# What is the Coriolis Force or Effect

The coriolis force (also called the coriolis effect) is a principle of physics whereby moving objects are deflected relative to a rotating reference frame.  A child throwing an object to another on a spinning carousel, for instance, will perceive it as curving leftward in flight if the carousel is rotating clockwise, and rightward if it is rotating counter-clockwise.

The term is derived from Gustave-Gaspard Coriolis, a French scientist who expressed the force mathematically in 1835 in a paper about the energy yield of machines with rotating parts.

The coriolis force is one of three forces that alter motion in a rotating reference frame compared to what it would be in an inertial reference frame.  The others are the centrifugal force and – if the rotation is not constant – the euler force.

The rotation of the Earth creates a coriolis force.  Moving objects in the northern hemisphere curve to the right compared to what their motion would be in the absence of the force.  Moving objects in the southern hemisphere curve to the left.

The coriolis force is relevant to meteorology, because it affects winds and currents.  Instead of flowing directly from areas of high pressure to areas of low pressure, winds and currents in the northern hemisphere flow to the right of this direction, and in the southern hemisphere they flow to the left.  The force has more effect the faster air is moving, and the farther it is from the equator.

The coriolis force, along with the pressure-gradient force, is one of the factors that determines the speed and direction of the spin of a storm, such as a hurricane.

There are many misunderstandings of the coriolis force.  Perhaps the most common is the notion that water will circle down a drain in a different direction depending on whether it is north or south of the equator.  This is a myth.  The effect the coriolis force would have on something like water in a drain is too miniscule to even be measured, and it is overwhelmed by the other forces causing motion in the water, stemming from how it was filled and how it has been disturbed since.  In other words, if you wash your hands in the sink, the way your hands happened to move in the water will determine whether the spin down the drain gets started clockwise or counter-clockwise.  The coriolis force created by the rotation of the Earth is too trivial to enter into it.  It is only noticeable for large-scale motions such as winds.

But there are other even more fanciful claims.  The Clark Gable character in the movie “Adventure,” for instance, asserts that the ringlets in a woman’s hair will curl in one direction in the northern hemisphere and in the opposite direction in the southern hemisphere!  One hopes this was intended facetiously by the filmmaker, but needless to say, it’s nonsense.