• Quick note - the problem with Youtube videos not embedding on the forum appears to have been fixed, thanks to ZiprHead. If you do still see problems let me know.

warm and cool breath

1984

ex-PiousWoo
1
Joined
Oct 14, 2005
Messages
343
Why is it that when I breathe air out through a wide open mouth, that air is warm? And when I blow air out through pursed lips, that air is cool?
 
1984 said:
Why is it that when I breathe air out through a wide open mouth, that air is warm? And when I blow air out through pursed lips, that air is cool?
Click to expand...

It would probably have something to do with air velocity, evaporation, the opportunity for heat transfer while the air passes out of the airway in to the mouth.
 
1984 said:
Why is it that when I breathe air out through a wide open mouth, that air is warm? And when I blow air out through pursed lips, that air is cool?
Click to expand...

"cool on what?" is the question. Skin, I'm assuming.

Try this trick in a sauna or steam room: you'll find that the faster the air comes out, the hotter it feels.

The reason is that the fast-flowing air on your skin helps moisture evaporate. Faster air flow makes for faster evaporation, and a cooler feeling.

But: if the air's already saturated with moisture, no more can evaporate, and you start to feel the effect of friction.
 
Thanks EagleEye, but I still need this expanded.

blutoski said:
"cool on what?" is the question. Skin, I'm assuming.

Try this trick in a sauna or steam room: you'll find that the faster the air comes out, the hotter it feels.

The reason is that the fast-flowing air on your skin helps moisture evaporate. Faster air flow makes for faster evaporation, and a cooler feeling.

But: if the air's already saturated with moisture, no more can evaporate, and you start to feel the effect of friction.
Click to expand...

Yes, it's skin. So is the air from inside my lungs actually changing in temperature? If so, how?
 
Faster evaporation from faster-flowing air does seem to account for much of the difference. However, it seems like there's more to it, perhaps the air from the mouth cooling as it travels. I've just tried this experiment - held up two fingers pressed together, so that the bottom finger was tilted slightly away from my mouth. When I blew at the line between the fingers, there was a noticeable, though small, difference in the temperature experienced by the two fingers (the closer one feeling warmer). I don't think there could be that much difference in the force of air across such a small distance, or could there be?
 
1984 said:
Thanks EagleEye, but I still need this expanded.



Yes, it's skin. So is the air from inside my lungs actually changing in temperature? If so, how?
Click to expand...

No, the air is not changing temperature. Repeat the experiment with a thermometer.

I'm assuming you're blowing on your forearm or something. The skin on your arm has moisture on it, from inside your body. It comes out in sweat glands. As you blow across it, the water evaporates. Evaporation draws heat from your skin, and makes it feel cooler. This is the point of sweat glands. And running through the sprinkler in July.

Hmm. Sprinkler's lookin' good right now.
 
Avita said:
I don't think there could be that much difference in the force of air across such a small distance, or could there be?
Click to expand...

It's probably the intensity, yes. Right up against your mouth, 100% of the air flow is sucking moisture off of a spot about the area of the opening between your lips. Maybe an eighth of an inch in diameter. That's the zone that's heating up, because moisture can't be wicked out to the skin faster than it evaporates.

An inch away, air turbulence has fluffed the area to a couple of inches wide, and the sweat glands can keep up.
 
EagleEye said:
It would probably have something to do with air velocity, evaporation, the opportunity for heat transfer while the air passes out of the airway in to the mouth.
Click to expand...

I'm going to be stubborn and go with this, especially the latter part. Might have to do some more googling to put my mind at rest.
 
1984-You might look at Pascal,Charles, Boyle while you are Googling.
Any gas compressed will lose heat to immediate surroundings, upon expanding, the gas will pick up that much heat again. Variables: the gas; purity of the gas.
Hence, you compress the air in your mouth/body. Heat(small amount) is absorbed by you- the air is allowed to escape , expanding to ambient pressure and picking up the small amount of heat ,absorbed temporarily by you.
Get an air compressor and a blowgun. Let the compressor fill its' tank;note how hot the compressed lines and the tank will become. The compressed gas is losing its' heat to immediate surroundings, ie, the lines and the storage tank.
/Allow the air to escape thru the blowgun. (I hope you will be careful. Compressed air -compressed anything- can be dangerous>) Place your hand in the expanding airstream. Place a thermometer in the air stream . Note differences in temperatures, before and after.
Cleverly made, a vortex(not woo) tube can sort out the cold air molecules from the hotter molecules in the compressed air stream, with the very cold air coming out one end, the hotter from the other end.
Happy Google to you!
 
trvlr2

While I digest what you've said, here's food for thought. Sometimes on a cold night I can exhale and get condensation from my breath. But when I purse my lips and do it, no condensation. The air around me hasn't changed in temperature, so it must be the air in my breath? :confused:
 
My guess:
Yes the air from your breath have a lover temperature after it exits your mouth when you purse your lips. and the lower temperature means lower humidity and therefore it might make a difference with condensation.

