Hard Freeze/Wet Slides

I was planning on skiing a line today in CO, and planning for an early start. (Both because of warming temps and that I needed to work this afternoon/evening :) ). The CAIC weather stations don't seem to be "current" with their data (maybe it's a browser issue), so the best thing I had to go on was the Copper Mountain (at 12,400) NWS weather station...which isn't exactly close to the line I intended to ski...but again, the best thing I thought I had to go on (at least a very similar elevation).

Anyway...ambient air temps dropped down to 30 on Tues night/Wed morn, but didn't stay there too long. 34 was the low Wed night/Thurs morn. Last night, the low hovered between 37-39 (was going back and forth). I know ambient air temps don't tell the whole story. With clear nights, radiative cooling would be in effect, helping to lock up the snowpack. Which brings me to question 1:

Is there anyway to know how much of an effect radiative cooling would have? For example, can I take the ambient temp, and subtract some fixed number or some multiple to get an idea of what the snow surface temp might be? If not, what's the minimum ambient air temp you would want to see to indicate a "solid" freeze? Does daytime high temps have any effect on all of this? (In other words...is 30 degrees "better" if the daytime high was only 50 vs a daytime high of 60?)

Edit: How many hours do I want to see the temp below 32 degrees to assume a "hard freeze"?

I haven't seen the snowpack up close of what I was intending to ski so, though being a S-SE aspect, it could already be an isothermal pack (no more layers/all one temp/with good drainage channels for moisture in the pack), which isn't as big of a deal for wetslide issues...still would want to hit it EARLY so it's not total slop. If it's not isothermal yet, or just recently isothermal, Tremper calls 3 nights without a solid freeze a red light...and even 1 night is a yellow. Based on those ambient air temps, I was guessing it was likely 2 nights without a solid freeze...and without firsthand knowledge of the pack, had to assume it might not be isothermal yet. So, I decided to not go after it. (wasn't motivated enough to drive 1.5 hours+ in each direction to just go find out for sure) Question 2: does that seem like a good decision?

Question 3...is there a better way to figure out if there's been a solid freeze (apart from an eyewitness account? :)

pssstttt snotel 7 day data tables contain hourly temp data.

except for eyewitness accounts and snotel, IMO the best way to find out is to go. I have no help on predicting solid freezes for you although I will say that I've gone before expecting no freezing given remote data and found super frozen conditions (including icy puddles). :)

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Originally Posted by lemon boy

pssstttt snotel 7 day data tables contain hourly temp data.

except for eyewitness accounts and snotel, IMO the best way to find out is to go. I have no help on predicting solid freezes for you although I will say that I've gone before expecting no freezing given remote data and found super frozen conditions (including icy puddles). :)

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Ah, yes...forgot to mention snotel. The only nearby snotel sites were low in elevation (2000+ ft) compared to where I was going. And the closest one was one that only updates once per day.

Edit: just answered my question...could use adiabatic cooling "rule of thumb" I guess to translate...but that would be a rough estimate at best, given even a range there, and a variety of other factors that change temperature at various locales/elevations.

Yeah…now you're starting to crazy talk. :)

One thing you could do is compare NOAA point forecasts for your area against NOAA point forecasts + snotel for a substitute area.

THE question this time of year in B/C skiing.

What I found is that there is no way to know other than to go there and inspect the snowpack.

I've gone up thinking it was frozen and it wasn't.

I've gone up thinking it would be unfrozen and it was.

I use snotel and meso-west sites to get temperature trends. http://www.met.utah.edu/roman/states...ate_frame.html

If its been below 30 for more than a few hours thats a really positive sign. On the other hand super hot temps during the day or several nights prior with out cold temps can mean that even with a good freeze it may not freeze the entire pack, just the surface.

Also clear skies are positive as radiative cooling can really help lock up the pack.

