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rotation of earth on its axis

The Coriolis effect (also called the Coriolis force) is defined as 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 but it has an impact on moving objects across the globe.

The "apparent" portion of the Coriolis effect's definition is also important to take into consideration. This means that from the object in the air (i.e an airplane) the earth can be seen rotating slowly below it. From the earth's surface that same object appears to curve off of its course. The object is not actually moving off of its course but this just appears to be happening because the earth’s surface is rotating beneath the object.

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 is moving east under the object at a faster speed.

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

Another example of this idea of latitudinal variations in deflection would be the formation of hurricanes. They 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. It also has a significant effect on man-made items like 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 item’s increasing speed, it significantly deflects air flows and as a result the wind.

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 water 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. Though the circulation is not as significant as that in the air, the deflection caused by the Coriolis effect is what creates the spiraling pattern in these gyres.

Finally, the Coriolis effect is important to man-made objects in addition to these natural phenomena. 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 Coriolis effect, the pilot has to constantly correct for the earth's movement beneath plane. Without this correction, the plane would land somewhere in the southern portion of the United States.

Myth of the Coriolis Effect
One of the biggest misconceptions associated with the Coriolis effect is that it causes the rotation of water down the drain of a sink or toilet. This is not truly the cause of the water's movement. The water itself is simply moving too fast down the drain to allow for the Coriolis effect to have any significant impact.

Though the Coriolis effect does not in fact influence the movement of water in a sink or toilet, it does have a an impact on wind, the ocean, and other items flowing or flying over the earth's surface, making the Coriolis effect an important component of the understanding of many of physical geography's most important concepts.