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Lambone

Freeze-Thaw

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ya'll should note wind chill is a measure of how cold it feels to a warm human body, not a solid piece of rock or ice already at ambient environmental temperature (a.e.t.). The wind chill factor essentially describes how the skin is cooled more quickly with a wind than without (duh!). So something that's already at the a.e.t. will experience no cooling effect from the wind. In fact, something below a.e.t. will experince a wind warming effect! Surface moisture also plays a role, but not nearly as much as....

 

also, i'd agree that it best to be adventuresome and go back home only when you see it's shitty out from personal observations.

 

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good. someone actually had something relevant to contribute on this. I was suspicious myself but did not follow through.

 

i'd agree that it best to be adventuresome and go back home only when you see it's shitty out from personal observations

 

if you don't stack the odds of success in your favor, you for sure are in for a lot of "adventure".

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ya'll should note wind chill is a measure of how cold it feels to a warm human body, not a solid piece of rock or ice already at ambient environmental temperature (a.e.t.). The wind chill factor essentially describes how the skin is cooled more quickly with a wind than without (duh!). So something that's already at the a.e.t. will experience no cooling effect from the wind. In fact, something below a.e.t. will experince a wind warming effect! Surface moisture also plays a role, but not nearly as much as....

 

Wind chill is due to two things that happen when wind blows across your skin:

 

1. it disturbs or destroys the thin insulating layer of air, warmed by your body, above your skin.

2. it accelerates the evaporation of moisture, which is an endothermic (cooling) process.

 

These are basic physical phenomena and apply just as much to rock and ice as they do to people. But, the extent to which they affect different objects and surfaces does vary, and "wind chill" refers to the response of human skin specifically. So using a wind chill chart to predict snow conditions may not be very accurate.

 

(Probably everyone has experienced the effect of #1 on sun-exposed rock on a cold day; it will be much warmer than the surrounding air if the winds are calm. #2 is one reason why thawed snow can crust up quickly on windy nights even when the temperature is above freezing - radiational loss of energy on clear nights is the other reason)

 

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ouch! 2 flip-flops in a day about basic science, it does hurt.

 

so the difference between surfaces results from radiation?

 

 

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so the difference between surfaces results from radiation?

 

That's part of the total energy input/output of an object, but when it comes to the extent to which wind affects refreezing of snow, I think evaporation is the primary factor (and probably a minor one compared to nighttime radiation, which is independent of wind).

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Was up looking at Chair Peak yesterday (thursday). Although the Alpental telemetry site said that the temp warmed with elevation, the only place where the snow was soft was in the sun. The north face had very little coverage compared to usual, 1/2" of very clear ice...way thin, little snow. So in spite of what the temperature sensors were saying, the inversion didn't seem to be as intense as they said, although it was warmer than down in the valley. My thought is that the pressure-differential generated wind which keeps the passes cold by bringing air from the east side was also flowing up into the valley and reducing the inversion effects there.

 

cheers,

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We didn't get a sense of an inversion yesterday (Friday) at Chair Peak. The snow never got mushy, even on the summit.

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I got much firmer snow/ice down low on Guye than up higher. Maybe all of Chair Peak was in the warmer air, which wasn't that warm anyway?

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It was pretty cold on Chair, definately below freezing. The snow was either very crusty or very sugary, but not mushy.

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You could see where the inversion layer was on the trees heading to Chair. Since Chair is north-facing conditions would be colder. But on the approach you could see the snow on the trees and then it started to dissipate. I am not sure how high or low. Even on the approach to chair there we sections of snow that would of made great skiing that snow was in the inversion layer. I am guessing the inversion layer is between 3-7000 feet and it fluctuated as the week went on. So that was why the condition on guye were better lower then higher

Dave

 

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Second what Lambone said. It seemed pretty cold. The hard snow created easy climbing, but since the crust had powder 6 inches beneath it, much of it was unprotectable. Still, we had quite some fun.

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