Temperature & Wind Chill: Why Does It Feel Colder?

[Loanzac]

Hi all, so when you heat something up the molecules begin to move faster. This is seen when you steam a tea kettle. So my question is when its very cold outside and there is no wind, it is much more comfortable then when there is cold wind whipping about your body. If molecules moving faster are generally more energetic, thus 'hotter' why is wind chill a 'colder' phenomenon?
Thanks

 

[boneh3ad]

Temperature has to do with the Brownian motion of individual molecules, not the bulk motion of a continuous fluid like the wind.

So when it is cold and windy, the air is simply cold and the fact that it is moving means it convects heat away from you body more quickly than if it is still.

 

[rootone]

Yep, the exchange of heat is such that your body is donating energy to the passing cold wind at a much greater rate than the wind is donating energy to you as a result of friction.

 

[Chestermiller]

Yep, the exchange of heat is such that your body is donating energy to the passing cold wind at a much greater rate than the wind is donating energy to you as a result of friction.

Have you ever calculated how much energy the wind friction donates to your body (for say a 20 km/hr wind) and compared it to the heat that your body donates to the wind by convection?

Chet

 

[boneh3ad]

Have you ever calculated how much energy the wind friction donates to your body (for say a 20 km/hr wind) and compared it to the heat that your body donates to the wind by convection?

Chet

I just did a (very) rough calculation of the viscous dissipation rate in such a wind and its tiny (as was your point, I believe), but I think the more important thing to note is that heat transfer doesn't work that way. The wind doesn't actually add heat to your body while simultaneously removing it. Heat simply moves from the hot energy sink to the cold one. Viscous dissipation into heat, even if it was significant, would still just slow down how fast heat left your body into the wind unless it was large enough to heat the air above your body surface (or outerwear surface, rather) temperature.

 

[Chestermiller]

I just did a (very) rough calculation of the viscous dissipation rate in such a wind and its tiny (as was your point, I believe)

Yes. That was part of my point. The other part (thanks for your help) was wondering whether it was really worth it for Rootone to introduce viscous heating in this thread at all, considering that (a) it is not important at typical wind speeds, (b) it was likely to confuse the OP, and (c) it probably serves no purpose other than the show everyone how clever the person raising the issue is.

but I think the more important thing to note is that heat transfer doesn't work that way. The wind doesn't actually add heat to your body while simultaneously removing it. Heat simply moves from the hot energy sink to the cold one. Viscous dissipation into heat, even if it was significant, would still just slow down how fast heat left your body into the wind unless it was large enough to heat the air above your body surface (or outerwear surface, rather) temperature.

Overall, the effect is as you describe it, but, conceptually, there is nothing wrong with regarding the heat transfer as the linear superposition of the viscous heating and the convective cooling (as long as the boundary conditions for each individual situation are specified appropriately).

Chet

 

[rootone]

Hmm, no I wasn't trying to be smart, it was just an afterthought that a small amount of heat might be generated by friction, but not nearly as much as heat lost through convection.
No I didn't try to calculate it, because 'wind chill' is an obvious fact.
I was wondering though about the case where the wind is hot, andit's certainly possible to have a wind at a temperature higher than body temperature.
It seems logical that in that situation there would be no wind chill, and a person might even get hotter, (although they then would sweat more as the body tried to regulate the temperature.)
Most likely the air humidity would then become an important factor as that would determine how quickly perspiration could evaporate.

 

[Chestermiller]

Hmm, no I wasn't trying to be smart, it was just an afterthought that a small amount of heat might be generated by friction, but not nearly as much as heat lost through convection.
No I didn't try to calculate it, because 'wind chill' is an obvious fact.
I was wondering though about the case where the wind is hot, andit's certainly possible to have a wind at a temperature higher than body temperature.
It seems logical that in that situation there would be no wind chill, and a person might even get hotter, (although they then would sweat more as the body tried to regulate the temperature.)
Most likely the air humidity would then become an important factor as that would determine how quickly perspiration could evaporate.

Yes. You are correct that all these mechanisms play a role. This has all been modeled quantitatively by the meteorology people to derive indicies like the Temperature-Humidity index (for hot days) and the Wind-Chill Factor (for cold days).