Air Pressure

Air PressureAir consists of atoms And molecules Which both have mass.Because air has mass, it also has other properties, including density and pressure.

Air Pressure

Density is the amount of mass in a given volume.

You can calculate density by dividing mass by volume.

Air PressurePressure is the force pushing on an area or surface.

A denser substance has more mass than a less dense one.

So denser air exerts more pressure than less dense air.

Air Pressure

There is a column of air above you all the time. The weight of the air in the atmosphere causes air pressure.

Air pressure is the result of the weight of a column of air pushing down on an area.

Air PressureWhy doesn’t air pressure crush your desk?

The molecules in air push in all directions. So the air pushing down on the top of your desk is balanced by the air pushing up on the bottom.

Measuring Air Pressure

Have you ever heard a weather report say that the air pressure is falling?

Measuring Air PressureFalling air pressure usually indicates that a storm is approaching.

Rising air pressure usually means that the weather is clearing.

Measuring Air PressureA barometer is an instrument that is used to measure changes in air pressure.

There are two kinds of barometers: mercury barometers aneroid barometers

Measuring Air PressureA mercury barometer consists of a glass tube

open at the bottom end and partially filled with mercury.

Measuring Air PressureThe word aneroid means “without liquid.”

An aneroid barometer has an airtight metal chamber.

The metal chamber is sensitive to changes in air pressure.

As the shape of the chamber changes, the needle on the dial moves.

Measuring Air Pressure

Increasing AltitudeAltitude, or elevation, is the distance above sea level.Sea level is the average level of the surface of the oceans.

Air pressure decreases as altitude increases.

As air pressure decreases, so does density.

Altitude Affects Air Pressure Imagine a stack of ten books. Which book has

more weight on it, the second book from the top or the book at the bottom?

Air at sea level is like the bottom book.Air near the top of the atmosphere is like the

second book from the top.

Altitude Also Affects Density If you were near the top of a mountain and tried to

run, would you get out of breath quickly?Why would you have difficulty breathing at high

altitudes?As you go up through the atmosphere, air pressure

decreases As air pressure decreases, the density of air

decreases

Pressure and Density based on Elevation