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Why does blowing hard on our hands cool them, but just breathing on them warm them? © Getty Images

Why does blowing hard on our hands cool them, but just breathing on them warm them?

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Asked by: Eric Williams, Lancashire

This is essentially due to a difference in the speed of the airflow in each case. When we breathe out slowly on our hands, the warmth and moisture from our mouths has time to transfer to our hands, warming them up. But pursing our lips speeds up the airflow, and as this passes over our hands, it blasts away any air that’s been warmed by contact with our hands, cooling them. In addition, the fast-moving column of air draws in, or ‘entrains’, the air around it. This tends to be cooler than our breath, so it boosts the cooling effect.

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Authors

Robert is a science writer and visiting professor of science at Aston University.

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