Air Pressure Read the lesson title aloud to students.

Learning Objectives Identify some properties of air. Describe how barometers can be used to measure air pressure. Explain how altitude affects air pressure and density. Click to show each learning objective. Direct students to the key terms: barometer, altitude, and pressure. Ask: Have you heard these terms before? Where? Sample answer: Yes, I heard the term barometer on a weather report. I heard the term altitude on an airplane. Ask: What do you think they mean? Sample answer: I think a barometer is used to predict weather somehow. I think altitude means height. Remind students that “pressure” has a scientific meaning and an everyday meaning. Invite a volunteer to use the word “pressure” in a sentence. Sample answer: My mom puts pressure on me to get good grades. Ask: Is that the scientific meaning or the everyday meaning? Sample answer: the everyday meaning Explain that in science, pressure refers to the physical force of one object on another object with which it is in contact. Even though we think of air as being light, it is putting pressure on us all the time. Tell students that by the end of the lesson, they should be able to describe the properties of air, identify what affects these properties, and explain how the properties are measured.

Properties of Air Mass Density Pressure Air column Explain that air is made up of smaller particles—atoms and molecules. Because these particles have mass, air has mass as well as other properties of matter. Click to reveal “mass.” Tell students that air also has density. Click to reveal “density.” Explain that another property of air is pressure. Click to reveal “pressure.” Explain that air pressure is the force from the weight of a column of air pushing down upon an area. Tell students that there is a column of air above each of us that extends all the way up through the entire atmosphere. In fact, the weight of the column of air above each desk in the classroom is about the same as the weight of a large school bus. The air pressure doesn’t crush the desk because the molecules in air push in all directions—down, up, and sideways. The air pushing down on top of the desk is balanced by the air pushing up on the bottom of the desk. Invite a volunteer to come to the board and draw arrows in the cube to show how air is moving. Students should draw arrows pointing in all directions.

Properties of Air What is density? The amount of mass in a given volume of air Calculate by dividing its mass by its volume What is air pressure? Pressure- is the force pushing on an area or surface Air pressure- is the result of the weight of a column of air pushing on an area Explain that the amount of mass in a given volume of air is its density. You calculate the density of a substance by dividing its mass by its volume. If there are more molecules in a given volume, the density is greater. If there are fewer molecules, the density is less. Ask: How are mass and density related? Answer: If there is more mass in a given volume of air, then the density is greater. If there is less mass in a given volume of air, then the density is less.

What Instruments Measure Air Pressure? Barometer – is used to measure air pressure 2 types of barometer Mercury Barometer Consists of a long glass tube that is closed at one end and open at the other Aneroid Barometer An airtight metal chamber that is sensitive to change in air pressure

Mercury Barometers Closed end Air pressure Mercury Open end Explain to students that there are instruments that can measure air pressure. One instrument is a barometer. Tell students the two common kinds of barometers are mercury barometers and aneroid barometers. Direct students to the picture on the slide. Tell students this is a mercury barometer. A mercury barometer consists of a long glass tube that is closed at one end and open at the other. Click to highlight the open end of the barometer. Explain that the open end of the tube rests in a dish of mercury. Click to highlight the closed end. Explain that the closed end of the tube is almost a vacuum—the space above the mercury contains very little air. Click to highlight the air pressure. Explain that the air pressing down on the surface of the mercury in the dish is equal to the pressure exerted by the weight of the column of mercury in the tube. When the air pressure increases, it presses down more on the surface of the mercury. Greater air pressure forces the column of mercury higher. So, the level of the mercury in the tube shows you the pressure of the air that day. Mercury Open end Mercury barometer

Changes in Air Pressure Describe what happens when air pressure decreases. High air pressure: as air pressure increases, it pushes down on the surface of the mercury in the dish. This causes the mercury tube to rise Low air pressure: as the air pressure drops, the mercury in the tube falls, and the mercury in the dish rises Check students’ understanding of how a mercury barometer works. Ask: How does a mercury barometer work? Answer: Air pressing down on the surface of the mercury in the dish is equal to the pressure exerted by the column of mercury in the tube that is inverted in the dish. When the air pressure increases, it presses down more on the surface of the mercury in the dish. Ask: What does this increase in pressure do to the mercury? Answer: The increased pressure forces the column of mercury higher in the tube. This rise of the mercury in the tube indicates what the air pressure is at that time. Click to reveal the sentence and the blank barometer on the slide. Ask: The barometer on the left shows high air pressure. What would the barometer look like if the air pressure dropped? Invite a volunteer to shade an appropriate level of the mercury in the blank tube and in the dish and describe what is happening. Answer: As the air pressure drops, the pressure on the mercury in the dish decreases. The mercury in the tube will fall and the mercury in the dish will rise. The shading should show a higher level of mercury in the dish and a lower level of mercury in the tube than the barometer on the left. Click to reveal the answer.

