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  1. When I am taking a shower, the shower curtain slowly moves towards my legs.

  2. Also, it seems that the hotter the water, the faster it gets to my skin.

Why is that?

alex_d
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Gohu
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    Ig Nobel prize was awarded on research of topic ;) You can start at https://en.wikipedia.org/wiki/Shower-curtain_effect and follow links from there. – Artur Biesiadowski Jun 28 '17 at 10:14
  • https://www.scientificamerican.com/article/why-does-the-shower-curta/ – valerio Jun 28 '17 at 10:57
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    I have found if instead of using the curtain to completely seal the bath you are able to leave a gap at the end furthest away from the shower (where you're unlikely to spray water anyway) this can significantly reduce or even stop this annoyance. – Matthew1471 Jun 28 '17 at 13:49
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    @Matthew1471 I also had always had a gap on both ends, and never noticed this phenomenon until I used a different shower. It's very minimal in my shower now because I do leave a gap on the end. – JMac Jun 28 '17 at 14:32
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    Related: https://lifehacks.stackexchange.com/q/15407/20114 – DonQuiKong Jun 28 '17 at 19:11
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    Considering that there are so many hypotheses (see WP article linked above and the answers) and a number of other factors (shower geometry, curtain geometry, person geometry, ambient temperature, humidity and pressure, spray characteristics, materials, water chemistry ...), I would assume that all mechanisms (convection, vortex, Bernoulli, electrostatic etc.) have some sort of effect, whose strength depends on all the other parameters. – Raketenolli Jun 29 '17 at 07:21
  • come on this is from air current isn't it? – rhill45 Jul 01 '17 at 07:19

4 Answers4

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It is due to the convection in the shower, as mentioned in Steeven's comment.

The water is significantly above room temperature. Obviously it is going to heat the air (and you can feel the warm air/vapour yourself).

The air in the shower is both warming up, and becoming more humid. Both of those factors lower the density of the air, making it raise up.

When the air flows up, it creates a low pressure zone in the shower. Once the pressure gets low enough compared to the room outside, the pressure difference pushes the curtain into the shower until enough air can get in to account for the air lost to convection.

Warmer water will cause greater convective flows, so the effect should be more noticeable.

Disclaimer: Note that the heat and humidity are not the only cause of convective flows; and convective airflow is likely only part of the situation. As some comments (and the wikipedia page on this subject) mentioned, the flow of the water downwards may actually generate an air vortex in the shower. This vortex creates a low pressure zone, which drives the convective process regardless of water temperature.

There are also other potential reasons, and without experiments and potentially (multi-physics) simulations, it's very unclear what the primary cause of this phenomenon is.

I still feel this answer properly addresses the second question about why warmer water makes the process more noticeable.

JMac
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EDIT on reflection, my answer, as noted in the comments below, is not correct (to be specific, it's dead in the water), it simply would not produce the mass of less dense air required. END EDIT

The Bernoulli Principle is probably the main cause. This states that an increase in velocity, of the air associated with the falling water, produces a reduction in air pressure around your body, and then ambient air pressure in the bathroom pushes the curtain towards you.

Another possible reason is the Coanda Effect, which you can observe when you hold a spoon near a stream of water from a tap and as the water moves over the curved back of the spoon, the spoon will move inwards toward jet center.

I have to say I include this Coanda Effect for interest only, I believe the first reason to be far more significant.

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    If the increase in velocity due to falling water was the significant factor, it should work irrespective of the water's temperature. Also, even if I haven't crunched the numbers, I think that this effect is many orders of magnitude too small to have a real effect. I think that Steeven's explanation is the right one (see comments). – valerio Jun 28 '17 at 09:13
  • @valerio92 thanks, that's a fair point. Although, I would (half heartedly :) argue that the increase in temperature effect is something I would have to check myself. –  Jun 28 '17 at 09:58
  • I can experimentally confirm Countto10 theory about the air velocity due to the failing water, regardless of the water temperature (at least into the range [10°C;40°C]). – YSC Jun 28 '17 at 13:46
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    @valerio92 Where is “Steeven’s explanation”? – Emil Jeřábek Jun 29 '17 at 13:31
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    @EmilJeřábek It has been deleted. It was similar to JMac's one, though. – valerio Jun 29 '17 at 13:35
  • Ah, all right. ${}$ – Emil Jeřábek Jun 29 '17 at 13:59
  • So you are saying if we turn the shower on as we normally would and then observe the inside of a shower via a camera, rather than stepping into it, the curtain would not rise/pull up or in at the bottom? That it's our bodies (or the effects around our bodies) causing it? – TylerH Jun 29 '17 at 15:56
  • @TylerH No, nothing to do with our bodies, the correct answer is given by jmac (and Steeven earlier). My answer is incorrect, my idea did not take account of rising, low density air. –  Jun 29 '17 at 16:02
  • @Countto10 I'm no longer convinced my answer is even that right. I think it answers question 2 pretty well, but the process itself seems really complicated. – JMac Jun 29 '17 at 21:33
  • @JMac When I wrote my answer, all my searches turned up aerodynamic effects, and a lot of hedging about the causes, but nobody mentioned what you wrote about. I think if this was tested under controlled conditions, most of the causes mentioned would contribute, but as the curtain mostly pulls in towards your hips, where the air is warmest, (imo), your answer is still the most valid. –  Jun 29 '17 at 22:23
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There are numerous theories and this has been a subject of eternal debate. However, given that we now have mountains of evidence, I believe at this point we can state conclusively that the mechanism is as follows:

