Apr 23, Lundin/Snoqualmie area

author=PeterC link=topic=4575.msg19464#msg19464 date=1145903163]
If anyone has a good hypothesis for what is causing the ball-bearing layer I'd like to hear it.

I forget what causes that granular, rotten layer, but during the transition from a winter snowpack to a spring snowpack I have often found such a layer sandwiched between various crusts. It hasn't concerned me much when it is near the surface, but when buried deeper it has led me to abort lines I intended to ski, though on a couple of climbs have pushed through to find better conditions higher up. I think this layer is persisting a little later this year since we really haven't had many true, spring-like days and more important nights when the snowpack refreezes. As the melt/freeze cycle becomes more persisitent and natural slides release during the melt phase, this rotten layer should disappear. However, I agree that a pit is almost always worthwhile since if a layer like this is contacting ground and supporting the whole snowpack, a climax avalanche is ripe for the making.

Other ideas or a more specific analysis?

I saw the same stuff where we toured last weekend on SE exposures.
At the very summit a pole could not penetrate the surface.
At the 5500 ft level there was some slight punching through the hard layer.
Lower elevations even more predominant surface crust and punching.
As we climbed up from lower elevations the snow would collapse with the weight of the ski compressing mostly around the skis. Some times areas (4x4ft) would drop and as described earlier in the thread, that was weird.

Although, not an engineer or scientist this is my logical take:
The persistant wind pulled H2O evaporation from the snowpack, reducing the percolation of water thru the snowpack.
Clearing overnight increased the ability for free dispersion of radiational energy stored in the snow keeping the surface cold.
The surface of the snowpack froze (temps in the teens Fri night- 20's Sat night). In our case 3 to 4 inches of frozen layer at 4500 to 5500 ft, solid freeze higher.
The makings of an isothermal (granular) layer of snow was in the making on the SE exposures during the preceding week.
This layer was not real deep, but deep enough that just below the frozen layer the layer continued to settle overnight and during the day.
This produced space between the frozen layer and settling isothermal snow (granular).

Overall-This created awesome firnspiegle ski conditions on Sat and most all the firnspiegle was gone on Sunday.

Just my thoughts feel free to rip it to shreds.

One thought that continued to cross my mind was-
If the snowpack continues this seperation at this elevation (it did not and varied on aspect) and we continue to laterally break trail across a slope will the surface tension of the ice slab be reduced enough to create massive failure?
To test this I tried to break free snow on the shallow angle of a switchback and nothing budged.
We did not find out thank goodness.
But, for a while I was nervous and intrigued by the variable properties on multiple aspects.

Joe

Was it ball bearings (round bits) or more like typical faceted snow (aka "sugar snow") which has no cohesion? If the latter, then my next question is whether it was in a shallow area? or did you find it across a broad area, regardless of snow depth? It's pretty typical to find cohesionless sugar snow in shallow areas near ridgelines due to the higher temperature gradient found in shallow snow (if I'm getting the cause exactly right). Bascially the same thing as depth hoar.

At least in the locations that I checked.
The snow was isothermal down below the crust (large grained wet).
I guess what Jim is talking about is more dry conditioned sugar corn.
What I saw was wide spread, but hard to predict exact locations- deeper isothermal wet with a ice crust layer on top seperated by a slight air pocket.
Strange.

My theory is this.  Below the surface frozen layer, the snow has remained largely unfrozen and liquid water is slowly draining from the grains and reducing mass.  This may be untypical, but then the fact that prior to the last relatively cold and deep snows of two weekends ago, the snowpack was warm and wet.  This old wet spring snow never really got a chance to be exposed to the freeze-thaw of these spring days, because they got buried by that insulating layer of fresh snow.  So as my theory wanders on... these days that hard layer on top is acting somewhat like the glass in a greenhouse.  The sunlight can penetrate the frozen layer for some hours before thawing the top completely and it further promotes liquid water percolation out of the near surface corn layers.  If the crust holds structure till the afternoon and evening refreezing, then that layer reconsolidates and seals the lower layer's heat in throughout the night.  I think that several cycles of this could cause a complete detachement of the crust from the lower layers.  I noticed the crust would break and settle as large plates under my weight when I was skiing Sat. on Lichtenburg.  I think that this snowpack is in its earliest stages of becoming what we call stable spring conditions and I was not expecting to find spring conditions this weekend literally a week after so much new deep snow had fallen at remarkably cold temperatures for this time of year.  Skiing at Steven's both Sat and Sunday the week before last easily matched any fluff that had fallen on the many other powder days we had this year.  Especially Sunday.  This type of snowfall is a great insulator for the layers beneath and dealing with the slalom and GS course at the hill was weird as skiers broke through the new packed powder into the slush below.  Certainly no freezing going on underneath, that is for sure.  The freezing of water, even water at near 32 degrees releases a lot of heat energy in itself.  If any insulating layer exists above a wet snow pack to prevent radiative or conductive heat loss, then it is unlikely any freezing of the deep wet snowpack will occur at all.  The snowpack is draining from the top down and will continue to consolidate.  I saw no sign of hoar in these layers just ball bearings.

