Coriolis force

In a non-rotating inertial frame, the Coriolis force is not a separate physical interaction: it is an apparent force introduced when motion is described from a rotating frame. In a frame rotating with angular velocity Ω, a moving object has a Coriolis acceleration commonly written as −2Ω × v, where v is its velocity in that frame. The sign depends on the convention used for defining the rotating-frame equation; the cross product indicates that the effect acts perpendicular to the motion and the rotation axis. On Earth, the apparent horizontal deflection is to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

The Coriolis effect depends on the object's motion as well as Earth's rotation. For horizontal motion at latitude φ, the magnitude of the horizontal Coriolis acceleration is 2Ωvsin φ, where Ω is Earth's angular rotation rate and v is speed. This horizontal component is zero at the equator and increases toward the poles. The effect becomes more consequential when motion covers long distances or persists long enough for the deflection to accumulate; it is generally too small to determine the direction of water draining from an ordinary sink or toilet.

Air moving over Earth's surface is affected by the Coriolis effect, which helps shape large-scale circulation. In the Northern Hemisphere, air around large low-pressure systems tends to circulate counterclockwise and around high-pressure systems clockwise; the directions are reversed in the Southern Hemisphere. The force does not start storms or supply their energy. Rather, it helps organize and deflect airflows, alongside pressure gradients, friction and other atmospheric processes.

Winds drive many surface ocean currents, and Earth's rotation deflects those moving waters to the right in the Northern Hemisphere and left in the Southern Hemisphere. The resulting circulation interacts with coastlines, seafloor features, density differences and friction. Ocean currents redistribute heat and influence regional climate; they are not caused by the Coriolis effect alone.

The effect is named for French mathematician Gaspard-Gustave de Coriolis, who analyzed motion in rotating systems in the 19th century. It is often introduced using a rotating merry-go-round: a ball may travel approximately straight when viewed from outside, but appear to curve to riders on the rotating platform. Likewise, Earth-based observers describe long-range moving objects using a rotating frame. A commonly repeated claim that the effect determines which way a bathtub or toilet drains is misleading: basin geometry, plumbing and small disturbances dominate at that scale.