What Is the Coriolis Effect?

THE PRESSURE-GRADIENT FORCE drives airflow in the atmosphere, but winds do not blow in exactly the direction we would predict if we only consider pressure gradients. All objects — whether air masses, ocean waters, or airplanes — moving across the surface of the Earth display an apparent deflection from the objects' intended path. The cause of this deflection is the Coriolis effect. Why does this apparent deflection occur?

What Is the Coriolis Effect?

1. The Coriolis effect refers to the apparent deflection in the path of a moving object in response to rotation of the Earth. The easiest way to envision this is by considering air that is moving from north to south or south to north. Earth's atmosphere, including any moving air, is being carried around the Earth by rotation.

2. The blue arrows show how much distance the surface rotates in an hour. The arrows are longer near the equator, indicating a relatively long distance that these areas have to travel, and therefore faster velocities. The distances traveled and the linear velocities gradually decrease toward the poles.

3. At the poles, the distance traveled and velocity are both zero — the surface has no sideways velocity due to the rotation. In 24 hours, an area directly at the pole would simply spin 360°, whereas an area at the equator would have moved approximately 40,000 km (the circumference of the Earth).

4. As air moves toward the poles, it possesses the eastward momentum that it had when it was closer to the equator. So, it appears, from the perspective on Earth's surface, to be deflected to the right (to the east).

5. The opposite occurs as air moves toward the equator and encounters areas with a faster surface velocity. The air appears to lag behind, deflecting to the west as if it were being left behind by Earth's rotation. Note that in the Northern Hemisphere, air deflects to the right of the flow (not necessarily to the right as you look at it on a map), irrespective of which way it is moving (toward the pole, away from the pole, or in some other direction).

6. In the Southern Hemisphere, air moving toward the pole travels from faster rotating areas to slower ones, so it appears to be rotating faster than the surface — it deflects to the left. Moving air in the Southern Hemisphere deflects to the left irrespective of which way it is moving.

Why Do Moving Objects Appear to Deflect Right or Left on a Rotating Planet?

To visualize why moving objects on a rotating planet appear to deflect left or right, examine these overhead views of a merry-go-round that is rotating counterclockwise (in the same way as Earth when viewed from above the North Pole). One person located at the center of the merry-go-round throws a ball to a second person standing near the outside edge of the merry-go-round. The path of the ball can be measured relative to two frames of reference: the two clumps of trees, which are fixed in our perspective, or from the persons on the merry-go-round, which is moving.

1. The person at the center of the merry-go-round slowly tosses a purple ball toward an outer person, in the direction of the upper two trees. The intended path of the ball is shown by the yellow arrow. The outer part of the merry-go-round moves faster than the center.

2. After a short time, the ball is heading toward the two trees, but, relative to the intended path (yellow arrow) or from the perspective of the thrower, the purple ball seems to be veering away to the right, because the thrower rotated.

3. With each passing time period, the intended receiver moves farther away from the ball as the ball goes toward the two trees. As viewed from the thrower, the ball deflected to the right relative to the intended path.

4. In the last figure, the ball's path traced upon the moving framework of the merry-go-round (open purple circles) reveals an apparent deflection to the right (shown with a dashed red line) of the intended path. However, relative to the fixed reference of the upper two trees, the ball has actually followed a straight line. This view is similar to one of the rotating Earth viewed from above the North Pole. The thrower and receiver are two locations  at different latitudes, and the ball represents an air mass moving from the slow-moving pole toward the faster-moving equator.

Will Deflection Occur if Objects Move Between Points on the Same Latitude?

A similar deflection occurs if an object moves parallel to latitude on a rotating planet. To visualize why this is so, we return to the merry-go-round, which is still rotating counterclockwise, like Earth viewed from above the North Pole. As before, it is key to consider movements in terms of a fixed reference frame and a reference frame that is moving.

1. The person throwing the ball is on the outside of the merry-goround along with the receiver. The intended path of the ball is shown by the yellow arrow. Since the players are the same distance out from the center, they are moving at the same rate.

2. After a short time, the ball is heading along its original path (the purple path) relative to the upper two trees (the fixed reference frame). In the intervening time since the throw, however, the thrower and receiver have both moved (a moving reference frame).

3. From the moving frame of reference of the thro wer, the ball appears to be deflected to the right of the intended path, with the deflection shown by the orange dashed line.

4. This example represents the apparent deflection of air (or any other object) moving parallel to latitude. So regardless of whether objects are moving in the north-south (meridional) or the east-west (zonal) directions, the objects appear to be deflected from their intended path. Moving objects have an apparent deflection to the right of their intended path in the Northern Hemisphere and to the left in the Southern Hemisphere. This left or right deflection due to the Coriolis effect accounts for the directions of prevailing winds, the paths of storms, and the internal rotation within hurricanes.

What Affects the Strength of the Coriolis Effect?

Since the Coriolis effect is related to the rate at which areas of the surface move during rotation of the Earth, we would suspect the strength of the effect may vary with latitude. It is also influenced by how fast objects are moving.

1. When viewed from above the poles, the parallels of latitude constitute a series of concentric circles increasing in circumference from the poles to the equator. Moving from one latitude to another, like from the pole to 80° N, the percentage increase in circumference is much greater at high latitudes than nearer the equator. The difference in circumference for every 10° difference in latitude is plotted as orange triangles that represent the changing “slope” of the Earth's surface. Thus, the Coriolis effect is greatest at high latitudes, where the velocity of the moving reference frame changes most rapidly relative to the moving object.

2. The Coriolis effect is expressed daily in many ways, including the shape of storms as viewed by satellites and featured on the daily weather report, the rotation of hurricanes, and the changes in wind directions as a large storm approaches and then exits your town.

3. The Coriolis effect is stronger for an object with a large amount of mass, like a huge storm, or for an object that is accelerating (moving faster with time). In the case of a rotating storm, the acceleration can be related to movement of the entire storm across Earth's surface, rotations within the storm, and other motions. In any case, the Coriolis effect deflects faster moving objects more than it does slower moving objects.