Why does wet skin feel chilly?

Dear Tom,
Why does wet skin feel chilly?
Lynn Malfort, Chicago
Dear Lynn,
It is a characteristic of every substance that the molecules of which the substance is composed vibrate. What we call “temperature” is actually a measure of the average vibrational energy contained by the molecules of the substance. We perceive the molecular vibration as the sensation of heat. When a liquid (like water) is heated, its molecules vibrate more rapidly and its temperature rises. But the vibrational motion of water molecules actually consists of a spectrum of speeds: Some molecules are moving less rapidly than the average speed and some are moving more rapidly. The fastest molecules, if they possess enough energy, will break through the water’s surface — they evaporate. This leaves the remaining liquid with a lower average energy content. Consequently, evaporation is a cooling process.

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