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Snowpack question

Original Post
Shepido · · CO · Joined Aug 2014 · Points: 50

I have some questions about the current state of the snowpack as it relates to stability and some confusion about some of the seemingly contradictory things I have read and personally observed.

In the spring I often see posts from people on message boards saying we need a good freeze and clear skies to ensure stability before attempting a route.

I have read similar remarks on the CAIC website, and do not doubt their validity.

However in my own limited personal observations the actual air temperature does not seem to matter as much. Last year in late June and July I climbed several couloirs starting around dawn and all of them were at about pencil to knife hardness and stayed that way as long as they were in the shade. I camped near each of the routes and left water out and each time the temperature did not drop below freezing. Similarly, in late May I attempted a route after a tepid refreeze (just below freezing, clear skies, and mild winds) and the snowpack was very wet and soft even before dawn. This seems very contradictory to what I often read and leaves me at a loss to explain what I have observed.

I guess my question is assuming a clear sky and calm winds, when is a good freeze necessary and when is it not?

Rick Blair · · Denver · Joined Oct 2007 · Points: 266

June through July is different than May through June. This year has been a cool spring with late snow so things are slower to consolidate.

Shepido · · CO · Joined Aug 2014 · Points: 50

I presume the wet slab cycle has to run for this to happen first?

Rick Blair · · Denver · Joined Oct 2007 · Points: 266

I would recommend posting up at lists of john and 14ers.com as well . Both sites have some prolific hiker/skiers. ask about the specific area you are concerned with.

Travis Haussener · · Unknown Hometown · Joined May 2012 · Points: 60

I believe it has to do with the percolation of water through the snowpack, if it reaches a certain level of depth but no more than that, you get a massive weakness and can get some serious wet slabs. Once all this consolidates (so no more water going through) the snow retains a much more robust character, like the one you're familiar with. However, spring/summer is a fickle beast; when temps were forecasted to reach high 70's I've skied powder and vice versa. The same applies for a corn cycle...i.e. sometimes a good refreeze isn't dictated by overall air temperature.

Taylor-B. · · Valdez, AK · Joined Oct 2009 · Points: 3,186

Oh my, what a complex question with a complex answer!

I think the "LaChapelle 200 inch theory" kind of explains your question. It takes approximately 200 inches of storm snow in a dry intermounatin and continental snowpack before it starts to settle out due to its own weight and weak layers bond better or a big avalanche cycle occurs. The 200 inch mark does not happen in Colorado until late in the season, like March/April if your lucky. The bonding and settling process is sped up when melt freeze cycles and rain occur, especially rain. Liquid water in the snow pack can knock out layers and create a more homogeneous and dense snowpack and depending on the temperatures this can be good or bad for stability.

By the time late June and July hits in Colorado the monsoon thunderstorms start to roll in and all that rain can really help to create the perma-neve we all love. The perma-neve is dense snow and is usually sun cupped or runneled. River levels are a good indicator of spring snow/avalanche cycles. That's some of the equation in a nut shell. And good luck finding copious amounts of the elusive Colorado perma-neve.

This picture is a good visual if I just confused you and said a bunch of dangerous stuff?!?!

Snow Science BS:)

Scott McMahon · · Boulder, CO · Joined Feb 2006 · Points: 1,425

It's also about the relative "big picture" for the week. How many nights above or below freezing were those routes? One? It takes a few days of freeze / thaw to help consolidate that pack. Maybe the day before was exceptionally hot, or no freeze, or rain. Aspect? Was there wind? What time of year is it?

Right now it's not JUST about the lack of a freeze. It's a ton of snow that was wet to begin with, warm days, lots of sun, no freezes which equates tons of melting water lubricating and moving the snow.

One of the big take aways from learning more about travelling safely in avy terrain is its about the bigger picture, not just the day. What was the week like? The month? How is the 30+" bonding with that old layer we had back in the beginning of April? What did THAT look like based on what happened over the winter?

That's one of the reasons most people have gotten away from digging tons of pits. That's just a small measure at one particular spot. I'll dig one just to get a look at a relative place especially if I haven't been there, but after that big picture tools. You take all the science and try to put it together and HOPE that you are reading it correctly.

At the end of the day it's either 100% safe or you are just making assumptions.

Dave Bn · · Boise, ID · Joined Jul 2011 · Points: 10

The primary mechanism by which the snow pack cools/refreezes at night is by emitting long wave (ie thermal) radiation. In fact, radiation trasfer typically dictates between 60 and 90% of the snow pack energy balance. Some of the remaining 10-40% is made up by sensible heat exchange (ie conduction via air to snow temperature differences) but as you can see it is smaller in comparison to the effects of radiative transfer.