/Hans
 
Constant Enthalpy.

The pressure differential created when pursing ones lips cools the exhaled air more than the open mouth scenario.

This is just a guess.
 
EHocking said:
It is caused by the difference in air expansion *after* the air leaves the mouth. Here's a quick science experiment for children that explains: http://www.kids.union.edu/hotAndColdBreath.htm
Click to expand...

That's not right. The cooling effect of just increasing the distance between molecules would be given by the Joule-Thompson coefficient, which is trivial for air, something like 1/500th of a degree under the pressure from your lungs. The air leaves your mouth with some velocity at atmospheric pressure anyway, like a nozzle. Also, the only way to have a significant pressure drop after opening to the atmosphere is to have choked flow, which certainly isn't possible with lung pressure.

An isentropic process, on the other hand, would explain a temperature drop of around 5 degrees, which is still too warm to explain why your hand feels cold. I'm in roughly 22 degree air, and my hand still feels cold when I blow on it.
 
I would've said it's because the air is expanding, the same as when you fill a spray bottle up with hot water, the spray mist that comes out is cool, because of the expansion caused by the nozzle.
 
EHocking said:
It is caused by the difference in air expansion *after* the air leaves the mouth. Here's a quick science experiment for children that explains: http://www.kids.union.edu/hotAndColdBreath.htm
Click to expand...

This is just bad science, though. Like the bird's wing / Bernoulli effect stuff. Bill Bennetta would just shake his head if he saw this. See my comments above re: perspiration. If you repeat this experiment with a thermometer, the 'cooler/warmer' sensation is not observed.

re: condensation... this is just an effect of the concentration of moisture particles when exhaled. If you exhale with a great deal of force, the air is cooled as it is expelled, and spreads over a large area. There isn't much chance for moisture particles to condense. If you exhale slowly, the moisture cools in-place, and condenses briefly before it mixes with the surrounding air and dilutes so much that the mist thins out of visibility.

The quantity of compression required to see these effects is not achievable with the lungs and face. That's why we don't fill car tires or footballs by mouth.
 
blutoski said:
This is just bad science, though. Like the bird's wing / Bernoulli effect stuff. Bill Bennetta would just shake his head if he saw this. See my comments above re: perspiration. If you repeat this experiment with a thermometer, the 'cooler/warmer' sensation is not observed.
Click to expand...
Unfortunately I don't have an adequately sensitive thermometer to confirm if there is an *actual* difference in temperature. But blowing on each of my fingers, alternating between open and pursed mouth, I still find I have the *sensation* of temperature difference. Also, when I held my fingers within 1mm or so from my mouth, I *couldn't* sense any discernable difference in temperature.

I'm not sure that the perspiration explanation explains the sensation, but without actual temperature readings....
re: condensation... this is just an effect of the concentration of moisture particles when exhaled. If you exhale with a great deal of force, the air is cooled as it is expelled, and spreads over a large area. There isn't much chance for moisture particles to condense. If you exhale slowly, the moisture cools in-place, and condenses briefly before it mixes with the surrounding air and dilutes so much that the mist thins out of visibility.
Click to expand...
If air, in my experience so far, is expelled at the same rate, though, the sensation of temperature difference is still there.
The quantity of compression required to see these effects is not achievable with the lungs and face. That's why we don't fill car tires or footballs by mouth.
Click to expand...
But it's not compression or pressure drop across an orifice that is being used as an explanation here, but different rates of expansion *after* the air has passed from the orifice at the same rate. And the effect (or sensation of an effect) is really only discernable at a distance greater than an inch from the mouth.

I don't really know - the URL I posted was merely a Google search. My attempt at a brief response has obviously given the impression that I'm defending the explanation.
 
Here, perhaps is another explanation:

http://www.uh.edu/engines/epi1796.htm
...Cooling soup is another matter. We purse our lips to raise the pressure of the air in our mouth. The temperature of exiting air drops and the air accelerates. That fast-moving air draws the surrounding room temperature are into it. The resulting jet is therefore close to room temperature when it passes over the much hotter soup. ...
http://www.uh.edu/engines/epi1796.htm
 
You must log in or register to reply here.

Back
Top Bottom