At some point in our lives, most of us have heard that water spins down a drain in different directions depending on which hemisphere we’re in. The fact is, the Coriolis force has virtually nothing to do with which direction water spins down a drain.
The Coriolis force is “real” and does have an affect on large, long-lived systems that travel great distances. However, water draining from toilets, sinks, and bathtubs are virtually immune from its affects. The Coriolis force is much too weak to have any affect on such small, short-lived rotating bodies of water under normal conditions.
The Coriolis force is an “apparent force” meaning it doesn’t really exert a force on any object. It just looks like it’s applying a force on objects traveling great distances. Objects “freely” in motion relative to the Earth, such as air and water, have the appearance of being deflected (if you want a more detailed tutorial, visit this site).
At the equator, the Coriolis force is non-existent. North of the equator (northern hemisphere) the deflection is to the right. South of the equator (southern hemisphere) the deflection is to the left. Without getting into the complicated math and physics, this deflection is a result of the Earth’s rotation and has different magnitudes depending on how close or far you are away from the poles and the equator. As you move closer to the equator, the Coriolis force weakens and becomes unnoticeable. When you cross over the equator, it reverses.
This is the reason why hurricanes can’t cross the equator. They’d have to stop and then spin in the other direction.
Sinks and Toilets
The deflection produced by the Coriolis force in a sink or toilet is a couple orders of magnitude too small. It’s not that the Coriolis force doesn’t exist, it is just far too weak to affect water going down a 2″ sink drain or a 4″ toilet drain over 10 seconds. This is in stark contrast to a hurricane which is many hundreds of miles wide traveling thousands of miles over a period of weeks.
What does affect the direction of spin is how the water jets into the bowl. Chances are the water enters at an angle. And one the water started spinning, it will accelerate as it empties down the drain. The same principal applies to an ice skater who pulls their arms in towards their body as they spin; they spin faster. The shape of the toilet and sink is also important as it can bias the initial direction of the draining water.
If you ever find yourself at the equator and there’s a demonstration, watch how they introduce a tiny amount of spin to the water or the object they put in the water. It only takes a very small force to get things spinning in the direction you choose.
Other Objects Affected by the Coriolis Force
Some other things affected by the Coriolis force are artillery shells, long range sniper rifles, ballistic missiles, and ocean currents. The affect is much smaller than a hurricane, but still needs to be taken into consideration. A two foot difference could have disastrous results for a sniper. Ballistic missiles travel thousands of miles at great speeds. Ocean currents span the entire globe, and as a result they too are affected by the Coriolis force. Interestingly, a tornado is not affected by the Coriolis force. In fact, in the northern hemisphere, about 10% of tornadoes spin clockwise while about 90% spin counter-clockwise. However, the mesocyclones, supercells, and low pressure systems which are responsible for spawning tornadoes are affected by the Coriolis force.