# The shower curtain question **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [February 5, 2015, 8:05pm UTC](https://boards.straightdope.com/t/the-shower-curtain-question/711920 "2015-02-05T20:05:29Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![bcfd36](https://avatars.discourse-cdn.com/v4/letter/b/e9c0ed/32.png) [@bcfd36](https://boards.straightdope.com/u/bcfd36)\ **Post date:** [February 5, 2015, 8:05pm UTC](https://boards.straightdope.com/t/the-shower-curtain-question/711920/1 "2015-02-05T20:05:29Z")
I cannot comment on the science of the question, but I can comment on what happens with large quantities of spraying water. By trade I am a software engineer and that doesn’t do much to shed any light on the issue. But I am also a firefighter and we have been known to spray water on all sorts of things. One feature of our nozzles is that the stream pattern is variable, from straight stream to completely fanned out. Just like some of your garden hose nozzles. With the fanned out pattern on the fire hose, you immediately notice a nice little breeze coming in behind you. If you are standing in front, you also notice a nice little breeze coming in with the water. We can actually vent a room like this in a pinch. I think the same thing happens in the shower. Whether or not this is the Bernoulli Effect or not makes no difference. It is the moving water pulling the air in behind it that creates the breeze. ---
**Author:** ![justmeetee](https://avatars.discourse-cdn.com/v4/letter/j/898d66/32.png) [@justmeetee](https://boards.straightdope.com/u/justmeetee)\ **Post date:** [February 5, 2015, 10:09pm UTC](https://boards.straightdope.com/t/the-shower-curtain-question/711920/2 "2015-02-05T22:09:57Z")
Sounds reasonable to me. Whatever it is I wish I could stop it from making the curtain stick to my butt. ---
**Author:** ![Fear\_Itself](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fear_itself/32/19637_2.png) [@Fear\_Itself](https://boards.straightdope.com/u/Fear_Itself)\ **Post date:** [February 5, 2015, 11:32pm UTC](https://boards.straightdope.com/t/the-shower-curtain-question/711920/3 "2015-02-05T23:32:42Z")
Relevant columns: [Why does the shower curtain blow up and in instead of down and out?](http://www.straightdope.com/columns/read/426/why-does-the-shower-curtain-blow-up-and-in-instead-of-down-and-out) [Why does the shower curtain blow in despite the water pushing it out (revisited)?](http://www.straightdope.com/columns/read/1369/why-does-the-shower-curtain-blow-in-despite-the-water-pushing-it-out-revisited) ---
**Author:** ![salinqmind](https://avatars.discourse-cdn.com/v4/letter/s/3ec8ea/32.png) [@salinqmind](https://boards.straightdope.com/u/salinqmind)\ **Post date:** [February 5, 2015, 11:40pm UTC](https://boards.straightdope.com/t/the-shower-curtain-question/711920/4 "2015-02-05T23:40:58Z")
This was the very first question asked of Cecil that I ever read, many moons ago. I was instantly and forever hooked on the SD 😃 ---
**Author:** ![Askance](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/askance/32/8281_2.png) [@Askance](https://boards.straightdope.com/u/Askance)\ **Post date:** [February 6, 2015, 12:04am UTC](https://boards.straightdope.com/t/the-shower-curtain-question/711920/5 "2015-02-06T00:04:45Z")
I thought it had been settled that the Bernoulli effect has nothing whatsoever to do with making planes fly? ---
**Author:** ![Kedikat](https://avatars.discourse-cdn.com/v4/letter/k/ed655f/32.png) [@Kedikat](https://boards.straightdope.com/u/Kedikat)\ **Post date:** [February 6, 2015, 1:14am UTC](https://boards.straightdope.com/t/the-shower-curtain-question/711920/6 "2015-02-06T01:14:16Z")
Does it happen during cold and hot showers? I am late to this question, so I may be repeating. In a hot shower, the air inside would rise. Making negative pressure across many square feet of curtain. ---
**Author:** ![Measure\_for\_Measure](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/measure_for_measure/32/557_2.png) [@Measure\_for\_Measure](https://boards.straightdope.com/u/Measure_for_Measure)\ **Post date:** [February 6, 2015, 7:21am UTC](https://boards.straightdope.com/t/the-shower-curtain-question/711920/7 "2015-02-06T07:21:55Z")
> [@Kedikat](#): > > Does it happen during cold and hot showers? > I am late to this question, so I may be repeating. > In a hot shower, the air inside would rise. Making negative pressure across many square feet of curtain. I recommend reading the articles. One of my favorites. According to Cecil and my own shower observations, the effect is invariant to temperature. I’ve noted that it does vary somewhat with water volume. For most showerhead positions, a running shower will create a circular flow of air inside the stall. In the center of the circular flow air pressure drops - just like a hurricane. That lower air pressure sucks in the shower curtain. Solutions include lowering the water volume (maybe with an energy-efficient showerhead), positioning yourself and the spray so as to disrupt the spiral airflow, weighting down the curtain, or perhaps not closing the curtain fully, so as to leave some space for the air to rush in. Ok, I haven’t tried that last one. ---
