Twisting the Air Away - The Coriolis Effect

This lesson plan was developed by NSTA master teacher Jerry D. Roth through NSTA's partnership with NOAA.

Subject Area:

Geoscience or General Science that has an Earth science component.

Standards Alignment-National Science Education Standards:

• Earth Science
• Energy in the Earth system
• Heating of the Earth's surface and atmosphere by the sun drives convection within the atmosphere and ocean, producing winds and ocean currents. (9-12)
• Earth Science
• Energy in the Earth system
• Global winds are part of a pattern of air circulation across the Earth and include the trade winds, westerlies and the polar easterlies.

Time Required:

The entire lesson requires about one-half hour from beginning to the end.

Overall Lesson Goal:

The student will understand that one of the effects of the Earth’s rotation on the air and oceans is called the Coriolis Effect and it causes objects including air and water to move to the east in the Northern Hemisphere and to the west in the Southern Hemisphere.

Individual Learning Objectives:

The student should be able upon completion of the lesson to predict in which direction an object will move when being affected by the Coriolis effect.

Prerequisite Knowledge; Misconceptions/Preconceptions:

Most students have observed movies of the Earth rotating. Ask them if an area near the equator is rotating at the same speed as an area near the poles, and you may be surprised at their answer. All things have the same rotational velocity, but an object at the equator is traveling far faster than an object at the poles. This is not the case for all objects in the Solar System If you have access to the SOHO Web site you can see that the sun actually has something called differential rotation where the equator of the sun rotates with greater rotational velocity than the slower areas toward the poles.

Because the air and the ocean act as fluids they actually appear to drag behind the rotation of the planet. This causes the real direction of an object to be altered. A very good graphical representation of this is found at The Coriolis Effect. From Getting Around the Coriolis Force we can view in which direction the air is moved by the Coriolis effect. Objects in the Northern Hemisphere moving north actually show net movement to the east, and objects in the Southern Hemisphere moving south appear to move toward the west. The change in direction is called the Coriolis effect. It is a very small effect but the Earth is large and has been rotating for a long time. It has an impact on how winds and ocean currents move.

Please do not fall into the trap (no pun intended) regarding toilets and drains and the Coriolis effect. See the primary site listed for this lesson but also have a good read at Bad Coriolis to find what the Coriolis effect does not do. Just keep repeating to yourself… design issues cause this. I recommend reading down to the bottom of the Bad Science Web page and try some fakery yourself.

Procedures/Instructional Strategy:

Step 1: Ask the students to predict in which direction a straight line will turn when drawn from top to bottom on a page spinning clockwise.
Ask them to predict which direction the line will turn when drawn in the same direction if the paper is rotating in a counterclockwise direction. Have the students draw their predictions in their notebook.

Step 2: Break the class into groups of four students. Give each group a piece of poster board and two different colored markers-one for each side. I’ve tried this with photocopy paper but larger paper works better. Use paper that is clean on both sides. That way, when you reverse rotation you can work on the reverse side. Also, it really takes a little practice to get the procedure correct.

Step 2 (a): After the students have recorded their predictions, let them move to any available free wall or black/whiteboard.

Step 2 (b): One student should hold the paper at the center using either a finger or capped pen firmly enough so that it does not drop but loose enough so that another student can spin the paper around the center point on the paper.

Step 2 (c): Have the second student practice spinning the paper clockwise at a steady rate.

Step 2 (d): While the second student is turning the paper, have the third student practice drawing a line from top to bottom on the paper with the nonwriting end of the marker.

Step 3: After the students have synchronized their movements and are comfortable, have the student with the marker turn the marker to the writing position and while the paper is spinning, draw a line downwards from the top to the bottom of the paper.

Step 4: Have the fourth student label the sheet with the direction of travel and the start and end points of the line.

Step 5: Repeat Step 1 through Step 4 after turning the paper over and reversing the direction of spin. The only difference is the direction of the spin. All students should repeat their roles.

At the completion of the activity explain that the Coriolis effect causes an object to be deflected to the east (right), in the Northern Hemisphere and to the west (left) in the Southern Hemisphere. For your edification, the paper turning counterclockwise represented the Northern Hemispheric direction.

When this activity is completed, ask the students if their predictions agreed with their results. Ask the students to describe the lesson and write a paragraph or two describing how the procedure modeled the effect of the spin of the Earth on the ocean and air masses. Ask the students which of the two directions represented movement related to the Coriolis effect in the Northern Hemisphere.

Outcome/Assessment:

This lesson is fairly simple to assess. Once the students have mastered the turning and drawing, make sure their understanding has followed by reading through the Web sites and looking at their models to see how well they agree with the readings. The writing assignment need not be a major undertaking as long as the students are confident that they understand the direction things appear to move when related to the equator. Be very careful though, when the students use right and left in terms of the directions, it is better to have them use toward the east or toward the west relative to the equator.

Extension:

If the Coriolis effect causes air moving toward the poles to move to the east in the Northern Hemisphere and to the west in the Southern Hemisphere, how is it possible to explain to your classmates why the winds in the mid-latitudes of the Unites States are prevailing westerlies?

Internet Resources

Atmospheric Pressure and Winds are the keywords. The URLs include:

Classroom Resources:

A large sheet of paper 18” by 24” or larger for each group of four students.
Wall space to work.
2 different colored markers, one for each side of the sheet of paper