Air Pressure

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.

Properties of Air

• Mass

• Density

• Pressure

Air column

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

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

Open end

Closed end

Mercury

Mercury barometer

Air pressure

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

Aneroid Barometers

Lever

Needle

Airtight metal chamber

high pressurelow pressure

Units of Air PressureMost weather reports use inches of mercury as the unit of air pressure.

30 inches of mercury

40 inches of mercury

15 inches of mercury

Altitude and Air PressureAltitude, or elevation, is the distance above sea level.

Altitude and Density