Water Drain Rotation

Question:

I understand that the flow of water is different in the northern & southern hemisphere's. For example, water drains in a clockwise direction in the northern and hemisphere and counter clock wise in the south. Can you explain this phenomenon ? Is it related to the correalis effect ?

Thank You

Answer:

First a couple corrections to your question. In the northern latitudes the rotation would be counterclockwise. Correalis is actually Coriolis . (Named after the French mathematician Gustave Gaspard Coriolis 1792-1843).

Consider a basin filled with still water on a non-rotating planet. If we opened a smooth sided, perfectly symmetrical drain in the bottom of the basin the water would flow straight in from the sides to the location of the drain to replace that water which fell out the hole.

Now if we conducted the same experiment but had some initial rotary motion of the water in the basin so that the water as a whole has some angular momentum about the drain axis, the path of a water molecule coming in from the edge of the basin to the hole would be a spiral wrapping around the drain in the direction of the initial rotation. And, as the moment of inertia of the water mass decreases due to more of it residing in the drain pipe, the angular velocity of the remaining mass increases as required by the conservation of angular momentum, making the rotation more noticeable.

Next let's transport our basin to the surface of the Earth in the northern latitudes. There if we start with no initial motion between the basin and the water, is there any angular momentum relative to an inertial reference frame? Well... yes. The basin and its water complete one revolution counterclockwise each 24 hours. In principle this will cause a counterclockwise vortex in the draining water. This effect is stronger, the farther north we go, being maximum at the north pole.

The problem with this is that this rotational effect is very weak. It is easily overcome by other effects like the residual motion left from filling the basin or convection currents coming from temperature changes in the fluid. In practice you must have a very large, stable basin and wait for a loooong time after filling before you can get consistent results.

The water motion described here is a manifestation of the Coriolis effect.

Regards,

JDJones