In which direction is the water flowing

Do water eddies always turn in the same direction in the northern hemisphere?

If water is drained from a bathtub, a small vortex will eventually develop above the drain, in which the water rotates and disappears through the drain pipe. But how is it decided in which direction the water is moving? And does it always turn in the same direction in our area?

The answer is: no! Contrary to what many people think, bathtub vortices do not always rotate in the same direction in one hemisphere. Rather, it is a coincidence whether the water flows clockwise or counter-clockwise. The reason for this is the size of the eddies: they are far too small to be influenced by the rotation of the earth.

Coriolis force

Animation: Coriolis force effect

It is different with ocean currents or the air eddies of atmospheric high and low pressure areas. In the northern hemisphere, such eddies actually always rotate in the same direction and always opposite to those in the southern hemisphere. The reason for this is the so-called Coriolis force. Because of the earth's rotation, the earth turns away under the water or the air, which means that bodies are deflected perpendicular to their direction of movement and to the direction of rotation of the earth. As the earth rotates to the east, right-rotating water eddies and high pressure areas are created in the northern hemisphere. A low pressure area, on the other hand, rotates to the left, as the air flows from the outside in and from the bottom to the top in the vortex due to the pressure gradient. In the southern hemisphere, water and air rotate accordingly in opposite directions.

The influence of the earth's rotation is described by the Rossby number, a dimensionless number. It depends on the flow velocity, the rotation of the horizon plane and the length of the vortex. The larger the Rossby number, the smaller the effect of the Coriolis force on water or air currents. If one compares the values ​​of a low pressure area in the atmosphere with those of a water vortex in the bathtub, the value for the Rossby number of the bathtub vortex is about a million times larger than for the low pressure area. The influence of the earth's rotation on the water eddies in the bathtub is therefore very small.

Water in the drain

Video: water vortex in the drain

The direction in which the water drains is influenced more by how symmetrical the tub basin is, what residual movement the water has after filling and how the drain is shaped. If you let water in the bathtub or a wash basin, the water starts moving. Even if you wait a few minutes for the water to settle, it will retain a little bit of residual movement after the tap is turned off. This can be made visible with a little sprinkled coffee powder or other floating objects. If you pull the plug out of the basin, many small eddies continue to arise, which rotate in different directions. Eventually, a main eddy forms through which the water runs off. The direction of rotation of the main vortex depends on so many factors that it is more or less random. We tried it out in our editorial sink and recorded it on video.

Swirls of water in the bathtub can flow away to the right or left of any place on earth. There is no preferred direction. If the rotation of the earth had a decisive influence on the direction of rotation, the water would have to flow off in a straight line directly at the equator. However, such a phenomenon has never been observed.