shower scum

Shower-curtain effect

In physics, the shower-curtain effect is the phenomenon in which a shower curtain gets blown inward with a running shower. The problem of the cause of this effect has been featured in Scientific American magazine, with several theories given to explain the phenomenon but no definite conclusion.


Buoyancy theory

Also called Chimney effect or Stack effect, observes that warm air (from the hot shower) rises out over the shower curtain as cooler air (near the floor) pushes in under the curtain to replace the rising air. By pushing the curtain in nearer to the showeree, the (short range) vortex and Coanda effects become more significant. See also Cooling tower

Bernoulli effect theory

Bernoulli's principle states that an increase in velocity results in a decrease in pressure. This theory presumes that the water flowing out of a shower head causes the air through which the water moves to start flowing in the same direction as the water. This movement would be parallel to the plane of the shower curtain. If air is moving across the inside surface of the shower curtain, Bernoulli's principle says the air pressure there will drop. This would result in a pressure differential between the inside and outside, causing the curtain to move inward. It would be strongest when the gap between the bather and the curtain is smallest - resulting in the curtain trying to wrap you when you get close to it.

Horizontal vortex theory

One computer model of the typical bathroom found that the spray from the shower-head drives a horizontal vortex. This vortex has a low-pressure zone in the centre, which sucks the curtain.

David Schmidt of University of Massachusetts was awarded the 2001 Ig Nobel Prize in Physics for his partial solution to the question of why shower curtains billow inwards. He used a computational fluid dynamics code to achieve the results. Professor Schmidt is adamant that this was done "for fun" in his own free time without the use of grant or other public monies.

The Coandă Effect

The Coandă Effect, also known as "boundary layer attachment".


A hot shower will produce steam that condenses on the shower side of the curtain; lowering the pressure there. In a steady state the steam will be replaced by new steam delivered by the shower but in reality the water temperature will fluctuate and lead to times when the net steam production is negative.


  • Ensure the roof of the shower enclosure is enclosed rather than mimicking the operation of a commercial cooling tower. By preventing the very inefficient escape of warm humid air, and thus retaining the comfortable warm humid showering atmosphere, the enclosure fills, (like a hot air balloon)and the shower enclosure is prevented from behaving like a thermal chimney. This measure in turn conserves comparatively vast quantities energy and water. Of course the shower curtain will billow outwardly unless a cold shower is being obtained and the surrounding air is warmer, and no billowing if the inside showering temperature and outside temperature are at an equilibrium.
  • Use a shower curtain with magnets strung along the bottom that affix to the side of a metal bathtub.
  • Use of a second curtain, that remains outside of the tub/rim during the shower to block or slow inflowing air.
  • Leaving the curtain partially open with a gap at one or both sides to allows for pressure equalization, decreasing air inflow.
  • Application of heat outside of the shower to minimize buoyancy effects.
  • Lowering of water pressure and/or temperature differential to decrease billowing.
  • Use of a curved shower curtain rod, commonly found in some hotels. The rod itself has a bend in it to hold the shower curtain out and away from the person.
  • Application of a cloth, soaked in water, along the rim of the bathtub while inside the shower to secure the bottom of the curtain, with cloth saturation (weight) and length corresponding to an increase in effectiveness.
  • Install sliding doors instead.

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