- What causes wind? Where do air masses come from and where do they go?
OverviewAnyone 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.
ActivityConvection 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
- 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. Reader's Digest, p. 82.
Schaefer, V. & Day, J. (1981) A field guide to the atmosphere (a Peterson field guide). Boston: Houghton Mifflin.
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The Wind: Our Fierce Friend