**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\ **Post date:** [February 6, 2015, 3:38pm UTC](https://boards.straightdope.com/t/the-shower-curtain-question/711920/8 "2015-02-06T15:38:51Z")
> [@Askance](#): > > I thought it had been settled that the Bernoulli effect has nothing whatsoever to do with making planes fly? It absolutely does apply to wing lift, it’s just that the common explanation involving Bernoulli is wrong. The common one is that since the air going over the top and bottom of the wing have to arrive at the trailing edge at the same time, therefore the air over the top goes faster, therefore lower pressure according to Bernoulli. The problem with this explanation is that the air streams _do not_ arrive at the trailing edge at the same time. The one over the top actually arrives first. ---
**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [February 7, 2015, 7:15am UTC](https://boards.straightdope.com/t/the-shower-curtain-question/711920/9 "2015-02-07T07:15:55Z")
I think we can all agree that (1) The air pressure is lower inside the curtain, and (2) The effect is not a thermal effect due to hot water. Questions remain unclear: (a) If the air is moving next to the inside of the curtain - for whatever reason - , isn’t it the force on the curtain described as “the Bernoulli Effect” ? (b) When the curtain jumps that last inch towards you, is this the same cause & effect as the cause and effect that caused the curtain to blow in /swing in? © When the curtain clings to you, is this the same effect? (clearly, no). ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [February 7, 2015, 10:40pm UTC](https://boards.straightdope.com/t/the-shower-curtain-question/711920/10 "2015-02-07T22:40:40Z")
> [@Askance](#): > > I thought it had been settled that the Bernoulli effect has nothing whatsoever to do with making planes fly? No, Bernoulli absolutely has to do with how airplanes fly, it’s just the common explanation of Bernoulli applied to airplanes is incorrect, as **CurtC** explained. > [@Melbourne](#): > > (a) If the air is moving next to the inside of the curtain - for whatever reason - , isn’t it the force on the curtain described as “the Bernoulli Effect” ? Sort of, but not exactly. 😉 Bernoulli is how you measure the pressure difference due to a moving airflow. However, the direction and pattern of airflow, and the cause of the airflow, are different because of the different explanations. The chimney effect posits that the thermal expansion causes the warmer air to rise, which drives it out the top. Cooler air gets pushed down and in at the bottom. Except it happens with cold water, too. The Coanda Effect explanation posits that it is not the air flowing past the curtain that causes the inflow of the curtain, but rather the water pulls the air along, and that air then pulls the curtain in. Coanda explains why the air is pulled along, but the fact that the curtain is then pushed in is due to the Bernoulli effect of the airflow. The vortex explanation says that the air is pushed by the water, which then flows out of the way, pushing the “air packets” in the way up, pushing the “air packets” above those over, which leads to a rotation of airflow in the stall. That rotation of airflow creates a low pressure inside the vortex, which is what pulls the air in at the bottom. I don’t think that is actually the Bernoulli effect, but I need to refresh my understanding. ---
**Author:** ![Askance](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/askance/32/8281_2.png) [@Askance](https://boards.straightdope.com/u/Askance)\ **Post date:** [February 9, 2015, 2:50am UTC](https://boards.straightdope.com/t/the-shower-curtain-question/711920/11 "2015-02-09T02:50:42Z")
> [@CurtC](#): > > It absolutely does apply to wing lift, it’s just that the common explanation involving Bernoulli is wrong. The common one is that since the air going over the top and bottom of the wing have to arrive at the trailing edge at the same time, therefore the air over the top goes faster, therefore lower pressure according to Bernoulli. The problem with this explanation is that the air streams _do not_ arrive at the trailing edge at the same time. The one over the top actually arrives first. Yes, I know that’s the wrong explanation. But I don’t see what else Bernoulli might have to do with it either; is it not just that the air striking the bottom of the wing (as it’s tilted into the airstream) pushes it up, like holding your hand flat out a car window? ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [February 9, 2015, 3:29am UTC](https://boards.straightdope.com/t/the-shower-curtain-question/711920/12 "2015-02-09T03:29:21Z")
