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Chapter 5Chapter 5

Air PressureAir Pressure

AMS Weather StudiesAMS Weather Studies Introduction to Atmospheric Science, 4Introduction to Atmospheric Science, 4 thth Edition Edition

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Case-in-PointCase-in-Point Mount EverestMount Everest

– World’s tallest mountain – 8848 m (29,029 ft)World’s tallest mountain – 8848 m (29,029 ft)– Same latitude as Tampa, FLSame latitude as Tampa, FL– Due to declining temperature with altitude, the summit is Due to declining temperature with altitude, the summit is

always coldalways cold January mean temperature is -36 January mean temperature is -36 °°C (-33 C (-33 °°F)F) July mean temperature is -19July mean temperature is -19° ° C (-2 C (-2 °°F)F)

– Shrouded in clouds from June through SeptemberShrouded in clouds from June through September Due to monsoon windsDue to monsoon winds

– November through February – Hurricane-force windsNovember through February – Hurricane-force winds Due to jet stream moving down from the northDue to jet stream moving down from the north

– Harsh conditions make survival at the summit difficultHarsh conditions make survival at the summit difficult Very thin airVery thin air Wind-chill factorWind-chill factor

– Most ascents take place in MayMost ascents take place in May

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Driving QuestionDriving Question

What is the significance of horizontal and What is the significance of horizontal and vertical variations in air pressure?vertical variations in air pressure?– Air pressure is an element of weather we do not Air pressure is an element of weather we do not

physically sense as readily as temperature and physically sense as readily as temperature and humidity changeshumidity changes

– This chapter examines:This chapter examines: The properties of air pressureThe properties of air pressure How air pressure is measuredHow air pressure is measured The reasons for spatial and temporal variations in air The reasons for spatial and temporal variations in air

pressurepressure

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Defining Air PressureDefining Air Pressure Air exerts a force on the surface of all objects it contactsAir exerts a force on the surface of all objects it contacts

– The air is a gas, so the molecules are in constant motionThe air is a gas, so the molecules are in constant motion– The air molecules collide with a surface area in contact with airThe air molecules collide with a surface area in contact with air

The force of these collisions per unit area is pressureThe force of these collisions per unit area is pressure Dalton’s Law – total pressure exerted by mixture of gases is Dalton’s Law – total pressure exerted by mixture of gases is

sum of pressures produced by each constituent gassum of pressures produced by each constituent gas Air pressure depends on:Air pressure depends on:

– Mass of the molecules and kinetic molecular energyMass of the molecules and kinetic molecular energy Air pressure can be thought of as the weight of overlying air Air pressure can be thought of as the weight of overlying air

acting on a unit areaacting on a unit area– Weight is the force of gravity exerted on a massWeight is the force of gravity exerted on a mass

Weight = (mass) x (acceleration of gravity)Weight = (mass) x (acceleration of gravity) Average sea-level air pressureAverage sea-level air pressure

– 1.0 kg/cm1.0 kg/cm2 2 (14.7 lb/in.(14.7 lb/in.22)) Air pressure acts in all directionsAir pressure acts in all directions

– That is why structures do not collapse under all the weightThat is why structures do not collapse under all the weight

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A mercury thermometer employs A mercury thermometer employs air pressure to support a column air pressure to support a column of mercury in a tubeof mercury in a tube

Air pressure at sea level will Air pressure at sea level will support the mercury to a height of support the mercury to a height of 760 mm (29.92 in.)760 mm (29.92 in.)

Height of the mercury column Height of the mercury column changes with air pressurechanges with air pressure

Adjustments required for:Adjustments required for:– The expansion and contraction of The expansion and contraction of

mercury with temperaturemercury with temperature– Gravity variations with latitude and Gravity variations with latitude and

altitudealtitude

Air Pressure MeasurementAir Pressure Measurement

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Air Pressure Air Pressure MeasurementMeasurement

An aneroid barometer is less An aneroid barometer is less precise, but more portable than precise, but more portable than a mercury barometera mercury barometer

It has a chamber with a partial It has a chamber with a partial vacuumvacuum

Changes in air pressure Changes in air pressure collapse or expand the chambercollapse or expand the chamber

This moves a pointer on a scale This moves a pointer on a scale calibrated equivalent to mm or calibrated equivalent to mm or in. of mercuryin. of mercury

New ones are piezoelectric – New ones are piezoelectric – depend on the effect of air depend on the effect of air pressure on a crystalline pressure on a crystalline substancesubstance

Home-use aneroid barometers Home-use aneroid barometers often have a fair, changeable, often have a fair, changeable, and stormy scaleand stormy scale– These should not be taken These should not be taken

literallyliterally

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Air Pressure MeasurementAir Pressure Measurement

