It's all down to the interplay of convection and evaporation.
Although your breath is generally warmer than the air, blowing on a hot cup of tea does cool it a little. As water molecules evaporate from the surface, the average kinetic energy of the tea drops, as does the temperature. The molecules condense in a steamy fog over the cup, which lowers the tea’s evaporation rate from the surface.
Blowing replaces the hot, moist air with cooler, drier air, which then increases evaporation. Stirring will help to cool the tea by speeding up the process of convection, which brings the hottest liquid at the bottom of the cup up to the top. Creating a vortex through stirring will also increase the surface area to boost evaporation and cooling.
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Asked by: James of Bridgwater, via email
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Dr Emma Davies is a science writer and editor with a PhD in food chemistry from the University of Leeds. She writes about all aspects of chemistry, from food and the environment to toxicology and regulatory science.
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