From the May 11th, 2007 CAIC forecast:

Snowpack Discussion
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Issued on May 11, 2007 at 2:00 pm (toepfer)Night skies should clear for the next several nights, which helps with a freeze, but be aware that the southwesterly air mass moving our way is going to bring on a quicker melt-down each day. Something to think about as temperatures start a steady climb upwards over the next several days. During the last super melt down, a few natural deep wet slabs ran. Reports indicate they were running around 5 PM. These ran on N, E andWaspects above treeline, with fractures running to the ground. There are areas were deeper instabilities still persist,, and would be most suspect on the more NW-N-NE aspects. Our overnight freezes will be a little weaker each of the next several nights. So getting off terrain before the snowpack melts will be key to avoiding the deeper instabilities. Another good thing about getting off early is that it keeps the snow surface smoother for the next riders that come through. Kind of an etiquette thing. A couple new obs....Several more wet loose naturals (WLN) observed around Summit County on all aspects above treeline. One very small soft slab noted on the south side of Peak 10 Friday. Many of the slidesare starting to run on the dust layer, several inches down. Some of the WLNs that run into gully/terrain traps accumulated a lot of debris, so watch the tube riding. From the 10th....Around 11:30 am Hoosier Pass skiers were able to ski-cut/trigger some good sized wet sluffs. Their ob was to be off slopes by no later than 11:30 amGot a call from one of the guys on Peak 2 Wednesday (thanx very much for the info, hugely appreciated). They left trailhead at 5 am, summited Peak 2 at noon, skiing by 12:30. They intentionally triggered several wet sluffs and would then ski on the bed surface/dust layer which was very supportable, and skiied nicely. Sounds like good protocol to me. They skiied in the area of the Silver Couloiron Buffalo the next day, started at 5 am, skiing out by 8:30am where lower elevations were getting pretty manky. N aspects have a less consolidated snowpack, andare rotting out faster then others that have gone through several melt freeze cycles, especially near and below treeline.


Avalanche Danger
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Avalanche danger isLOW in the early morning hours By afternoon look for increasing chances of human triggered wet loose and wet slab avalanches as the danger increases to MODERATE. This pattern should not change too much over the next couple of days. Even though there are flowers in the valleys, keep thinking about avalanches when you head into the high country. Most obs are pointing towards being off snow no later than 1 pm or earlier. This is a three-day forecast. The next update will be on the afternoon of May 14th. 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.

Dawson's 14ers guide has a pretty good discussion about these questions.

In the end, just go and then you'll know. ;) I mean, the level of info available through the internet is pretty amazing (snotel, Tr's, CAIC, NOAA, etc) but it's still a crapshoot. As much as it sucks to drive to a trailhead and find poor conditions, that was all you could do just 10 years ago and it's still all you can do. The NOAA forecast for the last couple of days in the Sangres was for temps in the upper 30's every night at 12-13,000'- we reluctantly decided to give it a go anyway and found that the temps were in the low 20's and if anything the snow (in the sun) was still too frozen at 10-11am.

We skied Argentine (13,738 feet) off Guanella Pass today. Snow had frozen over night, but it was sloughing pretty good on a NE aspect by 10am or so, especially once we got under 13k. I'd definitely be off easterly aspects in this area by 10 or 11ish and off westerly aspects pretty quick after that as well, definitely down from steeper slopes no later than 11 or 12.

I skied Grays this morning at sunrise. It was frozen for sure, but not a super hard freeze. When I left the Bakerville TH this am, my thermometer read 30 degrees. When I started up in the middle of the night it was 39 degrees. It still felt frozen to me even at 39 degrees. Keep in mind that freezing point raises to a higher temp also with altitude b/c there is less pressure... for the same reason that water boils at a lower temp at higher alt.

Keep in mind that freezing point raises to a higher temp also with altitude b/c there is less pressure... for the same reason that water boils at a lower temp at higher alt.[/QUOTE]


Might be time to revisit your high school physics book.

So hard to predict freezes because wind, RH and cloud cover all affect radiational cooling. Below freezing temps are always a good sign, but you can get a decent freeze at 40F, especially at the Colorado upper elevations.

