15.2 Air Pressure Key Concepts: 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? Key terms: Density, pressure, air pressure, barometer, mercury barometer, aneroid barometer, altitude

Properties of air Because air has mass, it also has other properties, including density and pressure Density = mass/volume Things that are heavier (more mass) and smaller (less volume) are more dense.

Pressure The force pushing on an area or surface is known as pressure. The weight of the atmosphere exerts a force on surfaces. Air pressure is the result of the weight of a column of air pushing down on an area. Air is super heavy, actually. The reason we aren’t crushed by it is because it is balanced on all sides of us. Air pushes on us down, up, side to side.

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

Mercury barometers: Consist of a glass tube open at the bottom end and partially filled with mercury. Air pressure pushes down on the surface of the mercury in the dish, causing the mercury in the tube to rise.

Aneroid barometers The word aneroid means “without water” Has an airtight metal chamber. The chamber is sensitive to changes in air pressure.

Units of air pressure Uses inches or millibars. One inch of mercury is equal to millibars

Altitude and properties of air Altitude – elevation – distance above sea level Air pressure decreases as altitude increases. As air pressure decreases, so does density. How does altitude affect air pressure?

Altitude vs. air pressure As altitude/elevation increases, there is less air pressure because the air is less dense. This means that the gas molecules are farther apart at high altitudes than they are at sea level. Because of this, air pressure is less at high elevations. It is also harder to breathe at elevation because oxygen molecules are farther apart. At sea level, air pressure is greater because of the weight of all of the air above it.