draft at the bottom of the shower curtain

September 21, 2004 10:32 PM

Why does the shower curtain act like there's a draft at the bottom of it when I take a hot shower? I don't feel a breeze down there. This has always made me feel vaguely uneasy--cue Psycho music--and it's time to get an answer.
posted by WolfDaddy to science & nature (12 comments total)

IANAP, but perhaps the updraft from the newly-heated air creates a (relative) low-pressure zone at the bottom of the curtain?
posted by Kwantsar at 10:35 PM on September 21, 2004

UMass professor Dr. David Schmidt was mentioned in several major newspapers a few years ago because he had computer-modelled the situation and came up with an answer. Something to do with the Bernoulli effect.
posted by bobo123 at 10:40 PM on September 21, 2004

The Straight Dope on shower curtain physics.
posted by falconred at 10:42 PM on September 21, 2004

The rushing water creates a fast moving air current. This helps lowering the air pressure inside  the tub. The air on the other side of the curtain pushes it forward.

See also: Toilet Science, towards the bottom of the page.

At one point, someone modelled the air currents that cause this, but I can't find the site on which that was published.
posted by majick at 10:43 PM on September 21, 2004

Scientific American's "Amateur Scientist" looked at this years ago and concluded something similar to, but differnet, than majick's quote above. That is, the spray of the water pushes air along with it, creating an area of low pressure waist level and above, and high pressure below in the tub. The low pressure higher up causes the curtain to billow inward, pulling it away enough from the side of the tub to allow a flow of air from the tub (which is at a higher pressure) to escape out the side. This was discovered using smoke to trace the air currents. The temperature of the water didn't make a noticable difference, but the fineness of the spray (how fine the spray and the greater the flow of water, the more the effect) did.

So, short version: air is not being sucked into the tub, but rather, air is being pushed outward. That's why the curtain billows inward with the bottom of the curtain closer to the tub wall than the billowing portion above it. If air was being sucked in from outside the tub, then you'd expect the curtain to be pushed most inward at its bottom. But it's not.
posted by Ethereal Bligh at 11:05 PM on September 21, 2004

As linked to by bobo123, you can experiment yourself:

The air begins moving in a stable circle, called a vortex, "just like a dust devil indoors. This one, unlike a dust devil, doesn’t die out because it’s continuously driven by the shower."

Anyone can try to test Schmidt’s model. "The best way to see it is to turn on the shower – cold water will do fine. Use a light, thin shower curtain and a strong showerhead. Stand outside the shower, stick your head in, and blow in smoke."
posted by vacapinta at 11:59 PM on September 21, 2004

If it bothers you, you can put a stop to it by using a double curtain. The curtain sets I'm thinking of have a cloth curtain that hangs outside the tub and a (rather better quality, i.e. heavier, than usual) plastic curtain than hangs inside. Looks nice and you don't get that wet-plastic-shroud embrace when you're trying to wash your bod.
posted by jfuller at 3:36 AM on September 22, 2004

You could also just buy a shower curtain with small weights (or magnets) sewed into the bottom of the fabric.
posted by Civil_Disobedient at 5:39 AM on September 22, 2004

I read an article a while ago about a guy who noticed this phenomenon in his aunt's shower or something, and made a computer sim about this very question, modelling each water drop etc. I forget the conclusion, but it's probably the same as the Straight Dope one, because as we all know Cecil's never wrong.
posted by abcde at 8:08 AM on September 22, 2004

I just knew someone had said it already. Never mind.
posted by abcde at 8:10 AM on September 22, 2004

Dr David Schmidt, it may be worth noting, received the 2001 Ignoble Prize for physics for his work in this field. Well done, fella.
posted by Monk at 11:44 AM on September 22, 2004

great question (and answers)!
posted by jacobsee at 3:30 PM on September 22, 2004