> [@Fear\_Itself](#): > > [Why does the shower curtain blow in despite the water pushing it out (revisited)?](http://www.straightdope.com/columns/read/1369/why-does-the-shower-curtain-blow-in-despite-the-water-pushing-it-out-revisited) Wait, is that the same [David Schmidt who won an igNobel prize](http://www.improbable.com/ig/miscellaneous/ig-2001-winners.html) for his shower curtain research? It seems to me, having stayed in a fair number of motels and hotels before and after that time, the was a very noticeable trend thereafter toward installing those bowed-out shower curtain rods in their bathrooms, which seemed to reduce or eliminate the problem. Probably just a coincidence, though. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [February 9, 2015, 4:39am UTC](https://boards.straightdope.com/t/the-shower-curtain-question/711920/13 "2015-02-09T04:39:25Z")
**Askance** , deflected air is the cause of the airplane staying in the sky, but more air is actually deflected by the top surface of the wing than the bottom surface of the wing. Once the airstream separates at the leading edge of the wing, there is nothing that demands that the air packets that separated at the front reach the rear of the wing together. In fact, the air is pulled over the top faster, and curved downward. That pushes it down, which causes lift. [Bernoulli’s principle](http://en.wikipedia.org/wiki/Bernoulli's_principle) says that if the airflow is faster, the pressure is less. Try reading this about [aerodynamic lift](http://en.wikipedia.org/wiki/Lift_(force)). ---
**Author:** ![Mr\_Swordfish](https://avatars.discourse-cdn.com/v4/letter/m/b38774/32.png) [@Mr\_Swordfish](https://boards.straightdope.com/u/Mr_Swordfish)\ **Post date:** [February 10, 2015, 9:38pm UTC](https://boards.straightdope.com/t/the-shower-curtain-question/711920/14 "2015-02-10T21:38:01Z")
> [@Askance](#): > > Yes, I know that’s the wrong explanation. But I don’t see what else Bernoulli might have to do with it either; is it not just that the air striking the bottom of the wing (as it’s tilted into the airstream) pushes it up, like holding your hand flat out a car window? Bernoulli’s principle is a small part of the big puzzle that is aerodynamics. It’s involved, but not in the way that the popular incorrect explanation describes it. Think about it this way: when a fluid flows from a region of high pressure to a region of lower pressure there’s more pressure behind than in front. That exerts a net force on the fluid that causes it to speed up. Bernoulli’s equation quantifies this idea. The only way that a plane can stay in the air with gravity pulling down on it is for there to be a pressure difference between the bottom and top of the wings, with lower pressure above. When the air flows through this region of lower pressure it speeds up. Bernoulli’s principle doesn’t explain why there’s lower pressure on top of the wing, only that this lower pressure is accompanied by a higher speed. Bernoulli’s principle concerns itself with pressure changes _parallel_ to the air flow so it’s not a very good explanation of why there are pressure differences _perpendicular_ to the airflow. Fortunately, there is a nice formula for that which was discovered by Euler around the time Bernoulli published Hydrodynamica: **dp/dz = rho x v^2 / R** dp/dz is the pressure difference in the vertical direction (or differential if you want to be more precise) , rho is the density of the air, v is the velocity, and R is the radius of curvature. This formula says that whenever a fluid follows a path that is curved there is a pressure difference with higher pressure on the outside of the curve and lower pressure on the inside. Faster flow or tighter curves make for bigger pressure differences, and for straight flow (R-\>infinity) there is no pressure difference. Air flowing past an airplane wing is deflected downward and follows a curved path, this is associated with pressure differences according to the formula above. (you could also cut to the chase and use Newton’s laws to say that since the air is forced down by the wing the wing must be forced up by the air, skipping the discussion of pressure altogether - it depends on what you want to emphasize). This downward deflection / curved path occurs both above and below the wing, so it’s not _just the air striking the bottom of the wing_. The curved airflow above the wing contributes strongly to the lift. Returning to the shower curtain, the vortex in the stall means air is flowing in a curved path. That means there is lower pressure on the inside of the vortex and this lower pressure “sucks” the shower curtain inward. I think we all know that a vortex has lower pressure on the inside, the formula above just quantifies it. ---