Snow is a surprisingly good emitter of long wave (emissivity of 0.95- 0.99 - ie damn near black body) but atmospheric conditions will dictate how much of that radiation is reflected back downwards.

Clouds (water vapor) have pretty high albedo (reflectance) in the long wave spectrum so on cloudy nights much of the long wave emitted by the snow pack is reflected right back down leading to less net energy lost from the snow, and less cooling of the snow pack.

On clear nights, all of the long wave emitted by the snow pack can escape into the atmosphere and the snow pack will cool rapidly and with a larger magnitude.

So, long story short, it's very common to have snow pack freezes when the air temperature is above freezing but the sky is clear.

There are a lot of factors that go into snow pack energy balance - I work with a group of scientists who devote their work hours to refining models to estimate just this. Anyways, it's good to develop an idea of what temp thresholds at what elevation and for what aspect need to be surpassed to get a good freeze on clear and cloudy nights. This mostly comes from experience that comes from getting out and being willing to turn around in the absence of a freeze.

Taylor-B. · · Valdez, AK · Joined Oct 2009 · Points: 3,186
Dave Bn wrote:"........."
Well said Dave!

14ers.com.
Alexey Dynkin · · Bozeman, MT · Joined Oct 2014 · Points: 0

Great topic! Special thanks to the folks who have the expertise to explain these things.

To chime in a bit: as I see it, there are actually two separate questions here that factor into "stability". One is the overnight freeze, as described by Dave above - which has to do with the surface of the snow. The other is consolidation of the entire mass of the snowpack - this is more related to the mechanism described by Taylor-B. In short, the situation right now is such that the overall snowpack hasn't consolidated yet, so even if an overnight freeze occurs, it's essentially just a thin crust forming over an otherwise unstable snowpack. If you can get up and down your route before that crust breaks down, then good...but, really, it's playing with fire.

Scott McMahon · · Boulder, CO · Joined Feb 2006 · Points: 1,425
Taylor-B. wrote: Well said Dave!
Great stuff right there. Thanks Dave!
Andrew Blease · · Bartlett, NH · Joined Apr 2010 · Points: 470

I was up in RMNP recently and found 6-8" of frozen crust over poorly consolidated wet slush over ice. No Bueno. We bailed, even early in the morning we didn't think it stable enough. Definitely needs time. It's pretty prime for slabs.

Shepido · · CO · Joined Aug 2014 · Points: 50

Correct me if I am wrong, but based on the replies it seems then the biggest issue currently is the amount of free water in the snowpack. This water is a result of all of the sun and warm weather we've been having. Because this water is present a combination of both clear skies and cold temperatures is needed to refreeze the snowpack. Once the water in the snowpack has drained away a solid freeze is more dependant on cool but not necessarily freezing temperatures and clear skies. The drainage of water of the water away from the snowpack normally occurs at the conclusion of the wet slab cycle. Is that a good summary?

Thanks for all the replies thus far.

Rick Blair · · Denver · Joined Oct 2007 · Points: 266

So, a summary of Dave's excellent post above would be...

You don't know what the snow will be like in a given area until you see it. Go check it out and gather info from those who have been there. Be ready to turn around.

Death Cookies · · Denver, CO · Joined Jan 2015 · Points: 0

If you have doubts about conditions, just wait it out and go rock climbing. You'll start to see a flood of trip reports/posts on 14ers.com when the conditions are good. If you're dying to go snow climbing, pick a ridge route rather than a couloir. You'll get an idea of current conditions and hopefully it's safer than going up some steep sketchy chute.

Sorry for the non-science answer. Just trying to give practical advice rather than getting hung up on all the factors that create stable snow.

Dave Bn · · Boise, ID · Joined Jul 2011 · Points: 10
Shepido wrote:Correct me if I am wrong, but based on the replies it seems then the biggest issue currently is the amount of free water in the snowpack. This water is a result of all of the sun and warm weather we've been having. Because this water is present a combination of both clear skies and cold temperatures is needed to refreeze the snowpack. Once the water in the snowpack has drained away a solid freeze is more dependant on cool but not necessarily freezing temperatures and clear skies. The drainage of water of the water away from the snowpack normally occurs at the conclusion of the wet slab cycle. Is that a good summary? Thanks for all the replies thus far.
Yes, the large issue currently is the amount of melt water present in the snow pack. We've had 10+ days were the energy going into the snow pack during the day is greater than what is being lost at night. That accumulation of energy results in the melting of more ice and snow crystals. This melt water is either held in place in the snow pack if pore space matric potential is high enough or it infiltrates to lower layers.

With this in mind, melt water infiltration has two main impacts with regards to snow pack stability:

1. Melt water increases the density of snow lower in the profile which in turn can add additional shear stress at that level in the remaining snow pack. This added shear stress can sometimes be enough to overcome present stability levels, resulting in wet slabs.

