Chapter 12 Section 2AIR Pressure

Essential Questions

1.What are some of the properties of air?

2. What instruments are used to measure air pressure?

3. How does increasing altitude affect air pressure and density?

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.

Properties of Air

Because air has mass, it also has other properties.

•Density•Pressure

DensityThe 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

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.

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.

PressureAir 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?

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.

BarometersMercury Aneroid

BarometersMercury

• 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

Units of Air PressureWeather 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?

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).

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).

High altitude

Air pressure decreases, so does density

Sea level

Air pressure increases and,so does density

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.

Altitude Affects DensityThe 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.

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

Quiz1. Air pressure is

typically measured with a ___________.

2. The force pushing on an area or surface is known as ___________.

3. As density of air decreases, its pressure ____________.

4. Air pressure is often measured in _____________.

Word Bank• increases• decreases• air pressure• altitude• millibars• barometer• thermometer• inches

Essential Questions Answered

1.What are some of the properties of air?Volume, density, pressure,

mass2. What instruments are used

to measure air pressure?Barometers – mercury and

aneroid

Essential Questions Answered

3.How does increasing altitude affect air pressure and density?As altitude increases, air

pressure and density decrease. As altitude decreases, pressure and density increase.