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 Post subject: Understanding this season's snow conditions
PostPosted: Sat Mar 10, 2012 11:32 am 
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Hey Alll, I mostly lurk on this board but wanted to get some input from the collective. I was talking with some friends recently about what a crappy and tragic year it’s been in the backcountry this year, but I was at a loss to explain what specifically it’s been about the weather patterns this year that has created these conditions. I think it has something with the early snowpack degrading (although I’m not sure in what way – thru melting? Evaporation?) over the long dry spell making for an unstable base for the later snowpack to load up on. It seems like a reasonable explanation to me in sort of a hand-wavey kind of way, but I’m pretty much pulling that outta my butt. Can anyone drop some knowledge on me about this?


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 Post subject: Re: Understanding this season's snow conditions
PostPosted: Sat Mar 10, 2012 11:56 am 
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The dynamics vary from place to place, but your general description matches more or less what we have going on in Tahoe (at least as far as my non-professional understanding goes).

In Tahoe we have a (rare for us) persistent weak layer that formed as a result of a long dry spell and some extended periods of cold following early season snows.

That series of events created a temperature gradient in the snowpack where the snow that is buried was actually warmer than the outside temperature. Over a period of time, this drew the relatively warm moisture from the buried snow to the surface causing the bonds holding the surface snow crystals to weaken and break. That surface snow then recrystalized as a layer of large and poorly bonded snow crystals. These large poorly bonded crystals are called facets or hoar.

This faceted layer sits on top of some pretty hard rain and sun crusts that formed during shorter periods of warm weather and some high elevation rains. Put them together and we have a weak layer (facets) on top of a strong bed surface (wind/rain crusts). Where there are strong cohesive layers (like a wind slab) on top of facets and crusts there is the potential for large and destructive avalanches.

Of course there is a lot spatial variability based on aspect, terrain, elevation, etc that means you don't have all of these elements everywhere. The hard part is figuring out where they exist. The really hard part is acknowledging that you really can't be 100% sure where they are when there is fresh snow on the ground and you haven't made any pow turns for a month.

Other Tahoe folks (or the many others out there who know far more about snow science than I), please chime in to add your knowledge/correct me where I'm wrong either about our local pack or my explanation of facet formation.


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 Post subject: Re: Understanding this season's snow conditions
PostPosted: Sat Mar 10, 2012 5:57 pm 
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Silver pretty much nailed it. A big temperature gradient (difference in temperature divided by the distance in a snowpack is perfect for growing buried facets, rounded snow crystals below the surface, that don't bond well (sugar snow). In addition, long, cold periods between storms with the right humidity grows surface hoar (like corn flakes on the surface) When those get buried, they provide a weak layer and when you pile new snow on top of those weak layers you create conditions ripe for avalanches. That is what we have experienced in Utah this year, much more so than usual, and generally around the west. What all that science means is shallow snowpack bad and long time between storms generally bad.

I highly recommend getting a copy of Bruce Tremper's book, Staying Alive in Avalanche Terrain, and read it at least every year for a more detailed explaination.


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 Post subject: Re: Understanding this season's snow conditions
PostPosted: Sun Mar 11, 2012 12:08 am 
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Yeah, Silver did a nice job of putting that into words pretty accurately. If ever there is a good year to take an avy course, it's this year. The worse the snowpack is, the more you can learn with a good instructor.

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 Post subject: Re: Understanding this season's snow conditions
PostPosted: Sun Mar 11, 2012 9:19 am 
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We should note that the seasonal average for avalanche deaths in the US is 28 and we are sitting at 27. So if some how we luck out and there are no other deaths, then it will still be a below average season. Being that there is almost 3 more months were these accidents typically happen, it is fairly safe to say we are going to be above average.

This persistent weak layer is a real problem. It's even unusual here in Colorado. The biggest problem I see is that people are just not willing to dial it back enough. When they see the set ups that have worked in the past they go for it. We've had some tragic results because of it.

For me, I am pretty sure the amount of alpine lines I'll get done this year is minimal to none. I'm still having fun, but it's not a year for me to be goal oriented. For the most part I've thrown this season out the window, in Colorado at least, and I am looking forward to next year. Sucks to lose a year like this, but I am also pretty sure there will be a season next year. Lot's of history to prove that will happen. Sometimes you just got to bag it.


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 Post subject: Re: Understanding this season's snow conditions
PostPosted: Sun Mar 11, 2012 1:12 pm 
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hey nice work, good information given here....
i dont mean to be 'that guy' but ive studied too much science and plenty of snow, i dont mean to be anal but i do value precision and accuracy, and i like for my brethren to be on our A game when droppin science... im a bit of a dinosaur from when shredders were perceived as the dumb young punx on the scene and i think its commendable how far we all have come from that.

temp gradient, as mentioned, is a measure of how temperature changes thru the depth of the snowpack. the ground is a source of warmth (typically 0 deg celcius / 32 f) and the air above the snow is typically well below freezing (esp in early winter when these layers set up). like paulster said, shallow snow and cold air temps make for strong temp gradients. strong gradients drive moisture thru the snowpack, from warm to cold, and the vapor typically sublimates in crystalline forms called facets (remember hydrogen bonds b/w the 'mickey mouse' shaped water molecules). strong gradients for long periods of time make larger facetted crystals, depth hoar, which are long lasting aka persistant. also, temp variations and crusts within the snowpack can affect moisture transport thru the pack, which can lead to increased facetting in specific layers.
In contrast, deep snow and mild air temps result in weak temp gradients in which moisture transport is more diffuse, random, does not form crystalline structures but rather 'rounds' them out (aka sintering) which is associated with strongly bonded or settled snowpack, or maybe just layers within the pack.

Surface hoar (aka 'V'), as mentioned, is the other typical villian in snow stability. Cold clear nites cause the snow surface to cool, and if theres moisture in the air it can condense on the cooled surface, again forming crystalline (facetted) structures. Valley fog / cloud on cool nights is a perfect scenario for this, as well as creek beds which can supply moisture. V can be fairly delicate if exposed to sun or wind, and its interference in bonding new snowfalls to the layer below is affected by how large the V crystals have formed, and also by the composition of the layer it sits on (crust? pow?). This means V related instabilities are often most significant in sheltered locations at and below treeline, but are very much subject to spatial variability.

clearly, i should be doing my taxes. :roll:

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 Post subject: Re: Understanding this season's snow conditions
PostPosted: Sun Mar 11, 2012 1:21 pm 
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Thanks Karkis. I for one was really hoping "that guy" would show up on this thread.


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 Post subject: Re: Understanding this season's snow conditions
PostPosted: Sun Mar 11, 2012 8:04 pm 
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This is great info. Thanks for cluing me in. It’s been a while since my Avy 1 class. While I’ve caught a few refresher lectures at Neptune’s Mountaineering here in Boulder now and again, I couldn’t come up with an explanation for this year’s snowpack beyond my high-level explanation. This definitely helps. I don’t want to cut-off the discussion if anyone has anything else to add, but I did want to make sure I thanked those of you who have been generous with the 411 so far.

And yes, thanks for the reminder to dig up my avy book for a refresher read – I’ve got a copy of “Snow Sense” lying around here somewhere but will pop in at Neptune’s to check out “Staying Alive…” as well to see if there’s any additional coverage to warrant adding it to the collection.

Cheers!


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