CC.12 The Coriolis Effect

ESSENTIAL TO KNOW

• When set in motion, freely moving objects, including air and water masses, move in straight paths while the Earth continues to rotate independently.
• Because freely moving objects are not carried with the Earth as it rotates, they are subject to an apparent deflection called the “Coriolis effect.” To an observer rotating with the Earth, freely moving objects that travel in a straight line appear to travel in a curved path on the Earth.
• The Coriolis effect causes an apparent deflection of freely moving objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The deflection is said to be cum sole, or “with the sun.”
• The Coriolis deflection is greatest at the poles and decreases at lower latitudes. There is no Coriolis effect for objects that move directly east to west or west to east at the equator.
• Regardless of their speed, freely moving objects at the same latitude appear to complete a circle and return to their original location in the same period of time. This period, called the “inertial period,” is 12 h at the poles and increases progressively at lower latitudes. The inertial period is 24 h at 30°N or 30°S, and it approaches infinity near the equator.
• For objects moving within 5° on either side of the equator, the Coriolis effect often can be ignored because the deflection is small and the inertial period is very long.
• Freely moving objects moving at the same speed appear to follow circular paths with smaller radii at higher latitudes.
• Freely moving objects at the same latitude appear to follow paths with larger radii if they are moving at higher speeds.
• The magnitude of the Coriolis deflection (the rate of increase of distance from a straight-line path) increases with increasing speed.