2. Melt water typically doesn't percolate through the snow pack uniformly but instead follows preferential flow paths which quite often terminate at ice lenses which formed earlier in the season (especially this season with several warm cycles mid-winter). Melt water can pool in these areas, flow laterally within the snow pack, and provide a lubricant for the bed surface or act as a week spot for point or crown initiation.

I'm not super familiar with the snow characteristics that trigger the end or reduction in wet slides seasonally, but I would assume that at some point in early summer the remaining snow pack becomes homogeneous in terms of density and grain size which should eliminate all layers and interfaces that can act as weeknesses or bed surfaces. But, the snow pack does continue to melt and that melt water will continue to flow through the remaining snow pack.

What we need to end the current cycle is several nights of solid freezes to refreeze the entire snow profile so we can enter into the more typical spring time freeze-thaw cycle where melt water is only in the upper portions of the snow pack. Although, wet slides will still be an issue in the PM, but more easily avoidable and predictable in the AM.
Alexey Dynkin · · Bozeman, MT · Joined Oct 2014 · Points: 0
Dave Bn wrote:but I would assume that at some point in early summer the remaining snow pack becomes homogeneous in terms of density and grain size which should eliminate all layers and interfaces that can act as weeknesses or bed surfaces.
Not to get too in-depth here (although, this is an in-depth topic, so why not?) - but, does this really ever happen? I would think that grain size and density would be, to some extent, a function of depth, simply due to the different amount of load at different points in the snowpack. In a very simple sense, I'd guess that density increases with depth. With grain size I don't really know, but I'd imagine that the surface grain structure will ALWAYS be different from the bulk (speaking as a materials scientist here, not a snow scientist). It DOES make sense, however, that interfaces would disappear in favor of a more continuous distribution of properties within the bulk of the snowpack.
Taylor-B. · · Valdez, AK · Joined Oct 2009 · Points: 3,186
Alexey Dynkin wrote: Not to get too in-depth here (although, this is an in-depth topic.
nws.noaa.gov/oh/hrl/nwsrfs/…
Shepido · · CO · Joined Aug 2014 · Points: 50

I believe in Staying Alive in Avalanche Terrain, that tremper says it does eventually become homogenous, and the stratigraphy goes away. This coincides with drainage channels forming in the snow.

Dave Bn · · Boise, ID · Joined Jul 2011 · Points: 10
Alexey Dynkin wrote: Not to get too in-depth here (although, this is an in-depth topic, so why not?) - but, does this really ever happen? I would think that grain size and density would be, to some extent, a function of depth, simply due to the different amount of load at different points in the snowpack. In a very simple sense, I'd guess that density increases with depth. With grain size I don't really know, but I'd imagine that the surface grain structure will ALWAYS be different from the bulk (speaking as a materials scientist here, not a snow scientist). It DOES make sense, however, that interfaces would disappear in favor of a more continuous distribution of properties within the bulk of the snowpack.
The documentation for the Utah model that Taylor-B linked to is some great information. That model is somewhat of a standard in terms of predicting snow metamorphism and snow melt dynamics. There is also another model out of Davos Switzerland that has offered some improvements in terms of 3d grain size and density distribution and was originally created for avalanche forecasting.

slf.ch/ueber/organisation/s…

Anyways, I think the wet slide crux is that the density profile during snow melt can often be inverted with the heavier denser snow on top of the pack. I've seen saturated snow at >500 g/m^3 sitting on top of slightly wet depth hoar crystals with no more than 80-100 g/m^3 density. This is that strong-over-week layering that provides the classic density profile for avalanches whether in wet or dry snow.

Contrast that to late season snow pack where water has fully infiltrated and numerous freeze-thaw cycles have cause crystal aggregation and you see an increasing density gradient with depth but you'll likely find 400-500 g/m^3 at the surface with something along the lines of 550-600 g/m^3 towards the base. So I guess homegeneity may not be exactly true but there is a smaller gradient in density and it increases with depth which, I think, is the key component.
Rick Blair · · Denver · Joined Oct 2007 · Points: 266

Dave,

On a slight tangent, there was an avalanche fatality at Arapahoe Basin in 2005. I never read a good report on what exactly were the mechanics behind it. Obviously an extremely popular run, 1st Alley into Pali, skied all season, avy controlled, packed and late spring conditions. I had heard someone mention flowing water under the snowpack possibly separating the snow from the bed surface. Could not find it on COAIC database. What exactly went on there? This presents another method avalanches can be created, although extremely rare.... hopefully.

Guideline #1: Don't be a jerk.

Colorado
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