Why Does Toilet Water Move When It's Windy Outside?

Reader Erin from Baltimore wrote in wondering why the water in her toilet bowl moves on windy days. This is one of those questions – like the one Jason asked about his dog’s popcorn-scented paws – that makes me pause and wonder if I’m getting trolled, or if I’m missing out on a strange phenomena that everyone else seems to be aware of.

So, I did a little Googling and even hung out in the bathroom staring at the toilet bowl one morning (yes, folks, this is the exciting life of a science writer). Turns out Erin isn’t messing with me. Perhaps I'm one of the last people on Earth to realize toilet water moves with the breeze. Way to go, Soniak.

So, what causes this commotion in the latrine?

In many homes (at least in the U.S.), part of the plumbing system is a pipe that runs up and out to the roof. This outlet, called a “vent stack,” allows sewage gases to vent outside instead of through the toilet, sink or tub—which would make the house reek. The stack also allows air to move through the pipes, which makes wastewater drain smoothly and keeps obnoxious gurgling to a minimum.

When the wind blows over the vent stack outlet on the roof, the air pressure in the pipe is lowered. This is Bernoulli’s principle (“as the speed of a moving fluid increases, the pressure within the fluid decreases”), the same thing that gives airplanes their lift, in action in your bathroom. The lowered pressure in the pipes creates a slight suction effect throughout the plumbing system, pulling on water in the toilet below. As the wind kicks up and dies down, the suction gets stronger and weaker, and the water in the bowl sloshes around accordingly.