Blowing speeds up evaporation
The key is evaporative cooling: when water molecules escape from a liquid into the air as vapor, they carry energy away. The molecules that escape are especially likely to be among the faster-moving ones, so the remaining coffee loses heat and cools.
Above a still cup, the air quickly becomes warm and humid. That layer makes it harder for more water vapor to escape. A gentle breath sweeps some of it away and brings in air that is usually cooler and less humid, allowing evaporation to continue faster.
Moving air carries heat away, too
Blowing also increases convection—the transfer of heat between a surface and moving air. Air flowing over the coffee carries away heat, while cooler air takes its place. Evaporation and convection work together, though evaporation can be especially important when the coffee’s surface is exposed.
That is why blowing can cool coffee even though your breath itself is warm. The airflow and the drier air it brings to the surface can remove heat faster than the breath adds it. The result depends on conditions such as humidity, airflow, and how much of the coffee’s surface is exposed; a covered cup, for example, offers less surface for evaporation.
