Have you ever noticed the water in your toilet moves on windy days? In many homes in the US, 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 waste-water drain smoothly and keeps 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 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.