3.7 Coriolis Effect

Can you tell which hemisphere you're in?

People say that Coriolis effect determines the direction that water flushes down a toilet or sink. If that's true, then which hemisphere is this toilet in? It looks like it's in the Northern Hemisphere, because the spiral arms are going the same direction as a Northern Hemisphere hurricane. Unfortunately, there are too many other factors that determine the direction toilet water flushes, such as friction and the power of the flush. So we don't know where this toilet is.

Coriolis Effect

The Coriolis effect describes how Earth’s rotation steers winds and surface ocean currents (Figure below). Coriolis causes freely moving objects to appear to move to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The objects themselves are actually moving straight, but the Earth is rotating beneath them, so they seem to bend or curve. That's why it is incorrect to call Coriolis a force. It is not forcing anything to happen!

An example might make the Coriolis effect easier to visualize. If an airplane flies 500 miles due north, it will not arrive at the city that was due north of it when it began its journey. Over the time it takes for the airplane to fly 500 miles, that city moved, along with the Earth it sits on. The airplane will therefore arrive at a city to the west of the original city (in the Northern Hemisphere), unless the pilot has compensated for the change. So to reach his intended destination, the pilot must also veer right while flying north.

As wind or an ocean current moves, the Earth spins underneath it. As a result, an object moving north or south along the Earth will appear to move in a curve instead of in a straight line. Wind or water that travels toward the poles from the Equator is deflected to the east, while wind or water that travels toward the Equator from the poles gets bent to the west. The Coriolis effect bends the direction of surface currents to the right in the Northern Hemisphere and left in the Southern Hemisphere.

Coriolis effect is demonstrated using a metal ball and a rotating plate in this video. The ball moves in a circular path just like a freely moving particle of gas or liquid moves on the rotating Earth (5b).

Summary

• Earth rotates beneath freely moving objects like water and air. Compared with a spot on the planet, the freely moving objects appear to be moving.
• Freely moving objects appear to move right in the Northern Hemisphere and left in the Southern Hemisphere.
• Coriolis is an effect rather than a force because it is not forcing a motion, it's just an appearance of a change of motion.

Explore More

Use this resource to answer the questions that follow.

1. Why does some land on Earth move faster than other land? Where is the fastest and slowest land?
2. If you are at the equator and try to throw a ball to your friend at the north pole, what happens to the ball? What if your friend is at the south pole?
3. What would happen to the winds if Earth didn't rotate? What do they do instead?

Review

1. If an airplane flies from east to west in the Northern Hemisphere without changing latitude at all, in which direction will it appear to curve?
2. If an airplane flies from south to north in the Southern Hemisphere, in which direction will it appear to curve?
3. If freely moving objects are only appearing to curve their paths, why is this important?