Chapter 15 Section 2

Goals  Identify some properties of air.  Name instruments that are used to measure air pressure.  Explain how increasing altitude affects air pressure and density.

What We Know!  How many of you have ever been on an airplane?  Did your ears pop?  Why do you think that is? (turn and talk)

Properties of Air  Because air has mass, it also has other properties, including density and pressure.  Density- the amount of mass in a given volume of air  Density= Mass/Volume  Pressure- the force pushing on an area or surface.  Air Pressure- the result of the weight of a column of air pushing down on an area.  How are mass and air pressure related?

Air Pressure  Turn to page 517.  The atmosphere is HEAVY. The weight of the column of air above your desk is the same as the weight of a large school bus!  Why do you think it doesn’t crush us?

Measuring Air Pressure  Barometer- an instrument used to measure air pressure.  Two types of barometer include mercury and aneroid barometers.

Mercury Barometer  Consists of a glass tube open at the bottom and partially filled with mercury.  The space in the tube above the mercury is almost a vacuum.  As air pressure pushes down, the mercury level goes up!

Aneroid Barometer  Contains an air tight metal chamber. When air pressure increases, the chamber walls are pushed in.  As the air pressure changes, the needle moves.

Altitude and Properties of Air  At the top of a mountain, the air pressure is less than at sea level.  Air pressure decreases as altitude increases.  As air pressure decreases, so does density.  Altitude-elevation