The water falls in the front of the shower. The falling water carries air downward with it. The air that moves downward at the very front of the shower moves upward further back. This pattern of downward moving air in the front and upward moving air in the back closes to form a stable, horizontal vortex. The vortex has a low-pressure zone in the middle, which produces a constant, low-pressure against part of the curtain. The higher air pressure on the other side of the air curtain therefore pushes it inward.

David Schwartz
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I have personally never been convinced of any explanation that involves a difference in air-pressure causing this effect.

When I first used a shower that suffered from this problem - about 25 years ago - I tried various things to understand it. One of those things was to make sure I left a large gap at one end of the curtain to allow the air to freely flow around the end of the curtain. This should allow cold air to enter the shower and warm air to leave with little obstruction. It made virtually no difference.

I fail to see how the pressure difference between the outside and inside can be so great that an opening of perhaps half a square metre cannot allow equalisation to take place.

The temperature of the room compared to the water also seemed to make little difference.

Unfortunately, I have never reached a conclusion, but my pet theory is that it may be electrostatic in nature. Indeed, the very small difference you do observe when creating an opening in the curtain is far closer amount you would expect from decreasing the surface area of the curtain than the very much larger change in the pressure difference that would result from the opening.

EDIT: Have just remembered the rest of my reasons for leaning toward electrostatic forces and away from pressure differences.

I noticed that the effect was much smaller before I got into the shower - long after the water got to full temperature. It worsened after I entered. If the effect is caused by some interaction between the water and air, surely it would DECREASE as enter the shower, not INCREASE...? @DavidSchwarz says the effect is caused by a "vortex" but when I am in the shower, the falling water column is hitting my HEAD and is therefore about 5 or 6 times shorter than when it is unoccupied.

Also, I noticed that my wife (with a significantly smaller body than mine) never complained about the problem at all. Observation showed is was much reduced for her than for me.

If the cause was electrostatic, then my larger body would increase the "charge" - AND I would be closer to the curtain to make the effect much larger. As observed.

Lefty
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  • See my answer. This is now well-understood. The air pressure cannot equalize because the vortex is stable. – David Schwartz Jun 28 '17 at 17:15
  • @DavidSchwartz Can you cite the evidence please? I'm willing to be convinced but I'd like to know why such a small pressure difference resists equalisation so effectively. – Lefty Jun 28 '17 at 17:29
  • The pressure difference resists equalization because the vortex is stable. So long as the vortex remains, so will the pressure difference. It's somewhat like the eye of a horizontal hurricane. – David Schwartz Jun 28 '17 at 17:41
  • That's a nice theory - but you mention evidence...? – Lefty Jun 28 '17 at 18:20
  • Evidence. @DavidSchwartz I strongly disagree that one guy's CFD simulation on his home computer represents "well understood". – user121330 Jun 28 '17 at 18:35
  • @user121330 Even if that simulation earned him an award issued in the name of Nobel? – David Schwartz Jun 28 '17 at 23:57
  • @DavidSchwartz I assume you are joking. I've read the Scientific American article that's mentioned on the Wikipedia page, it's not exactly a peer reviewed paper is it? The point I'm trying to make is that the physics of this phenomenon are not well understood and accepted by the scientific community. The op of this question has accepted an answer as if the debate is settled, when, in fact, the science hasn't really begun because this is seen only as a humorous curiosity. – Lefty Jun 29 '17 at 05:43
  • @DavidSchwartz in the name of Nobel? I think you should look up puns. – user121330 Jun 29 '17 at 15:34
  • @Lefty There are always going to be deniers who insist we need more evidence even though the science is clearly settled. – David Schwartz Jun 29 '17 at 18:10
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    Everything would be some much more settled if it weren't for those damned deniers and their so called "scientific method". – Lefty Jun 29 '17 at 22:06