The Coriolis effect is the apparent deflection of moving objects observed from a rotating frame of reference, such as Earth. It is most important for large-scale motion because Earth turns underneath moving air and water. This effect helps shape global wind belts, ocean currents, and the spin of major storm systems.
It does not create motion by itself, but it changes the path that motion appears to follow on Earth’s surface.
The size of the Coriolis effect depends on speed, latitude, and Earth’s rotation rate. It is zero at the equator and strongest near the poles because of the geometry of Earth’s rotation. In the Northern Hemisphere, moving air and water are deflected to the right of their motion, while in the Southern Hemisphere they are deflected to the left.
Meteorologists and oceanographers use this effect to explain trade winds, jet streams, gyres, and the rotation of hurricanes and cyclones.
Key Facts
- Coriolis acceleration: a_c = 2Ωv sinφ, where Ω is Earth’s angular speed, v is object speed, and φ is latitude.
- Earth’s angular speed is Ω = 7.29 × 10^-5 rad/s.
- The Coriolis effect is zero at the equator because sin0° = 0.
- The Coriolis effect is strongest at the poles because sin90° = 1.
- In the Northern Hemisphere, moving objects are deflected to the right of their direction of motion.
- In the Southern Hemisphere, moving objects are deflected to the left of their direction of motion.
Vocabulary
- Coriolis effect
- The apparent deflection of moving objects caused by observing them from a rotating reference frame.
- Rotating reference frame
- A viewpoint that is turning, such as an observer standing on the rotating Earth.
- Latitude
- The angular distance north or south of the equator, measured in degrees.
- Angular speed
- The rate at which an object rotates, usually measured in radians per second.
- Geostrophic wind
- A large-scale wind that flows nearly parallel to isobars because pressure-gradient force and Coriolis force balance.
Common Mistakes to Avoid
- Saying the Coriolis effect is a real force pushing objects sideways is wrong because it is an apparent effect that appears in a rotating reference frame.
- Using the same deflection direction in both hemispheres is wrong because motion bends right in the Northern Hemisphere and left in the Southern Hemisphere.
- Applying the Coriolis effect strongly to sinks, toilets, or small containers is wrong because friction, container shape, and initial motion dominate at small scales.
- Forgetting the latitude factor is wrong because a_c = 2Ωv sinφ, so the effect is zero at the equator and increases toward the poles.
Practice Questions
- 1 A wind current moves at 20 m/s at latitude 30° N. Using a_c = 2Ωv sinφ and Ω = 7.29 × 10^-5 rad/s, calculate the Coriolis acceleration.
- 2 An ocean current moves at 1.5 m/s at latitude 60° S. Calculate the magnitude of its Coriolis acceleration using Ω = 7.29 × 10^-5 rad/s.
- 3 A parcel of air moves northward from the equator into the Northern Hemisphere. Explain why its path appears to curve and identify the direction of the deflection.