I observed this as when I want to cool my soup I blow like a whistle and when I want to warm my hands I open my mouth more as my breath is warmer now.

  • $\begingroup$ I've seen a remarkable number of different explanations of this, from the Joule-Thompson effect, to the effect of entraining more outside air when you purse your lips and blow, to adiabatic expansion, to evaporative effects due to increased airspeed, or simply increased convection. I'm not entirely convinced by any of them, and I'm very curious to see the answers here. $\endgroup$ – Brionius Feb 26 '15 at 14:03
  • $\begingroup$ @Brionius how does joule thompson effect explain this? $\endgroup$ – humble Feb 26 '15 at 14:38
  • $\begingroup$ possible duplicate of Why the breath sometimes warm and sometimes cold? (2 different explanations!) $\endgroup$ – John Rennie Feb 26 '15 at 15:00
  • $\begingroup$ Possible duplicates: physics.stackexchange.com/q/7868/2451 $\endgroup$ – Qmechanic Feb 26 '15 at 15:05

Your breath is the same temperature either way. The difference is how much ambient air is brought along with the breath by the time it reaches the object.

Emitting a thin and fast stream of air will cause a lot of other air to follow along with it. When you are blowing on the soup to cool it, what you're really doing is using your breath to move a lot of ambient air, which is what cools the soup.

When you breath on your hands to warm them, you are emitting a wide and slow stream and hold your hands close to it. The result is that your hands get mostly that original wide and slow stream without a lot of ambient air mixed in yet.

  • $\begingroup$ That is my thought too. None of the other potential answers that Brionius mention sound convincing. This one does. $\endgroup$ – mmesser314 Feb 26 '15 at 14:32
  • $\begingroup$ @rjmess: "Ambient" means the stuff that is all around. In this case you can take ambient air to mean the not-breath air. $\endgroup$ – Olin Lathrop Feb 26 '15 at 15:00
  • $\begingroup$ @OlinLathrop is it due to viscosity that the ambient air flows with our breath more when we blow hard? $\endgroup$ – humble Feb 26 '15 at 15:05
  • $\begingroup$ @rjmess: You can think of it various ways, which ultimately should lead to the same result. Viscosity is one way to think about it. Bernoulli's principle is another. $\endgroup$ – Olin Lathrop Feb 26 '15 at 15:13
  • $\begingroup$ Then why is the slow air hot in the first place? $\endgroup$ – Babu James Mar 3 '17 at 9:08

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