Hard Freeze/Wet Slides

oh great. what did i get myself into. you are actually making me think about that stuff i read in my clever books :rolleyes: ;)

Quote:

---

Originally Posted by pechelman

Not sure what R is as Ive never used that number, but it sounds like it would be pretty constant, and K also sounds like its now greater because of the increased wind velocity and increased in delta T.
So if all those numbers are greater, why would the flux be lower?

---

It's not. The sensible heat flux will be greater. That and the radiative heat flux are two different things.

edit: hm, you may well have a point about the wind not being that important for snow surface temp. I am used to thinking about what radiative cooling at night will do for the air temp at about 2m above ground and the wind will definitely prevent it from getting as cold as it coud there.

Quote:

---

Originally Posted by pechelman

Conceptually, Id also think radiative cooling would be the primary method for cooling, but Im not to the same place agreeing that radiative cooling is most effective with stagnant air thats allowed to heat in a manner it forms warm-to-cold layers. Why isnt introducing a new\fresh cold air more effective as I explain?

---

Basically you are making the air colder like that, not so much the snow. Ofcourse really cold air will eventually affect the snow but to a lesser extent than what the snow can do to itself be giving off radiation.

Side note: no idea about your part of the world but if we get strong cold wind, it usually brings clouds and humidity with it, it doesn’t just show up to make a nice overnight freeze. We often get a very warm wind, which eats away the snow so maybe I just associate wind with warm weather and sucky snow.

Quote:

---

Originally Posted by pechelman

Also, Im curious, what is your job\degree in?
Some of the things you typed remind me of one of the classes I took called Earth and Atmospheric Sciences.

---

i am attempting to get a degree in meteorology but not quite there yet. ;)

Quote:

---

Originally Posted by pechelman

edit: I may have overlooked one thing, where you said the atmosphere doesnt absorb much of the LW Rad in the 8-13um range that the snow emits.
If thats the case, then 1 of 2 things would seem to be implied depending on how much this is true. That you could either neglect the air temperature and all of its properties all together and assume its temperature is that of the vaccum of space or that the most important factor in how much energy the snow surface can radiate is how large the temperature gradient exists. The former seems like more what you're saying and the latter is where I am.

---

Stefan boltzmann law: s x e x T^4 (your formula in edit 2 should have different values e for Ta and Ts). This gives the emitted radiation of a body in Watt per m^2 of surface area. How many molecules of air would you need to get enough surface area to make a big difference in the net flux of radiation on the surface? Don't forget the particles emit in every direction, not just towards the ground.

What amount of radiation the surface emits does not depend on the difference in temperature with the surroundings. The snow will send out radiation regardless of what is around it (be it rocks, trees, “air particles”). On angled slopes, under trees or clouds it may well also receive radiation from these sources, significantly changing the net flux of radiation and potentially preventing freezes.

Quote:

---

Originally Posted by pechelman

Klar, is another thing Im neglecting that with more winds also has the potential for more vapor transport and a possibility for local rises in humidity?
Im asking honest questions here, and hopefully you dont think Im arguing with you. I got the feeling the above poster thought I was. Either way, I just want to get to the bottom line.

---

the latent heat flow will increase with windspeed (coefficient of turbulent heat transfer). I don’t know how strongly this would affect humidity, doubt it would be much.

And I am trying to figure out answers but have an exam tomorrow and need to go to bed now. :)

Quote:

---

Basically you are making the air colder like that, not so much the snow. Ofcourse really cold air will eventually affect the snow but to a lesser extent than what the snow can do to itself be giving off radiation.

---

I think we're still dealing with a disconnect.

Im not at all saying that convective cooling is cooling the snow, but cooling the "T infinity" to where the snow is radiating its energy. The greater that delta T between the air\atmosphere and snow surface, the larger the radiative cooling effects....especially since we're dealing with FOURTH order temperatures in KELVIN. A 10 degree change in ambient\surrounding\"T infinity" temperature will increase your radiative cooling potential by ~90%. (ie almost double within this temp range)

Quote:

---

Side note: no idea about your part of the world but if we get strong cold wind, it usually brings clouds and humidity with it, it doesn’t just show up to make a nice overnight freeze. We often get a very warm wind, which eats away the snow so maybe I just associate wind with warm weather and sucky snow.

