A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Why does the shower curtain blow up and in instead of down and out?

June 7, 1985

Dear Cecil:

Ahem. This is coming several years late, but once I finally get this in the mail I'll be able to rest easier, knowing that another great issue has been laid to rest. I read your column regularly during 1978-80, and suppose it was during 1979 that the great pigeon controversy appeared in the Straight Dope. It went on for several weeks, and revolved around why pigeons bob their heads as they walk. The proposals included the need to maintain center of gravity over the one foot on the ground, stability of head position for clear vision while the body moved forward, and many other creative and well thought out alternatives. The question was never resolved. I don't propose to resolve it here.

The reason for this letter is a question which followed soon after — that is, why does the shower curtain come inward and cling to your legs when you would expect it to be forced outward by the rushing water? Your answer boiled down to, "I don't know." I was shocked to find that a group intellectually curious and creative enough to debate pigeon head-bobbing for weeks would let you get away with this lame response. In order to rectify this failure of six year's standing, and in the hopes that the original questioner has not pined away and died of intellectual malnutrition in the meantime, I offer the following:

The shower curtain attacks you for the same reason airplanes can fly. You all remember those drawings of airplane wing cross-sections. The air travelling over the top of the wing has farther to go, therefore it travels faster, resulting in lower air pressure. This provides the wing's lift. Similarly, the air inside the shower is being driven past the shower curtain by the water, resulting in localized lower air pressure relative to the still air on the other side. The shower curtain quite naturally flies into your legs. The result is annoying, but if it didn't happen we would still require two weeks to get to Europe by boat, so it has its compensations. In the hopes this puts other troubled minds at ease (and next time don't give up so easily!), I remain, yours truly …

Cecil replies:

There are several issues I wish to address here, D.: (1) your unspeakable impertinence, (2) your whereabouts, and (3) your question. We will take these one at a time.

(1) Your impertinence. You not only think there's a question I didn't know the answer to, you think this was it? If so, please disabuse yourself. As you surely know, I have done more to advance the cause of human knowledge than any single person since Galileo. Nonetheless, sometimes I am overcome with an exquisite languor, causing me to blow off a query. Forgive me.

(2) Your whereabouts. Despite the Fremont return address, your letter is postmarked Brazil. Do you so fear my well-deserved wrath that you have decided to flee the country, lest I have you snuffed? Or have you committed some other crime against nature that has compelled you to take it on the lam? In any case, now that you're out, have fun trying to get back in. I have friends.

(3) Your question. The preceding notwithstanding, the answer you supply is correct. Shower curtains and airplane wings do what they do because of Bernoulli's principle, which says that as the velocity of a fluid (liquid or air) increases, its lateral pressure decreases. Surrounding air (or whatever) moves in to fill the partial vacuum thus created. You can demonstrate Bernoulli's principle by dangling a sheet of paper in front of you so that it's perpendicular to the plane of your face and blowing across it. The paper will be deflected toward, not away from, the airstream. The situation in the shower is slightly more complicated. The falling water pulls air along with it, a process known as "entrainment." The moving air creates a low-pressure area that causes the curtain to fly inward. Similarly, when a truck or car moves at high speed down the highway, it creates a partial vacuum in its wake, whence the expression, "let's suck his doors off, Elroy."

I hope you have learned something from this, D. Try to be a better person in the future.

More ravings from the Teeming Millions

Dear Cecil:

Imagine my shock upon learning that you had been fooled by such a ridiculously unscientific notion as the Bernoulli theory to explain why shower curtains are sucked inward when you're taking a shower. The real reason is much simpler. Haven't you ever noticed that hot air rises? The air in your shower stall is being heated by the warm water and therefore rises out of the opening at the top. This air must be replaced by something, so the shower curtain is pushed inward as outside air flows in at the bottom. If you want to prove this, take a cold shower.

Cecil replies:

Matthew, did it occur to you to test this idea before you mailed it into the newspapers? If you had, you would have found, as I did, that the shower curtain is sucked in no matter what the temperature of the water. Admittedly the effect is exacerbated by hot water, but the airflow patterns can get pretty complicated. For instance, even when the shower is running with hot water, air flows out at the bottom of the curtain. Try blowing cigarette smoke around and you'll see.

The crowd gets ugly

Dear Cecil:

Cut the crap about shower curtains! Try this — take a cold (100%) shower and the curtain goes out; a hot shower and it comes in. Cold air (cooled by the water spray) settles, causing pressure outward, and hot air (as in a hot shower) rises, creating a mild vacuum (suction).

Cecil replies:

You been eating those lead paint chips again, Dillo? I have just spent the last 15 minutes in the bathroom repeating my earlier experiment, and the results are the same. The curtain blows in no matter how cold the damn water is. I have, however, been discussing this question with Jearl Walker, who in addition to writing the Amateur Scientist column for Scientific American is chairman of the physics department at Cleveland State University. While agreeing that adherents of the chimney-effect school of shower research are missing a couple bolts below the waterline, he also thinks it unwise to appeal to the Bernoulli principle, which, from the standpoint of explanatory power, he regards as on a par with water sprites. (Jearl is not a man to mince words.)

Jearl says the shower curtain phenomenon is caused by something akin to the Coanda effect. Strictly speaking, the Coanda effect is the tendency of a moving fluid to adhere to an adjacent wall — e.g., water clinging to the inside of a pipe as it drains. It seems reasonable to suppose, however, that if the adjacent wall is sufficiently diaphonous, as in the case of a shower curtain, the Coanda effect can work the other way around — the wall will cling to the fluid. What happens is that air is entrained by the turbulent "boundary layers" of the water and pulled along with it. This creates a localized suction, because other air has to rush in to replace the entrained air. The suction is what pulls in the shower curtain. The Coanda effect can also be used to explain the blowing-across-the-sheet-of-paper-held-in-front-of-the-mouth phenomenon.

Cecil is sorry if he initially steered the Teeming Millions wrong on this, but he notes with a certain satisfaction that Jearl and the Cleveland State physics department spent three hours arguing about it before they came up with the right angle. When you're working on the frontiers of science, you have to expect the occasional wrong turn.

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