At the equator – testing the Coriolis effect on water

In the Northern Hemisphere water runs clockwise down the drain. In the Southern Hemisphere water runs counter-clockwise (anti-clockwise) down the drain.

So what happens at the Equator where the Southern Hemisphere and the Northern Hemisphere meet?

Testing this at the Equator in Kenya, locals demonstrate the Coriolis effect of water going down a drain and a funnel.

It is true that north of the Equator, water does run clockwise.

It is also true that south of the Equator, water does run counter-clockwise.

And at the Equator, exactly on the Equator, water does not rotate either clockwise or anti-clockwise, and it runs directly down the drain or funnel.

In physics, for the clockwise rotation, the Coriolis force acts to the left of the motion of water, and for the counter-clockwise rotation, the Coriolis force act to the right. It is called the Coriolis effect due the the French scientist Gaspard-Gustave de Coriolis who, in 1835, recognized this effect on water wheels.

The Coriolis force causes moving objects on the surface of the Earth to be deflected to the right (with respect to the direction of travel) in the Northern Hemisphere and to the left in the Southern Hemisphere, such as with the movement of cyclones.

There are two reasons for this phenomenon: first, the Earth rotates eastward; and second, the tangential velocity of a point on the Earth is a function of latitude (the velocity is essentially zero at the poles and it attains a maximum value at the Equator). Thus, if a cannon were fired northward from a point on the Equator, the projectile would land to the east of its due north path. This variation would occur because the projectile was moving eastward faster at the Equator than was its target farther north. Similarly, if the weapon were fired toward the Equator from the North Pole, the projectile would again land to the right of its true path. In this case, the target area would have moved eastward before the shell reached it because of its greater eastward velocity.

At the Equator the forces are equal, and water runs directly down the drain – neither clockwise nor counter-clockwise.

MARTINA NICOLLS is an international aid and development consultant, and the author of:- Similar But Different in the Animal Kingdom (2017), The Shortness of Life: A Mongolian Lament (2015), Liberia’s Deadest Ends (2012), Bardot’s Comet (2011), Kashmir on a Knife-Edge (2010) and The Sudan Curse (2009).