What Is the Coriolis Effect?

The Coriolis effect (also known as the Coriolis force) refers to the apparent deflection of objects (such as airplanes, wind, missiles, and ocean currents) moving in a straight path relative to the Earth's surface. Its strength is proportional to the speed of the Earth's rotation at different latitudes. For example, a plane flying in a straight line north will appear to take a curved path when viewed from the ground below.

Key Takeaways: Coriolis Effect

• The Coriolis effect occurs when an object traveling in a straight path is viewed from a moving frame of reference. The moving frame of reference causes the object to appear as if it is traveling along a curved path.

• The Coriolis effect becomes more extreme as you move further away from the equator toward the poles.

• Wind and ocean currents are strongly affected by the Coriolis effect.

Coriolis Effect Definition

The Coriolis effect is an "apparent" effect, an illusion produced by a rotating frame of reference. This type of effect is also known as a fictitious force or an inertial force. The Coriolis effect occurs when an object moving along a straight path is viewed from a non-fixed frame of reference. Typically, this moving frame of reference is the Earth, which rotates at a fixed speed. When you view an object in the air (such as a plane) that is following a straight path, the object will appear to curve off its course because of the rotation of the Earth. The object is not actually moving off its course. Again, the effect is only apparent—the object appears to be following a curved path because the Earth is rotating beneath it.

Causes of the Coriolis Effect

The main cause of the Coriolis effect is the Earth's rotation. As the Earth spins in a counter-clockwise direction on its axis, anything flying or flowing over a long distance above its surface is deflected. This occurs because as something moves freely above the Earth's surface, the Earth moves east under the object at a faster speed.

As latitude increases and the speed of the Earth's rotation decreases, the Coriolis effect increases. A pilot flying along the equator itself would be able to continue flying along the equator without any apparent deflection. A little to the north or south of the equator, however, and the pilot would be deflected. As the pilot's plane nears the poles, it would experience the most deflection possible.

Another example of latitudinal variations in deflection is the formation of hurricanes. These storms don't form within five degrees of the equator because there is not enough Coriolis rotation. Move further north and tropical storms can begin to rotate and strengthen to form hurricanes.

In addition to the speed of the Earth’s rotation and latitude, the faster the object itself is moving, the more deflection there will be.

The direction of deflection from the Coriolis effect depends on the object’s position on Earth. In the Northern Hemisphere, objects deflect to the right, while in the Southern Hemisphere they deflect to the left.

Impacts of the Coriolis Effect

Some of the most important impacts of the Coriolis effect in terms of geography are the deflection of winds and currents in the ocean. There is also a significant effect on man-made items such as planes and missiles.

In terms of affecting the wind, as air rises off of the Earth's surface, its speed over the surface increases because there’s less drag as the air no longer has to move across the Earth's many types of landforms. Because the Coriolis effect increases with an object’s increasing speed, it significantly deflects air flows.

In the Northern Hemisphere these winds spiral to the right and in the Southern Hemisphere they spiral to the left. This usually creates the westerly winds moving from the subtropical areas to the poles.

Because currents are driven by the movement of wind across the waters of the ocean, the Coriolis effect also affects the movement of the ocean’s currents. Many of the ocean's largest currents circulate around warm, high-pressure areas called gyres. The Coriolis effect creates the spiraling pattern in these gyres.

Finally, the Coriolis effect is important to man-made objects as well. One of the most significant impacts of the Coriolis effect is a result of its deflecting planes and missiles. Take, for example, a flight leaving from San Francisco, California, that is heading to New York City. If the Earth did not rotate, there would be no Coriolis effect and thus the pilot could fly in a straight path to the east. However, due to the Coriolis effect, the pilot has to constantly correct for the Earth's movement beneath the plane. Without this correction, the plane would land somewhere in the southern portion of the United States.