Why do we blow cooler air when we purse our lips?

Why do we blow cooler air when we purse our lips and warmer air when we widen our mouths?

You don’t. Moving air feels cooler, because it evaporates water off your skin. It’s the same reason air from a fan feels cooler than room temperature.

but, when air is forced through a small opening, it gets colder, right?

which is pretty much how your a/c in your car works, although it uses freon instead of air, because air doesnt get THAT much colder…

think about it… if youve ever used an air compressor… you know air compressors get pretty hot compressing that air, but when it comes out of a nozzle its definitely cooler than it should be, after being compressed…

The air we blow when we widen our mouths is moving too and it feels warmer.

Purse your lips right now and blow. You can blow cooler air than when you widen your mouth.

Well, that doesn’t sound right to me, but I’m not familiar with air compressors. I looked up air conditioners on How Stuff Works, and it says that it compresses the freon to heat it up, not cool it down. This is what I would expect, pressurized gas being hotter. However, I don’t know if you can pressurize air enough with your mouth to make a difference.

Try this to convince yourself that blown air is in fact warm, just like breath. Blow as hard as you can on your shirt sleeve or some piece of cloth or something. Then touch your lips or fingertips or something sensitive to the spot. It should feel warmer than ambient, not cooler.

You must have misunderstood HowStuffWorks. When the refrigerant is compressed it sure enough heats up (but that’s not why it’s compressed). Then it is put through a heat exchanger with forced air cooling so that the heat of comression and the heat from the box interior are dissipated into the exterior air of the room. The rerigerant was compressed and then cooled so that when it is expanded into the cooling coils inside the box it is really cold.

What are you using to measure the temperature of the air?

If it is your hand, then you can’t say it is cooler, only that it feels cooler to the skin. If you have a fast acting thermocouple device, I think you would find they are at remarkably similar tempertures.

Ah, no I didn’t misunderstand, but I thought that something clever was talking about the compressor, not the expansion valve. That makes more sense now. But I’m not sure of one thing: is it actually the moving through the small orifice in the expansion valve that cools the liquid? HSW seems to suggest that it only cools because it’s moving from a high-pressure area to a low-pressure area.

The difference of course is how fast it’s moving. Faster air feels cooler; this is the idea behind “wind chill”. And even if you’re exhaling the same volume of air, it will be moving faster if it has to go through a smaller opening.

To convince yourself that air from pursed lips is faster, blow on a pinwheel. See if you can get it up to as fast a speed with your mouth wide open as you can blowing. (I think this should work. I don’t have a pinwheel handy to try it.)

It’s the drop in pressure that lowers the temperature, not the size of the orifice.

The refrigerant below its critical temperature so it is easily liquified. The stuff comes out of the compressor at high pressure and temperature. Then it is cooled in the heat exchanger and becomes a liquid under high pressure. Then it goes through an orifice the size of which controls the rate of flow but it is the expansion into low pressure that cools it down.

A quick way to test this is with wet and dry bulb thermometers. I have a digital weather thermometer from radio shack with a small probe that has fast response time. With the bulb dry there should be little change in temp no matter how you blow. With a bit of damp tissue around you’ll see a big difference. Dry you measure temp, wet you measure wind chill

sorry, i didnt explain it very well…

i mentioned AC systems in cars and then just normal air compressors :slight_smile:

my example, i have a 50psi air compressor that you plug into the wall, used for airing up bicycle tires… top of the compressor says WARNING, SURFACE GETS VERY HOT, DO NOT TOUCH.

everyone knows air gets hot when it gets compressed, hence the need for intercoolers on turbocharged cars (at ~20psi the air coming out of the turbo on my car is above 250*f )

anyway… this little compressor doesnt have a tank, it just constantly compresses (heats) the air and expells the extra, if you hook up the attachment to air up a car tire and press the button, the air coming out is very cold, not just because its moving fast, but its COLD. remember, the top of the machine is hot enough to burn your hand after a few minutes of operation, yet the air coming out of the line is still way colder than the outside air.

AHA! its the pressure drop that makes the air get colder:


DOH! sorry… i typed all that mess while David was responding, guess im too late. anyhow :slight_smile:

The best way to see what’s going on with the blowing thing is to blow through your lips with the same small space and start blowing as slowly as possible and then gradually increase the force of the blow. It will start feeling cooler as you use more force. More force, more pressure.

The warm effect is probably increased somewhat when you blow slowly because the air is mostly the warmer air that was close to warm flesh for a while.

If a gas is allowed to expand suddenly – say, because it has left your high pressure mouth – it will gladly do so. The molecules, though, have to do real work to push everything outward during the expansion. In exchange for this macroscopic motion, the molecules lose some of their molecular motion. If no heat is allowed to enter the gas during the expansion – either because of insulation or because there isn’t enough time for significant heat exchange (your case) – the gas will end up cooler.


I know the answer to this, but let’s see how you do.

The role of pressure has been addressed, but let’s eliminate that. Why does a breeze feel cooler than still air? Temperature is proportional to the average kinetic energy per molecule, i.e., (1/2)mv^2, where v is the speed. Given that the average speed of molecules for a given temperature is v, then the same air moving in a 20 mph breeze will have average speed relative to you of v + 20 mph (or thereabouts), and has a higher temperature. Why does it feel cooler?

Mmmm… because it evaporates water off your skin? And the evaporating water takes thermal energy with it?

Ah, you betcha, there Achernar. Wind chill is basically a physiological response.

And if you get the air moving fast enough the local temperature goes way up. I think there was an example of this recently…

All this talk of compression/expansion is a red herring wrt the op.
This is an adiabatic process. Cooling from expansion is balanced by heating from compression. Unlike the compressor in the gas station air pump, your lips do not get hot.

As the original question has been pretty well answered by Achernar (in the very first reply), I’d like to point out that the OP is in fact not a bad question, as questions go. Instead of discussing the finer points of adiabatic processes, I’d like to point out that Anaximenes was on the same track 2500 years ago, so don’t give up quite yet.

He made the same discovery, namely that slowly exhaled air, with the mouth open feels hot, whereas puckering the mouth makes it feel cold.
He then built up a theory where this ‘compression’ and ‘rarefication’ of air is expanded, to include fire, as the extreme case of rarefied air, and clouds, giving rise to water are composed of compressed air. The water was then compressed further by plants, into solid matter.

Later on the same ideas were added to by such illustrous minds as Heraclitus and Aristotle.

To add to what pasta said…

The air is cooler when we purse our lips because it exits at a higher pressure. When a gas expands (ie it’s pressure drops) it’s temperature is lowered as well. When the high pressure air exits our lips it normalises with the atmospheric pressure and therefore it’s temperature drops as well.

When we blow with our mouth open the pressure of the air exiting our mouth is much lower (because the cross area of our mouth is larger). This air also cools but only by a smaller amount.

Try blowing slowly with your mouth open. Then increase the force at which you blow out (but keep your mouth open) and you will feel the air stream temperature drop. Also try exhaling very slowly with pursed lips. The temperature of the air exhaled will be warm.