ATM OCN 100 Summer 2003 1 ATM OCN 100 – Summer 2004 LECTURE 2B ATMOSPHERIC PRESSURE: A Fundamental Weather Element (con’t.) u A. Practical Problem u B

ATM OCN 100 Summer 2003ATM OCN 100 Summer 2003 22

MADISON’S CURRENT WEATHERMADISON’S CURRENT WEATHERMadison Weather at Madison Weather at 1000 AM CDT MON 23 JUN 2003 Temperature: 25.6°C (78°F) Dewpoint: 15.0°C (59°F) [RH = 52%] Pressure (altimeter): 29.93 inches Hg (1013.6 mb)[Sea-level pressure: 1012.9 mb] Winds: from the S (170 degrees)

at 13 MPH (11 knots; 5.7 m/s) Visibility: 10 or more miles (16+ km) Ceiling: at least 12,000 feet AGL Clouds: sky clear below 12,000 feet AGL Present Weather: no significant weather observed


Current Surface Weather Map Current Surface Weather Map with Isobars (“iso” = equal & “bar” = weight), Fronts & Radarwith Isobars (“iso” = equal & “bar” = weight), Fronts & Radar

Isobars


Deadly Tornado in NebraskaDeadly Tornado in Nebraska



Isobars


Clouds

Current Visible Satellite ImageCurrent Visible Satellite ImageCurrent Visible Satellite ImageCurrent Visible Satellite Image


Tomorrow AM Forecast MapTomorrow AM Forecast Map


The Summer SolsticeThe Summer Solstice was Saturday @ 2:10 PM CDTwas Saturday @ 2:10 PM CDT

See Fig. 3.10 Moran (2002)See Fig. 3.10 Moran (2002)


Rule of Thumb Rule of Thumb

The tighter the packing of the The tighter the packing of the isobars, the stronger the winds!isobars, the stronger the winds!


AnnouncementsAnnouncements

Homework 1 Homework 1 – Get from our webpageGet from our webpage

http://www.aos.wisc.edu/~hopkins/aos100/homework/ http://www.aos.wisc.edu/~hopkins/aos100/homework/

– Due Thursday 26June 2003Due Thursday 26June 2003



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Did You know that –Did You know that –Sunday is summer solstice (7:57 PM CDT)?Sunday is summer solstice (7:57 PM CDT)?



ATM OCN 100 – Summer 2004ATM OCN 100 – Summer 2004LECTURE 2BLECTURE 2B

ATMOSPHERIC PRESSURE:ATMOSPHERIC PRESSURE:A Fundamental Weather ElementA Fundamental Weather Element

A. PRACTICAL PROBLEMA. PRACTICAL PROBLEM– The pump problem: Why can’t The pump problem: Why can’t

water be lifted more than 30 ft?water be lifted more than 30 ft?


B. BASIC CONCEPTS:B. BASIC CONCEPTS:ATMOSPHERIC PRESSUREATMOSPHERIC PRESSURE



Fundamental conceptsFundamental concepts

Importance of air pressureImportance of air pressure

Pressure Weight ( Force)Area

Weight = Mass gravity

or

where





Isobars


Explaining Differences inExplaining Differences in Air Pressure Air Pressure

Low PressureLow Pressure High PressureHigh Pressure

LL HH


Isobars -Isobars -- - lines of equal barometric pressurelines of equal barometric pressure- use sea level corrected pressure- use sea level corrected pressure

HHHH

LL


Display of Pressure Differences on a Display of Pressure Differences on a Weather Map - IsobarsWeather Map - Isobars

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C. BAROMETRYC. BAROMETRY

The Science of BarometryThe Science of Barometry– How is air pressure measured?How is air pressure measured?

Types of BarometersTypes of Barometers– Mercury BarometersMercury Barometers– Aneroid BarometersAneroid Barometers where ...where ...


MERCURY BAROMETERSMERCURY BAROMETERS

Torricelli invention in Torricelli invention in 1643;1643;

A self-supported A self-supported column of mercury;column of mercury;

A balance between A balance between weight of atmosphere weight of atmosphere & weight of mercury & weight of mercury in tube.in tube.


