The research admirably answers the question “Why does a shower curtain get sucked into an EMPTY shower?”, but what about a shower with a large human moving around in it? I’m skeptical that the vortex shown in the simulation would form, or if it did form, be large enough and powerful enough to suck in the curtain in an occupied shower.
good point bcat, but wasn’t the problem presented upon the observation of those who were taking a shower? i don’t think many people could relate to it, nor would the question even be asked if a lot of people haven’t experienced this. i myself have many times, when the curtain seems to blow inward and stick to my wet body. one of the most annoying anomalies in my everyday life, if you ask me.
A link to the column is appreciated. Why does the shower curtain blow in despite the water pushing it out (revisited)?
As is a link to the thread wherein this is already under discussion. 
http://boards.straightdope.com/sdmb/showthread.php?threadid=79353
Welcome to the SDMB, BCat, and thank you for posting your comment.
Please include a link to Cecil’s column if it’s on the straight dope web site. To include a link, it can be as simple as including the web page location in your post (make sure there is a space before and after the text of the URL).
Cecil’s column can be found on-line at the link provided by bibliophage.
moderator, «Comments on Cecil’s Columns»
I agree with Bcat. The simulation, I’m sure, was executed as well as you can with a computer. But I believe convection plays a major role in this hum-drum conundrum. I thought I comfortably knew the answer until I see Cecil’s article, and now I can’t think of anything else! I must know the True Answer to The Great Shower Curtain Puzzle!
Standing in the shower this morning with the curtain billowing, I realized that in a computer model it may be a low-pressure issue, but IRL it’s heat. I pull the curtain open enough so that it quits billowing and I can feel the cold air getting sucked in and see the steam go out the top. And I have noticed that on colder days, the shower curtain billows more.
Did the scientist add the variable for the temperature difference between each droplet of water and the effect it’s passage through the air has on the shower curtain? And if the air is colder, it is more dense so shouldn’t the pressure difference formed be less and so the curtain is affected less?
I know, it doesn’t effect the shower curtain itself, but you know what I mean. I think. No! I’m sure that you know what I think I am saying. Or typing rather… oh I give up… 
Try it again Cecil! That isn’t the answer! Don’t try to cheap out on us! Call NASA! Call the JPL!
On the shower curtain issue, I come down on the convection theory of shower curtain motion. When you begin a hot shower in a cold bathroom, the heat from the hot water creates an updraft in the confined space of the shower, lowering the atmospheric pressure inside. The curtain, being the only part of the enclosure that would be affected by small pressure changes, is pushed in by the higher pressure outside. This air is also colder, and as such, pushes more strongly on the bottom of the curtain. As this air enters the shower area, it is warmed by the hot water passing through it, and begins to rise, thus continuing the convection cycle. Your readers should notice that once the bathroom reaches a steamy thermal equilibrium (i.e. all the air is uniformly warm, the curtain returns to its normal vertical position.
Is it possible for the vortex and convection to have an effect at the same time? I’m thinking the theories are not necessarily mutually exclusive.
Yeah, have the convection-hypothesis supporters tried running a cold shower to see if the shower is still drawn in?
Yeah, have the convection-hypothesis supporters tried running a cold shower to see if the shower is still drawn in?
I don’t know if they have, but Cecil certainly did . . . twice, as a matter of fact. He mentions that in the column. The curtain was still pulled in, though not as strongly.
RR
Suggesting a combination of the heat and the convection effects.
Years ago, when I lived with my parents, they had an old water-guzzler shower head that emitted a gentle stream of large drops of water. The shower curtain there sucked in badly on chilly winter mornings, and less after a long hot shower when the bathroom heated up. The curtain sucked in when the shower ran hot and hung straight when the shower ran cold. I was convinced that the convection theory was at least the most important part of the explanation.
More recently, I moved into an apartment building. The shower here uses a super-high-pressure water-saver shower head, which spews the water out in a stinging fury of microscopic drops. Here, the shower curtain gets sucked in pretty strongly no matter what I do. So the fluid-dynamics guys aren’t completely nuts, after all.
I don’t think the explanations we’ve all read have done enough to acknowledge that there are plenty of factors that vary from one shower to another, and could change which effect dominates in sucking the shower curtain in. The water pressure, water velocity, water droplet size, water temperature, air temperature, shower size, shower shape, and so on will all surely effect the outcome. Perhaps the truth of the matter is unknowable on more than a shower-by-shower basis.
The problem with the vortex theory is that, unlike the eye of a hurricane, each vortex is contained entirely within the shower. Therefore, isn’t each low pressure area exactly offset by the higher pressure of the air swirling around it? I’m no physicist, so perhaps this is an oversimplification, but it does seem pretty obvious.
On the other hand, when the water goes down the drain, wouldn’t it drag air down with it? Would that effect create enough of a pressure difference to lift the shower curtain? The pressure difference can’t be that great in the first place, or the effect on the curtain would be much more dramatic. One could test this theory by plugging the drain when the shower is running. Our shower has a glass door, so I will let someone else take it from here.