Coriolis force

The Coriolis force is the second of the two fictitious forces that appear when Newton's laws are written in a rotating reference frame. Unlike centrifugal force, which depends only on position, the Coriolis force depends on velocity: its direction is perpendicular to both the rotation axis Ω and the velocity v. On a rotating planet, an object moving north in the northern hemisphere gets deflected east; moving south, west. The deflection is the same size regardless of direction.

The magnitude is 2mΩv·sin(latitude) — proportional to the planet's rotation rate, the object's speed, and the sine of latitude. At the equator the Coriolis deflection vanishes. At the poles it is maximal. Trade winds, hurricane spin directions, the rotation of Foucault pendulums, the drift of long-range artillery, and the systematic deflection of freely falling objects all trace back to this one term.

Because the Coriolis force is fictitious — an artifact of describing physics from a rotating observer's perspective — it vanishes in an inertial frame. From space, a hurricane is simply air with conserved angular momentum flowing in toward a low-pressure center; the spin is the consequence. From the ground, it looks like an invisible lateral push. Both descriptions are correct. Which is simpler depends on whether you want to compute the orbit of a satellite or forecast tomorrow's weather.

Gaspard-Gustave de Coriolis derived the force in 1835 while analysing rotating machinery, not weather. Meteorologists rediscovered it for atmospheric circulation later in the century; the name "Coriolis force" for the meteorological effect was not standard until the 1920s.