Principles of a Mercury BarometerPrinciples of a Mercury Barometer See Fig 5.1, Moran (2002)See Fig 5.1, Moran (2002)


MERCURY BAROMETERS MERCURY BAROMETERS (con’t.)(con’t.)


ANEROID BAROMETERSANEROID BAROMETERS

Perfected in Perfected in 1840’s;1840’s;

An evacuated An evacuated canister;canister;

A balance between A balance between force of force of atmosphere & atmosphere & force of spring force of spring inside canister.inside canister.

See Fig 5.2 A, Moran (2002)See Fig 5.2 A, Moran (2002)


Aneroid BarometerAneroid BarometerSee Fig 5.2 B, Moran (2002)See Fig 5.2 B, Moran (2002)


BAROGRAPHBAROGRAPH See Fig 5.3, Moran (2002)See Fig 5.3, Moran (2002)


A pressure trace from a barographA pressure trace from a barographSee Fig. 5.6, Moran (2002)See Fig. 5.6, Moran (2002)


C. BAROMETRY C. BAROMETRY (con’t.)(con’t.) Placement of BarometersPlacement of Barometers Pressure UnitsPressure Units

– Inches Inches oror millimeters of mercury; millimeters of mercury;– Pounds per square inchPounds per square inch oror millibars.millibars.

The MillibarThe Millibar– Traditional meteorological pressure unitTraditional meteorological pressure unit

(a metric unit of force per unit area); (a metric unit of force per unit area);– Sea level pressure Sea level pressure 1000 millibars 1000 millibars

( (oror 1 bar). 1 bar).


C. BAROMETRY C. BAROMETRY (con’t.)(con’t.) Placement of BarometersPlacement of Barometers Pressure UnitsPressure Units

– Inches Inches oror millimeters of mercury; millimeters of mercury;– Pounds per square inchPounds per square inch oror millibars.millibars.

The MillibarThe Millibar– Traditional meteorological pressure unitTraditional meteorological pressure unit

(a metric unit of force per unit area); (a metric unit of force per unit area);– Sea level pressure Sea level pressure 1000 millibars 1000 millibars

( (oror 1 bar). 1 bar).


D. CLIMATOLOGY ofD. CLIMATOLOGY ofATMOSPHERIC PRESSUREATMOSPHERIC PRESSURE

Spatial variation in Spatial variation in Sea-level air pressure Sea-level air pressure– Extremes Extremes – Typical range: 980 mb to 1040 mbTypical range: 980 mb to 1040 mb such as … such as …


Average Sea Level Air Pressure & Average Sea Level Air Pressure & Atmospheric CirculationAtmospheric Circulation


D. CLIMATOLOGY ofD. CLIMATOLOGY ofATMOSPHERIC PRESSURE ATMOSPHERIC PRESSURE (con’t.)(con’t.)

Temporal variation of Temporal variation of Sea-level air pressure Sea-level air pressure– Typically less than 1 mb per 3 hours,Typically less than 1 mb per 3 hours,

but… but…


A pressure trace from a barographA pressure trace from a barographSee Fig. 5.6, Moran (2002)See Fig. 5.6, Moran (2002)

5-6 mb in 3 hrs5-6 mb in 3 hrs



Why Do Variations occur in Air Why Do Variations occur in Air Pressure?Pressure?

Piling On Piling On oror taking off taking offby:by:– Local Heating (convection)Local Heating (convection)– Dynamic EffectsDynamic Effects


Vertical Cross-Section of Vertical Cross-Section of Air Converging AloftAir Converging Aloft See Fig. 8.20, Moran (2002)See Fig. 8.20, Moran (2002)

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Map View of Flow around High Pressure Map View of Flow around High Pressure Fig. 1.3A, Moran (2002)Fig. 1.3A, Moran (2002)


Vertical Cross-Section of Vertical Cross-Section of Air Diverging AloftAir Diverging Aloft Fig. 8.21, Moran (2002)Fig. 8.21, Moran (2002)

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Map View of Flow around Low Pressure Map View of Flow around Low Pressure Fig. 1.3B, Moran (2002)Fig. 1.3B, Moran (2002)



Vertical Variations in air pressureVertical Variations in air pressure The higher up, the lower the The higher up, the lower the

pressure.pressure.– Why?Why?


