Dear Tom,
When I get out of the shower, it feels cold. So the question is, why does wet skin feel cold?
— Shawn Albertson
Dear Shawn,
It’s due to the behavior of water molecules. They’re in constant motion, and the warmer they are, the faster they’re moving. We sense that motion as heat.
In liquid form, and as an airborne gas, water molecules are vibrating and tumbling around because they’re not bonded together the way they are in ice. In order for a water molecule to escape from the liquid into the air — to evaporate — it must obtain greater speed (more heat), and this happens through random collisions with other molecules in the water.
If, after a collision, a particular molecule already at the water’s surface gains enough speed to break into the air, it takes its energy (its heat) with it. That’s heat lost from the liquid, which therefore cools, and that’s why water evaporating from our skin gets cooler.