Forecasting uses Forecasting uses air pressure and air pressure and tendency valuestendency values– changes over timechanges over time

Barometers may Barometers may keep a record of keep a record of air pressureair pressure– These are called These are called barographsbarographs

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Air Pressure Air Pressure UnitsUnits

Units of lengthUnits of length– Millimeters or inchesMillimeters or inches

Inches typical for TVInches typical for TV

Units of pressureUnits of pressure– Pascal – worldwide standardPascal – worldwide standard

Metric scaleMetric scale Sea-level pressure = Sea-level pressure =

– 101,325 pascals (Pa)101,325 pascals (Pa)– 1013.25 hectopascals (hPa)1013.25 hectopascals (hPa)– 101.325 kilopascals (kPa)101.325 kilopascals (kPa)

– Bars – U.S.Bars – U.S. A bar is 29.53 inches of mercuryA bar is 29.53 inches of mercury A millibar (mb) is the standard used on weather maps, meaning A millibar (mb) is the standard used on weather maps, meaning

1/1000 of a bar1/1000 of a bar– Usual worldwide range is 970 – 1040 mbUsual worldwide range is 970 – 1040 mb– Lowest ever recorded - 870 mb (Typhoon Tip in 1979)Lowest ever recorded - 870 mb (Typhoon Tip in 1979)– Highest ever recorded – 1083.8 mb (Agata, Siberia)Highest ever recorded – 1083.8 mb (Agata, Siberia)

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Variations in Air Pressure w/AltitudeVariations in Air Pressure w/Altitude Overlying air compresses the atmosphereOverlying air compresses the atmosphere

– the greatest pressure is at the lowest elevationsthe greatest pressure is at the lowest elevations Gas molecules are closely spaced at the surfaceGas molecules are closely spaced at the surface Spacing increases with altitudeSpacing increases with altitude At 18 km (11 mi), air density is only 10% of that at sea levelAt 18 km (11 mi), air density is only 10% of that at sea level

Because air is compressible, the drop in pressure with Because air is compressible, the drop in pressure with altitude is greater in the lower tropospherealtitude is greater in the lower troposphere– Then it becomes more gradual aloftThen it becomes more gradual aloft

Vertical profiles of average air pressure and temperature Vertical profiles of average air pressure and temperature are based on the standard atmosphere – state of are based on the standard atmosphere – state of atmosphere averaged for all latitudes and seasonsatmosphere averaged for all latitudes and seasons

Even though density and pressure drop with altitude, it is Even though density and pressure drop with altitude, it is not possible to pinpoint a specific altitude at which the not possible to pinpoint a specific altitude at which the atmosphere endsatmosphere ends– ½ the atmosphere’s mass is below 5500 m (18,000 ft)½ the atmosphere’s mass is below 5500 m (18,000 ft)– 99% of the mass is below 32 km (20 mi)99% of the mass is below 32 km (20 mi)– Denver, CO average air pressure is 83% of Boston, MADenver, CO average air pressure is 83% of Boston, MA

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Average Air Average Air Pressure Pressure

Variation with Variation with Altitude Altitude

Expressed in mbExpressed in mb

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Horizontal Variations in Air PressureHorizontal Variations in Air Pressure

Horizontal variations are much Horizontal variations are much more important to weather more important to weather forecasters than vertical forecasters than vertical differencesdifferences– In fact, local pressures at In fact, local pressures at

elevations are adjusted to elevations are adjusted to equivalent sea-level valuesequivalent sea-level values

– This shows variations of This shows variations of pressure in the horizontal pressure in the horizontal planeplane

– This is mapped by This is mapped by connecting points of equal connecting points of equal equivalent sea-level equivalent sea-level pressure, producing isobarspressure, producing isobars

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Horizontal Variations in Air PressureHorizontal Variations in Air Pressure

Horizontal changes in pressure Horizontal changes in pressure can be accompanied by can be accompanied by significant changes in weathersignificant changes in weather

In middle latitudes, a continuous In middle latitudes, a continuous procession of different air procession of different air masses brings changes in masses brings changes in pressure and weatherpressure and weather– Temperature has a much more Temperature has a much more

pronounced affect on air pronounced affect on air pressure than humiditypressure than humidity

In general, the weather becomes In general, the weather becomes stormy when air pressure falls stormy when air pressure falls but clears or remains fair when but clears or remains fair when air pressure risesair pressure rises

Air pressure varies continuously

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Horizontal Variations in Air PressureHorizontal Variations in Air Pressure Influence of temperature and humidityInfluence of temperature and humidity

