"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: Yes you can, I just did.

[ilanin.livejournal.com]

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.

Re: Yes you can, I just did.

[cartesiandaemon.livejournal.com]

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?

Re: Yes you can, I just did.

[1st-law.livejournal.com]

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.