this post was submitted on 29 Apr 2025
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Well, essentially, it's that the coriolis effect, while a real thing, is much weaker than most other factors in play. If everything else is neutralised or near to it, the coriolis would indeed be the remaining decider, but that's very unlikely in practice.
The coriolis effect has nothing to do with this. The coriolis 'force' is not a real force, it's just the product of things trying to move in a straight line on a rotating surface which to observers on that surface looks like a curve which implies a accelerating force. Usually this applies to things flying through the air, because the are moving independent from the ground. Something that is not a force can not influence something like the water in a thub.
What people confuse the coriolis force with is the centrifugal force of the earth's rotation. But this force increases radialy but is tangetialy evenly distributed, which means it's symmetrical so it doesn't matter which hemisphere you're in. It doesn't point 'left' or 'right' it only points 'out' or 'up'. Unless you're right on one of earths rotational axis none of those effects matter.
Otherwise people on the equator, their toilet water wouldn’t spin at all? It would just go straight down, no spin.
it spins both ways at the same time
Water swirls because it's the course of 'least action' for the draining process. It creates a laminar and continuous volume exchange through a hole. Pipes are usually filled with air which has to be exchanged with water to drain. Physics just optimizes itself to be as efficent as possible with this. You can't have a perfect draining surface so so currents in one direction will always be a little bit stronger that in the others. Gravity applies a constant acceleration, so this small difference in initial direction will be amplified over time creating a swirl.
In case of a toilet though, the water is already introduced in a swirl during flushing. So none of the above even matters.