Objects moving freely across the Earth appear to curve because the planet itself is rotating beneath them. This apparent deflection is the Coriolis effect. In the Northern Hemisphere the deflection is to the right of the direction of motion; in the Southern Hemisphere it is to the left.

The effect is negligible at the equator and increases toward the poles. It shapes the large-scale circulation of both the atmosphere and the ocean, steering winds and currents into the familiar patterns of trade winds, westerlies, and the great subtropical gyres. Without the Coriolis effect, ocean basins would lack the western boundary currents that transport vast amounts of heat poleward.

The force is proportional to velocity and to the sine of latitude, so it influences fast-moving or long-traveling features most strongly. Everyday intuition can mislead because the effect is too weak to notice in household-scale motions. In oceanography and meteorology, however, it is a fundamental organizing principle.

Every major current system on the planet bears the imprint of Earth’s rotation.