Saturday, March 17, 2007

You're all wet

And you're just standing there, naked, in the cold night air, freezing.

I won't ask why this happened - that's between you and your rabbi. In keeping with the canons of modern Western science, I'll only explain what's happening and how. It's an amusing application of the last posting's discussion of heat capacity and thermal conductivity.

Water is a decent conductor of heat, 25 times better than air, but a far better retainer of heat, 3000 times better. Heat capacity is a volume effect of a material (so many calories of heat per volume), while heat conduction is a surface effect (so many calories of heat flowing through a surface per surface area per time).

When you're standing there shivering in the cold, reflect on the fact that the water on your skin is mainly having a surface effect. It's conducting heat away from your skin 25 times faster than the air would. That's why you're shivering. Your body isn't used to it and responds by generating more internal heat. When you're not wet, the body mainly loses heat by infrared radiation, not conduction. If you shiver long enough, you tax your body's heat-producing ability to the point where you get sick.

Of course, the water itself also has some heat, and, if it's warm water, you won't shiver quite as much because you'll benefit by absorbing some of it. But water on the skin is mainly a surface effect. Even a thin film of water will dramatically change the heat conduction at your skin, while adding little heat content.

OTOH, suppose you're lounging peacefully in tub of cozy, warm water. (Remember, we won't ask any personal questions here - it's just about thermodynamics.) The water is warm, but now there's a lot more of it, not just a thin skin layer. Your body gratefully absorbs some of that heat from the water. You won't shiver - quite to the contrary. You're enjoying the volume effect of heat capacity.
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POSTSCRIPT: This is just too good to pass up: a "global warming walk" started this past Friday in Massachusetts - the same day as the worst snowstorm of the winter. This is like the well-known "Gore effect."

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