http://www.straightdope.com/classics/a4_178.html
I was just reading some past columns and came across this one. It is mentioned within it by a misinformed reader that your breath is a different temperature depending on how you blow. Now, I know it isn’t for the reasons he described, but anyone can tell by blowing on their own hand that an “open mouth” blow (hehe) feels hotter than one with puckered lips. How come?
It seems obvious to me (so it’s probably wrong) that it’s a matter of wind-chill factor.
Would you believe, I’ve never noticed that before, but you’re right. On experimentation, there seems to be a very discrete jump from hot-feeling to cold-feeling: That is to say, there is no intermediate lip configuration which just produces “lukewarm”. I’m going to take a guess that it’s a matter of turbulence: The puckered lips produce a faster, turbulent flow, while the open lips produce a slower, laminar flow. I’m not sure, though, why this would cause an apparent difference in temperature… Fluid dynamics isn’t my strong suite.
Open mouth exhalation is at body temperature. There is nothing to cool it down. Check.
Why is it cool when you pucker your lips? Possible factors…
Puckering your lips causes a Venturi, which speeds up the air velocity through your lips, and then the air expands as it exits the Venturi. Expanding air cools off. Ergo, cooler air.
Possible entrainment of surrounding air. The air around your mouth gets pulled in and thus cools off. No, because you can put your fingers right next to the opening and it is cool. No room for entrainment.
Turbulence would only matter if it is leading to the entrainment mentioned above.
Okay, so I’m thinking it’s explanation 1 above. Anyone else?
It is not that the breath comes out at different temperatures or is affected by the pressure differences as one reader suggested.It is percieved as cooler because the volume of air is directed at a SMALLER surface. For instance, blowing on a burned finger feels much cooler when you purse your lips and direct the entire breath at a small part of your finger than say blowing open mouthed into your palm.There is simply more air at that particular spot to carry heat away. Obviously liquids are somewhat different because the “spot” is always moving and being replaced with new liquid. When you blow directly onto the surface of a liquid you not only are concentrating the air which provides more heat dissipation, you also make the effective surface area of the fluid much larger by causing the fluid to form ripples and waves which also adds to the cooling effect.
Lick your partner on the small of the back.
Slow open-mouth blow. - Ooohh.
Quick pucker-mouth blow. - Whee.
Slow open-mouth blow - Ooohh.
Quick pucker-mouth blow. - Whee.
Repeat.
I wouldn’t believe it either. I thought for sure I read it in Jearl Walker’s Flying Circus of Physics (with answers), but I can’t find it in my copy.
I think Rhapsody’s explanation sounds the most plausible but I’m still not sold… yet. On an unrelated note, does anyone else find this board is INCREDIBLY slow to get around? I’m new so I was just wondering if its me. I mean really slow people. Uber-slow.
Yeah, it gets that way sometimes, and there’s not much that can be done about it. You can try at different times (American nighttime usually works pretty well), if possible.
I disagree, Rhapsody. Hold your hand six inches from your mouth. Pucker your lips. Blow slowly. And I mean very slowly. It feels cooler, even though you feel it on your whole palm, just like open mouth exhalation. It is not the size of the target that matters. It has something to do with the airstream.
From chemistry, expanding gases cool down. That’s how air conditioners work - the coolant goes through an expansion valve to cool down. http://www.howstuffworks.com/ac1.htm The expansion valve is a Venturi.
A definition relating to automobile carburators:
A description from a fluid flow-meter manufacturing site:
http://www.flow-dyne.com/venturi.htm
So, I will spell it out. Puckering your lips provides a restriction to the flow that open-mouthed exhalation does not. This restriction in flow is a venturi. It speeds up the velocity of the flow inside the restriction, which causes a pressure drop. As the flow leaves the venturi, the velocity slows, but the pressure does not increase.
Now, from chemistry, we know that when the pressure decreases, the volume increases and/or the temperature decreases.
P[sub]1[/sub] V[sub]1[/sub] P[sub]2[/sub] V[sub]2[/sub]
------- = -------
T[sub]1[/sub] T[sub]2[/sub]
Thus, we have a decreasing pressure, which causes the air to cool.
As cecil states in the column there simply is not enough compression/expansion to work like you say. There me be some effect but not enough to provide an explanation.
Anyway I still feel (and from my own simplistic experiments) that it has a lot more to do with the increase of airflow over a smaller space causeing the cooling effect of a fast blow. As for the warming your hands part, a major component missing from this discussion is the fact that when you are trying to warm your hands you really don’t just blow on them or breath on them. You CUP your hands and TRAP the warmer air from your lungs. blowing fast on cold or wet fingers does not warm them and neither does blowing slowly except maybe when held very close to the mouth. It is the cupping action though that helps to preserve the heat, just as hair and feathers trap air to provide warmth.
I respectfully disagree. Oh, I concede that blowing air has a cooling effect - convection cooling and all - but I maintain that puckering the lips also provides a cooling effect. Unfortunately, I don’t have any data to back this up, because I don’t have equipment to measure transient air temperatures lying around my apartment, and it is exceedingly difficult to blow enough air to effect a standard body temperature thermometer.
You are correct in describing how to cup your hands to trap the heat when trying to warm your hands.