---

apparently youve never been to colorado ;)
Wind is the norm, even when its bluebird or not a cloud in the sky at night.
You do make a good point and it is something I think we should all think about when we see high winds, especially if its overcast and was especially warm\humid during the day.

Quote:

---

Stefan boltzmann law: s x e x T^4 (your formula in edit 2 should have different values e for Ta and Ts). This gives the emitted radiation of a body in Watt per m^2 of surface area. How many molecules of air would you need to get enough surface area to make a big difference in the net flux of radiation on the surface? Don't forget the particles emit in every direction, not just towards the ground.

---

its true the emissivities will be different, but im not wanting to get too nitpicky and solve anything with any accuracy right now, I just want to understand whats going on. Emmissivities for snow seem to be anywhere from .85 to almost unity, whereas air can range from .75 to unity depending on moisture content. Im willing to bet\argue that for typical colorado conditions with weather and snow, we could remove the emissivity factor completely and still get a semi accurate answer.

Im not following you at all when you ask about air molecules. Calculating radiative flux has nothing to do with surface area. Quite simply, it only has to do with your delta T.

Sure the air molecules radiate in every direction, but for our discussion its very reasonable to explain and assume that everything is radiating in one direction. From Snow, up to air, through the atmosphere, and finally to space.
If you know each steady state temp, you can plug and chug each scenario to find out what each interfaces radiative flux is, but we're only concerned with snow, since thats what we ski on and need to be stable.

Quote:

---

What amount of radiation the surface emits does not depend on the difference in temperature with the surroundings. The snow will send out radiation regardless of what is around it (be it rocks, trees, “air particles”). On angled slopes, under trees or clouds it may well also receive radiation from these sources, significantly changing the net flux of radiation and potentially preventing freezes.

---

This is my first 100% disagreement.
I dont know where you got that fact, but radiation is goverened by one thing and pretty much one thing only.
The temperature difference of the surface of the object and whatever the object is radiating its energy into.
In our scenario, snow surface vs sky temp.

I also just found a very very similar example in my heat xfer book, and they calculate it this same way Im describing. Worthy of mentioning is that theyre assuming 2 different numbers for the "Air Temp" and "Sky Temp".
Im not really sure where theyre drawing a distinction between what temperature you use for calculating radiative fluxes. Interesting. Maybe you have a text that specializes in this or have some insight or can ask a prof?
Id be interested to know where this distinction is drawn, since it might just be that its not the nearsurface air temp I should be thinkign about, but moreso the "Sky Temp", whatever and whereever that is.


Ive also been working this with the assumption that the air is colder than the snow.
Which brings me to another question which I think I know the answer to, but will ask it anyway cause its not making sense completely.

If the air is warmer than the snow, and both are above freezing on a clear night, will the snow radiate off enough energy to actually drop in temperature?

Quote:

---

Originally Posted by klar

the latent heat flow will increase with windspeed (coefficient of turbulent heat transfer). I don’t know how strongly this would affect humidity, doubt it would be much.

And I am trying to figure out answers but have an exam tomorrow and need to go to bed now. :)

---

oh cool

by all means take care of that
no hurries :)

good luck!

Haven’t we got a nice little private wannbe geek party here? :biggrin: I know there are some more qualified people around, do feel free to enlighten us.

Quote:

---

Originally Posted by pechelman

I think we're still dealing with a disconnect.

---

Yes. I think I see where you are coming from now. Our main point of disagreement is that you think the snow cares about the air temperature and I don’t.

Quote:

---

Originally Posted by pechelman

Im not following you at all when you ask about air molecules. Calculating radiative flux has nothing to do with surface area. Quite simply, it only has to do with your delta T.

---

You come in wet and cold from eating shit all day on the hill, do you stand next to the big fire or the little one to warm up? If both fires have the same temperature, ie no delta T, does that make them somehow less hot?

So, say you had two objects of the same T. Object 1 has a surface area of 100 m^2, object 2 has a surface area of 50 m^2. We calculate how much they emit with (sigma x T^4). This gives us a value of X in W/m^2. Object 1 has 100m^2 to emit x from, thus emits 100X, object 2 emits only 50X.

