Coriolis Effect
Dana Desonie, Ph.D.
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Printed: July 9, 2014
AUTHOR
Dana Desonie, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Coriolis Effect
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Coriolis Effect
• Explain the Coriolis effect and distinguish between an effect and a force.
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 1.1). 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.
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FIGURE 1.1
The Coriolis effect causes winds and currents to form circular patterns. The direction that they spin depends on the
hemisphere that they are in.
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): http://www.youtube.com/watch
?v=Wda7azMvabE (2:04).
MEDIA
Click image to the left for more content.
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.
Practice
Use these resources to answer the questions that follow.
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Chapter 1. Coriolis Effect
MEDIA
Click image to the left for more content.
1. What is the Coriolis effect?
2. What is subject to the Coriolis effect?
3. What is the direction of deflection in the Northern Hemisphere?
4. What is the direction of deflection in the Southern Hemisphere?
Test your skills on the Coriolis effect.
http://www.montereyinstitute.org/noaa/lesson08/l8ex1.htm
5. What happens if pilots do not correct for the Coriolis effect?
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?
References
1. Laura Guerin. The Coriolis effect causes winds and currents to form circular patterns. CC BY-NC 3.0
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