Alan

Joe - what I was describing was not any sort of "corn" as I know it - rather I was wondering if what Peter found (mentioning being near the ridge where I assume rocky areas may be near surface, creating shallow snowpack) was TG snow - depth hoar that might perhaps be starting to melt now and then w/o necking/bonding. Sounds like you're describing non-TG metamorphism that's creating corn pellets w/o a lot of bonding between pellets quite yet?

Thanks for all the observations and hypotheses.  The layer I found was definitely large rounded grains, not faceted crystals and although we were near the top of the ridge, the snowpack was quite deep and we weren't near rocks.  The grains were almost spherical and were solid, so I don't think they started life as depth hoar at all.  After reviewing Tony Daffern's "Avalanche Safety..." book, I've concluded that these ball bearings were definitely the product of rounding in an isothermal snowpack (as surmised earlier in this thread) and was surprised that Daffern talked only of rounding as STRENGTHENING the snowpack, where here it had clearly done the opposite.  Upon reflection, I think something along the lines of BigSnow's comments is going on.  In my (unprofessional) opinion, the snow from 1+ weeks ago has formed a thick crust layer due to melt-freeze and at the same time the snow below has been settling due to rounding.  Since the surface crust isn't melting completely, this top layer isn't sinking and an air pocket has formed between the layers as the lower snow settles.  Since there is no pressure on this layer, these grains are not bonding together like they normally would, creating the ball bearings I found. 

It's interesting that jonthomp has seen this before.  I wonder if it's a feature unique to the springtime W. Washington snowpack.  Have other people seen this previous years?  Perhaps if someone else finds these grains they could take a picture of some and post it for others to see.

I hope it goes away soon!

-Peter

author=PeterC link=topic=4575.msg19548#msg19548 date=1146079756]
I wonder if it's a feature unique to the springtime W. Washington snowpack.

Upon reading BigSnow's hypothesis and reflecting further on when I actually saw this before, one climb up the N. Face of Shuksan comes to mind from a couple years ago. We had been enjoying springlike conditions in the weeks leading up to the climb then experienced a rapid return to winter with a dumping of new snow the week of the climb, not unlike what we experienced recently this year. Between about 5K-6K we found new snow atop a solid, though breakable crust, atop the ball bearing layer which we almost had to swim through to make progress up. It certainly didn't strike me that whatever conditions were creating this were creating a more stable snowpack and, in fact, almost sent us packing for home. Higher up the crust became more solid and the ball bearing layer disappeared. My guess is that it never had formed higher up since the snowpack would have remained more winterlike and, therefore, if BigSnow's hypothesis is correct, the new snow would not have provided the same kind of insulating effect since the snowpack was not saturated with spring melt.

I suppose this is pretty typical of W. WA's snowpack for at least a short period of time as we make the transition into stable spring conditions, though it would be reliant on not having enough melt/freeze days to push toward a springpack coupled with a pretty good amount of new snow.

I wonder how many other regions experience this or is spring weather in other areas more consistently sunny (or snowy) so as to avoid producing this buried layer?

I found the same conditions near Hurricane Ridge last Saturday. It was on a SE slope in a narrow band around 5000'. The previously mentioned settling gave me pause so I jumped on it and board cut and was satisfied when the snowpack stayed put. Weird to see what I perceived as a layer of AIR under the surface crust.