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Originally Posted by panchosdad

Keep in mind that freezing point raises to a higher temp also with altitude b/c there is less pressure... for the same reason that water boils at a lower temp at higher alt.

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Might be time to revisit your high school physics book.



[/QUOTE]

Do you think something I said was wrong? Can you elaborate? Do I need to send you a phase diagram for water so you can see how pressure relates to temperature and the boundaries at which the phase changes occur??????

Sure, many other things have effects on MP/FP but I do not think what I said was wrong. Pressure has more of an affect than we realize.

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Originally Posted by mtnbikerskierchick

Keep in mind that freezing point raises to a higher temp also with altitude b/c there is less pressure... for the same reason that water boils at a lower temp at higher alt.

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Originally Posted by mtnbikerskierchick

Do you think something I said was wrong? Can you elaborate? Pressure has more of an affect than we realize.

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mbsc

check your equations... unlike the change in boiling point of water with pressure, the change freezing point is miniscule especially for the dP we are talking about

dT(fusion(H2O))/dP = -0.0074degK/atm

stick to chemistry ;)

2nd' what summit said
melting point changes are pretty small with pressure

theres more variation in melting point due to things like dust layers

what adiabatic rule of thumb are you referring to?
there are various charts and formulae (angström/brunt) that give approximations for the radiative flux at surface level, my understanding is that they are complicated and you get different results depending on what you use.

maybe kind of useful:
water vapour absorbs a lot of long wave radiation, which prevents radiative cooling. i.e.: you want low air humidity.

clouds absorb and reflect the lw radiation from the ground even more effectively and pretty much throw it all back at the ground, so a hard freeze is unlikely.

with radiative cooling, the temps fall slowly, moving upward from ground level, sometimes to the point of an inversion. this means you get a very stable atmospheric layering and there is hardly any thermal exchange through turbulence. if it's windy that gets messed up and the air near the ground doesn't get as cold as it could during a calm night.

so, no wind, no clouds and dry air is good.

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Originally Posted by Summit

mbsc

check your equations... unlike the change in boiling point of water with pressure, the change freezing point is miniscule especially for the dP we are talking about

dT(fusion(H2O))/dP = -0.0074degK/atm

stick to chemistry ;)

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Okay, yes, pressure has less of an effect on FP than I remembered. I checked the slope of the line for crystallization on an H2O phase diagram. The triple point lies at .01 deg C, 0.006 atm and the FP is 1 deg C at 1 atm. I don't think we will ever live anywhere where the pressure is 0.006 atm ;) Maybe you could though Summit! You'd find a way!

That means freezing at night is primarily due to evaporative and radiative cooling.

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Originally Posted by klar

with radiative cooling, the temps fall slowly, moving upward from ground level, sometimes to the point of an inversion. this means you get a very stable atmospheric layering and there is hardly any thermal exchange through turbulence. if it's windy that gets messed up and the air near the ground doesn't get as cold as it could during a calm night.

so, no wind, no clouds and dry air is good.

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Interesting. I always assumed, as an educated guess, that wind was good, thinking that wind would evaporate any "free" water in the snowpack, meaning less water would have to freeze than if all that water was still in the snowpack. I guess now I'm not so sure, because I hear what you're saying, too.

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Originally Posted by klar

what adiabatic rule of thumb are you referring to?
there are various charts and formulae (angström/brunt) that give approximations for the radiative flux at surface level, my understanding is that they are complicated and you get different results depending on what you use.

maybe kind of useful:
water vapour absorbs a lot of long wave radiation, which prevents radiative cooling. i.e.: you want low air humidity.

clouds absorb and reflect the lw radiation from the ground even more effectively and pretty much throw it all back at the ground, so a hard freeze is unlikely.

with radiative cooling, the temps fall slowly, moving upward from ground level, sometimes to the point of an inversion. this means you get a very stable atmospheric layering and there is hardly any thermal exchange through turbulence. if it's windy that gets messed up and the air near the ground doesn't get as cold as it could during a calm night.

so, no wind, no clouds and dry air is good.