Quote:

---

Originally Posted by pechelman

Sure the air molecules radiate in every direction, but for our discussion its very reasonable to explain and assume that everything is radiating in one direction. From Snow, up to air, through the atmosphere, and finally to space.
If you know each steady state temp, you can plug and chug each scenario to find out what each interfaces radiative flux is, but we're only concerned with snow, since thats what we ski on and need to be stable.

---

ok, now I have some questions for you. What is the difference between air and atmosphere? Where is the boundary? What is the temperature of the air? What is the temperature of the atmosphere? Is it the same temperature everywhere? If everything radiates into only one direction, namely to space, why isn’t the earth perpetually frozen solid? (Also, if everything radiates in one direction, what is the point of our argument? I thought you were saying that there is less cooling through radiation because the “air” radiates energy back into the snow?)

my point is that the air particles do not radiate off enough on their own to significantly slow down the cooling of the snow surface, which I think is what you were implying?

Quote:

---

Originally Posted by pechelman

This is my first 100% disagreement.
I dont know where you got that fact, but radiation is goverened by one thing and pretty much one thing only.
The temperature difference of the surface of the object and whatever the object is radiating its energy into.

---

If you have a situation where there are several fluxes in different directions, you can write the NET result, the sum of incoming and outgoing radiation, as a function of the difference in temperatures of the emitting (idealized) bodies.

A body will emit a certain amount of radiation regardless of its surroundings. I am looking at the OUTGOING FLUX from the snow. This depends only on the temperature of the snow.

Back to the room with the fire where you are trying to get warm after skiing, will the fire be less warm because you got so much snow down your jacket that you also turned the radiator on?

Quote:

---

Originally Posted by pechelman

I also just found a very very similar example in my heat xfer book, and they calculate it this same way Im describing. Worthy of mentioning is that theyre assuming 2 different numbers for the "Air Temp" and "Sky Temp".
Im not really sure where theyre drawing a distinction between what temperature you use for calculating radiative fluxes. Interesting. Maybe you have a text that specializes in this or have some insight or can ask a prof?
Id be interested to know where this distinction is drawn, since it might just be that its not the nearsurface air temp I should be thinkign about, but moreso the "Sky Temp", whatever and whereever that is.

---

Not really sure what this is about. The temperature obviously changes throughout the atmosphere, for the vertical temp gradient it is mainly a function of pressure/altitude. For models where you look at radiation balances, you often simplify everything by ignoring the latent and sensible flux and considering only the solar and terrestrial radiative fluxes at the ground and the atmosphere, both of which can be then considered as planes. Maybe the temp of this “atmosphere plane” is your sky temperature. I’m just guessing.

For microclimates, you would also take into account a boundary layer between the ground and the free atmosphere, where you have continuous turbulence, mostly cross-isobaric wind and large energy dissipation, whereas in the free atmosphere everything is much more boring. This boundary layer can be anything between from about 100m-3km high.

Quote:

---

Originally Posted by pechelman

Ive also been working this with the assumption that the air is colder than the snow.
Which brings me to another question which I think I know the answer to, but will ask it anyway cause its not making sense completely.

If the air is warmer than the snow, and both are above freezing on a clear night, will the snow radiate off enough energy to actually drop in temperature?

---

The snow in Colorado can be warmer than 0°C? Interesting. ;)

In a typical spring scenario, the air will indeed be warmer than the snow and yes, both will drop in temperature.

get a room already you two, jesus.

From the CAIC:

