jack

That was mostly rhetorical, actually. I don't think we actually did, though, and I couldn't tell you any details at all, but I thought my qualitative explanation was mostly correct, did I look that much at sea?

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dragonwoodshed.livejournal.com

My boat gets really REALLY cold. Does that help at all?

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beckyc.livejournal.com

Others have discussed wind chill, so I'll talk about something else. Have you ever studied kinetic theory and thermodynamics? Do you know what absolute zero, heat, temperature etc actually mean?

Consider (for simplicity) a gas of molecules of some sort. At temperatures above absolute zero, these molecules whizz about and exert pressure on their surroundings. If you give it more heat (energy), they speed up and whizz about faster. Conversely, if you remove heat (energy) from the system, they slow down. At absolute zero, all atoms and molecules are in their ground state. Where do you think this energy is going to come from to give to the surroundings at this point? (which, incidentally are going to have substantially more energy, it's actually really hard to move heat from a colder body to a hotter, just listen to Flanders and Swann singing about it)

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Have you ever studied kinetic theory and thermodynamics? Do you know what absolute zero, heat, temperature etc actually mean?

Up to a point. I have a qualitative idea of bouncing molecules and absolute zero is when they would stop (uh, for our purposes). But the connection with individual molecules and entropy and the connection of different sorts of entropy and the physics of low temperatures seemed very interesting but I wasn't really explained and I've still not gone back to, unfortunately.

Where do you think this energy is going to come from to give to the surroundings at this point?

Well explained, thank you, but I didn't think I *was* doing that: what part worried you? X is at a positive temperature, Y is marginally above absolute zero, the "wind chill" is entirely theoretical...

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Negative Kelvin temperatures do actually have a physical meaning, in that the density of states of a system can become inverted and such a system could be characterised as having a negative temperature. Consequently, the temperature in question would definitely not be negative.

An actually negative wind chill temperature is an interesting concept. It implies the wind is somehow causing a population inversion. Maybe you could have a negative solar windchill if you tried hard enough.

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Negative Kelvin temperatures do actually have a physical meaning,

This is one of the things I read about, but didn't quite understand. You don't happen to have a link to a clear explanation handy, do you?

It implies the wind is somehow causing a population inversion.

What? I agree, if wind actually cooled something to below 0k, that would require [weird entropy stuff], but I have no idea if that's remotely plausible, and certainly didn't postulate in my example anything at or below absolute .

Maybe you could have a negative solar windchill if you tried hard enough.

Oh?

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You don't happen to have a link to a clear explanation handy, do you?

No, not really if you don't understand Wikipedia's. The basic idea is that normally energy states fill up from the bottom (low-E) end - occupation of a state being proportional to exp (-1/kT) or something close thereto depending on exactly what it is that is filling the state up. Obviously if T is negative they would start filling up from the top (high-E) instead. This is good if you want to make a laser.

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Well, thank you. I may understand wikipedia, I haven't tried to refresh my brain on it very recently. I'll let you know if I do :)

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That sort of "negative temperature" really isn't really relevant to heat flow at all.

As thermal conduction is normally proportional to the difference in temperature if you're defining a wind chill scale based on the rate of thermal conduction from the human body there's no reason why you can't have negative Kelvin but you'd have to be pretty damn cold! Or I suppose you could cheat and define "wind chill" to also cover things like being completely immersed in freezing water.

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You can't define negative Kelvin to have a different meaning depending on what field you're in. A Temperature is a Temperature, and thermodynamics can only have one meaning thereof. A temperature in negative Kelvin represents a population inversion.

If you want a heat flow reason you can't have a temperature in negative Kelvin then the answer is "obvious violation of the third law of thermodynamics."

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Yeah, so, that was me.

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This may sound a little dijointed, sorry, I'm trying not to be snarky :) You sound to be denying the concept of wind chill completely.

I think what I'm trying to say is that wind chill isn't a temperature, it's just related and measured in the same units.

Surely by your argument a temperature of 285K can only ever mean "molecules moving at this speed". But in fact, weather forcasts all over the world do report wind chill, NOT to mean "this has a temperature of 280 K" but "this has a rating on the wind chill scale of 280K." Have I misunderstood you?

