metamorphosis. I think it's something like a gradient of >1 deg C/10cm leads to faceting, less than that rounding. Think of the ground as a constant... always right around freezing. But the air dips way below freezing. In a thin autumn snowpack with those frigid nights, the gradient is quite large between the ground and the air. That's why an early season snowpack often consists entirely of facets/sugar snow. Near surface faceting occurs with those subsequent clear days and nights where a large gradient occurs right at the surface of the snowpack with the diurnal temperature swings.

Late spring/early summer you have nothing but rounds. The whole snowpack is warm (around 32F). It's losing mass. It's melting and freezing on repeat. That's your corn snow and your isothermal snowpack. I know less about spring snow maturing into future firn because I'm not as worried about it killing me.