The Coriolis effect
Which way does the water drain from your sink? Or if you care to watch, from your toilet? Clockwise or anticlockwise? Here’s the thing - it has absolutely nothing to do with which hemisphere you live in!!!
I’m not sure where this myth - that water spirals out the drain one way in the Northern Hemisphere and the other way in the Southern Hemisphere - first originated, but I do know that I come across lots of students who believe it. So it’s time to set the record straight.
Is there an effect that could be actual real science going on here? Oh yes, of course. It’s called the Coriolis effect. The thing is, it’s only observable on a large scale.
If the Earth were a non-rotating planet, winds and currents would tend to flow from an area of high pressure to an area of low pressure. They would flow in a nice straight line. However, because the planet does rotate, moving objects appear to veer to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere.
This is the Coriolis effect - the apparent deflection of moving objects when they are viewed from a rotating reference frame (such as the Earth).
There are two reasons for the Coriolis effect to be, well, an effect. Firstly, the Earth rotates eastward (that’s why the Sun rises in the East and sets in the West). Secondly, the tangential velocity of a point on the Earth is a function of latitude. This is a funky way to say that you are moving faster at the equator than you are at the poles. Tangential velocity is the speed at right angles to the surface of the Earth. It is essentially zero at the poles (because all they’re doing is spinning on the spot, 360° over the course of a day - technically a little more than 360° but we won’t get into that now) and has a value of 1674 km.h–1 at the Equator.
So let’s say you go old school and set up a cannon - one capable of firing a cannon ball a really really long way. If you fire it northward from a point on the Equator, the cannon ball would land to the east (to the right) of its due north path. It would appear to be deflected to the right. This is because the cannon ball was moving eastward faster (being at the Equator) than its target in the north was. If the cannon was fired toward the Equator from the North Pole, the cannon ball would again land to the right of its intended target. This time the target area would have greater eastward tangential velocity, and would have moved eastward before the cannon ball reached it. Keep in mind the cannon ball would have flown in a straight line in both cases, and would only have appeared to veer off to the side.
The same situation can be applied to the Southern Hemisphere, except in this case the cannon ball would appear to be deflected to the left.
So you see, the Coriolis deflection is related to the motion of the object, the motion of the Earth, and the latitude. It is proportional to the rotation rate, and because the Earth completes only one rotation per day, the Coriolis effect is actually quite small. It becomes noticeable only for motions occurring over large distances (hence your cannon would need to be able to fire a cannon ball a really really long way to observe the effect) and long periods of time (cannon balls take a while to arrive at their destination).
Large-scale movement of air in the atmosphere? Water in the ocean? Yes, they can be affected by the Coriolis effect. But the direction in which water drains from your sink/toilet? Nope, blame that on the shape of the basin/bowl, in addition to the angled placement of any water jets etc.