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you nerd





(lets go skiing)
edit: to others, i think klar actually studies this stuff so she might very well even be right

Thanks for the discussion, everyone. Though trenidng more technical than I think I was looking for, it's good. :) Unless you can create a website for me that will pull all the necessary data from NWS, mesowest, snotel, CAIC, etc. weather stations and do the math so I can just get a number that means something to me. ;)

A little more context for the questions. As I acknowledged in the original post, I realize the only way to know for sure is to go. Some days I may have the time to go and do that...where it doesn't matter if I get to ski or not...there is definitely "hands on" learning to be gained...which cannot be replaced by pure discussion. And, hey...a few hours in the mountains is a day in the mountains. Other days, if I'm not going to get to ski, I'd rather "save" the time for a "better" day in the near future...based on time contrainsts/other things I need to tend to. That was the case on Friday.

Another detail that factors in to my bc planning is I like to plan for a line that I have a fairly high level of confidence is going to be "green light" for the day. I feel like it's a good first step in staying safe. Always willing to turn around...but I like to plan in such a way that I'm not usually surprised by what I find when it comes to weather/snowpack/etc. in evaluating my final go/no-go decision. So...approaching it from a "I'm rather confident this is going to be a safe line THIS DAY, but of course, we'll see what we find when we get out there", as opposed to "I'll be surprised if the conditions are safe to do this line TODAY, but maybe we'll get to ski it anyway." I think I have to still be careful in that approach...I don't want over-confidence before stepping out of my car skew my evaluation when I actually get on/in the snow. But, I still like the idea of careful decision-making/evaluation before I head out. Why aim for a line that I'm already doubtful is going to be safe?

So...that's why I ask these questions...to save time when I don't have time to spare (a "turnaround" trip) and to increase the accuracy of "pre-field" planning/decision-making/evaluation. Plus...I find these discussions are just generally beneficial when it comes to overall bc knowledge...

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Originally Posted by klar

with radiative cooling, the temps fall slowly, moving upward from ground level, sometimes to the point of an inversion. this means you get a very stable atmospheric layering and there is hardly any thermal exchange through turbulence. if it's windy that gets messed up and the air near the ground doesn't get as cold as it could during a calm night.

so, no wind, no clouds and dry air is good.

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can you explain this last point?

it goes against pretty much everything Ive learned and have been taught and observed in physical testing.

mainly;
higher fluid velocity = higher convective heat transfer coefficient = faster cooling\heating (ESPECIALLY with turbulent flows)

edit: im with you 100% on wanting no clouds and low humid. its just the wind Im we're not seeing i2i

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Originally Posted by keksie

you nerd

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you have no idea :)
and yeah, let's go skiing.

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Originally Posted by pechelman

can you explain this last point?

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mmhhkinda, we are thinking about different things.

I meant cooling through radiation as that is what spthomson mentioned. This depends on the temperature (and type of material) of the surface. See Stefan/Boltzmann Law.
Of course you are right that more wind will incresase convective cooling.
These are two seperate effects.

Afaik, cooling through radiation is the most important factor to get a freeze thaw cycle.

Stuff that affects the energy balance of the snowpack:

- Energy given off through radiation: Q = outgoing long wave radiation - incoming lwr - incoming swr x (1-albedo)

A snowsurface will act similar to a black body in long wave radiation, emmiting at a maximum between 8 and 13 micrometers. This just so happens to be a window where the atmosphere abosrbs hardly any radiation, thus the snow can cool down effectively. Water vapour, ozone and co2 absorb a lot in the rest of the spectrum.


- The sensible heat flow, given by

Q = -d x c x K x( dT/dz + R)

Where d= density of air, c = heat capacity of air, dT/dz = temp. Gradient, R= dry adiabatic lapse rate, K = index of (unsure about correct english terminology) turbulent transport, taking into account windspeed, roughness of surface and difference in surface and air temp.