– Rising air temperature = rise in the average Rising air temperature = rise in the average kinetic energy of the individual moleculeskinetic energy of the individual molecules In a closed container, heated air exerts more In a closed container, heated air exerts more

pressure on the sidespressure on the sides– Density in a closed container does not changeDensity in a closed container does not change– No air has been added or removedNo air has been added or removed

The atmosphere is not like a closed containerThe atmosphere is not like a closed container– Heating the atmosphere causes the molecules to space Heating the atmosphere causes the molecules to space

themselves farther apartthemselves farther apart– This is due to increased kinetic energyThis is due to increased kinetic energy– Molecules placed farther apart have a lower mass per unit Molecules placed farther apart have a lower mass per unit

volume, or densityvolume, or density– The heated air is less dense, and lighter.The heated air is less dense, and lighter.

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Horizontal Variations in Air PressureHorizontal Variations in Air Pressure Influence of temperature and humidity, continuedInfluence of temperature and humidity, continued

– Air pressure drops more rapidly with altitude in a column Air pressure drops more rapidly with altitude in a column of cold airof cold air Cold air is denser, has less kinetic energy, so the molecules are Cold air is denser, has less kinetic energy, so the molecules are

closer togethercloser together

– 500 mb surfaces represent where half of the 500 mb surfaces represent where half of the atmosphere is above and half below by massatmosphere is above and half below by mass This surface is at a lower altitude in cold air vs. in warm airThis surface is at a lower altitude in cold air vs. in warm air

– Increasing humidity decreases air densityIncreasing humidity decreases air density The greater the concentration of water vapor, the less dense is The greater the concentration of water vapor, the less dense is

the air due to Avogadro’s Lawthe air due to Avogadro’s Law We often refer to muggy air as heavy air, but the opposite is We often refer to muggy air as heavy air, but the opposite is

truetrue– Muggy air only weighs heavily on our personal comfort Muggy air only weighs heavily on our personal comfort

factorfactor

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Horizontal Variations in Air PressureHorizontal Variations in Air Pressure Influence of temperature and humidity, Influence of temperature and humidity,

continuedcontinued– Cold, dry air masses are the densestCold, dry air masses are the densest

These generally produce higher surface pressuresThese generally produce higher surface pressures

– Warm, dry air masses generally exert higher Warm, dry air masses generally exert higher pressure than warm, humid air massespressure than warm, humid air masses

– These pressure differences create horizontal These pressure differences create horizontal pressure gradientspressure gradients Pressure gradients cause cold or warm air advectionPressure gradients cause cold or warm air advection

– Air mass modifications can also produce Air mass modifications can also produce changes in surface pressureschanges in surface pressures

– Conclusion: local conditions and air mass Conclusion: local conditions and air mass advection can influence air pressureadvection can influence air pressure

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Horizontal Variations in Air PressureHorizontal Variations in Air Pressure

Influence of diverging and converging windsInfluence of diverging and converging winds– Diverging = winds blowing away from a column Diverging = winds blowing away from a column

of airof air– Converging = winds blowing towards a column Converging = winds blowing towards a column

of airof air– Diverging/converging caused by :Diverging/converging caused by :

Horizontal winds blowing toward or away from some Horizontal winds blowing toward or away from some location (this chapter)location (this chapter)

Wind speed changes in a downstream direction Wind speed changes in a downstream direction (Chapter 8)(Chapter 8)

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Influence of Temperature and Influence of Temperature and HumidityHumidity

When air is heated, air When air is heated, air density usually decreases density usually decreases as a result in the increased as a result in the increased activity of the heated activity of the heated molecules. molecules.

Air pressure drops more Air pressure drops more rapidly with altitude in cold rapidly with altitude in cold air than in warm airair than in warm air

Increasing humidity also Increasing humidity also decreases the density of decreases the density of air, because water vapor air, because water vapor has a lower molecular has a lower molecular weight than dry airweight than dry air

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Influence of Diverging and Influence of Diverging and Converging windsConverging winds

If more air diverges at If more air diverges at the surface than the surface than converges aloft, the air converges aloft, the air density and surface air density and surface air pressure decreasepressure decrease

If more air converges If more air converges aloft than diverges at aloft than diverges at the surface, density the surface, density and surface pressure and surface pressure increaseincrease

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Highs and LowsHighs and Lows Isobars are drawn on a map as previously Isobars are drawn on a map as previously

discusseddiscussed– U.S. convention – these are drawn at 4-mb intervals U.S. convention – these are drawn at 4-mb intervals

(e.g., 996 mb, 1000 mb, 1004 mb)(e.g., 996 mb, 1000 mb, 1004 mb) A High is an area where pressure is relatively high A High is an area where pressure is relatively high

compared to the surrounding aircompared to the surrounding air A Low is an area where pressure is relatively low A Low is an area where pressure is relatively low

compared to the surrounding aircompared to the surrounding air Highs are usually fair weather systemsHighs are usually fair weather systems Lows are usually stormy weather systemsLows are usually stormy weather systems

– Rising air is necessary for precipitation formationRising air is necessary for precipitation formation– Lows are rising columns of air. Highs are sinking Lows are rising columns of air. Highs are sinking

columns of air.columns of air.