PS. If you want a heat flow reason you can't have a temperature in negative Kelvin then the answer is 'obvious violation of the third law of thermodynamics.' and It implies the wind is somehow causing a population inversion, make me think we're talking at completely cross purposes. Perhaps we should go back to the beginning, please? I thought I did describe a way it would make logical sense to have a negative "wind chill", but it keeps sounding like you didn't respond to that, but asserted nothing can actually have a temperature below absolute zero (excluding population inversion stuff) which I never meant to disagree with in the first place, and thought we all agreed on.

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To assert a negative windchill, using windchill as an effective temperature which is how it is supposed to be measured, for something implies that it would be able to cool something down to absolute zero. Which is a violation of the third law.

Sure, you can get heat fluxes faster than would be achieved from the human body at 0K in still air, such as in contact with CO2/acetone-cooled copper. I don't think characterising this as a negative "windchill" is helpful.

So, yes, basically, I am denying the concept, or at least I am denying that you can measure what "windchill" is supposed to be a measure of in Kelvin. Or at least, you can, but it isn't scientific. Heat flux is not measured in units of temperature, so to start talking about absolute zero in this context has no scientific meaning at all. Ergo, I submit your "physics" tag should be removed...

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No it doesn't because you aren't specifying that the person reaches equilibrium with the air or the "wind chill factor". If it did then with the numbers people frequently quote we would all be dead. The system being described is (hopefully) in a steady state. If you are going to assert temperature is only useful in thermodynamic equilibrium I'd have a look at your post further down the page.

How is it measured? Somebody already said you can do it with a damp thermometer. Alternatively you could consider heat flow from your ideal human.

Is it scientific? I think it's clearly defined and observable. Is it physics? It's to do with energy flows. I think it could probably be dervived from a physical model. I imagine that it might even be a useful concept if you were cooling heat sinks or something in the lab with airflow (though the scale would be different).

I do agree with you it's probably mostly of use for on more people friendly scale of degrees centigrade.

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I accept that you can define a number of degrees Kelvin as "the apparent decrease in the thermodynamic temperature experienced by a body at 310K due to an increased rate of heat flux caused by a change in the relative velocity of the body and the medium", and that this could produce a system where the apparent temperature was a negative number of Kelvin from heat flux equations. This doesn't have any meaning in actual thermodynamics. I can specify velocities faster than c using equally specious arguments but that doesn't give them a meaning outside of the concept of tachyons either.

I don't accept that this is in any way a useful thing to do, since it doesn't predict anything other than a heat flux for a specific temperature, whereas using dimensionless constants like the Reynolds number accounts for all temperatures *and* doesn't produce this absurdity.

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I don't accept that this is in any way a useful thing to do,

OK, I guess this is where we disagree (1st_law has stated what I thought very well, thank you).

I think we've agreed that we can assign a defined meaning to what we're talking about?

But I don't think we disagree that much. Right in the first paragraph I said "subjective" and "arbitrary" because this isn't a particularly useful thermodynamic concept. However, I do think wind chill is useful -- for exactly what is used for, reporting conditions in a simple evocative way.

There's a trade off, it's easier to understand in some ways, but misleading in others. I apologise, the human race isn't perfect :) I also think units like "kilotonnes (for explosions)" and "pounds-force" and "pounds-mass" and "light-years" and "even if we could go at the speed of light, it would take 4 (rest) years to reach the nearest star" can be confusing, but are useful for the analogy value.

Then, I extrapolate it downwards. OK, not very useful. But it wasn't really intended to be, mainly whimsical. Do you not think the "can such a scale exist" is an interesting question?

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I think the Reynolds number alone doesn't contain all the necessary information. Humidity is also likely to be a factor for example as evaporation is part of the cause of wind chill.

However more to the point as Cartsiandaemon just said it isn't evocative. Reynolds numbers aren't part of most people's personal experiences.

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I respond to that with an "I just did".