- Then we have the latent heat flow (--> evaporative cooling)

Q = -d x L x K x (dq/dz)

L= enthalpy change of evaporization, dq/dz = gradient of specific humidity

Heat exchange w/ the ground and (in spring) heat of fusion also play into the overall energy balance.

edit: found a picture!

particularly in the spring skiing months the sensible and latent heat flows are not very big.
Black = net energy flux through radiation, red = sensible heat flow, black dotted = latent heat flow

http://www.geo.unizh.ch/~hoelzle/E&M...g/ebalmurt.gif

i didnt make my point\question very clearly.

We agree that convective cooling will increase with higher velocity, so lets start there.

If the wind is stronger, it will move more mass of air around, mixing up the hot and the cold layers of air, increase the amount of cold air from the atmosphere, and then, the air temperature will see an overall net decrease in temp. Thats kinda where Im starting to ask questions, with the decrease in air temperature.

So now we have a lower air temp, which then brings a higher temperature gradient between the radiating snow surface and the air temp (what the snow first radiates its energy to).

Now, looking at your formula.
- The sensible heat flow, given by

Q = -d x c x K x( dT/dz + R)

Where d= density of air, c = heat capacity of air, dT/dz = temp. Gradient, R= dry adiabatic lapse rate, K = index of (unsure about correct english terminology) turbulent transport, taking into account windspeed, roughness of surface and difference in surface and air temp.

With colder air you have;
A Greater air density
A larger heat capacity
a greater delta T

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?

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?

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.


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.


edit2: The formula Im familiar with and would probably use, which might not be as accurate as what you typed, is much simpler

Qrad = e * S * (Ts^4 - Ta^4)

where e is epsilon which is surface emissivity
where S is sigma which is boltzmans const
where Ts is the snow surface temp
where Ta is the air temp \ surrounding air temp.

This is why Im so hung up on this fact, because delta T is the driver in radiative effects, and its a Fourth order driver in this method of heat flux.

I think you're making it too complicated. The reason that wind is bad is that the ambient (2m) air temp is frequently above freezing on spring nights. Thus the convective transport would actually warm the surface, not cool it. If the air temp is below freezing, then obviously the opposite occurs, but this might be bad for spring skiing as well, because the surface might not thaw come morning.

Klars points about low humdity and clear skies are right on.

Another factor is slope angle. Low angle slopes face more directly at the sky, and thus radiatively cool more effectively. Steep slopes tend to face other mountains, and thus pickup radiation from them. The more open, rockless and treeless a slope is, the more effectively it will cool as well.

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Originally Posted by panchosdad

I think you're making it too complicated. The reason that wind is bad is that the ambient (2m) air temp is frequently above freezing on spring nights. Thus the convective transport would actually warm the surface, not cool it. If the air temp is below freezing, then obviously the opposite occurs, but this might be bad for spring skiing as well, because the surface might not thaw come morning.

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The thread entitled "Hard Freeze" had me in the mindset we were discussing freezing, and as such, Ive been thinking through this with the thought air temp is below freezing. Obviously Im not making this too complicated because even you so state that "If the air temp is below freezing, then obviously the opposite occurs," however, that is NOT what klar is exactly saying, especially when the air is below freezing and with substantial winds.

And while a hard freeze "might be bad for spring skiing", it is most definitely good for stability.

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Klars points about low humdity and clear skies are right on.

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Yes. I know. I said I agree with them.

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Another factor is slope angle. Low angle slopes face more directly at the sky, and thus radiatively cool more effectively. Steep slopes tend to face other mountains, and thus pickup radiation from them. The more open, rockless and treeless a slope is, the more effectively it will cool as well.

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good observations
In the steep face facing a steep face, it would then seem that convective cooling of the air between the faces would be even more significant than on lower angle\flat slopes.



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.

Woah. Pechelman, I really wasn't going after any thing you said, just trying to keep it simple, and clarify why I thought convection was important.

I'm after the same thing as you, understanding the physics.

oh
ok :)

my appologies if I came accross as argumentative or anything.