The cooling power of fans
 Q: I've read on your site that a fan cools you via a wind-chill effect, but what does that mean? I seem to recall
from school that perspiration absorbs heat which is removed with evaporation thus cooling you, but my wife
disagrees.
A:
Fans create a wind that cools you for a couple of reasons. The warmth of your body heats a thin layer of air next to
your skin. Like someone ripping a blanket off, wind blows the warm air away. A breeze also blows water vapor away
from your skin, which lowers the humidity, increases the evaporation rate, and cools you, like stepping out of a humid
shower cools your body.
Right: [Corel] A wet surfer in a stiff breeze: chilly
The stronger the wind, the greater the evaporation and the cooler you feel.
Sweat evaporation cools because you lose hot sweat and keep the cool. Let's see how it works at the molecular level:
Consider a cup of water. Water molecules zip here and there, going every which way, at different speeds. Their average kinetic energy (i.e., the energy of their
motion) is closely related to the water temperature. Look! There, at the surface, a molecule from below bumps into a surface one, causing it to fly off into
the air. The top one evaporated.
It's like playing pool. The hit molecule, like a hit ball, increases its speed by the same amount that the one below loses speed. The top one now has enough
energy to escape surface tension and it flies off into space. The cup's energy drops because the cup loses the top molecule's energy and the energy of the left-behind one is less. Consequently, the water temperature in the cup drops. It cools.
By the way, wives are always right.
(Answered by April Holladay, science correspondent, July 4, 2001)
Further Surfing:
National Center for Atmospheric Research: Wind chill
Environment Canada: windchill fact sheet
Books:
Conceptual Physics by Paul G. Hewitt, 1997.
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