Blowin' in the Wind: Coriolis Force

The Coriolis force is a “fictitious” force, unrelated to interactions between particles or between particles and radiation. The Coriolis force is included in Newton’s Second Law (F=ma) to account for the acceleration of Earth’s frame of reference. (Another example of a “fictitious” force is the force you feel pushing you outward when you are riding in a car speeding around a curve.)

As Earth turns, the ground, and the air above it, has an easterly component of velocity due to the angular velocity of the surface (see diagram). This easterly component is an initial velocity for moving objects. In the northern hemisphere, wind blowing from south to north has the same initial velocity as the ground initially beneath it, but as it moves north, the ground beneath has less easterly angular velocity (the distance to Earth’s axis is less), so the wind curves to the right. In the same way, winds blowing from north to south in the northern hemisphere curve to the left. Although it’s not obvious, objects moving to the east or west experience the Coriolis force too—to learn more, see the first Link.

The arrows show the initial velocities of objects moving through the atmosphere, and of the ground as well, at the equator and a location in the northern and southern hemispheres. Since an object moving north from the equator has a larger initial eastward velocity than the ground to the north of it, the object will curve to the right in the Earth frame of reference. An object moving south from the equator will curve to the left.

Adapted from a drawing in “Getting Around the Coriolis Force” by Dave Van Domelen, Kansas State University

The Coriolis force is also responsible for bending the paths of hurricanes, typhoons and cyclones in opposite directions in the northern and southern hemispheres, as shown on the map. Note from the map that these storms do not form at the Equator, because the Coriolis force there has no horizontal component.

Curvature of the tracks of hurricanes, typhoons, and cyclones worldwide shows the opposite effect of the Coriolis force in the Northern and Southern hemisphere (image courtesy of SSAI/NASA-GSFC and Hal Pierce)