What is the Coriolis Effect?

Most people, if asked about the Coriolis effect, would probably say that it had something to do with the direction that water swirls down the sink or in a toilet. The basic principle is related, in that it involves rotation, but the truth is slightly different. The Coriolis effect works on a much larger scale.

Named for Gaspard-Gustave Coriolis, the French scientist who described the effect in an 1835 paper, the Coriolis effect is commonly defined as the apparent displacement, or movement, of an object from its path due to the rotation of the frame of observation. In this instance, the frame of observation is generally considered to be the Earth, although it can be any rotating body. The key word to consider here is “apparent.” The Coriolis effect does not actually move an object, nor does the effect depend on an outside force. At its most basic, the Coriolis effect can be said to be caused by inertia, or the tendency of an object to stay in the state of rest or motion it is already in.

To get an idea of how the Coriolis effect works, imagine a butterfly on a beach ball. The butterfly is sitting at a point near the top of the ball, and decides to fly to a little speck of pollen stuck on the horizontal center line of the ball, or the equator. If the ball is not moving, the butterfly will travel in a straight line to the pollen. However, if the ball is rotating, the butterfly will fly toward the pollen in a straight line, but by the time it gets to where the pollen was, the ball’s rotation will have moved it and the butterfly will appear to have taken a curved path. In actuality, the butterfly’s path was straight, but an observer watching the butterfly will see a curved path relative to the ball, which is rotating. This is the Coriolis effect in action.

The shift of an object’s path caused by the Coriolis effect depends on the position of the object relative to the rotating body. In Earth’s Northern Hemisphere, the Coriolis effect shifts objects to the right. In the Southern Hemisphere, objects shift to the left. Since these shifts are related to the rotation of the observation frame relative to the object, i.e., the Earth’s rotation, differences in latitude, or distance from the equator as measured along an imaginary line at right angles to the equator, can make a difference in the observed effect. This is due to the fact that the Earth’s rotational speed changes depending on how far from the equator the measurement is made. The speed of the object being observed also affects the observed displacement.

A number of scientific disciplines make use of the Coriolis effect and its permutations. Meteorology, or the science of atmospheric behavior and observation, takes the Coriolis effect into account in studying hurricane formation and movement, while astrophysicists, or scientists who study stars, see it in studying sunspots and other stellar phenomena. Navigators and gunners have to factor it into calculations, as do pilots. Any system that utilizes a rotating frame of reference will have to account for the Coriolis effect in one fashion or another.