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Understanding the Coriolis Force for Students

written by: KennethSleight•edited by: Donna Cosmato•updated: 7/8/2011

Why do hurricanes move from north to south and east to west instead of just from west to east like most other storms? The simple answer is because of the Coriolis force, but that’s only a simple answer if you know what the Coriolis force is. Here we explain the Coriolis force in hurricanes for kids

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    Hurricane Gelane The Coriolis force is the practical application of a mathematical expression. What does that mean?

    It means that there is an actual way to calculate the rotation of a large storm system and explain why it moves in the particular way that it does. The mathematical expression looks like this: Fc= -2mΩ x V where Fc is the Coriolis force, m is the mass of the object (in this case the storm system), V is the velocity of the system and Ω is the angular velocity vector which has magnitude equal to the rotation rate.

    You can thank French scientist Gaspard-Gustave Coriolis for this. He published a paper on it back in 1835. Don’t worry though, you won’t need to remember that unless you are going to be a meteorologist, and even then, you’ll have a computer that will help you with those calculations.

    Of course, none of this really means much if you don’t know how to figure out the mass of a storm system or the angular velocity vector. Since we don’t want to get bogged down in the actual math but rather just get a basic understanding of this force, we can do a few experiments to understand this a bit better.

    These will explain the basics of the two major factors that cause the Coriolis effect, which are the rotation of the Earth and the inertia of the storm system. Even though the Coriolis force is quite small when viewed in slow rotating systems, its effects become noticeable for systems that occur over large distances and long periods of time. So, here are two experiments that can be used to explain the Coriolis force in hurricanes for kids.

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    Experiment 1: Balloon and Marker

    You’ll need four things for this experiment: a round balloon, two different colored felt tipped markers and a partner. Blow up the balloon and tie it. Have your partner hold the balloon tight. Draw a line around the center of the balloon to form an equator. Now draw a straight line from the top of the balloon to the equator line. This will be your reference line or what the movement of a storm would look like if the Earth didn’t rotate.

    Use the second color marker for the next step. Now have your partner start to spin the balloon counterclockwise while you draw a line from the top to the equator. See the difference? One line is straight while the other curves off to the left. This is the rotational force of the Earth. Your hand is the storm, the balloon is the Earth, and the marker is the path the storm would take if only the force of the rotation of the Earth was working on the storm.

    When you spin a balloon on its axis, a point on the equator travels farther than a point near the axis in the same amount of time. So if one point goes farther than the other, it must be going faster. When we translate that to a global scale, we can say that the westward path of a hurricane results from the surface of the Earth rotating faster at the equator than it does at the poles. So as the storm moves toward the equator, the Earth beneath it is traveling faster and covering more distance. This is why we see the curve in the storm getting stronger the closer it gets to the equator.

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    Experiment 2: Bathtub Drain

    The bathtub drain experiment is something you’ve probably done and don’t even realize it. When you pull the plug or open the drain in your bathtub and watch it swirl down the drain, you are actually witnessing the angular acceleration of water due to gravity. While this might sound really technical, it just means that the speed at which the water goes down the drain increases the longer there is water to flow. While this doesn’t have much to do with the rotation of the Earth, it does have a lot to do with inertia.

    To do this experiment properly you need to fill a tub with water and a few small pieces of floating material (wood, ivory soap, etc.) and let it sit until there is little to no motion in the tub from it being filled. Once the motion has stopped, very slowly open the drain. Watch as the water drains down.

    For the first few minutes you probably won’t see a vortex form. The water will flow straight down the drain. Then, something interesting will start to happen. A small water tornado will form and rotate in a counter clockwise direction. This will happen wherever you do it in the world (although it was commonly believed that toilets and bathtub drains would work in the opposite direction in the Southern hemisphere).

    The water in the tub accelerates because as the pressure on the outside of the vortex gets lower, the Coriolis force gets stronger creating a feedback loop. The pressure gets lower as there is less water pushing on the system. The less pressure on the system, the faster the water drains.

    So what does this have to do with hurricanes? The same physics apply here. The lower the pressure on the outside of the vortex, the faster it will rotate. A stronger hurricane with higher winds will be produced by a storm with a very low center of pressure in the eye.

    When the pressure outside of the eye of the storm starts to rise, the rotation of the storm will slow. The reason that hurricanes can become so devastatingly powerful is because of this difference in pressure on the inside and outside of the central vortex of the storm.

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    Motion of a Hurricane from North to South

    The simplest motion of a hurricane to explain is the north to south motion. The speed of rotation near the equator results in a much lower Coriolis effect in this area. This is why hurricanes begin to form in the higher-pressure regions of northern waters far away from the equator and then move toward the lower pressure southern waters.

    The occasions where a hurricane turns in a northerly direction are a direct result of high-pressure systems building over an area of land and then pushing out over the east coast of a large land mass like the United States. This area of high pressure can push the storm north toward a pocket of lower pressure air and turn the storm in the northern direction. It will then head back out to sea where it will begin the process all over again. Although not directly related to the Coriolis force in hurricanes, for kids it is important to understand the basic fundamental movement from high pressure areas to lower pressure areas as well.

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    References; Mikkelson, Barbara; "Flush Bosh",

    Univeristy of Oregon: "Coriolis Effect",

    University of Illinois, "Coriolis Effect",

    Photo by NASA Goddard Photo and Video on Flickr;