At some point, most of us have heard that water spins down a drain in different directions depending on which hemisphere we happen to be in. The fact is, the Coriolis force (an apparent force as a result of the Earth’s spin) has virtually nothing to do with which direction water spins as it empties down a drain. Although this force is “real” and does have an affect on other large, long-lived systems that travel great distances, water draining from toilets, sinks, and bathtubs are rendered 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. 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 become unnoticeable.
So why does water spin down a drain? Is the Coriolis Force causing the water to spin? Let’s look at some other things first. As a demonstration, imagine that a merry-go-round represents the entire surface area of either the northern or southern hemisphere of the Earth. If you start in the center of the merry-go-round and walk towards the edge as it’s spinning, you will start to feel a deflection. You will find yourself drifting to one side or the other (depending on which way you spun the merry-go-round).
Now, if something moved a comparably equal distance in one of the Earth’s hemispheres (such as a hurricane does), it too will be deflected. The merry-go-round is an exaggeration because nothing moves over a comparable distance on the Earth in the same amount of time, but you start to see how insignificant and small a sink drain is in comparison to the Earth.
Hurricanes and pressure systems move hundreds of miles over a period of days, and they are most certainly affected by the Coriolis force. In a low pressure system, air flows towards the center and as it does so, it is deflected to the right in the northern hemisphere and to the left in the southern hemisphere. If the Earth were to stop rotating, there would be no deflection and the low pressure system would quickly equalize as the air rushed towards the center. However, on a rotating Earth the air keeps getting deflected away from the center which causes the low pressure system to spin counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere.
In a high pressure system, air flows out from the center. In the northern hemisphere, the deflection is to the right which causes the system to spin clockwise. In the southern hemisphere, the deflection is to the left which is why a high pressure system spins the opposite direction. The fact that the Coriolis force is “zero” at the equator is the reason why no hurricanes cross from one hemisphere to the other.
As for water emptying down a drain, the amount of deflection produced by the Coriolis force is a couple orders of magnitude too small compared to other influences. It’s not that the Coriolis force doesn’t exist, it is just too weak to have any influence on such a small amount of water in an 18″ toilet bowl or sink as compared to a hurricane which is hundreds of miles across and moves thousands of miles. The only things that affect which way the water empties down a drain is the shape of the container it’s draining from, the way the water was introduced into the container
So in the case of a toilet, take a look and observe which way the water jets into the bowl and see if your toilet bowl is perfectly round. Chances are the water enters at an angle, no matter how slight, and the bowl itself is elliptical. All you need is a small rotation of water spin at the start and as the water empties towards the bottom of the toilet/sink, it will begin to rotate faster due to the narrowing of the drain (same principal as when an ice skater pulls their arms in towards their body; they spin faster).
So the next time a you’re at the equator and there’s a demonstration to prove which way water spin down a drain, ignore it. If you can’t help yourself, watch how they intriduce a tiny amount of spin to the water or the object they put in the water.
Some other things affected by the Coriolis force are artillery shells and long range sniper rifles. The affect is much smaller than a hurricane, but still needs to be taken into consideration so the shells hit the right target. A 2 foot difference could have disastrous results for a sniper. Ballistic missiles must really take the Coriolis force into consideration because they will 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, about 10% of tornadoes spin clockwise while about 90% spin counter-clockwise in the northern hemisphere. However, the mesocyclones, supercells, and low pressure systems which are responsible for spawning tornadoes are affected by the Coriolis force, which in turn tend to bias tornado genesis.