Snowpack Discussion
--------------------
Issued May 16, 2007 at 4:30 pm (toepfer) Weather details for elevations around 12,000 to 13,000 ft. Several obs have come in recently, and thanx a lot to those that have. Most come from last weekend:Pikes Peak: Only a light freeze at around 12-13,000 ft over the weekend. Snowpit on E aspect at 13,000 ft showed a very weak layer about 8 inches deep. This was not wide-spread though. N aspects A very hard Melt Freeze crust about 2 feet deep. No easy shears or results from compression tests. There was some activity noted from the last storm cycle we had. Mostly new snow on east aspects. Nothing deep. Marble Area: Significant activity from last storm cycle, roughly half the paths that could slide did. Mostly new snow running on the old dust layer. (this dust layer looks as bad or worse than last year) Almost all activity was triggered by cornice fall or sluffs off cliff bands. There was a lot of debris in some areas. If no cornices or cliffs above a path, the snow stayed in place and was turning to very nice corn. Freezes were light, but clear nights were helping with a good freeze. There were 3 north aspect natural wet slabs that were triggered by wet loose sluffs, 2 ranon thethe old dust layer, one on the N aspect of Mt Justice just below treeline was much deeper, basically to the ground, but could not see how far it ran. Westerly aspects had only a 4 inch crust on top of wet ball bearings, need to exit off these slopes early. We could stay outto early afternoon if we had a well beaten path to exit out on lower elevations on N & E aspects. Skied Ant, Thompson & Daly. Handies Peak/San Juans:Perfect Corn...Signs of late day sluffs, but not big, shallow onlySilverton area: N-NE-E aspects several WLN involving newer snow. Near Ophir Pass on Saturday non-supportive crust with lots of large collapse, backed off to lower angle terrain. Savage Mtn south of Red Cliff:OK freeze below treeline, but near the summit in a NE couloir there must have been an inversion as the snow suddenly became very wet and slushy with no freeze crust. Watched a cornice peel and trigger a 150 ft wide wet slab at 12:30Longs Peak: Snowpack damp by noon, but not quite corn, no slides noted.Mt Bancroft, near James Peak in Front Range: One natural wet slab on ESE aspect above 12,000 ft. One natural wet slab to ground on NE aspect of Peak 1 in Summit County. Ran between 8 amand 4 pm Monday. About 100 ft wide, by 400 vertical, fracture about 3-4 ft deep estimated. Similar to slabactivity in Marble.Wednesday skied east aspect of Torreys and N aspect of Kelso (Front Range), great corn until 1 pm. Clear nights gave a very firm freeze overnight, though Loveland Pass and Berthoud Pass weather sites showed only a very light freeze. 25 degrees on my deck at 10,200 ft at 5 am.
Mesowest: http://www.met.utah.edu/cgi-bin/droman/raws_


Avalanche Danger
-----------------
The avalanche danger is LOW in the early morning hours. By midday the avalanche danger will increase to MODERATE. Human triggered wet loose and wet slab avalanches are possible on steep slopes. Ride on the snow early, then switch to a bike or kayak for the afternoons.This is a three-day forecast. The next update will be on the afternoon of May 18th. We will no longer be issuing daily forecast for the 2006-2007 operating season. We will be issuing SPRING FORECASTS for the Front Range and Vail Summit zones on Monday, Wednesday, and Friday afternoons through most of May. The forecasts will include observations from other areas as we receive them. This is an experimental service. If there is sufficient use, we will seek additional funding to expand the spring forecasts next season. We would like to thank everyone who used our products and especially those who have sent us observations. We certainly appreciate your help and support.

http://www.summitpost.org/images/large/293862.JPG

"slide on Tenmile Peak just above Frisco" 5-16-07



pic from kjkrow on splitboard.com

Snowpack Discussion
--------------------
Issued May 18, 2007 at 3:30 pm (toepfer) Weather details for elevations around 12,000 to 13,000 ft. A few more obs to relay:Berthoud Pass area has seen only weak freezes the last couple days. Snowpack becomes slushy and non-supportive by late morning. Observers could trigger some shallow wet sluffs with ease, even on shallow angle terrain late in the day while wandering around the Peter Rabbit hut. Abasin has lost 9" off their total snow stake over the last week. Warm days, and no new snow are eating away at the backcountry snowpack.Fewer areas where you can skin-up right at the trailhead. Mt Lincoln in the Mosquito: No sign of recent slide activity, but booting up near the ice climbing area below the amphitheater at 6 am was mostly supportable, but some postholing encountered. Crust is maybe 4-8 inches thick on N and E aspects, but got sloppy by around 10:30 coming down. Could trigger 8 inch deep wet loose slides. Good to be off by 11:30 am at the latest, 10:30 would be better. Fremont Pass, Bartlett Peaks: Near ridges, some big collapses noted on early morning solar exposures (easterly). These did not propagate far, but are a sign that a good freeze did not hit overnight. Crust was about 4 inches thick at 13,555 ft on NE aspect, and near any rock outcrops was not supportive. Lots of frozen chickenheads from earlier rollerballs, and these were quite frozen at 9:30 AM. As noted in weather synopsis, the 1/2 inch of new snow seemed to insulate the snow from a hard freeze. Area weather stations indicated a very weak freeze at best, but at my house at 10,200 ft it got to 25 degrees, with a hard frost. No new slides were seen, but a medium sized soft or wet slab was seen to the south on Mt Arkansas at around 12,500 ft. Looks like it ran Monday or Tuesday? N aspect, a couple feet deep. Recent thaws producing rollerballshave made clean lines a little hard to find. But they are out there. Narrow gullies/couloirs on the ridge from Fletcher to Clinton Peak had some large wet loose slides from what looked like several days ago that ran to ground, looked messy. These were on west aspects, starting at around 13,000 ft.


