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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.]

Hritik Narayan
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1 Answers1

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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.

Señor O
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