It doesn’t matter whether human skin is exposed or not. If I’m wearing a light jacket (covering my skin), I will still suffer hypothermia much faster at 20 deg F with absolutely no breeze, than at 20 deg F with a 50-knot gale blowing.
What does sweating have to do with anything? Believe it or not, you will lose heat from cold air whether you are sweating or not.
It’s not just “perception.” A human will really and truly suffer hypothermia and die faster at 20 deg F with absolutely no breeze, than at 20 deg F with a 50-knot gale blowing.
It’s true that a normal thermometer does not indicate any difference between the two situations–specifically, in both cases it will read 20 deg F. However, one can easily set up an objective experiment which will demonstrate that even inanimate objects will behave differently in the two situations. For example, one could measure the time it takes for a container of water to cool down from 75 deg F to 40 deg F.
“David Simmons, when you talk about “wind chill,” you are describing a situation where you are at body temperature, and a 30 deg F wind (for instance) is perceived by you as removing heat from you as fast as (for example) a 10 deg F stationary air mass.”
By that description then it would appear that there could be a 30[sup]o[/sup] wind chill factor on a body at 10 K. The heat is being removed at at rate equivalent to a 30[sup]o[/sup] temperature difference under no wind conditions, even though the body itself can’t go below o K.
The wind chill does not make any definitive statement regarding the temperature of the air, therefore it isn’t clear that no gasses can exist at negative Kelvin wind chills. If the air is at -50 C and blowing 40mph, it’s about a -80 C wind chill. Wind chill depressions get higher when the temperature gets lower, but top out at around 40mph wind speeds.
You can probably get wind chill depressions of 100 degrees when the “air” is super cold and fast moving. If you’re in 50K helium gas that’s blowing 40mph, you probably have a below 0K wind chill.
Since it’s a phoney baloney temperature, only telling you how it “feels” it isn’t bound in any way by physical constraints.
Perhaps it isn’t as important in wind chill calculations at lesser temperature differentials, but I’m pretty sure that at ambient temps close to 0K the ability of a warm-blooded body to heat the immediate surroundings should be taken into account.
Therefore, a mass of stationary “air” at around 0K would not chill someone as much as a mass of circulating “air” at the same temperature because the body itself would heat the surrounding air enough to create a temperature buffer, which would be reduced when the “air” is circulated. So I’d say a wind chill below 0K is possible.
Sorry to have ignored this thread for so long. To clarify my question and answer a few issues raised.
The general concept of wind chill is applicable in a non-gaseous environment, and even in one where sweat doesn’t evaporate. A warm body will lose heat at a faster rate if it is surrounded by a moving fluid at a lower temperature than it will if it is surrounded by a still fluid (at that same lower temperature). If we consider this generalized “fluid flow chill” factor, can it get to negative K?
For those who claim that you can’t put anything at a lower energy than 0 K, that’s not what I’m asking, and it’s not what wind chill claims to measure. It’s the rate that’s important. It’s not even necessary that the real temperature of the environment is anywhere near 0 K, just that the rate of heat loss for the warm body is equivalent to what it would be in a sub-0 K environment. I suppose the real question is whether the heat transfer equation breaks down at temperatures or rates near absolute 0.
For what it’s worth, the Futurama quote is likely referring to conditions on another planet, so exotic elements, pressures, and temperatures are valid considerations.
Commonly referenced “wind chill” charts apply only to an object who’s surface temperature is maintained at about 95F. (e.g. a human being). That doesn’t mean that the effect doesn’t occur at other temperatures, but that the strength of the effect depends on the existance of a temperature difference between the air and an object under consideration. Live mammels happen to be an object who’s temperature we can reliably assume, so the wind chill charts obscure this crucial fact. A universal wind chill chart would need a third axis for the temperature of the object under consideration.
Objects at the same temperature as the air do not experience any wind chill…heat only flows in response to a temperature gradient.
So to the OP: Wind chill will never lower the temperature of anything to less than the temperature of the air. Since no air will ever be colder than absolute zero, there is no way to exploit wind chill to produce a temperature below absolute zero.
As you pointed out, wind chill doesn’t measure the actual temperature of the air. It measures the rate at which a warm body will lose heat to the environment.
I’m not asking about using a moving fluid to lower the temperature of an object below zero. I understand that that’s impossible by the definition of absolute zero. I’m asking whether a cold moving fluid could cool a warm body at a rate faster than a still fluid at absolute zero would.
Note that neither the fluid nor the body need to reach anything close to absolute zero for this to occur. If a -40 deg F wind can cool a 98 deg F body the same way that -80 still air can, it doesn’t matter that the body won’t ever reach -80 deg, nor does it matter that the air will never reach -80 deg. Yet we still say that the temperature is “wind chill to -80 deg.”
