# Why do we blow cooler air when we purse our lips? **URL:** \ **Category:** Factual Questions\ **Created:** [February 10, 2003, 5:36am UTC](https://boards.straightdope.com/t/why-do-we-blow-cooler-air-when-we-purse-our-lips/154046 "2003-02-10T05:36:23Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Goldschlager](https://avatars.discourse-cdn.com/v4/letter/g/ea666f/32.png) [@Goldschlager](https://boards.straightdope.com/u/Goldschlager)\ **Post date:** [February 10, 2003, 5:36am UTC](https://boards.straightdope.com/t/why-do-we-blow-cooler-air-when-we-purse-our-lips/154046/1 "2003-02-10T05:36:23Z")
Why do we blow cooler air when we purse our lips and warmer air when we widen our mouths? ---
**Author:** ![Achernar](https://avatars.discourse-cdn.com/v4/letter/a/e274bd/32.png) [@Achernar](https://boards.straightdope.com/u/Achernar)\ **Post date:** [February 10, 2003, 5:41am UTC](https://boards.straightdope.com/t/why-do-we-blow-cooler-air-when-we-purse-our-lips/154046/2 "2003-02-10T05:41:05Z")
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. ---
**Author:** ![something\_clever](https://avatars.discourse-cdn.com/v4/letter/s/3bc359/32.png) [@something\_clever](https://boards.straightdope.com/u/something_clever)\ **Post date:** [February 10, 2003, 5:44am UTC](https://boards.straightdope.com/t/why-do-we-blow-cooler-air-when-we-purse-our-lips/154046/3 "2003-02-10T05:44:54Z")
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… ---
**Author:** ![Goldschlager](https://avatars.discourse-cdn.com/v4/letter/g/ea666f/32.png) [@Goldschlager](https://boards.straightdope.com/u/Goldschlager)\ **Post date:** [February 10, 2003, 5:59am UTC](https://boards.straightdope.com/t/why-do-we-blow-cooler-air-when-we-purse-our-lips/154046/4 "2003-02-10T05:59:50Z")
> [@](#): > > \*Originally posted by Achernar \* > \*\*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. \*\* 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. ---
**Author:** ![Achernar](https://avatars.discourse-cdn.com/v4/letter/a/e274bd/32.png) [@Achernar](https://boards.straightdope.com/u/Achernar)\ **Post date:** [February 10, 2003, 5:59am UTC](https://boards.straightdope.com/t/why-do-we-blow-cooler-air-when-we-purse-our-lips/154046/5 "2003-02-10T05:59:59Z")
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. ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [February 10, 2003, 6:08am UTC](https://boards.straightdope.com/t/why-do-we-blow-cooler-air-when-we-purse-our-lips/154046/6 "2003-02-10T06:08:30Z")
> [@](#): > > \*Originally posted by Achernar \* > \*\*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. \*\* 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. ---
**Author:** ![billy](https://avatars.discourse-cdn.com/v4/letter/b/ecd19e/32.png) [@billy](https://boards.straightdope.com/u/billy)\ **Post date:** [February 10, 2003, 6:11am UTC](https://boards.straightdope.com/t/why-do-we-blow-cooler-air-when-we-purse-our-lips/154046/7 "2003-02-10T06:11:20Z")
> [@](#): > > \*Originally posted by Goldschlager \* > \*\*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. \*\* 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. ---
**Author:** ![Achernar](https://avatars.discourse-cdn.com/v4/letter/a/e274bd/32.png) [@Achernar](https://boards.straightdope.com/u/Achernar)\ **Post date:** [February 10, 2003, 6:17am UTC](https://boards.straightdope.com/t/why-do-we-blow-cooler-air-when-we-purse-our-lips/154046/8 "2003-02-10T06:17:25Z")
> [@](#): > > \*Originally posted by David Simmons \* > 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. 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. > [@](#): > > \*Originally posted by Goldschlager \* > The air we blow when we widen our mouths is moving too and it feels warmer. 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.) ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [February 10, 2003, 6:28am UTC](https://boards.straightdope.com/t/why-do-we-blow-cooler-air-when-we-purse-our-lips/154046/9 "2003-02-10T06:28:00Z")
> [@](#): > > \*Originally posted by Achernar \* > \*\*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. > \*\* 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. ---
**Author:** ![Padeye](https://avatars.discourse-cdn.com/v4/letter/p/a9a28c/32.png) [@Padeye](https://boards.straightdope.com/u/Padeye)\ **Post date:** [February 10, 2003, 6:32am UTC](https://boards.straightdope.com/t/why-do-we-blow-cooler-air-when-we-purse-our-lips/154046/10 "2003-02-10T06:32:33Z")
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 ---
