Snowpack question
|
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. |
|
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. |
|
I presume the wet slab cycle has to run for this to happen first? |
|
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. |
|
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. |
|
Oh my, what a complex question with a complex answer! |
|
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? |
|
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. |
|
|
|
Great topic! Special thanks to the folks who have the expertise to explain these things. |
|
Taylor-B. wrote: Well said Dave!Great stuff right there. Thanks Dave! |
|
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. |
|
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? |
|
So, a summary of Dave's excellent post above would be... |
|
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. |
|
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. |
|
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. |
|
Alexey Dynkin wrote: Not to get too in-depth here (although, this is an in-depth topic.nws.noaa.gov/oh/hrl/nwsrfs/… |
|
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. |
|
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. |
|
Dave, |