"Wind chill" below absolute zero
Oct. 24th, 2006 02:21 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
jack1. What is wind chill temperature? A human in cold air in a wind feels as cold as a human in colder but still air. We have fitted some approximations to this and come up with an official scale, but it was originally based on subjective judgements, and depends on other conditions as well, eg. humidity, so isn't definitive, but is a useful measure.
2. How a human feels doesn't really have meaning when you get near *that* cold. Instead death feels much like instant death :)
3. However, you should be able to create a standardised measure, right? Have object X at temperature T K in medium Y at temperature 0.x K and pressure Z, flowing at speed v. Establish the rate of heat loss at that moment (which is at least theoretically calculable). Define "wind chill temperature" to be the temperature in still medium at which the rate of heat loss is the same.
4. Could X lose heat faster under some speed than in still near absolute zero medium? I don't see why not. Physics is weird down there, but it can still heat up the surroundings, etc.
5. Does that make a negative Kelvin wind chill? On the one hand, it implies a wind chill colder than absolute zero. On the other hand, it doesn't actually define a wind chill at all because there is no temperature to compare it to. If you have a nice non-asymptotic graph you could extend it, but does that have any meaning?
2. How a human feels doesn't really have meaning when you get near *that* cold. Instead death feels much like instant death :)
3. However, you should be able to create a standardised measure, right? Have object X at temperature T K in medium Y at temperature 0.x K and pressure Z, flowing at speed v. Establish the rate of heat loss at that moment (which is at least theoretically calculable). Define "wind chill temperature" to be the temperature in still medium at which the rate of heat loss is the same.
4. Could X lose heat faster under some speed than in still near absolute zero medium? I don't see why not. Physics is weird down there, but it can still heat up the surroundings, etc.
5. Does that make a negative Kelvin wind chill? On the one hand, it implies a wind chill colder than absolute zero. On the other hand, it doesn't actually define a wind chill at all because there is no temperature to compare it to. If you have a nice non-asymptotic graph you could extend it, but does that have any meaning?
Yes you can, I just did.
Date: 2006-10-24 02:40 pm (UTC)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.
Re: Yes you can, I just did.
Date: 2006-10-24 03:56 pm (UTC)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...
Re: Yes you can, I just did.
Date: 2006-10-24 05:10 pm (UTC)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.
Re: Yes you can, I just did.
Date: 2006-10-24 05:32 pm (UTC)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.
Re: Yes you can, I just did.
Date: 2006-10-24 07:37 pm (UTC)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?
Re: Yes you can, I just did.
Date: 2006-10-24 11:37 pm (UTC)However more to the point as Cartsiandaemon just said it isn't evocative. Reynolds numbers aren't part of most people's personal experiences.