Avalanche Danger
-----------------
The avalanche danger is LOW in the early morning hours. By midday the avalanche danger will increase to MODERATE. Human triggered wet loose and wet slab avalanches are possible on steep slopes. Ride on the snow early, then switch to a bike or kayak for the afternoons.This is a three-day forecast. The next scheduled update will be on the afternoon of May 21st. We will no longer be issuing daily forecast for the 2006-2007 operating season. We will be issuing SPRING FORECASTS for the Front Range and Vail Summit zones on Monday, Wednesday, and Friday afternoons through most of May. The forecasts will include observations from other areas as we receive them. This is an experimental service. If there is sufficient use, we will seek additional funding to expand the spring forecasts next season. We would like to thank everyone who used our products and especially those who have sent us observations. We certainly appreciate your help and support.

Snowpack Discussion
--------------------
Issued May 21, 2007 at 3:30 pm
All of the observations we have received since Friday are from the Front Range and Vail/Summit County zones. Observers report finding wet snow or sloppy conditions much earlier in the day than they expected. Several people said they found good corn snow conditions around 9:00 in the morning, but by midday they started to see avalanche activity (even on north facing slopes). A pair of alpine climbers in Rocky Mountain National Park questioned whether there was a hard freeze at elevations as high as 13,000’.
Most of the avalanches reported to us over the weekend were wet loose-snow avalanches or sluffs. These slides are occurring on all snow-covered aspects. Of course with any generalization there are some exceptions. An observer witnessed a “huge” avalanche coming off the E and NE side of Notchtop Peak. The slide released at 4:50 pm on Saturday. Another large wet slab avalanche released from the E side of Peak 1 in the Ten Mile Range. The avalanche was around 5’ deep and up to 300’ wide. It probably ran during the middle of last week.
Spring conditions currently exist in most mountain areas. The avalanche danger is generally low in the mornings, but increases rapidly with daytime heating. A good mantra to follow is “start early and end early”. With our persistent warm weather and marginal freezes each night, you need to start even earlier. Loose-snow avalanches have been prevalent, but there have been a few wet slabs reported.


Avalanche Danger
-----------------
The avalanche danger has been generally LOW in the morning and rising to MODERATE each afternoon. Over the next two days, daytime heating could be less intense, but new snow and wind may cause the avalanche danger to rise.
This is a three-day forecast. The next scheduled update will be on the afternoon of May 23rd.
We will issue our LAST FORECAST for the 2006-2007 season on Friday May 25. This will cover the Memorial Day weekend. Even through we will stop regular updates, we will continue to monitor conditions and issue Avalanche Watches and Avalanche Warnings as necessary. Please send us any observations when you are traveling in the high country. We would like to thank everyone who contributed to the program this year by sending information and donations. We could not survive without your support. Please send any comments, suggestions or observations to caic@qwest.net. Have a great summer and we will start operations for the 2007-2008 season in early November. See you at the Avalanche Jam on September 7th!

Hard Freeze report from Independence Pass this Sat and Sunday:

Perfect Corn. Frozen stiff where I skied from 13,700' down to 11,000' due to really good nighttime temperatures. I imagine stuff much lower might be less frozen, but up high it was very firm early in the morning.