HOW COME? Why bathroom curtains cling

How come the shower curtain seems to blow in to the shower, even though there's no draft in the room? asks a reader.

Some blame the shower's steamy-hot water for the annoying billow. The theory: Air inside the shower is heated by the flowing water. Since warmer air rises, the heated parcel exits into the bathroom above the curtain. Cooler air outside the curtain sinks. And a mini-wind is created, blowing toward the shower near the bottom. And suddenly, your legs are wrapped in a sticky (or worse, slimy) vinyl curtain.

But according to physicist Jearl Walker of Cleveland State University, there's a problem with the theory. Even if your shower is downright chilly, even colder than the bathroom air, the clingy curtain will make its unwanted appearance.

Others blame the billow on Bernoulli -- or, rather, the principle named after him. Dutch mathematician Daniel Bernoulli, who lived and worked in the 1700s, is known mainly for discovering a fundamental fact about fluids: The pressure of a fluid varies inversely with its speed. So the faster-flowing the fluid, the less its internal pressure.

What does the Bernoulli principle have to do with unruly shower curtains? A fluid flows under certain kinds of applied stress, so both liquids (like water) and gases (like air) are fluids. Obeying the Bernoulli principle, the air inside a shower, driven by the streaming water, flows faster along the curtain. Then, or so the explanation goes, the resulting pocket of low pressure on the curtain causes it to collapse into the shower.

But as it turns out, there's even more to the shower curtain effect than the Bernoulli principle can explain.

David Schmidt, an engineering professor at the University of Massachusetts, decided to create a computer model of an average shower. Schmidt's model divided the showering water into 50,000 tiny cells, analyzing how droplets behave as they change shape and break up on their way to the shower floor. Running for weeks, the computer simulation did more than a trillion calculations, emerging with a 30-second shower snapshot.

The Bernoulli effect, Schmidt found, did operate near the showerhead, with air speeding up and pressure dropping on the inside of the shower curtain. But the spray of water droplets created an even stronger effect.

How? As gravity causes droplets to accelerate toward the shower floor, they are slowed by friction with the air. The shower air reacts by beginning to move in a circle, like a mini-tornado. However, this shower twister is tipped on its side, and its low-pressure vortex is aimed at the innocent shower curtain. Presto: The curtain billows in, as if drawn by an invisible force.