Chapter 12 Section 2

1. Chapter 12 Section 2 AIR Pressure

2. Essential Questions • What are some of the properties of air? • What instruments are used to measure air pressure? • How does increasing altitude affect air pressure and density?

3. Properties of Air The atmosphere is constantly pressing its weight against your body. You may think that air has no mass, but in fact air consists of atoms and molecules. Atoms and molecules both have mass, so air must have mass.

4. Properties of Air • Because air has mass, it also has other properties. • Density • Pressure

5. Density The amount of mass in a given volume of air is its density. Density = Mass If there are more molecules in a given volume, the density is greater. If there are fewer molecules, the density is less. Volume

6. Density If volume stays the same, density increases as mass increases and density decreases when mass decreases. If mass stays the same, density increases when volume decreases and density decreases when volume increases.

7. Pressure The force pushing on an area is pressure. Air pressure is the result of the weight of a column of air pushing down on an area. The column of air that extends upward through the entire atmosphere.

9. Pressure Air pressure can change from day to day. A denser substance has more mass per unit volume than less dense one. So denser air exerts more pressure than less dense air. How does the density of air affect air pressure?

10. Measuring Air Pressure A barometer is an instrument that is used to measure air pressure. Two kinds of barometers are mercury barometers and aneroid barometers.

11. Mercury Aneroid Barometers

12. Mercury Consists of a glass tube open at the bottom end and partially filled with mercury Space above the mercury is almost a vacuum – little air Aneroid Aneroid means without liquid Has airtight metal chamber Chamber is sensitive to changes in air pressure Air pressure increases and the thin chamber wall’s are pushed in Barometers

13. Units of Air Pressure Weather reports use several different units • for general public they use inches of mercury • National Weather Service indicate air pressure in millibars • 1 inch of mercury is about 33.87 millibars • ( 30 inches of mercury) ( 1,016 millibars) What two common units that are used to measure air pressure?

14. Altitude and the Properties of Air At the top of a mountain, air pressure is less than the air pressure at sea level. Altitude, or elevation, is the distance above sea level (the average level of the surface of the oceans).

15. Altitude and the Properties of Air Air pressure decreases as altitude increases. As air pressure decreases, so does density. At sea level air has all the weight of the whole atmosphere pressing on it (=more pressure). The air at the top of the mountain has less weight pressing on it (= less pressure).

16. High altitude Air pressure increases and, so does density Air pressure decreases, so does density Sea level

18. Altitude Affects Density As you go up in the atmosphere, the density of air decreases. This means the gas molecules that make up the atmosphere are farther apart at high altitudes than they are at sea level.

19. Altitude Affects Density The air contains 21 percent oxygen, whether you are at sea level or on top of a mountain. So when you are at a higher altitude there is less oxygen per cubic meter. Same amount in the air just not compacted together. The oxygen molecules are more spread out. So you will give out of breath easily at higher altitudes due to not getting as much oxygen in each breath.

20. Atmospheric Pressure • There is more pressure in the atmosphere closer to the surface of Earth • Less pressure (and less particles of air) makes it harder to breathe in the mountains • Jets flying in the stratosphere must maintain pressurized cabins so people can breathe

21. Air pressure is typically measured with a ___________. The force pushing on an area or surface is known as ___________. As density of air decreases, its pressure ____________. Air pressure is often measured in _____________. Word Bank increases decreases air pressure altitude millibars barometer thermometer inches Quiz

22. Essential Questions Answered • What are some of the properties of air? • Volume, density, pressure, mass • What instruments are used to measure air pressure? • Barometers – mercury and aneroid

23. Essential Questions Answered • How does increasing altitude affect air pressure and density? • As altitude increases, air pressure and density decrease. As altitude decreases, pressure and density increase.