**Author:** ![Askance](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/askance/32/8281_2.png) [@Askance](https://boards.straightdope.com/u/Askance)\ **Post date:** [February 11, 2015, 5:21am UTC](https://boards.straightdope.com/t/the-shower-curtain-question/711920/15 "2015-02-11T05:21:28Z")
OK, this I understand > **[Lift (force)](https://en.wikipedia.org/wiki/Lift_(force)#mediaviewer/File:NASANewtons3rdGlennResearchCenter.gif)** > > When a fluid flows around an object, the fluid exerts a force on the object. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the force parallel to the flow direction. Lift conventionally acts in an upward direction in order to counter the force of gravity, but it is defined to act perpendicular to the flow and therefore can act in any direction. > If the surrounding fluid is air, the force is ca... Not only does the air smacking into the underside of the tilted wing push it up, but the shape of the wing throwing air downwards produces a Newtonian reaction force upwards. > [@](#): > > Bernoulli’s principle doesn’t explain why there’s lower pressure on top of the wing, only that this lower pressure is accompanied by a higher speed. So I’m right- Bernoulli has nothing whatsoever to do with the plane staying up. It purely describes the pressure-speed relationship of the airflow, which in and of itself has nothing to do with creating lift. The lower pressure on top of the wing creates some lift, AND Bernoulli describes why that air also speeds up - but that speeding up is irrelevant to the production of lift. So Cecil’s statement > [@](#): > > … the Bernoulli effect, the well-known principle that explains how airplanes fly: as the velocity of a fluid increases, its lateral pressure decreases is flat-out wrong. Bernoulli happens, but as a side-effect of phenomena that also cause lift, it is not the cause of lift itself. ---
**Author:** ![Mr\_Swordfish](https://avatars.discourse-cdn.com/v4/letter/m/b38774/32.png) [@Mr\_Swordfish](https://boards.straightdope.com/u/Mr_Swordfish)\ **Post date:** [February 11, 2015, 6:53pm UTC](https://boards.straightdope.com/t/the-shower-curtain-question/711920/16 "2015-02-11T18:53:45Z")
_Bernoulli has nothing whatsoever to do with the plane staying up._ While I think your explanation is basically right, there’s more than one way to (correctly) explain lift. You don’t need Bernoulli’s principle to explain how a plane flies, but saying it has nothing whatsoever to do with it is over-stating things. As I said earlier, Bernoulli’s principle is a small part of a large puzzle. Here’s where it fits in: The more or less standard treatment in aerodynamics texts is to start with conservation of momentum and conservation of energy at the infinitesimal level and model the airflow with differential equations. The result is the Navier-Stokes equations, which are unfortunately damn near impossible to solve. But if you make some simplifying assumptions and approximations they are. The solution is a vector field that indicates the speed and direction of the air at each point in space. Once you have that, you can use Bernoulli’s equation to calculate the pressure at each point on the wing, and if you add up all the pressure you’ll get the lift. That’s more or less how it’s taught to engineers, with Bernoulli’s equation being the next-to-last step in a long chain of mathematics. Once you know the velocity distribution of the air in the vicinity of the wing, Bernoulli’s equation tells you the pressure distribution. But determining the velocity distribution is no small task. My view is that this is more of a convenient trick that facilitates the calculations and not all that important unless you are actually doing the calculations. All the important physics is modeled early in the process when you write the differential equations - the rest is mostly just a bunch of math. “The wing pushes the air down, so the air pushes the wing up” is sufficient for most people. If you want to discuss pressure, noting that curved airflow implies pressure differences with lower pressure on the inside of the curve. Dragging Bernoulli into it without also discussing the massive amount of math needed to get to the point where his equation can be applied almost always results in a nonsensical explanation that defies the laws of physics. But that doesn’t invalidate his equation, or mean that it can’t be part of a careful (if complicated) explanation.