The Rotating Frame
Stand at the North Pole and throw a ball southward. As it flies, the Earth rotates underneath it. To you, standing on the rotating Earth, the ball appears to curve to the right — even though it is actually traveling in a straight line through space. This apparent deflection is the Coriolis effect, named after French mathematician Gaspard-Gustave de Coriolis who described it mathematically in 1835. It is not a real force but an artifact of our rotating reference frame.
The Coriolis Parameter
The strength of the Coriolis effect depends on latitude and Earth's rotation rate: f = 2Ω sin(φ), where Ω is Earth's angular velocity (7.292×10⁻⁵ rad/s) and φ is latitude. At the equator (φ=0), f=0 — no deflection. At the poles (φ=90°), f is maximum. This latitude dependence explains why hurricanes form in the tropics (enough heat) but not right at the equator (not enough Coriolis). The Coriolis acceleration is perpendicular to velocity: a = -2Ω × v.
Watching Deflection
This simulation shows a top-down view with an object moving from the center. The dashed line shows its intended straight path; the solid curved line shows its actual trajectory as seen from Earth's rotating surface. In the Northern Hemisphere, deflection is to the right. Increase latitude to see stronger deflection. Set rotation to zero to see the object move straight — confirming this is purely a rotational effect. Multiple trajectories at different launch angles create the characteristic spiral pattern of Coriolis-deflected motion.
Global Weather Patterns
The Coriolis effect shapes Earth's entire atmospheric circulation. Rising hot air at the equator flows poleward at altitude but is deflected east, creating the subtropical jet stream. Sinking air at ~30° latitude flows back toward the equator but deflects west, creating the trade winds — the reliable easterly winds that powered centuries of sailing commerce. At mid-latitudes, the prevailing westerlies blow from west to east. These three circulation cells (Hadley, Ferrel, Polar) in each hemisphere create Earth's climate zones. The Coriolis effect also deflects ocean currents, creating the great gyres that redistribute heat across the planet.