What Causes Wind? Exploration: How Does Air Move When Pressure Builds Up? 4.2 Explore. Predict

Transcription

1 4.2 Explore What Causes Wind? In Learning Set 1, you built an anemometer. You used it to measure wind speed and direction in your community. In the last section, you read about how wind and ocean currents on a larger scale were important to early explorers. Wind is air moving from one place to another. You will begin by doing an exploration about what causes air to move from one place to another. After you carry out the exploration, you will share your results and ideas with the class. Then you will read what scientists know about what causes wind. Exploration: How Does Air Move When Pressure Builds Up? You will be blowing into a bottle with a crumpled ball of paper partially blocking the neck of the bottle. The air inside the bottle represents an air mass. The air you blow into the bottle represents air added to the air mass, increasing its pressure. The paper represents a particle (molecule or atom) in the air mass in the bottle. There are a couple of things you need to know to interpret the results of this exploration. When you add air to a container, the pressure of the air inside the container increases. Also, increasing the pressure increases the force exerted by the air on the walls of the container. Recall that air pressure is the force the air is exerting on a given area of surface. Therefore, greater pressure means greater force. You will observe what the paper does. For this, you can infer how particles (molecules and atoms) that make up air move when there is a difference in air pressure. Predict Read the Procedure. Predict what will happen to the ball of paper when you blow on it. WW 174

2 4.2 Explore Procedure 1. Cut a piece of paper about 10 cm square. 2. Crumple the paper into a tight ball. 3. Place the paper ball in the opening of a bottle so that it fits loosely. 4. Hold the bottle in a horizontal position and blow on the paper. Materials 1 clean, dry 2-L bottle 10 cm square piece of paper 5. Observe what happens to the ball of paper when you try to blow it farther into the bottle. Record your observations and any ideas you have about what causes the paper to move. Stop and Think 1. When you add air to a container, what happens to the air pressure inside the container? 2. When you blow on the paper in the neck of the bottle, some air moves past the paper and enters the bottle. How does this affect the air pressure inside the bottle? 3. When you blow air into the bottle, how does the air pressure inside the bottle compare with the air pressure outside the bottle? 4. When you blow on the paper, you exert a force that pushes the paper toward the inside of the bottle. If the paper does not move in this direction, there must be an equal or greater force pushing the paper out of the bottle. What force could be pushing the paper out of the bottle? 5. The paper in the opening of the bottle represented a particle that makes up air. How do the particles that make up air move when there is a difference in air pressure? WW 175 WEATHER WATCH

3 Communicate Share Your Ideas To prepare to share your ideas make a poster demonstrating the results of your exploration. Draw a diagram of what you observed. Label areas of high pressure and low pressure. Draw arrows to show the direction in which forces acted or in which air moved. If air moved, label the moving air wind. Then record your answers to these questions: How do particles that make up air move when there is a difference in air pressure? What causes wind? When the posters are completed, select one member of your group to present the results of the exploration to the class. Present your observations and conclusions. Discuss how the results can be used to relate wind to air pressure. Listen carefully to each group s presentation. If another group made observations or conclusions that differ from those made by your group, or if you do not understand a group s presentation, ask questions. After all groups have completed their posters and made presentations, you will place your poster on the wall. Walk around the room and look at the results from other groups. Do all of the posters show the same flow of air that your group observed? If any of the results are different, discuss why the results might not have been the same for the two groups. On each poster, notice the relationship between air pressure and the direction in which the air moved (the wind). Reflect 1. Now that you have looked at the results from all of the groups, what can you conclude about the relationship between air pressure and wind? 2. Develop one or more claims about what causes wind. 3. What else to you think you need to know to explain what causes wind? Air Pressure and Wind Imagine blowing up a balloon. Instead of tying it, you pinch it closed with your fingers. The air pressure is higher inside the balloon than outside because the balloon material is squeezing on the air. The particles that make up air inside the balloon are crowded together. WW 176

4 4.2 Explore You can think of air pressure in terms of collisions of particles. When the particles strike the inside of the balloon, they exert a force on the balloon s walls. When more air is pushed into the balloon there are more collisions on the sides of the balloon, which means that there is more pressure. The balloon expands because the particles collide more often with the walls and push them outward. What happens when you slowly let air out of the balloon? First, it is important to know that the balloon material squeezes the air a bit. This keeps the pressure inside the balloon a little bit higher than the pressure of the air outside the balloon. When the balloon is opened, the particles that make up air move out of the balloon. They move from the region of higher pressure inside the balloon into the lower pressure outside the balloon. They can now move freely and spread apart from one another more easily than when they are in the balloon. If you put your hand by the opening of the balloon, you can feel the air moving out of it. Air molecules inside the balloon are crowded closer together than air molecules outside the balloon. Therefore, the air pressure inside the balloon is higher than the air pressure outside the balloon. When you stop pinching the balloon s opening, there is a net flow of air from high pressure (inside) to low pressure (outside). This same process causes wind in the atmosphere. Wind is the movement of air from areas of higher pressure to areas of lower pressure. You will probably not be surprised to know that the Sun s energy plays a big role in causing the winds. If you think about what you already know about air temperature, you may be able to figure out what causes winds. The particles that make up air move freely in all directions. As they move about, they collide with one another. WW 177 WEATHER WATCH