**Author:** ![something\_clever](https://avatars.discourse-cdn.com/v4/letter/s/3bc359/32.png) [@something\_clever](https://boards.straightdope.com/u/something_clever)\ **Post date:** [February 10, 2003, 6:33am UTC](https://boards.straightdope.com/t/why-do-we-blow-cooler-air-when-we-purse-our-lips/154046/11 "2003-02-10T06:33:06Z")
sorry, i didnt explain it very well… i mentioned AC systems in cars and then just normal air compressors 🙂 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: > [@](#): > > Now comes the magic! As the high pressure, warm liquid exits the drier, it passes through an expansion device. It can either be an expansion valve (which modulates refrigerant flow in an expansion valve/drier system), or a fixed expansion orifice tube in a cycling clutch/orifice tube system (which expands at the same rate all the time). The two systems have slight have slight functional differences, mainly in how the low temperature is maintained. Well any way, for illustration, the pressurized liquid passes through the expansion device, the pressure is reduced considerably; hence the temperature drops also. Output = Cold Liquid. The following paragraph describes the expansion valve > > CYCLING CLUTCH/ORIFICE TUBE SYSTEM - The exiting liquid is sent via a small liquid line DIRECTLY to an expansion ORIFICE TUBE. The orifice tube is fixed, therefore the proportional pressure drop across it will constant. This type of expansion device must work in conjunction with a clutch cycling switch. Because the pressure drop across the orifice tube is constant, the switch is used to maintain the system low pressure side in a certain operating range. The cycling clutch switch, through various pressure changes in the system, turns the compressor off and on during normal operation. A typical operation would have the switch turn the compressor on at about 45 psi and off at 25 degrees. This would maintain the evaporator refrigerant at temperatures around 35-45 degrees F. from: [http://members.tripod.com/acguy2/theory\_of\_operation.htm#Expansion%20Devices/Orifice](http://members.tripod.com/acguy2/theory_of_operation.htm#Expansion%20Devices/Orifice) ---
**Author:** ![something\_clever](https://avatars.discourse-cdn.com/v4/letter/s/3bc359/32.png) [@something\_clever](https://boards.straightdope.com/u/something_clever)\ **Post date:** [February 10, 2003, 6:34am UTC](https://boards.straightdope.com/t/why-do-we-blow-cooler-air-when-we-purse-our-lips/154046/12 "2003-02-10T06:34:14Z")
DOH! sorry… i typed all that mess while David was responding, guess im too late. anyhow 🙂 ---
**Author:** ![voltaire](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/voltaire/32/313_2.png) [@voltaire](https://boards.straightdope.com/u/voltaire)\ **Post date:** [February 10, 2003, 7:10am UTC](https://boards.straightdope.com/t/why-do-we-blow-cooler-air-when-we-purse-our-lips/154046/13 "2003-02-10T07:10:36Z")
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. ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [February 10, 2003, 7:35am UTC](https://boards.straightdope.com/t/why-do-we-blow-cooler-air-when-we-purse-our-lips/154046/14 "2003-02-10T07:35:12Z")
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. -P ---
**Author:** ![TGWATY](https://avatars.discourse-cdn.com/v4/letter/t/b5e925/32.png) [@TGWATY](https://boards.straightdope.com/u/TGWATY)\ **Post date:** [February 10, 2003, 7:53am UTC](https://boards.straightdope.com/t/why-do-we-blow-cooler-air-when-we-purse-our-lips/154046/15 "2003-02-10T07:53:30Z")
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? ---
**Author:** ![Achernar](https://avatars.discourse-cdn.com/v4/letter/a/e274bd/32.png) [@Achernar](https://boards.straightdope.com/u/Achernar)\ **Post date:** [February 10, 2003, 8:07am UTC](https://boards.straightdope.com/t/why-do-we-blow-cooler-air-when-we-purse-our-lips/154046/16 "2003-02-10T08:07:12Z")
Mmmm… because it evaporates water off your skin? And the evaporating water takes thermal energy with it? ---
**Author:** ![MonkeyMensch](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@MonkeyMensch](https://boards.straightdope.com/u/MonkeyMensch)\ **Post date:** [February 10, 2003, 8:16am UTC](https://boards.straightdope.com/t/why-do-we-blow-cooler-air-when-we-purse-our-lips/154046/17 "2003-02-10T08:16:40Z")
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… ---
**Author:** ![ticker](https://avatars.discourse-cdn.com/v4/letter/t/d07c76/32.png) [@ticker](https://boards.straightdope.com/u/ticker)\ **Post date:** [February 10, 2003, 10:31am UTC](https://boards.straightdope.com/t/why-do-we-blow-cooler-air-when-we-purse-our-lips/154046/18 "2003-02-10T10:31:44Z")
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. ---
**Author:** ![Popup](https://avatars.discourse-cdn.com/v4/letter/p/94ad74/32.png) [@Popup](https://boards.straightdope.com/u/Popup)\ **Post date:** [February 10, 2003, 10:56am UTC](https://boards.straightdope.com/t/why-do-we-blow-cooler-air-when-we-purse-our-lips/154046/19 "2003-02-10T10:56:22Z")
> [@](#): > > \*Originally posted by Goldschlager \* > \*\*Why do we blow cooler air when we purse our lips and warmer air when we widen our mouths? \*\* 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**. ---
**Author:** ![Skip](https://avatars.discourse-cdn.com/v4/letter/s/ebca7d/32.png) [@Skip](https://boards.straightdope.com/u/Skip)\ **Post date:** [February 10, 2003, 2:43pm UTC](https://boards.straightdope.com/t/why-do-we-blow-cooler-air-when-we-purse-our-lips/154046/20 "2003-02-10T14:43:59Z")
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. [Next page](https://boards.straightdope.com/t/why-do-we-blow-cooler-air-when-we-purse-our-lips/154046.md?page=2)