Why does your shower curtain attack you?

Hot air can't be the whole story

For a long time the favorite suspect was warm air. Back in 1938, Popular Science said that hot air rises in the shower and cooler air from outside the tub moves in to replace it, pushing against the base of the curtain⁵. The trouble is that curtains billow during cold showers as well⁵. David Schmidt, an engineering professor at the University of Massachusetts Amherst, put it plainly: the curtain sucks inward toward a cold shower, too¹.

Bernoulli is slipperier than it sounds

The next popular answer uses Bernoulli's principle. In this version the water drags air along, which lowers the pressure inside the curtain, so the air outside pushes the curtain in⁷.

Physics educator Peter Eastwell points out that Bernoulli's principle strictly applies to flow that is steady, free of internal friction (viscosity) and incompressible⁴. He argues that the handy rule "faster air means lower pressure" is often misused, because air does not lose pressure just because it is moving⁴. Schmidt also wrote that the Bernoulli explanation leaves out the water droplets, and that by his calculations it was not what moved the curtain¹.

A tiny whirlwind in your bathtub

Schmidt's day job was simulating sprays in diesel and aircraft engines, so he pointed the same tools at his bathroom¹. He used flow-simulation software extended with spray models, and he split a virtual shower, including the tub, showerhead, curtain rod and the room outside, into about 50,000 small cells^{1, 2}. His home computer took about two weeks to work out 30 seconds of showering^{1, 2}.

In the simulation, the air slows the falling drops down, and in turn the drops push that air into motion². The moving air settles into a steady, circling vortex², which Schmidt compared to a sideways dust devil¹. Unlike a real dust devil, it doesn't die out, because the shower keeps driving it². Its center, "much like the center of a cyclone," is a low-pressure region that helps pull the curtain in¹. In 2001 the work earned Schmidt the Ig Nobel Prize in Physics, "for his partial solution to the question of why shower curtains billow inwards"³.

Why heavy curtains stay put

The forces are weak. They are only strong enough to move light, thin curtains, which is why heavy plastic ones rarely misbehave¹. Schmidt's advice is to sew weights into the bottom hem, or to use magnets if your tub is metal¹.

Eastwell got to test the effect himself when car trouble left him stuck in a motel with an unusually adjustable shower^{4, 5}. With cold water falling a moderate distance from the curtain, the curtain moved inward. With the water right beside the curtain or far away from it, nothing happened⁴. Hot water made the pull stronger, which he put down to warm air rising⁴. So heat looks like an extra push, not the main cause.