Airtight metal chamber Aneroid Barometers Lever Needle Explain that another type of barometer is an aneroid barometer. Explain how the aneroid barometer works. Click to reveal the label for the metal chamber. Explain that the metal chamber is sensitive to changes in air pressure. When air pressure increases, the thin walls of the chamber are pushed in. When the pressure drops, the walls bulge out. Click to reveal the label for the lever. Explain that the chamber is connected to a dial by a series of springs and levers. As the shape of the chamber changes, the needle on the dial moves. Click to reveal the label for the needle. Ask: How does the shape of the metal container shown in the two images change? Answer: On the left, it is bulging. On the right, its walls are pushed in. Ask: Which aneroid barometer indicates high air pressure? Low air pressure? Click to reveal the correct answers. Tell students that the word “aneroid” means “without liquid.” Ask: Given the other type of barometer discussed, why is this an appropriate name? Answer: The aneroid barometer does not use the liquid mercury to measure air pressure. Airtight metal chamber low pressure high pressure

Units of Air Pressure Most weather reports use inches of mercury as the unit of air pressure. 40 inches of mercury 30 inches of mercury 15 inches of mercury Tell students that weather reports use several different units for air pressure. Most weather reports for the general public use inches of mercury. Click to reveal the sentence on the slide. Tell students: For example, if the column of mercury in a mercury barometer is 30 inches high, the air pressure is “30 inches of mercury” or “30 inches.” Click to reveal the air pressure reading. Tell students that on this specific barometer, each marking on the mercury tube indicates 5 inches of mercury. Ask: If the current measure of air pressure is 30 inches of mercury, where would 40 inches be on the barometer? 15 inches? To the left of the barometer, have volunteers draw a line or arrow indicating 40 and 15 inches of mercury, respectively, on the screen. Click to reveal the correct answers. Clarify any misunderstandings. Tell students that the National Weather Service maps indicate air pressure in millibars. The pressure of the atmosphere is equal to one bar. One inch of mercury is about 33.86 millibars, so 30 inches of mercury is equal to about 1,016 millibars.

Altitude and Air Pressure Altitude, or elevation, is the distance above sea level. Read the sentence on the slide aloud. Tell students that air pressure decreases as altitude increases. Tell students: Suppose you have a stack of books. Which book has more weight on it, the second book from the top or the book at the bottom? The second book from the top has the weight of only one book on top of it. The book at the bottom of the stack has the weight of all the books pressing on it. Explain that air at sea level is like the bottom book. Sea level air has the weight of the whole atmosphere pressing on it. Air near the top of the atmosphere is like the second book from the top. There, the air has less weight pressing on it and thus has lower air pressure. Click to highlight the three sets of lines that represent the air columns above the hikers at various altitudes. Reiterate that air pressure is the force from the weight of an air column pushing down upon an area. Ask: How does altitude affect the downward air pressure on the hikers at different altitudes? Answer: As altitude increases, the downward air pressure on the hikers decreases. Ask: Which hiker has the least pressure on him or her? Click to reveal the answer. Ask: Why is air pressure greater at sea level? Answer: Air at sea level has the weight of the whole atmosphere that is above it pressing down on it.

Altitude and Density Explain that as air pressure decreases, so does density. Click to highlight the three circles that represent the density of molecules in the air at various altitudes. Explain that the gas molecules that make up the atmosphere are farther apart at high altitudes than they are at sea level. Tell students: If you were near the top of a tall mountain and tried to run, you would quickly get out of breath. Why? The air contains 21 percent oxygen, whether you are at sea level or on top of a mountain. However, since the air is less dense at a high altitude, each cubic meter of air you breathe has fewer oxygen molecules than at sea level. So you would become short of breath more quickly at a high altitude. Ask: Why might someone have to carry a supply of oxygen when climbing to the top of a very high mountain? Answer: Because the air is less dense the higher you climb, there might not be enough oxygen to breathe at the top of the high mountain where the air is very thin.

Student Worksheet Answers Divide students into small groups. Tell students to write “mercury barometer” over the circle on the left and “aneroid barometer” over the circle on the right. Instruct each group to assign a reader to read the worksheet instructions aloud. Check to see that students have labeled the circles appropriately. Then have students complete the diagram to compare the barometers. Remind students that in a Venn diagram, the overlapping area should have features/characteristics in common and the outside area should have features/characteristics that are different. After a few minutes, create a master Venn diagram on the board or chart paper, using volunteers’ answers to complete the Venn diagram. Use this opportunity to identify and discuss any points of confusion or misconceptions that students have. Worksheet Answers: Student Venn diagrams should compare and contrast a mercury barometer and an aneroid barometer. Accept all Venn diagrams that logically summarize the similarities and differences between a mercury barometer and an aneroid barometer. Similarities may include: Both are tools; both measure air pressure; both have parts that respond to differences in air pressure. Differences may include: They have different parts and shapes; the mercury barometer uses the level of mercury in the tube to give an air pressure reading, but the aneroid barometer uses a needle on a dial to give a reading; the mercury barometer is open, but the aneroid barometer is closed; air pressure pushes on the mercury in the dish of the mercury barometer and on the airtight metal chamber of the aneroid barometer; the mercury barometer uses liquid and the aneroid barometer has only solid parts.