Why is your breath sometimes warm and sometimes cold

12 September 2010


When you blow air out of your mouth in a large O shape, you feel it as hot. When you purse your lips and you make the volume of your mouth smaller, the air moves from a smaller volume to a bigger volume. Since gas expanded form a smaller to a larger volume, the gas cools down from the 1st law of thermodynamics, and you feel it as cold.

No, that's not the explanation. It's a good thought, but it's not right. What is written above is the correct answer.

If friction also acts in open mouth case , then why doesn't it acts in tightened mouth case and makes it hotter , in which the friction should be more making air hotter than open mouth case ? Pls. Answer my diellima

Test this hypothesis by repeating the different mouth blowing techniques in ambient temperatures that are similar to the body's temperature. That is the entrained air stream has an identical temperature to the air leaving the mouth.

Evaporation of the air stream leaving the mouth is also related to the nature of the air flow. The open mouth stream is less turbulent and perhaps even near laminar flow which reduces the evaporation of moisture from the breath air to the surrounding room air.

If you can verify the effect of room temperature on the air entrainment influence on the skin sensation of heat/cold then the effect of evaporative cooing of the air stream cpdriven by turbulence can be determined.

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