Coriolis Force

Because Earth spins on its axis, the Coriolis force bends wind right or left from the direction of its flow. The Coriolis force, therefore, causes wind to deviate from a straight path. If Earth did not spin on its axis, wind would blow following the Earth’s curvature, with no deviation. A wind blowing from south to north would not deviate northeast or northwest. In 1835, French mathematician Gustave Gaspard Coriolis discovered the force that bears his name, and derived the mathematical equations that describe the Coriolis force.

The Coriolis force is weak, compared to other meteorological phenomena. At the equator, the Coriolis Force is nonexistent because wind does not rotate at the equator as it does at the poles. At the equator, wind follows the Earth’s curvature, without deviating right or left. The absence of the Coriolis force explains why hurricanes do not form at the equator, but rather in tropical waters, at least 5 degrees north and south of the equator.

Absent the Coriolis force at the equator, a hurricane or other tropical storm that formed in the north will not cross into the Southern Hemisphere, nor will one in the south cross into the Northern Hemisphere. The equator, therefore, acts as a barrier against the movement of tropical storms. The Coriolis force keeps tropical storms in the hemisphere of their origin. By contrast to the weakness of the Coriolis force as one nears the equator, the force gains strength as one approaches the poles and reaches a maximum at the poles, where winds experience the full effect of the Earth’s rotation on its axis. At the poles, wind rotates sharply right in the Northern Hemisphere or left in the Southern Hemisphere with maximum force.

The Coriolis force accounts for the trade winds that mariners used to navigate the ocean in past eras. So predictable are these winds in direction and force that Christopher Columbus used them in his later voyages to retrace the route of his first voyage. The winds originating in the Northern Hemisphere are called the Westerlies, whereas those that originate in the Southern Hemisphere are called the Easterlies. 

On land, the Coriolis force can form tornados. A tornado will rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The direction of rotation may seem counterintuitive, however, because wind flows to the right in the Northern Hemisphere, one might expect that it would rotate clockwise, rather than counterclockwise. The same rationale applies in the Southern Hemisphere, where the flow of wind to the left might be expected to produce counterclockwise rotation, rather than clockwise rotation. However, a countervailing force, the difference in air pressure at the center and at the periphery of a tornado, forces a tornado to rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

The counterintuitive rotation of tornados underscores the weakness of the Coriolis force. The Coriolis force could be compared to gravity; both are relatively weak, unseen forces, with existences that were undefined for centuries.