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The Gas LawThe Gas Law

We have discussed variability of temperature, We have discussed variability of temperature, pressure, and density pressure, and density → these properties are known → these properties are known as variables of state; their magnitudes change from as variables of state; their magnitudes change from one place to another across Earth’s surface, with one place to another across Earth’s surface, with altitude above Earth’s surface, and with timealtitude above Earth’s surface, and with time

The three variables of state are related through the The three variables of state are related through the ideal gas law, which is a combination of Charles’ law ideal gas law, which is a combination of Charles’ law and Boyle’s lawand Boyle’s law– The ideal gas law states that pressure exerted by air is The ideal gas law states that pressure exerted by air is

directly proportional to the product of its density and directly proportional to the product of its density and temperature, i.e. pressure = (gas constant) x (density) x temperature, i.e. pressure = (gas constant) x (density) x (temperature)(temperature)

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The Gas LawThe Gas Law Conclusions from the ideal gas lawConclusions from the ideal gas law

– Density of air within a rigid, closed container remains Density of air within a rigid, closed container remains constant. Increasing the temperature leads to increased constant. Increasing the temperature leads to increased pressurepressure

– Within an air parcel, with a fixed number of molecules:Within an air parcel, with a fixed number of molecules:– Volume can change, mass remains constantVolume can change, mass remains constant– Compressing the air increases density because its volume Compressing the air increases density because its volume

decreasesdecreases

– Within the same air parcel:Within the same air parcel:– With a constant pressure, a rise in temperature is With a constant pressure, a rise in temperature is

accompanied by a decrease in density.accompanied by a decrease in density.– Expansion due to increased kinetic energy increases volumeExpansion due to increased kinetic energy increases volume– Hence, at a fixed pressure, temperature is inversely Hence, at a fixed pressure, temperature is inversely

proportional to densityproportional to density

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Expansional Cooling and Expansional Cooling and Compressional WarmingCompressional Warming

Expansional cooling – when an air parcel Expansional cooling – when an air parcel expands, the temperature of the gas dropsexpands, the temperature of the gas drops

Compressional warming – when the pressure on Compressional warming – when the pressure on an air parcel increases, the parcel is compressed an air parcel increases, the parcel is compressed and its temperature risesand its temperature rises

Conservation of energyConservation of energy– Law of energy conservation/1Law of energy conservation/1stst law of thermodynamics law of thermodynamics

→ heat energy gained by an air parcel either increases → heat energy gained by an air parcel either increases the parcel’s internal energy or is used to do work on the the parcel’s internal energy or is used to do work on the parcelparcel

– A change in internal energy is directly proportional to a A change in internal energy is directly proportional to a change in temperaturechange in temperature

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Conservation of EnergyConservation of Energy

A. If the air is compressed, energy is used to do work on the air

B. If we allow the air to expand, the air does work on the surroundings

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Adiabatic ProcessesAdiabatic Processes

During an adiabatic process, no heat is exchanged During an adiabatic process, no heat is exchanged between an air parcel and its surroundingsbetween an air parcel and its surroundings– The temperature of an ascending or descending The temperature of an ascending or descending

unsaturated parcel changes in response to expansion or unsaturated parcel changes in response to expansion or compression onlycompression only

– Dry adiabatic lapse rate = 9.8 CDry adiabatic lapse rate = 9.8 C°/1000 m (5.5 °F/1000 °/1000 m (5.5 °F/1000 ft)ft)

– Once a rising parcel becomes saturated, latent heat Once a rising parcel becomes saturated, latent heat released to the environment during condensation or released to the environment during condensation or deposition partially counters expansional coolingdeposition partially counters expansional cooling

– Moist adiabatic lapse rate = Moist adiabatic lapse rate = 6 C6 C°/1000 m (3.3 °F/1000 °/1000 m (3.3 °F/1000 ft) → this is an average rateft) → this is an average rate

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Adiabatic ProcessesAdiabatic Processes

Illustration of dry and moist Illustration of dry and moist adiabatic lapse ratesadiabatic lapse rates

Dry adiabatic lapse rate describes the expansionalDry adiabatic lapse rate describes the expansionalcooling of ascending unsaturated air parcelscooling of ascending unsaturated air parcels