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I hope the question is clear enough and I'm sure that you can try this thing quite easily yourself. When I blow air from my mouth to my palm through a small opening, I feel cool in my palm, but its very warm when I do the same with an open mouth!

Does it have something to do with the speed at which the air is moving, and therefore it is cooler when the air blown is faster through a smaller opening? (same volume in both cases but smaller opening in the small hole case). or is it due to some observational flaw which I made? Is it due to the fact that the air momentarily undergoes an adiabatic expansion?

[I know there might be previous questions about this but I need a simpler explanation.]

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    $\begingroup$ Yes, I'm aware of all the duplicates. $\endgroup$
    – Hritik Narayan
    Commented Dec 9, 2014 at 13:32
  • $\begingroup$ Oh! I'm frankly new to this site. I wasn't aware of that, thanks. (for real) $\endgroup$
    – Hritik Narayan
    Commented Dec 9, 2014 at 14:48
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    $\begingroup$ @HritikNarayan: Well, we don't allow duplicates, so saying 'yes, I'm aware of all the duplicates,' is quite humorous to us because it translates to, 'yes, I know my question is going to be closed, but I posted it anyway.' But I'm guessing you didn't know duplicates aren't allowed? $\endgroup$
    – JamalS
    Commented Dec 9, 2014 at 14:50
  • $\begingroup$ Then should I proceed to delete the question or just let it get closed? $\endgroup$
    – Hritik Narayan
    Commented Dec 9, 2014 at 14:54

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Simple answer: When you blow harder, more surrounding air gets mixed in with the stream of air from your mouth.

The faster air moves, the lower pressure it has (Bernoulli's principle). So when you blow faster, your stream of air is lower pressure than the surrounding air. Thus the surrounding air fills in the stream. The surrounding air is obviously cooler than the air in your lungs. To test this - blow fast, but put your finger right on your lip - notice it's still quite warm because other air hasn't had a chance to mix in yet.

To visual why this happens, imagine the stream of air coming from your mouth as a freeway with all red cars on it. Every on ramp to this freeway has a long line of blue cars eager to get on. If traffic is moving slow, and the red cars (air from lungs) are very close together, not a lot of blue cars (surrounding air) can get on. But if the traffic is sparse and the flow quick, you'd see more of a mix of red and blue cars.

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    $\begingroup$ Also relevant is the evaporative cooling from the faster stream. $\endgroup$
    – Señor O
    Commented Dec 9, 2014 at 12:47
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    $\begingroup$ I second Senor O's point. The air comes out faster from a small opening than a large opening and faster moving air carries more water vapour/heat away than slow moving air. $\endgroup$
    – Jim
    Commented Dec 9, 2014 at 13:00
  • $\begingroup$ Yes that does make a lot of sense! but does it have anything to do with adiabatic processes? $\endgroup$
    – Hritik Narayan
    Commented Dec 9, 2014 at 13:13
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    $\begingroup$ @HritikNarayan I haven't run the numbers, but I don't think so. The pressure doesn't get all that high inside your mouth/lungs. Even if it did, by the time the air gets out of your mouth its back near to reg. pressure, and no work has been done by the air so that wouldn't cool it down. $\endgroup$
    – Señor O
    Commented Dec 9, 2014 at 13:17
  • $\begingroup$ Not a single mention of the Joule Thompson effect? $\endgroup$
    – Abdul Moiz Qureshi
    Commented Apr 22, 2020 at 14:33

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