Cariolis Effect and Its Role in Spinning the Direction of Hurricanes

What You Need to Know

Wind is the horizontal flow of air relative to Earth's surface. The Coriolis effect is the apparent deflection of free-moving things due to the rotation of the Earth. To deflect means to turn aside from a straight path. A hurricane is a tropical cyclone with a constant wind speed of 74 miles (119 km) per hour or more. A cyclone is an area of low atmospheric pressure around which spiraling winds blow. The tropics is a band about 3,000 miles (4,800 km) wide around the equator.

How Does the Coriolis Effect Work?

Differences in air pressure between two areas tend to push air in a straight path. But as the air moves from a high-pressure area to a low-pressure area, Earth rotates under it, making the wind appear to follow a curved path. In the Northern Hemisphere, wind deflects to the right of its direction of motion. In the Southern Hemisphere, wind deflects to the left. These apparent deflections are called the Coriolis effect.

What Does This Have to Do with the Spin Direction of Hurricanes?

The Coriolis effect causes winds to deflect to the right in the Northern Hemisphere, but in hurricanes wind spins counterclockwise around a low-pressure area north of the equator. This happens because while wind is deflected to the right by the Coriolis effect, air-pressure difference is pushing the air toward the center of the area of low pressure. The diagram below shows the counterclockwise movement of air around a low-pressure area in the Northern Hemisphere. The solid arrow represents the force on the air due to pressure differences. The spiraling solid line shows the resulting counterclockwise wind motion.

Fun Fact

The only difference between a typhoon and a hurricane is location. Typhoons occur west of the International Date Line and hurricanes occur east of it.

Real-Life Science Challenge

At this time, scientists can track only the direction and wind speed of an existing typhoon or hurricane. This information is broadcast to the public so that people in the storm's potential path can prepare. A storm is put in a category from 1 to 5 depending on its wind speed and the potential damage it may cause when it hits land. These categories are:

Experiment

Now, start experimenting with the deflection of a moving object due to rotation. Why do hurricanes spin counterclockwise north of the equator and clockwise south of the equator?

Hints

• A revolving surface, such as a lazy Susan, can be used to simulate the Earth's rotation.
• Moving a pencil or a pen in a straight line across a paper secured to a revolving surface is one way of recording the path of a moving object due to rotation.
• Research other methods of recording the deflected path of a moving object on a rotating surface.

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