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Begin the lesson by asking: What is wind? What do you think makes it blow? Where does the energy come from to power the wind?

Explain that wind is moving air, and the energy to drive it comes from the sun. To illustrate, conduct the following experiment: Place an empty balloon over the top of an empty, clean, 2-liter soda bottle. Ask the class: What do you think will happen to the balloon if we begin to heat the bottle with a hair dryer? Heat the bottle until the balloon inflates. Explain that when air is heated, it expands and becomes less dense. As a result, hot air rises. In the real world, the sun heats EarthÕs surface, which in turn heats the air.

Rising air is only part of the story of what makes the wind blow. Watch the NEWTON'S APPLE video so you can see the"big picture."

Anyone who has ever lived through the fury of a hurricane or witnessed the destructive power of a twister knows just how much punch the wind can pack! What many people don't realize is that when they see the wind blow, they're really watching the power of the sun!

On Earth, our surface is surrounded by an ocean of air called the atmosphere, which, like water, is quite fluid. Just like there are currents in the ocean, our atmosphere has wind currents controlled by many of the same factors, including temperature differences, density differences, and the spin of the planet. Most winds get started because of local changes in the density of air. As with most matter, when air is heated, it expands, causing it to become less dense. Just like in a hot-air balloon, warm air is buoyant. Cool air, which is more dense, moves in and literally pushes the warmer air up, or, in common terms, the warm air rises. We sense the motion as wind.

All of the energy to heat the air comes from the sun, but in general the sun heats the air indirectly. Solar radiation in the form of visible light penetrates our atmosphere and strikes Earth's surface, where it's converted into heat and, as described above, begins to rise. Since the surface of the Earth is quite variable in its makeup (rock, tree, water, and pavement), the air is not heated evenly. Instead, separate pockets of rising warm air masses called thermals are formed, ultimately driving the local wind directions.

While small variations in Earth's surface help to cause localized wind conditions, differential heating and cooling of the atmosphere generates global-scale winds as well. Hot air rising over the equator pushes northward and southward. Upper atmosphere cooling causes the hot air masses to become more dense and sink. Once near the warm Earth, the air heats up and rises again. A vertically circling flow of air, called convection cells, results. The cyclic motion of these air masses is further modified by Earth's own rotation, deflecting them to the east and west. Known as the Coriolis effect, this rotation deflection is what gives rise to the global wind belts including the polar easterlies, mid-latitude westerlies, and tropical trade winds.

How did the global wind belts control the routes of early explorers and traders who depended on the wind to move them around? Besides sailboats, how else was the wind used to power civilizations of the past?

THE HEAT IS ON
WIND BLOW: Student Activity
See how heat from the sun creates wind and how Earth's rotation changes its path.

Convection cells are circular currents of air that result when hot air rises into the upper atmosphere, cools and contracts, sinks down near the Earth's surface, heats up and expands, and then rises again. The rotation of Earth causes these air masses to move in the form of wind. In this activity, you'll create small convection cells and watch their patterns as you put the spin on them.

Materials

• 2 aluminum pie pans
• opaque, white dishwashing liquid
• red or green food coloring with a dropper
• candle in a holder and matches
• watch with second hand

• 1. Fill one of the aluminum pie plates with a half inch of dishwashing liquid. Fill the other plate with a half inch of water.

2. Using the plate with the dishwashing liquid, place several drops of food coloring about halfway between the center and the edge of the plate.

3. Light the candle and place it in its holder. Hold the plate over the candle so that the drops of food coloring are directly over the flame. Wait about 45 seconds and observe what happens to the drops of food coloring. Describe how they look and how they move.

4. When heated, your drops of food coloring act like convection cells that form near Earth's surface. To see how the Earth's rotation might affect these cells, place the first pie plate into the pie plate containing water and spin it for about 30 seconds. Describe the patterns that are formed.

Questions

1. How did the shape of the convection cells change as you heated the plate?

2. The patterns that formed when you spun the plate can be compared to the wind patterns in Earth's atmosphere. Do you think wind patterns flow the same around other planets? Why or why not?

Books and articles

Demillo, R. (1994)
How weather works.
Emeryville, CA: Ziff-David Press.

Gipe, P. (1995)
Wind energy comes of age.
New York: John Wiley & Sons, Inc.

Greeley, R. (1994, Jan 21)
Wind streaks on Venus: Clues to atmospheric circulation.
Science, p. 358.

Onish, L (1995)
Wind and weather.
New York: Scholastic Voyages of Discovery.

Rennicke, J. (1995, Nov)
Why the wind blows.

Schaefer, V. & Day, J. (1981)
A field guide to the atmosphere (a Peterson field guide).
Boston: Houghton Mifflin.

Organizations

NOAA Environmental and Information Services
1315 East West Highway, Room 15400
Silver Spring, MD 20910
(301) 713-0575

Web sites

National Weather Service
www.nws.noaa.gov

The Wind: Our Fierce Friend
sln.fi.edu.tfi/units/
energy/wind.html

How fast does the wind blow in your neighborhood? Try your hand at designing and building your own wind vane and anemometer to measure the direction and speed of the wind. (You could use a bicycle wheel for your anemometer.) Set up a mini-weather station and record the data for a month. Are there any local wind patterns?

How does Earth's surface control the wind? Set up an experiment using two identical cups. Fill one with water and leave the second empty. Place each under a light bulb for 15 minutes. Use a thermometer to see which heats faster and then turn off the light to see how fast they cool. How does your experiment help explain offshore winds ?

Long before there were fossil fuels to power our vehicles, people used the wind. Design and build a tabletop wind racer that goes with the flow. Using a fan, challenge your friends to see who wins the race.

NEWTON'S APPLE video cassettes and educational materials provide further information about this and other topics. Call 1-800-588-NEWTON.