The swimming pool paradox

It’s another warm day, but the breeze isn’t helping much, so off I go to the inviting outdoor swimming pool (banner picture) at the other end of campus. It’s an unheated pool (well, there’s no artificial heat source), which means one thing: It’s going to feel cold when I get in*. I should just jump in and get on with it, but that’s easier said than done when you know what the temperature is like. Eventually I get in (brrr!) and have a good swim, by which time I’m not feeling so cold anymore.

But I can’t stay in all afternoon. Work to do. So it’s time to get out. And here’s the total injustice of it all. When I get out I feel cold as well. That’s just not fair. It’s cold when I get in, and cold when I come out. How does that work?

The cold on going into the pool is a result of conduction of heat. Water is a much better conductor of heat than air. So when you are surrounded by water, energy can flow from your body (conduction of heat) much more quickly than it would if you were surrounded by air at the same temperature. In fact, even if the water is warmer than the air, you might feel cold when getting in to it.

But the feeling cold when coming out is a result of another process. In this case it’s down to evaporation of water from your body. When you come out, your body is wet. On a hot day, especially if the humidity is low and there’s a wind, this water quickly evaporates. But it needs energy to do so (‘latent’ heat).  And guess where that comes from. That’s right, it’s you. And so you feel cold on the way out as well.

In terms of energy transfer then, when you are in the water energy is leaving you and going into the water (and the water heats up, though with an Olympic size pool one person doesn’t raise its temperature by much at all) and when you get out, energy again flows from you and into the water you’ve taken with you, in order to evaporate it.

The good part of the story is that you get to enjoy the time in the middle, between the in and out events.


*To be fair, by the end of January the temperature of the pool isn’t too awful. Considerably better than the beginning of December, I can tell you!




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