Activity Time: 20 minutes
Recommended Grades: Middle School to High School
Objectives: In this experiment, we explore how Earth’s rotation affects wind direction.

• Balloon
• Two different color markers. Permanent markers are best because they don’t rub off of the balloon.
• Optional: “My Radar,” a free app that shows Earth’s winds in real time.

1. Blow up a balloon. Try to make it as round as possible, as it is a model for Earth.
2. Draw in the equator and label the North Pole and South Pole.
3. Hold the balloon at eye level with the North Pole on the top and rotate it from right to left. This models the Earth’s rotation from west to east.
4. While one person rotates the earth balloon, the other person examines the movement of Earth from the North Pole perspective and from the South Pole perspective.
5. As you look from the North Pole toward the equator: Is the balloon spinning clockwise or counterclockwise?
6. As you look from the South Pole toward the equator: Is the balloon spinning clockwise or counterclockwise?
7. While one person continues to rotate the balloon steadily from left to right, the other slowly tries to draw a line straight from the North Pole, south to the equator. Did the line curve to the right or left?
8. Now use a different colored marker and while your Earth continues to rotate, try to draw a straight line from the South Pole, north to the equator. Did the line curve to the right or left?
9. Draw short lines towards the equator that match the direction of the lines you drew from the poles. The winds from the north and south hemispheres meet near the equator. This is called the Intertropical Convergence Zone.

Gustave-Gaspard Coriolis, a French physicist, noticed that if an object was spinning and another object traveled straight across it, like a tennis ball across a moving merry go round, the object appeared to curve to one side. This is because the larger circumference (the outside of the merry go round) has to move faster than the smaller circumference (the center of the merry go round) in order to stay as one piece. On Earth, the speed of rotation at the equator is 1037 miles per hour, at the poles is 0 miles per hour, and halfway in between (latitude 45) 733 miles per hour.

We see this with our balloon. As we tried to draw a straight line from the top of the rotating balloon towards the center, the wider part of the balloon had to travel faster than the smaller top part. When the marker got to the wider part, that part had moved farther than the area just above it, resulting in a curved line. In the northern hemisphere, the line curves to the right. In the southern hemisphere, the line curves to the left.

Low-pressure air masses rotate inwards and high-pressure air masses rotate outwards. The Coriolis Effect is what causes the low-pressure air masses in the northern hemisphere to rotate counter-clockwise and high-pressure air masses to rotate clockwise. In the southern hemisphere, the opposite is true.

If you have downloaded the free app “My Radar,” go into the layers and select “winds.” You can see Earth’s current winds. High pressure systems gracefully spiral outwards, and low-pressure systems coil tightly inwards. Compare the direction of rotation above and below the equator. You can find the Intertropical Convergence Zone near the equator where the winds from each hemisphere meet.

Key Terms:

Equator - an imaginary line drawn around the widest part of Earth, dividing the planet into the northern hemisphere and the southern hemisphere.
Circumference - the distance around an object.
North Pole - the northern axis of rotation and the northernmost point on Earth.
South Pole - the southern axis of rotation and the southernmost point on Earth.
Intertropical Convergence Zone - an area near the equator where winds from the northern hemisphere and winds from the southern hemisphere meet.
High-pressure air mass - area of air that has higher pressure than the air surrounding it.
Low-pressure air mass - area of air that has lower pressure than the air surrounding it.
Coriolis Effect - a moving object seems to veer toward the right when going across something rotating counter-clockwise and toward the left when going across something rotating clockwise.