5 Air in the atmosphere moves in air masses. Some air masses have warmer air, and some have cooler air. Particles in warm air have more energy than particles in cool air, so that they move faster, on average, than particles in cool air. The molecules in warmer air also collide more often and harder than molecules in cooler air. As they collide, they spread out. This means there tends to be more space between particles in warm air than between particles in cool air. Because the particles are farther apart in warm air than in cool air, warm air masses exert lower pressure than cool air masses. Air particles move from areas with higher pressure to areas with lower pressure. This means that wind will tend to blow from areas with higher pressure to areas with lower pressure. local winds: winds that blow over short distances near Earth s surface. You know that the molecules in warm air are farther apart than molecules in cool air, so warm air is less dense than cool air. Since warm air is less dense than cool air, it tends to rise, while cool air tends to sink. Liquids or gases that are free to flow will arrange themselves according to their density. The less dense warm air rises and the denser cool air sinks. Near Earth s surface, air that is lower in the atmosphere is warmer than air that is higher in the atmosphere. This is because Earth s surface heats air that is close to it. Liquids are free to flow and will arrange themselves by density. The liquid on the top is salad oil, which is less dense than the vinegar on the bottom. You can now begin to understand how the Sun s energy affects winds. The Sun heats Earth s surface, which in turn heats the air above it. Warm air masses near the surface are low-pressure masses, while cool air masses above the surface are higher-pressure masses. Near Earth s surface, this causes winds that blow over short distances, from high-pressure areas to low-pressure areas. These are called local winds. Local winds occur in the lower part of the atmosphere, close to Earth s surface. WW 178

6 4.2 Explore Effects of Land and Water on Local Winds While investigating temperature, you read that land and water heat up and cool down at different rates. You know that this has an effect on air temperatures. Areas closer to large bodies of water have different climates from areas at the same latitude that are farther from large bodies of water. The heating of land and water also has an effect on local winds. You know that during the day, land heats up more quickly than water. This means the air above land is warmer than air above the water. This sets up a pressure difference. Warm air over the land rises and is replaced by the cooler air moving from high pressure over the water toward the low pressure over the land. During the day, when air moves from the water to the land, it is called a sea breeze. At night, when the land cools off faster than the water, the air over the land is cooler than the air over the water. The air now fl ows from the land to the water. This is called a land breeze. WW 179 WEATHER WATCH

7 Stop and Think 1. How is warm air different from cool air? 2. How are collisions of air molecules with the walls of a balloon related to air pressure? 3. What are two reasons why an area would have high air pressure? 4. Why does air move from areas with higher air pressure toward areas with lower pressure? 5. Why is warm air less dense than cool air? 6. Why do land breezes and sea breezes move in opposite directions? Explain You have already made a claim about what causes wind. You know that air moves because of differences in air pressure. Now you will use evidence from your exploration. You will also use the information from the reading to support your claim about what causes wind and to explain how the motions of air molecules and the Sun s energy can account for the wind. Remember that a good explanation has this structure: your claim your evidence your science knowledge a statement connecting your claim to your evidence and the science you know Use a Create Your Explanation page to guide you. Begin by recording the question you are answering: What causes wind? Then, if you need to, work with your group to revise the claim you already made about what causes wind. Record your claim. Record evidence that supports your claim. Your evidence might come from the explorations you did. It might also come from investigations you did earlier in the Unit. Include in your evidence what you observed about air pressure. WW 180

8 4.2 Explore The science knowledge that supports your claim should come from what you read in this section. You can also use what you have read in previous Learning Sets. You should clearly state the role the Sun plays in warming air masses, the role of density, and what you know about how particles within air masses move. Make sure the claim, evidence, and science knowledge you record are all consistent with one another. Add any evidence you remember as you are recording science knowledge. Develop an explanation statement that combines what you know about how the Sun s energy causes winds. Your explanation statement should include your claim and the evidence and science knowledge that support your claim. It should help others to understand your claim and convince them that your claim is trustworthy. Your explanation statement should show the reasoning you used to connect together all of the science knowledge that helps you understand what causes winds. Discuss how accurate you think your explanation is. What else would you need to know to make a more complete explanation? Use a Create Your Explanation page to guide you. You have seen that air moves because of differences in pressure. Think about what causes differences in pressure between two places in the atmosphere. Use the evidence from your explorations and the information from your reading to explain how the motions of particles that make up air can account for the wind. Communicate Share Your Explanation When it is your group s turn, share your explanation with the class. Tell the class what makes your claim accurate based on the evidence you have and your science knowledge. Tell the class what else you think you need to know to make your explanation better. As each group shares its explanation, pay special attention to how other groups have supported their claims with science knowledge. Ask questions or make suggestions if you think a group s claim is not as accurate as it could be or if the group has not supported its claim well enough with evidence and science knowledge. WW 181 WEATHER WATCH

9 Revise Your Explanation After all of the groups have made their presentations, discuss the strengths and weaknesses of each explanation. Create a class claim and use it, along with evidence and science knowledge, to create a class explanation of what causes wind. Update the Project Board Return to the Project Board and add what you know about what causes winds to change to the What are we learning? column. Be sure to include a description of how the Sun s energy causes differences in air pressure, and how winds are related to changes in weather. Put your evidence from the exploration and your reading in the What is our evidence? column. In the What do we need to investigate? column, add questions you need to answer about factors that you were not able to include in your explanations. What s the Point? Wind is the movement of air from one area to another caused by a difference in air pressure. When there is a pressure difference between two air masses, more particles that make up air move from the high-pressure area to the low-pressure area, so there is a net movement of air from high pressure to low pressure. Pressure changes in Earth s atmosphere occur as air is warmed by energy from the Sun. In warmer air, molecules are farther apart than in cooler air. When molecules are less crowded, the rate of collisions decreases and the pressure decreases. Warm air rises because it is less dense than cool air. Land and water absorb energy at different rates, and this causes temperature differences in the atmosphere. During the day, a sea breeze occurs when air moves from cool areas over the water (high pressure) toward warm areas over the land (low pressure). At night, a land breeze moves in the opposite direction because air over the land is cooler (high pressure) than air over the water (low pressure). WW 182