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Winds & Pressure
Background Information: Wind is moving air. It is caused by differences in air pressure. Air
rushes from high pressure areas to lower ones. Since the Earth is rotating, however, the air does not flow directly from
high to low pressure, but it is deflected to the right (in the Northern Hemisphere; to the left in the Southern
Hemisphere), so that the wind flows mostly around the high and low pressure areas. This is known as the Coriolis Effect.
The pressure exerted by a gas like air changes as it becomes denser or less dense. When air is warmed, it expands and
becomes less dense. As the air becomes less dense, its air pressure decreases. This occurs because molecules in warm
air have greater kinetic energy (energy of motion) than in cold air. As the molecules move faster they spread out. As the
molecules move farther apart the volume of the air increases and its density decreases. When air is cooled, its molecules
move closer together. The air contracts and becomes denser. As air becomes denser, it contains more mass per unit
area and exerts greater pressure. Therefore, cooler, denser air have a higher air pressure than warmer, less dense air. As
air warms it becomes less dense than the cooler surrounding air and rises as a result.
As the sun warms the Earth's surface, the atmosphere warms too. Some parts of the Earth receive direct rays from the
sun all year and are always warm. Other places receive indirect rays, so the climate is colder. Warm air, which has less
mass than cold air, rises. Then cool air moves in and replaces the rising warm air (convection). This movement of air is
what makes the wind blow. During the day, the air above the land heats up faster than the air over water. The warm air
over the land expands and rises, and the heavier, cooler air rushes in to take its place, creating wind. At night, the winds
are reversed because the air cools more rapidly over land than over water.
When air of high pressure is near air of low pressure, air will move from the high pressure area to the low pressure area.
Draw a diagram to show how wind is produced below:
Purpose: To investigate how changes in pressure create wind
Materials:
Long balloon
Fishing line
Straw
M. Poarch 2015
science-class.net
Permission to copy granted for non-profit, educational use only.
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Procedure:
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Thread the fishing line through the straw.
Tie the ends of the fishing line to the legs of two different chairs.
Spread the chairs apart so that the fishing line is taut.
Blow up your balloon, but do not tie the end off.
With your partner’s help tape the balloon to the straw.
Pull the balloon to one end of the fishing line.
Let go of the balloon and observe what happens. Record your observations.
Repeat two times.
Data:
Observations of Balloon
Trial 1
Trial 2
Trial 3
Conclusions:
1.
Before you let the balloon go, the air inside the balloon was being squeezed by the rubber or latex of the
balloon. The air outside of the balloon was not being squeezed. Where was the air pressure the greatest?
Explain your answer.
2. Explain how this investigation models what happens when wind is produced. What are limitations of this
model?
M. Poarch 2015
science-class.net
Permission to copy granted for non-profit, educational use only.