i really can’t figure out how this works, so I’d be glad if you could offer me any tips.
it’s a bit hard to explain, but bear with me. if you blow air out of your mouth, it usually is pretty cold, such as when you blow on childrens’ wounds and such, while if you just sort of “breathe” out it is usually pretty hot. why is this? i figgered pressing the air out would create some sort of friction but in that case it would be vice versa… any clues?
I thought this was going to be a question about Aesop’s fables.
Your breath is warm from having been inside your lungs, when you blow out through pursed lips, you’re creating a fast stream of air, which will cool down objects such as hot soup or sweaty skin, partly by evaporation.
When you ‘huff’ your breath out with an open mouth, you’re creating a fairly slow-moving pocket of warm air that transfer heat to a colder object (or attenuate the rate of heat loss from a warmer one)
A hairdryer will both melt an ice cube and cool a cup of coffee on the same warm setting.
While your hair dryer analogy is not exactly incorrect, it fails to point out that not only is your breath cooler through pursed lips, it is moving at different speeds, involves different volumes of air, and a changing moisture content.
When you breathe out to warm your hands you usually produce much more volume but a loss less speed than when you blow at a focused spot.
Are you sure it’s significantly cooler through pursed lips? - it certainly feels cooler when you blow on your hand, but that could just be down to more evaporation of sweat because of the increased rate of air flow. Most of the heat in your breath is coming from your lungs; why would it be cooler simply because it is being expelled from a different-shaped nozzle? (Or is there a process going on that is analogous to the compressor in a fridge?)
Cecil isn’t a fan of the nozzle explanation:
Why do you blow on coffee to cool it, but on your hands to warm them?
When you blow through pursed lips, you’re forcing the air through a small opening, so the pressure inside your mouth is increased. After leaving your mouth, the air undergoes adiabatic expansion. When a gas expands from a high pressure region (you mouth) into a lower-pressure region, its temperature drops.
This is the same principal that air conditioners and refrigerators exploit.
OK, I just tried a quick experiment with a thermocouple.
Ambient temperature is 22 [sup]o[/sup]C.
Blowing on it, the temperature reads 29 [sup]o[/sup]C.
Breathing slowly on it, I can get the temperature up to 31 [sup]o[/sup]C.
So there is some small, measurable difference.
thanks, that was sort of a eureka for me. thanks to the rest of you lot too 
I don’t have much time to dig though the muck, but THINK about it for minute.
Keep it simple: A cup of coffee is hotter than your lungs, or I should hope it is! So, if you blow on the coffee (and forget about added effects of evaporation) you do COOL down the coffee! As for warming up your hands, you are blowing the same air onto your cold hands, so of course they will warm up! Mechanically, the temperature difference (delta-T) is the gradient that drives the system.
Just like a ball running down a hill, the greater the slope (gradient) of that hill (i.e.: large delta T), the better the energy exchange- good enough for the layman.
Shouldn’t there be a corresponding increase in temperature somewhere else? (like inside your mouth?)
What’s happening is that when you blow through pursed lips, the high turbulence quickly mixes ambient air into the stream, cooling it. You can prove this by blowing on the back of your hand, putting right up to your lips. As you move it away, you can feel the airstream getting cooler. Exhaling through open lips doesn’t cause as much turbulent mixing.
Well, sure. If your lungs can put out 5 or 10 lb/sq in pressure this effect might be measurable. But they can’t. I am sure enough this effect is negligible for blowing that it’s not even worth doing the math. Sorry to ruin the Eureka! Sanguine.
The most likely causes are a combination of several things-
(Largest contibutor) faster moving air entrains room air in it’s flow, thus providing a cooler and drier air stream. Try this, blow hard into your palm at 3 or 4 inches away. Feels a little cool. Now blow hard into your palm at 1/2 inch from your mouth. Surprise! You get a warm spot on your hand.
faster moving air increases heat and mass transfer, thus allowing the cooler drier fast air to pick up more moisture off your hand to cool it, (as Mangetout mentioned)
Time of contact- the Thermocouple experiment that Desmostylus so ingenously performed failed to rule out many variables to prove whether or not the air from your mouth was hotter or colder, such as entrained air and time of contact. TC are not instantaneous response devices. If you gently huff on something, that cloud of warm air remains on the TC much longer than the fast stream from a good blow.
But I think that entrained air is the biggest reason. Another experiment, blow through a straw with the end 1/2 inch from your palm, you once again get a warm spot.
Gaah! That’s what I get for getting a bite to eat before hitting Submit. Take my long answer and change it to, “What Q.E.D. said.”