Windy Curtain

In continuation with the previous article (Spooky Doors), we're developing the theme of weird phenomena in the bathroom. Today, we'll talk about something that has famously become one of mankind's most hated pet peeves. While enjoying a calm and peaceful shower, the curtain has been known to billow in and attack people. If you're one of the few to have been blessed with a glass door shower, refer to Figure 1 for a better understanding. Today's article will explain why this happens.

I'd like to preface the explanations with the fact that an exact reason for this phenomenon is not agreed upon. There are many theories, and whether it's one of them or a mixture of them is for you to decide. In this article, I'll describe the three prevailing theories on the reason for this phenomenon.

Theory 1 (Convection Currents): This theory gives a pretty simple explanation for the shower-curtain effect. We assume that the person is taking a hot shower. As a result, there is warm air inside the shower, whose molecules are more excited and spread farther apart. In contrast, the air near the ground outside the shower is cold. The molecules here are relatively tight and packed together.

These conditions create what we call convection currents, in which warm air rises and cold air sinks. When the warm air inside the shower goes up, the cold air from the ground is pulled in to fill the space (Figure 2). As the cold air is pulled in, the curtain is trapped in between and pulled towards the person in the shower. While this is a viable explanation, we know that it can not be the only factor because the shower-curtain effect remains even in a cold shower.

Theory 2 (Bernoulli Effect): The Bernoulli effect describes the variation of air pressure due to the speed of the air's movement. This is best explained using an example: lift of an airplane. The airplane's wing is shaped such that the top of the wing has a greater curve, making the air across the top move faster (Figure 2). Bernoulli's equation describes the energy of fluids (air is also classified as a fluid). The equation states that the faster the air moves, the lower the air pressure will be (Figure 3). As a result, the air at the top of the wing has a lower pressure than the bottom of the wing. Naturally, high-pressure zones will push towards low-pressure zones, creating what we know as "lift".

The same concept applies in the shower. Inside, the stream of water pushes the surrounding air, increasing its speed. The faster-moving air inside the shower has lower pressure than the air outside the shower. As a result, air tries to move in towards the shower from outside, bringing the curtain along with it.

Theory 3 (Horizontal Vortex): This is the most prevailing theory at the moment. It's derived from research by David Schmidt at the University of Massachusetts. He simulated sprays for industry purposes, specifically for automotives. In his research, Schmidt created a computer model of a shower, which acts very similar to a spray. He found that the shower creates a horizontal vortex. The vortex is similar to a hurricane: the center of the vortex is a low-pressure zone. This low-pressure zone attempts to balance out with the normal air pressure outside of the shower. Once again, the shower curtain is pulled in towards the shower as a result of a pressure variation.

With three different reasons for this effect, one may be afraid that the shower-curtain effect would be impossible to overcome. Don't fret. There are many possible solutions to fix this. One common workaround is the use of suction cups. Many shower curtains will have suction cups that can stick to the side of the tub to exert a restraining force stronger than that of the above reasons. Similarly, other shower curtains sport magnets at the bottom that can stick to the side of the tub. All in all, the shower-curtain effect is no match for humanity's ingenuity.