Dear Tom,
Why does it feel so cool on your skin when you get out of a shower or when you get out of a swimming pool in the summer?
Todd, Chicago
Dear Todd,
The familiar chill that we feel when our skin is wet is rooted in the rules that explain how heat moves from one substance to another. Water (in this case, water on our skin) evaporates because some of its molecules break through the water’s surface and enter the air as water vapor gas. “Within-liquid” water molecules are constantly colliding. As a result of those collisions, some molecules acquire above-average speeds–speeds that allow them to break into the air, that is, to evaporate–taking their above-average energy with them. Those molecules left behind possess below-average energy and that registers on our skin as a decrease in temperature. That’s why our skin feels chilled when water evaporates from it.
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Tom Skilling is chief meteorologist at WGN-TV. His forecasts can be seen Monday through Friday on WGN News at noon and 9 p.m.
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