 Poster: A snowHead
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| rjs wrote: |
| Bode Swiller, The humidity could also come from people breathing out. |
Exactly
Get your kids to do this – its fun
Have a go at this when you get home.
Highly scientific gear you’ll need.
A hot tap and a cold tap
Two glasses – same size
Bag of sugar
A spoon
Fill up one class with really cold water
Fill the other with really hot water
Put one spoon of sugar into the cold glass and one into the hot glass
Stir well after each spoon
Keep doing this until you see the sugar start to collect in the bottom of the glass (it “will” be the cold glass). Stop putting sugar into the cold glass
Now keep doing this until you get some sugar forming in the water in the hot glass as well.
The difference between the amounts of sugar required to saturate the water in each glass will depend on the difference in temperatures.
It’s the same with the air (clouds and lack of them)
Warm air can absorb more water molecules before it become saturated (at this temp and pressure it's the dew point) but using sugar lets you see it
So there you go.
As the people (bodies, breath, etc) warm up the air in your big fridge the air warms slightly and so can hold in suspension more vapour which you see as mist. When the people leave (the vapour doesn’t, it stays there) but the heat source does, so the air cools down and the air can no longer hold the vapour in suspension so it falls down and the mist disappears.
Test it for yourself
Put your eye as near to the ground in snow as you can (that’s head on the floor looking sideways) you’ll notice a layer with no vapour.
Why ?
The snow is cooling this layer so it can’t hold as much moisture in suspension and you don’t see any mist.
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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| holidayloverxx wrote: |
Mr HL says that sublimation is to change from solid to gas without becoming a liquid. Carbon Dioxide does this ice/snow will always go through a liquid phase before becoming water vapour.
er.... questions and cries of disbelief direct to Mr HL please (4 years Hons degree in materials science). Also, BTW, the fogginess is out of balance air con (he worked for the company that did the ice rinks and bobsleigh runs at Lillehammer winter olympics) |
Ask him what the triple point of water is, and what triple point means. Can you clarify exactly what Mr HL did in this company. Given that he doesn't know that water can sublime, I am hoping he was making the tea and not involved in any important decisions.
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Well, the person's real but it's just a made up name, see?
Well, the person's real but it's just a made up name, see?
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Thornyhill, He knows very well what triple point means. Sublimation is a phase change from solid to gas with no liquid (he says).
If you think he's wrong then clarify how water sublimes and I'll tell him. (BTW, he wasn't actually making the ice rinks!)
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I don't know anything about the physics but I do know that snow disappears (out on the mountain, not in a fridge) without any noticeable melting. It just goes up, up, up and away. On some days it does it so fast you can see it "steaming" (and yes, I do know that that is water vapour). But it still happens, whether you can see it or not. and when the dry, warm, foehn wind blows for a day across a section of off piste which has been tracked the tracks end up like railway lines, sticking up above the surrounding snow.
Whether that's sublimation or evaporation, and what the difference is, who knows? It's disconcerting, though. You can almost see the snow depths dropping.
It happens when there's no foehn too (ie the air is not so warm and dry), just slower.
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Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
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pam w, there isnt a roof over most mountains though, plus, they tend to be higher than sea level 
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You'll need to Register first of course.
You'll need to Register first of course.
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I believe the ski centre at Hemel physically removes the 'old' snow from the botom of the slope (to where it gradually gravitates) and stick it in a big bin outside to melt & drain away. I overheard the piste-basher driver telling someone in the bar.
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| holidayloverxx wrote: |
Thornyhill, He knows very well what triple point means. Sublimation is a phase change from solid to gas with no liquid (he says).
If you think he's wrong then clarify how water sublimes and I'll tell him. (BTW, he wasn't actually making the ice rinks!) |
Mr HL is right in normal everyday circumstances
At normal pressure and temps water will always go from ice to water to floaty stuff.
So in his working environment he is correct.
This all changes when you go up a hill.
For sublimation happen you need a few things. Low pressure, shed loads of energy and cold.
Of course you get this up hills
Low pressure as there is less air up there (see item 4 for why)
Energy, on a slope this is from the sun – too complex for here but basically in the sun stuff goes bang and energy is released (see 1st para for why)
Everyone knows it’s colder up a hill (see 2nd paragraph and item 4 for why)
So, on a ski slope
If it was warmer then the ice would just melt and then the water would evaporate.
If it was dark there would be no energy to speed up the atomic mass (so vibrating the molecules enough to break bonds that hold the ice together)
If the pressure was higher the triple point would change (drop) so you’d need more (or is that less – who knows) cold and/or more energy.
Mind you this still doesn’t tell you why the snow disappears at Hemel, not clue about that one. Maybe someone pinches it each night.
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Wayne, thanks for today's science lesson - never my strong point!
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You'll get to see more forums and be part of the best ski club on the net.
You'll get to see more forums and be part of the best ski club on the net.
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skimottaret, I'm not too sure what the lack of a roof has to do with it. If the air is less than 100% saturated, and is warmer than the snow, then surely the snow will evaporate? Slower, no doubt, than on a mountain with a foehn blowing, but it will disappear gradually, nonetheless.
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| holidayloverxx wrote: |
| Mr HL says that sublimation is to change from solid to gas without becoming a liquid. Carbon Dioxide does this ice/snow will always go through a liquid phase before becoming water vapour. |
I'm afraid that is wrong - for a multi-component system, such as water plus the components of air.
