[Cumulonimbus]

Most people have been led to believe that water drains in opposite directions in the Northern and Southern Hemispheres. For this to be the case, the Coriolis Force would need to have a major influence on the draining water. However, for the Coriolis Force to be important, the flow (be it air or water), would have to persist for a significant fraction of the day, at least a few hours. Does this sound like your sink or bathtub? Probably not.

Therefore, the answer to this age old question is as follows: the water draining in a tub or sink drains in either direction in either hemisphere because it is influenced more strongly by other forces, such as the centrifugal force, than by the Coriolis force. It does the same at the Equator, as well. Water goes down the drain too quickly and the sink or tub basin is far too small for the Coriolis force to have an appreciable effect.

This point can be made even more dramatically throught the use of some simple mathematics. Meteorologist quantity which forces are important in a weather system by calculating their ratios. The ratio of the centrifugal force and the Coriolis force is a special quantity known as the Rossby Number, abbreviated to (Ro) in the equation that follows and named after the famous pioneer of research into atmospheric motion, Carl-Gustaf Rossby:

where U is the speed of the system in question, f is the Coriolis Parameter, and L is the spatial size of the system.

When the Rossby Number is very small, the Coriolis force is much larger than the centrifugal force. In these cases, wind and water motions change directions depending whether you are in the Northern or Southern Hemisphere.

When the Rossby Number is very large, the Coriolis Force is much smaller than the centrifugal force. In these cases, wind and water motions are not "affected" by the Coriolis force and spin in which ever direction other forces may dictate.

Now, let's do the key calculation: What is the Rossby Number for your sink or bathtub? The size of this number will tell us if water draining in your sink switches directions across the equator due to influence from the Coriolis force (Large number = no, Near-zero number = yes).

To make this calculation, let's come up with some ballpark estimates for the speed of the draining water and the size of a sink. Let us assume a standard N. Hemisphere mid-latitude Coriolis Parameter of .0001 per sec, a sink basin size of 1 meter (large) and a draining speed of 1 meter/sec (fast). Using these numbers, we find:

Ro= U/fL = (1 m/sec)/(.0001 1/sec×1m) = 10000

The Rossby Number for your sink is 10000! Clearly this number is much larger than zero. This estimate tells us with mathematical certainty that the Coriolis force is nowhere close to being a factor in the direction water spins in your sink, so water in your sink can spin in either direction in the Northern or Southern Hemisphere. In reality, the direction of drainage is dependant on the shape of the sink/tub/toilet basin and how the water is directed into it.

[KandaPanda]

fuck, i knew it!

[peterdragon]

someone likes waxing intelligence.

[Lion-hearted Girl]

Cbus! Thank you, I have wondered this for a long time, and have tried to investigate when I've been travelling but I never did get a satisfactory answer! There is actually no end to your awsomeness, is there? You rock!

[Lion-hearted Girl]

Originally Posted by peterdragon ↗

someone likes waxing intelligence.

Hey peterdragon? Eat shit and die. Seriously, please do, you fucking moron.

[Cumulonimbus]

Thank you, LHG. I'm happy I was able to answer that for you, my friend :)

The professor who taught me about this was an Aussie named Jenni Evans. She had a real pet peeve about resolving misinformation regarding weather systems in different hemispheres.

[Seven Of Nine]

Do you have any idea how glad I am that we have met here on DR? You've offered me a great deal of insight in to so many things I can't help but want to hug and squeeze the stuffin' outta you ...;) Hope your day is as wonderful as you!

[Cumulonimbus]

Thank you so much, WA. You are a wonderful friend, and it has been my pleasure to share this with you :)

[peterdragon]

Ok Cumulonimbus, since this thread, i have stumbled upon many other post by you. As an engineer, i appreciate what you participate to this site. I apologize, and I retract my smart ass comment. No hard feelings to you or any others. again, i was wrong to respond so harshly.

[Nite]

Originally Posted by LHG ↗

Hey peterdragon? Eat shit and die. Seriously, please do, you fucking moron.

someone has their pissy pants on to-day