Temperature the thermodynamic variable we normally talk about normally applies to things with Boltzmann distributions (or Fermi-Dirac or Bose-Einstien...). The population inversion you are talking about normally only applies to a few of the systems energy levels while the rest follow more conventional statistics, mostly because they aren't being optically pumped etc. In this way your definition of negative Kelvin is equally artificial but more to the point it is irrelevent to the problem at hand (though interesting).

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My definition of negative Kelvin has some basis in statistical physics. Yours doesn't. It is true that no macroscopic system in nature has yet been detected with a negative temperature, but that in no way makes the definition "artificial". You are just defining an increased Reynolds number to be functionally equivalent to a decrease in temperature and saying "abracadabra, negative Kelvin!". This may be useful journalistically, but it isn't science.

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The definition is clear. The scale is an easy one to understand and is relevant to those who use it. The numbers can be theoretically calculated (though I imagine they probably use some sort of simple convention). I don't see what's not scientific or useful about wind chill as a concept.

Similarly a population inversion is defined by fitting an exponential to the two energy levels you are interested in and considering the densities of states appropriately. A useful and clearly defined concept....for measuring the efficiency of a laser.

I can also define other parameters, like the theta temperature for the excluded volume of a polymer, in Kelvin. I don't see any physically reason why this can't also be negative. Would you require me to invent some other convention simply because might involve me specifying a negative number of Kelvin?

There is no reason why there can only be one true definition of negative temperature. You surely aren't going to argue that anyone is going to confuse wind chill and population inversions.

Now you might argue that wind chill factors are more of and an experimental parameter than the negative tempertures for a population inversion, slightly less ivory tower, but population inversions have as much relevance in statistical physics as wind chill factors have to heat conduction equations. Even if this wasn't so, it would hardly be relavent to this particular conversation.

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You can certainly have a negative theta temperature - the guy on the other side of my office uses them all the time in his modelling calculations. This has an accepted statistical meaning in terms of polymer-solute interaction (it could, I think, be represented in other ways but in general it makes the modelling program easier to write if you use temperature which was probably a variable anyway) and, like a population inversion, actually represents a high-energy system.

Applying this logic to windchill falls over, because windchill is an increased Reynolds number. You can relate this to an increased temperature difference, but that isn't a useful thing to do because it only applies to any single start temperature, since the constant is multiplicative and not additive. If you could, I doubt anybody would have bothered with a Reynolds number in the first place. This most obviously breaks down when you get near absolute zero since you start defining temperatures which imply you could cool something to 0K.

Seriously, why do you want to measure an increase in flux using units of temperature?

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Negative theta temperature is no more or less "absurd" than a negative temperature due to wind chill even if it is somewhat less likely to be encountered in practice. It is a negative number that drops naturally out of the way we've chosen to define the scale. A population inversion is similarly an artificially defined scale in which you pick only the energy levels of interest. Both the other examples are hardly concerned with thermodynamics though negative temperature in a population inversion is a case of fitting a Boltzmann distribution to something doesn't actually have one (though two points do always make a line).

As an aside you'd presumably have to assume that the air didn't liquefy....or you could have a scale in which it did but I think it's simpler assuming it remains an ideal gas....

I think your point about additive and multiplicative constants is a valid consideration but only if you want to apply the scale to non-humans, which the vast majority of weather forecasters don't.

Wind chill doesn't define something that would cool anything at all. The scale is defined by a constant body temperature. Similarly it's not a measure of heat flux. A normal human can't produce much more than ~100W of thermal energy sustainably. If they find themselves needing to produce more they should put some more clothes on which is the whole point of knowing the wind chill factor.

Why measure heat loss in terms of a temperature? If you were a mountaineer climbing Everest would you rather know the Reynolds number?

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BTW, Hi :) Thanks, I'm Jack; pleased to meet you.

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Hi! I'm Jack and I'm pleased to meet you too.

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*waves* Hi. Oh dear, I'm getting a headache and it's only tuesday... Do you mind me asking if you're a Jack I know the surname of? :)

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I'm Jack Levell current CURS President if that helps.

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That was my first guess; sorry, I wanted to check :) Hello.

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*Waves* I imagine I've probably bumped into you at some event or other.

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