Mr hlxx is probably remembering the single-component phase diagrams for CO2 and H2O, for which the statement is correct at anything approaching a pressure equivalent to one atmosphere.
I don't know if Mr hlxx ever came across the "phase rule"; but that explains (at least the logic leading to it does) the possibility of transitions between multiple pairs of phases in a multi-component system at any given temperature and pressure.
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snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
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| pam w wrote: |
| If the air is less than 100% saturated, and is warmer than the snow, then surely the snow will evaporate? . |
Hmmmmmm.
Not really – if the snow was (for example) at -10’ and the air temp was at -5, the snow would not melt. Unless you took it down a really deep mine shaft.
For way to boring reasons if it was the other way around ie. the snow was -5 and the air was -10 you’d get sublimation.
Oh yeah, the mine shaft stuff is like this. When most stuff is frozen and you apply heat to it will normally expand (so the volume pushes outwards) as it melts. The higher the pressure around our stuff, the more energy is required for this push outwards, so the temp at which it melts is (ever so slightly) higher.
But with snow it’s different. The volume of snow/ice contracts when it melts so any increase of pressure will make it easier to contract and the melting point is (again, ever so slightly) reduced. So take a block of ice down a pit and it’ll melt quicker.
laundryman, I think (maybe wrong) that Mr HL was talking about ice/water/floaty stuff - not independent binaries, so he's right in everyday experiences. At least thats what they have dumbed down to now at A level (F=0 with pure water)
So a bit of slack may be awarded to a fellow  - or at least the over'arf of a
All this has nowt what-so-ever to do with the theft of the snow at Hemel, so that's me out of this enormous thread drift.
I live therefore I snow-plough 
Last edited by snowHeads are a friendly bunch. on Wed 11-08-10 18:31; edited 1 time in total
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And love to help out and answer questions and of course, read each other's snow reports.
And love to help out and answer questions and of course, read each other's snow reports.
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pam w, i was just trying to get the point across that in the dome we dont get sunshine, which adds a lot of the energy needed for sublimation...
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You know it makes sense.
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Otherwise you'll just go on seeing the one name:
Otherwise you'll just go on seeing the one name:
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Is this going to be another 1,00 post thread, complete with A level physics, diagrams, personal abuse and trolling? (opens popcorn)
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Poster: A snowHead
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Spud9, ooh, I do hope so! (Passed physics "O" level thirty odd years ago and never had cause to do any since  .)
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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Come in, GrahamN, your time is up.
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Well, the person's real but it's just a made up name, see?
Well, the person's real but it's just a made up name, see?
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This thread could go from the sublime to the ridiculous.
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You need to Login to know who's really who.
You need to Login to know who's really who.
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Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
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Hurtle,
GrahamN and I were taught this stuff by the same crew, so I'd be surprised if we differ.
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You'll need to Register first of course.
You'll need to Register first of course.
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Wayne has it sussed with the pit thing. I could go into about 100 pages of this, just for the proof, but I fear I would die of boredom at the same time as everyone else. You don't need low pressure or loads of energy for sublimation of water. In fact, it is almost impossible to create a scenario where snow/ice won't sublime if the conditions are right for it to melt. Every time you make a turn, you are expending energy..... where does this go? Into the snow you have just turned on. The snow can basically either gain heat, potential energy or kinetic energy from it. If you are throwing the snow uphill, it will have gained potential energy. If you just throw it to the side, it will have converted your energy to kinetic energy. If you push it down the hill, it will momentarily have more kinetic energy than it had, but by the time it lands it will have the same kinetic energy, but less potential energy. The energy you expended in the turn will seem to have disappeared, along with a small portion of the potential energy. We all know that energy in a closed system can't just disappear, so it can only have been converted to heat. (This is how ice skating works - the pressure of your blades melt the ice below your skate so you can glide along on an incredibly small skin of water.) The heat required is fairly large in proportion to the other energies involved, so if we take P1V1/T1=P2V2/T2, T2 has increased substantially so P2V2 must also increase for the snow under your turn. As you have just melted a minuscule amount of snow, V2 will be smaller, so P2 will have to be greater than the increase of T2 and loss of V2 combined. Once you leave that bit of snow, the reverse happens. Suddenly T1 and V1 have been reinstated, so P2 will drop under P1 by the amount of energy you have expended making the turn. During the course of 1 turn, you will exert a relatively massive amount of pressure and temp on the snow. As the snow jumps from being frozen to melted and back to frozen, a tiny amount of it will have evaporated, without ever becoming water. You have to trust me on the triple point thing though. (You don't even need to make a turn..... the friction related to skiing straight down will do the same thing.)
So every time someone comes down the slope, they evaporate a tiny bit of it. Mr HL could have a point that the fogginess is caused by underpowered AC, but his dismissal of sublimation would mean that in a cold enough snow dome or ice rink, you wouldn't ever need any energy to make new snow or ice
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Thornyhill, I'm sure loads of us were all thinking exactly that.
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| Bode Swiller wrote: |
| Thornyhill, I'm sure loads of us were all thinking exactly that. |
What... That you would die of boredom?
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You'll get to see more forums and be part of the best ski club on the net.
You'll get to see more forums and be part of the best ski club on the net.
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snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
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Boredsurfing, You are just plain evil
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And love to help out and answer questions and of course, read each other's snow reports.
And love to help out and answer questions and of course, read each other's snow reports.
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Well, we are back from hols (very nice time, thanks for asking). Mr HL might wish to come back in, if he can be ars*d
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FAQ
