Anyone who owns a shower curtain knows one thing for sure: It’s going to engulf them as soon as they turn the water on. This strange phenomenon is called the shower-curtain effect, and while there’s no conclusive explanation for why running water causes shower curtains to billow inward, there is one prevailing theory put forth by David Schmidt, assistant professor of mechanical and industrial engineering at the University of Massachusetts Amherst. Here’s how it goes:
The water spraying from the showerhead creates a horizontal vortex, sort of like a sideways tornado. But unlike tornadoes, this vortex doesn’t die out, because it’s continuously driven by the running water. The center of this vortex, similar to the center (or the “eye”) of a cyclone, contains a low-pressure zone that sucks the shower curtain inward.
The actual force generated by this airflow is pretty weak, so it can only feasibly pull lightweight shower curtains inward, which explains why people with heavier shower curtains don’t typically experience the shower-curtain effect. The same goes for those with poor water pressure—they probably won’t see the shower curtain billow inward, because the low water pressure isn’t able to create a horizontal vortex.
If you don’t particularly enjoy your shower curtain hugging you every morning, there are a few simple ways to prevent the shower-curtain effect. Hanging the curtain rod higher or lower—just further away from the shower head in general—can reduce the effect. If that’s not an option, sewing a few weights to the bottom of your shower curtain will keep it in place, since, as we mentioned, the actual force of the vortex is pretty weak.
On the other hand, maybe you enjoy starting your morning off with a shower and a cold, clammy hug. If that’s the case, you should also appreciate the fact that there’s a frickin’ tiny sideways tornado showering right there with you.