Contrary to popular belief
, the Coriolis effect has almost no effect on the direction in which water rotate
s as it drain
s from your sink or toilet
The reason it affects the rotation of storms is due to the massive size of weather systems. A hurricane, for example can be several hundreds of kilometers in diameter. This means that one end of the storm is significantly closer to the equator to the other. Because of the fact that the surface of the Earth is moving faster at the equator than at areas to the north or south of it, large systems such as hurricanes tend to rotate. The direction of the rotation is counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. As you can no doubt see, the distance to the equator does not differ significantly between the north and south ends of your typical sink. The Coriolis effect, while present, is immeasurably small in this case.
The direction in which the water rotates is governed by much stronger local variables, such as the shape of the sink and the direction from which the water is flowing into it. Ditto for the toilet. You can prove this to yourself in a large public rest room. If you fill several similar sinks with water and then pull the plugs on all, chances are that in at least one the water will be rotating in a different direction than in the others.