The real question (I think) is whether there’s something about Fourier’s Law that breaks down at very cold temperatures or very high differences in temperature.
Surely you meant “I will still suffer hypothermia much SLOWER at 20 deg F with absolutely no breeze, than at 20 deg F with a 50-knot gale blowing.”?
Further, altho sweat will make a difference to perceived temperature, I agree that it isn’t factored into the wind chill factor. But while neither sweat nor clothing insulation are factored into the official wind chill, certainly either will make a difference to real life perception of, and actual change in temperature to the affected human life form. With a jacket on, you will still suffer hypothermia more quickly with wind blowing than not, but you will suffer less wtih the jacket at the same wind speed.
Well if wikipedia is anything to go by, then “wind chill” seems to have a fairly precise definition:
(bolding mine)
So, it would seem that all the talk tangential to the OP of super-cooled fluids and micro-Kelvin does not fall within the scope of the definition of wind chill.
Now, as to the issue of whether a heat transfer coefficient could exceed that between a 98F human and a hypothetical fluid at 0K - this seems like a no-brainer. I could imagine an extremely low-density, still fluid at near-0K, and it would not be able to extract much thermal energy from an exposed patch of skin. However, I could have a jet of liquid Nitrogen that would achieve an instantaneous heat transfer coefficient much greater than that possible from the low-density fluid. In any case, if this is the thrust of the OP, then he/she seems to have addressed it in that latest post.
The answer depends on your calculation. The current wind chill factor formula, which was revised for accuracy in 2001 is listed [URL=http://www.msc.ec.gc.ca/education/windchill/science_equations_e.cfm]here](http://boards.straightdope.com/sdmb/showpost.php?p=7891431&postcount=32). Under that formula, at -100°C it would take a wind speed of just over 10,000 km/h to get you to feel like you are at absolute zero. At -150°C it would take about 535 km/h to get you there. However, according to web based wind chill calculators, this is only meant to work for temperatures of -50 to 10°C and wind speeds of above 4.8 km/hr and below 177 km/hr.
If the wind speed were less than the speed of normal calm air, the perceived temperature would actually be warmer, and in terms of actual heat loss, still air can act as an insulator.
This formula is an improved version of the original formula (derived from the time that water in a plastic cylinder took to freeze under different conditions of wind and temperature) that takes into account the difference between wind speed where it’s measures (10m) and where the human face is, the calm wind threshhold (average walking speed), and consistent standard for skin tissue resistance to heat loss. Neither formula currently takes into account solar radiation. There are other formulas that take into account humidity.
So technically, the wind chill as defined by the equation can dip below absolute zero without the variable contraints, but not within them.
If we are talking about actual heat transfer, I’m not sure. It would depend on how conduction and convention. Is there an equation for the temperature change of a solid based on ambient air temperature and pressure?
I don’t understand all the people who are saying “No” - so what is the lowest possible windchill adjusted temperature then?
Windchill, theoretically, is an adjustment factor that is meant to compensate for the effects of convection. You could have a windchill be positive if the ambient temperature is higher than the temperature of the body in question (usually the human body). It depends on pressure, relative temperatures, absolute temperatures, shapes, wind speed and direction, elasticity and a whole other big sheband of things.
We have a pretty solid grasp of how fast (at least a ballpark) an object made of a given material and of a given surface area would lose heat if surrounded by magically liquid air (pure helium for instance) at ~0K. That speed is not infinity. Now, if the medium suddenly starts moving and the medium temperature is say 50K or 20K or whatever, with convection the same body could possibly lose heat faster than in the 0K case. I fail to see what principle prevents this from happening.
In other words, your medium, even if it’s liquid helium at ~0K is not magic. When the object will start losing heat the medium will start gaining heat and the temperature delta will drop resulting in the corresponding decrease in the rate of heat loss. With convection this effect is reduced with the ‘ideal’ convection behaving exactly like a magic medium that is an infinite heat sink. Liquid helium at ~0K is NOT an ideal heat sink so it can have a windchill factor that makes it approach one. That can be expressed as a temperature below ~0K although that’s not useful to any degree.
Northern Piper, I think you’re agreeing with me. It’s rare for people in general to experience very cold conditions, though if you are Northern enough of course it’s not rare for you. So, I’m saying that if for example you brought someone into -20 C with a strong wind, they’d think it was terribly cold, and then if you brought them to -35 C with no wind, they would find they much prefer it, and would be surprised that it was 15 C colder than the previous experience. We’re agreeing, right?