Arggh, Why Does the Shower Curtain Attack Me?

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It strikes when we're cold, tired, naked. We may try to dodge it -- but we're cornered.

The shower curtain. Why does it bow inward when we turn on the faucet? NPR's Joe Palca investigates from the scene of the crime: his bathroom. He talks with engineer Liz Marshall from Fluent, Inc., in New Hampshire, a company investigating the curtain effect.

Marshall says there are three explanations. But the simplest, she says, is full of hot air.

Traditional thinking went like this: the hot spray heats the air around it. As the hot air rises, it pulls cooler air into the shower from outside. With that cooler air comes the shower curtain. But this theory is lacking, Marshall says, because it overlooks one important fact. The curtain does the same thing in a cold shower.

Scientists have since turned to the "Bernoulli Effect," which states that when fluids accelerate, the pressure around them drops. So when we turn on the shower, the spray is surrounded by lower air pressure. The pressure outside the shower curtain stays roughly the same. That difference in pressure on either side of the curtain makes it bow in.

That theory held until about five years ago, Marshall says. Then David Schmidt, an engineer at the University of Massachusetts, simulated the shower scene on his computer. His model predicts that when the shower sprays, the air inside the shower becomes a kind of spinning vortex. The pressure at the center of this vortex is very low, as it is at the eye of a hurricane. And that low pressure, Schmidt says, could be what sucks the shower curtain in.

Marshall hopes that further modeling and field studies will settle this steaming hot question.