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Thread: Cornices

  1. #1
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    Default Cornices

    I've always wondered why the White Mountains don't get cornices like you see in Colorado or Utah. I have some theories, but nothing to explain the complete lack of cornices in New England when they are so prevalent in the west.

    Every now and then I hear people talking about cornices in the northeast, but their definition is highly skewed. Often, a steep wall of snow passes as a cornice, but there is never anything like this:



    Here are some ideas, but most of these just deepen the mystery.

    Snowfall: Certainly Utah has more snow than NH, but in a normal year Colorado is right in line with what the higher summits get in the Whites. And Colorado gets plenty of cornices, they are just as common here as they are in Utah.

    Wind: No arguments here. It's just as windy in the Whites as it is out west. It's probably even windier. The increased wind speed would also make up for any lack of snow.

    Snow Density: Utah gets the world's best snow with densities of 6-10%, CO gets high altitude dust at 3-6%, and NH gets just about everything. From Sierra Cement to Utah powder. I'm not sure this matters either. The sludge that falls in California and Washington also has no problems forming cornices.

    Treeline: The big argument here is that places out west have much larger areas above treeline to transport snow. This is a dead issue. Cornices out west are everywhere. Above treeline, in the trees, on house eaves, porches, lift towers, everywhere.

    Terrain: I think this is the answer, sort of. It's just steeper out west. Ridge lines are sharp with steep drop-off's. In the northeast the mountains tend to more rounded. There are steep valleys, but their are no sharp ridges above them, everything just rolls into steepness.

    Except...there are steep ridges. Lion's Head, above the Great Gulf, the east side of Monroe, Katahdin, and many more. Why are there no monster cornices in these places?
    Bill
    Next up: Vermont City Marathon: May, 2011
    EasternLight

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    Well having been to Utah on multiple occasions I would have agree and say it is the terrain. Everything is steep out there, in a lot of cases 90 degrees steep. Elevation is much higher in the west as well. For example main st in Park City UT, is 7000 feet and that is the base of the ski slopes.

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    I don't think its the elevation. The density of the air should have little effect on the formation of cornices. Plus, I have seen cornices at low elevations in the Pacific northwest. 5-6,000 feet in British Columbia, about the same in Washington and much lower in the coastal mounains of Alaska...like a few hundred feet above sea level.

    Terrain still tops the list, but its not the final answer. Like I said before, cornices are everywhere in the western mountains. Above 90 degree cliffs, and above 30 degree ski slopes. We have terrain like that on the east coast, not as prolific as the west, but we should see cornices on those slopes.
    Bill
    Next up: Vermont City Marathon: May, 2011
    EasternLight

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    Variability of wind direction? Winds that are two strong, and thus create more powerfull erosive eddies.

    -Neil

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    Quote Originally Posted by NLareau
    Variability of wind direction? Winds that are two strong, and thus create more powerfull erosive eddies.

    -Neil
    Probably a huge factor.

    Just for fun let's do some comparisons. If 20% of ridges or terrain features in the Wasatch are capable of forming a cornice I'd bet that less than 1% of ridges and terrain features in the Whites are capable of holding a cornice.

    Take that 1% and maybe 50% of those have too much wind like Neil said. Another quarter don't get enough snow, and the rest just have everything else working against them.

    I wonder if some of the big snow years like 68/69 were a different story.

    If a cornice forms in the fog above the Great Gulf and nobody is there to see it, did a cornice really form?
    Bill
    Next up: Vermont City Marathon: May, 2011
    EasternLight

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    Bill, there is a great article in an old Appalachia magazine that we have here on the summit that examines some of the avalanche paths that ran in 68/69. There was a 30ft deep fracture line on the north Wall of King's Ravine that was allegedly a cornice break. It would be great to find some photos to confirm that. Maybe someone at the RMC, like Mike Pelchat or Doug Mayer, would have some insights. Also, there is at least one picture that i've seen here on the summit that shows a true cornice on a portion of Mount Clay. No Doubt it didn't stay put for very long.

    Furthermore I was thinking about the wind dynamics in a bowl versus a ridge. There are so many aspects feeding into bowls and also a great deal of eddying within the bowl. Very different than the long narrow extended ridge set up in many other locations.

    I would be interested in seeing if anything ever forms on the Knife's Edge of Katahdin. I'll be there in March and I'll have a look for you...

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    what about having these on a smaller scale?

    Im always on the lookout during and after large snowstorms for these drifts (I'll call them drifts for this small scale comparison) and only seem to notice them on steep peaked hills and not nearly as much, if at all, on smaller rolling hills. For instance, there is a large barrier wall of dirt along this stretch of road that surrounds an old land fill and the top is sharply peaked. The "drifts" will sometimes stretch feet past the peaks of the hill and Im always thinking "wow thats a decent sized drift" while driving or walking past. And since these drifts always form in the same direction it would appear for the most part the wind seems to be blowing in the same in that spot on a constant basis (at least during and after storms).

    So based on my elementary comparison Id say its one part terrain, one part wind.

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    Quote Originally Posted by NLareau
    Furthermore I was thinking about the wind dynamics in a bowl versus a ridge. There are so many aspects feeding into bowls and also a great deal of eddying within the bowl. Very different than the long narrow extended ridge set up in many other locations.

    I would be interested in seeing if anything ever forms on the Knife's Edge of Katahdin. I'll be there in March and I'll have a look for you...
    Which feature produces more cornices? A u-shaped ridge (a bowl) or a straight ridge? Off the top of my head I'd say I have seen cornices on both, with a slight tendency to see more above a bowl.

    As for Katahdin, maybe not. It is steep on both sides of the ridge so the wind would tend to jump over the knife-edge. Depositing its snow farther down the slope in a large pillow.

    This gets me thinking that the main factor is the eddy. You need a relatively gentle (strong, but not turbulent) wind that creates a stable eddy in the lee of the ridge. A cornice builds from the top, but snow is also packed on the under side by the eddy.
    Bill
    Next up: Vermont City Marathon: May, 2011
    EasternLight

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    Quote Originally Posted by Ryan
    what about having these on a smaller scale?

    ......

    So based on my elementary comparison Id say its one part terrain, one part wind.
    Certainly drifts, and even cornices form quite frequently on the small scale. After reading this, I'm sure you'll start to notice them a lot more.

    Drifts are a no brainer. Anywhere there is wind and loose snow there are going to be drifts. Spring skiing in Tuckerman's relies on the process of drifting, avalanching, and more drifting.

    The cornice is different because an eddy allows it to form, building a delicate snow bridge with no support. It takes a perfect combination of wind and terrain to stack snowflakes in a fragile structure that can grow to the size of a house and become hard enough to walk on.
    Bill
    Next up: Vermont City Marathon: May, 2011
    EasternLight

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    I would agree with Neil's thoughts that variable wind direction limits cornice growth. During the winter of 2003-4 a small cornice did develope on the east side of Mount Clay, and broke off after a rain storm. I have also seen them above Jefferson Ravine.

    Highly variable wind direction, temperature, snow density, winter rain, along with vast quanities of wind transported snow result in the White Mountain snow pack containing many layers with variable bond stranghts between layers. Throw in 100 mph winds, and many potential cornices break off of the ridge lines before they have much of a chance to grow!

    Pete

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