Air PressureAir Pressure

Ch 15 Sec. 3Ch 15 Sec. 3

By:Irma KrupicBy:Irma Krupic

T-3 T-3

Contents Contents

Guide for reading Guide for reading Properties of airProperties of air Measuring air pressure Measuring air pressure

Properties of AirProperties of Air Atoms,mass,moleculesAtoms,mass,molecules

, density,pressure., density,pressure. Density:amount of Density:amount of

mass in a given mass in a given volume of air.volume of air.

The more molecules in The more molecules in a given volume of air a given volume of air the higher the the higher the density.Fewer density.Fewer molecules =low molecules =low density.density.

Force pushing on an Force pushing on an area is pressure. area is pressure.

Denser Denser substance=more substance=more mass per unit mass per unit volume than a less volume than a less dense one.dense one.

Air pressure: weight Air pressure: weight of a column of air of a column of air pushing down on an pushing down on an area.Molecules in air area.Molecules in air push in all directions.push in all directions.

Instrument Used to Instrument Used to Measure Air PressureMeasure Air Pressure

Mercury Mercury barometers+aneroid barometers+aneroid barometers arebarometers are

used to measure used to measure changes in air changes in air pressure.pressure.

Falling air Falling air pressure=storm is pressure=storm is coming.coming.

Rising air Rising air pressure=weather is pressure=weather is clearing.clearing.

Mercury BarometerMercury Barometer

Was the first barometer invented. Was the first barometer invented. Glass tube open at bottom. Partially filled Glass tube open at bottom. Partially filled

with mercury. Space above the mercury with mercury. Space above the mercury has no air.Open end of tube is in a dish of has no air.Open end of tube is in a dish of mercury.Air pressure pushing down on mercury.Air pressure pushing down on surface of mercury in dish is equal to surface of mercury in dish is equal to weight of column in the tube. When air weight of column in the tube. When air pressure increases ,it presses down more pressure increases ,it presses down more on the mercury. Greater air pressure on the mercury. Greater air pressure makes the column of mercury higher. makes the column of mercury higher.

Aneroid BarometerAneroid Barometer Has airtight metal Has airtight metal

chamber,metal chamber chamber,metal chamber is sensitive to changes in is sensitive to changes in air pressure.air pressure.

As air pressure As air pressure increases,thin walls of increases,thin walls of chamber push in. As chamber push in. As pressure drops,walls pressure drops,walls come out. come out.

Chamber is connected to Chamber is connected to dial be series of springs dial be series of springs +levers.When shape of +levers.When shape of chamber changes needle chamber changes needle on dial moves. on dial moves.

Units of Air PressureUnits of Air Pressure

Inches of mercury.Inches of mercury. National weather National weather

service maps use service maps use millibars to measure millibars to measure air pressure.air pressure.

One inch of mercury One inch of mercury =33.87 millibars.=33.87 millibars.

30 inches of 30 inches of mercury=1016 mercury=1016 millibars.millibars.

Increasing AltitudeIncreasing Altitude

Air pressure at the top of mount Air pressure at the top of mount McKinley is more than 6 kilometers McKinley is more than 6 kilometers above sea level, that’s less than half above sea level, that’s less than half the air pressure above sea level.the air pressure above sea level.

Altitude-distance above sea level.Air Altitude-distance above sea level.Air pressure decreases as altitude pressure decreases as altitude increases. When air pressure increases. When air pressure decreases,so does density. decreases,so does density.

Altitude Affects DensityAltitude Affects Density Air pressure is Air pressure is

greatest at sea level.greatest at sea level. Near the top of the Near the top of the

atmosphere it has less atmosphere it has less weight pressing on it.weight pressing on it.

As you go up through As you go up through the atmosphere, air the atmosphere, air pressure decreases, pressure decreases, and as air pressure and as air pressure decreases,the density decreases,the density of air decreases too. of air decreases too.

Web ResourcesWeb Resources

http://www.grc.nasa.gov/WWW/K-http://www.grc.nasa.gov/WWW/K-12/airplane/pressure.html12/airplane/pressure.html

http://kids.earth.nasa.gov/http://kids.earth.nasa.gov/archive/air_pressure/archive/air_pressure/