Why do shower curtains billow inward?
The short answer
Shower spray moves the air around it and can create a circulating flow beside the curtain. When the pressure on the shower side becomes lower than the pressure outside, the surrounding air pushes a light curtain inward.
In 30 seconds
Water from a showerhead does not simply fall through still air. Friction and drag between the droplets and the air move some of that air along with the spray. This can create a circulation, or vortex-like flow, around the shower. Pressure near the center of that flow can become lower, so the higher pressure outside pushes the curtain toward the water.
Air has pressure
Air is invisible, but its molecules are moving and pushing against walls, people, and curtains. That push is called air pressure. If the pressure is the same on both sides of a curtain, it has no strong reason to move one way.
If the pressure becomes slightly lower on one side, the stronger push from the other side moves the curtain. So although the curtain may seem to be “sucked” toward the shower, it is more accurate to say that the pressure outside pushes it inward.
Spray moves the air
Droplets leaving the showerhead travel through air. As they transfer force to the air, nearby air begins to move with them. Because the spray continues, this airflow is continuously driven rather than being a one-time gust.
A fluid simulation of a typical shower showed that the spray can drive a vortex-like circulation near the curtain. The center of that circulation has a lower-pressure region, which provides a way for the airflow to pull a light curtain inward.
Is hot water the cause?
Hot water can add another effect. Air warmed by the shower tends to rise, and cooler air may move in below it. This circulation can also affect the curtain.
But hot water is not the whole explanation. A curtain can move inward with cold water too. The force from the spray, the shape of the bathroom, ventilation, and the position of the door all influence the airflow.
It is not just Bernoulli’s principle
People sometimes explain the effect using Bernoulli’s principle: faster-moving fluid can be associated with lower pressure in the right conditions. This principle is useful for thinking about fluid flow and pressure.
However, a shower includes both air and water droplets, as well as walls and a flexible curtain. The real motion is a combination of airflow, force transferred from droplets to air, temperature differences, and ventilation. Applying Bernoulli’s principle alone does not explain every bathroom in the same way.
Why does it vary from one bathroom to another?
The force is fairly weak. A thin, light curtain is easier to move, while a heavy curtain or one with weights along the bottom is harder to move. A showerhead with weak flow may produce little visible movement.
The bathroom’s size, the distance between the spray and the curtain, an exhaust fan, and an open door or window can also change the airflow. That is why the same kind of curtain may billow in one bathroom but remain almost still in another.
A common misunderstanding
“Hot air rises, so the curtain must always move inward” is incomplete. The curtain can move with cooler water, and the motion of the spray and the air it drives is important. The water is not mainly pulling the curtain directly; a pressure difference in the surrounding air is doing most of the pushing.