Coriolis effect

The Coriolis effect describes how the path of a moving object appears curved when its motion is viewed from a rotating reference frame. For Earth-based observers, the reference frame rotates with the planet. The apparent deflection is not caused by a new fundamental interaction; it is a term used to describe motion in that rotating frame. From an inertial frame outside the rotating system, the object's path can be described using ordinary motion and the other forces acting on it.

In Earth science, the effect matters chiefly for motion over large distances or long periods. The direction of deflection depends on the hemisphere and on the direction of travel: on Earth, motion is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, relative to its direction of travel.

Earth rotates eastward, but the ground's eastward speed is not the same at every latitude: it is greatest at the Equator and falls toward the poles. When air, water or a projectile moves across latitudes, it carries its existing motion as the ground beneath it changes speed. Viewed from Earth's surface, this difference contributes to the apparent sideways deflection.

For many large-scale horizontal motions, scientists express the latitude dependence using the Coriolis parameter, f = 2Ω sin φ, where Ω is Earth's angular rotation rate and φ is latitude. The parameter is zero at the Equator, changes sign between hemispheres and reaches its greatest magnitude at the poles. This latitude dependence is one reason the effect is central to geophysical fluid dynamics but weak near the Equator in its usual horizontal-flow form.

The Coriolis effect helps shape the paths of winds and ocean currents over large areas. In combination with pressure differences and other influences, it contributes to broad circulation patterns and to geostrophic flow, in which a pressure-gradient force and rotational deflection balance one another. It does not by itself set the speed or direction of every current or wind.

The rotation of large low-pressure storm systems illustrates the hemispheric pattern: tropical cyclones generally rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The Coriolis effect helps organize that circulation; storm development also depends on other atmospheric and oceanic conditions. Because its horizontal component becomes very small near the Equator, tropical cyclones rarely form there.

The effect becomes important when motion covers large distances or persists long enough for small deflections to accumulate. It is not a practical explanation for the direction in which water drains from an ordinary sink, bathtub or toilet. Drain geometry and how water was introduced have much greater influence in these small systems. The Coriolis effect is real as a feature of rotating-frame descriptions, but its importance depends strongly on the scale and context of the motion.

The effect is named for the French engineer and mathematician Gaspard-Gustave de Coriolis. In 1835 he described the apparent force that must be included when expressing motion using a rotating reference frame. His analysis concerned mechanics in rotating systems generally; later geophysical science applied the same framework to motions in Earth's atmosphere and oceans.