Why Is Your Breath Hot and Cold?
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The answer
A thin, fast "hoo" jet drags in room air on the way, so it arrives cool and pulls heat off your skin. A slow "haa" barely mixes and lands warm.
Sources (3)
- Robert Matthews (physicist), BBC Science Focus, "Why does blowing hard on our hands cool them, but just breathing on them warm them?": it is "essentially due to a difference in the speed of the airflow"; slow breath has time to pass its warmth and moisture to the hand; fast pursed-lip air blasts away warmed air, and the fast column "draws in, or 'entrains', the air around it", which is cooler. https://www.sciencefocus.com/science/why-does-blowing-hard-on-our-hands-cool-them-but-just-breathing-on-them-warm-them
- The Naked Scientists (Cambridge), "Why is your breath sometimes warm and sometimes cold": at ~15 cm the pursed-lip jet has entrained mostly room air, which removes heat from the hand by conduction and evaporation; right at the lips the same jet still feels warm (so the breath itself starts warm). https://www.thenakedscientists.com/get-naked/experiments/why-your-breath-sometimes-warm-and-sometimes-cold
- Turbulent jets entrain surrounding air, and the entrained mass grows with distance: Ricou & Spalding (1961), "Measurements of entrainment by axisymmetrical turbulent jets", J. Fluid Mech. 11:21–32. https://doi.org/10.1017/S0022112061000834
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