"Wind chill" below absolute zero

[jack]

1. 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?

Comments

Re: Physics Attacks!

[1st-law.livejournal.com]

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.

Re: Physics Attacks!

[ilanin.livejournal.com]

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?

Re: Physics Attacks!

[1st-law.livejournal.com]

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?