Posted by Bonita Fote on April 15, 2007

Q: How exactly does the water on the arm pull the heat from the body? Why does the body not maintain the heat?

State: New York

A: Bonita,

Good questions.

The water does not exactly "pull" heat from the body. Heat simply moves from where there is more of it to where there is less of it, in accordance with the Second Law of Thermodynamics. In this case the motion of heat is called conduction, which is the transfer of heat energy through the motion and collisions of the atoms and molecules that make up the skin and the water in contact with it. (Heat is conducted faster into the water than it would be to air alone because water molecules are packed closer together than air molecules.)

As long as the arm continues to generate heat and stay warmer than its surroundings heat continues flow out and some of the water on the arm changes from liquid to gas. That's evaporation. Especially if there is more energy input (in the form of someone fanning the arm, causing the water vapor to dissipate as it is produced), water in contact with the skin will evaporate. The moistened arm will feel cooler than the dry arm. The process is all driven by the availability of heat energy.

Mark Baldwin ( April 16, 2007)