AIR PRESSURE CLIMATOLOGY AIR PRESSURE CLIMATOLOGY (con’t.)(con’t.)Vertical VariationVertical Variation


Pressure fallsPressure falls

Pressure @ top of building Pressure @ top of building ~ 5-6 mb less than ~ 5-6 mb less than

@ street level@ street level

So how tall is the building?So how tall is the building?


VERTICAL PRESSURE PROFILEUS STANDARD ATMOSPHERE, 1976

01020304050

0 200 400 600 800 1000

AIR PRESSURE [millibars]

ALTI

TUDE

[km

]AIR PRESSURE CLIMATOLOGY AIR PRESSURE CLIMATOLOGY (con’t.)(con’t.)


50% of surface


AIR PRESSURE CLIMATOLOGY AIR PRESSURE CLIMATOLOGY (con’t)(con’t)

Variation of air pressure with heightVariation of air pressure with height

– Lower atmosphere: Lower atmosphere: 1mb per 10 m height rise1mb per 10 m height rise;;

– ororSea Level 1000 mb

5,500 meters 500 mb11,000 meters 250 mb


E. ALTIMETRYE. ALTIMETRY

BackgroundBackground– Pressure Altimeters Pressure Altimeters

Aneroid Barometers that indicate Aneroid Barometers that indicate altitude rather than pressurealtitude rather than pressure

ComputationComputation


VERTICAL PRESSURE PROFILEUS STANDARD ATMOSPHERE, 1976

012345

500 600 700 800 900 1000

AIR PRESSURE [millibars]

ALTI

TUDE

[km

]AIR PRESSURE CLIMATOLOGY AIR PRESSURE CLIMATOLOGY (con’t.)(con’t.)

See Fig. 5.4, Moran (2002)See Fig. 5.4, Moran (2002)

1 mb drop for 10 meter height rise


ALTIMETRY ALTIMETRY (con’t.)(con’t.)

Sources of errorSources of error– Standard or model atmosphere Standard or model atmosphere

does notdoes not always apply! always apply!


Altimetry & Air TemperatureAltimetry & Air Temperature(see Figure, pg. 107, Moran (2002))(see Figure, pg. 107, Moran (2002))

ColdColdWarmWarm


Flying with a pressure altimeterFlying with a pressure altimeterSee Fig, pg. 107, Moran (2002)See Fig, pg. 107, Moran (2002)

ColdColdWarmWarm


Sea-Level Pressure ReductionSea-Level Pressure Reduction

Desire –Desire –– To view only horizontal pressure To view only horizontal pressure

variations on weather map;variations on weather map; Need –Need –

– To eliminate effects of topography from To eliminate effects of topography from station pressure.station pressure.


Sea-Level Pressure Reduction Sea-Level Pressure Reduction (con’t.)(con’t.)

ResultResult– Assume that pressure increases at fixed Assume that pressure increases at fixed

rate down to sea level.rate down to sea level.

Station PressureStation Pressure

Station PressureStation Pressure

Sea level Pressure readingsSea level Pressure readings

≈ ≈ 1 mb per 10 m1 mb per 10 m≈ ≈ 1 mb per 10 1 mb per 10 mm


Isobars -Isobars -- - lines of equal barometric pressurelines of equal barometric pressure- use sea level corrected pressure- use sea level corrected pressure

HHHH

LL


Current Surface weather map 24 June 2003Current Surface weather map 24 June 2003



Isobars


Atmosphere in Perspective Atmosphere in Perspective (UCAR)(UCAR)

Documents

ATM OCN 100 Summer 2003 1 ATM OCN 100 – Summer 2004 LECTURE 2B ATMOSPHERIC PRESSURE: A Fundamental Weather Element (con’t.) u A. Practical Problem u B