13. air pressure and winds

8/8/2019 13. Air Pressure and Winds

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Air Pressure and WindsAir Pressure and Winds

J.T.II OlivarJ.T.II Olivar

Faculty of Arts and LettersFaculty of Arts and Letters

University of Santo TomasUniversity of Santo Tomas


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Outline of the LectureOutline of the Lecture

Atmospheric PressureAtmospheric Pressure

Measuring Air PressureMeasuring Air Pressure Pressure ReadingsPressure Readings

Why the Wind blows?Why the Wind blows?

Forces that affect the horizontal movement ofForces that affect the horizontal movement ofairair

Wind DirectionWind Direction


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Atmospheric PressureAtmospheric Pressure

The mass of air above a given level.The mass of air above a given level.

As we climb in elevation above the EarthsAs we climb in elevation above the Earthssurface, there are fewer air molecules abovesurface, there are fewer air molecules aboveus; hence, atmospheric pressure alwaysus; hence, atmospheric pressure alwaysdecreases with increasing height.decreases with increasing height.


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Measuring Air PressureMeasuring Air Pressure

BarometersBarometers

Are instruments that detect and measureAre instruments that detect and measurepressure changes.pressure changes.

It literally means an instrument that measuresIt literally means an instrument that measuresbars.bars.

Bar is a unit of pressure that describes a forceBar is a unit of pressure that describes a forceover a given area.over a given area.


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Millibar (mb)Millibar (mb)

The unit of pressure most commonly found onThe unit of pressure most commonly found onsurface weather maps.surface weather maps.

One millibar is equal to oneOne millibar is equal to one--thousandth of athousandth of abar.bar.

1013.2 mb = 1013.2 hPa = 29.92 in. Hg.1013.2 mb = 1013.2 hPa = 29.92 in. Hg.


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Atmospheric Pressure in inchesAtmospheric Pressure in inches

of Mercury (in. Hg.)of Mercury (in. Hg.)


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Mercurial BarometerMercurial Barometer


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Evangelista Torricelli invented theEvangelista Torricelli invented themercurial barometer in 1643.mercurial barometer in 1643.


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Aneroid BarometerAneroid Barometer


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BarographBarograph


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Pressure ReadingsPressure Readings

Station pressureStation pressure

The barometer reading at a particular locationThe barometer reading at a particular locationand elevation.and elevation.


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Altitude CorrectionsAltitude Corrections

Altitude corrections are made so that aAltitude corrections are made so that abarometer reading taken at one elevationbarometer reading taken at one elevation

can be compared with a barometercan be compared with a barometerreading taken at another.reading taken at another.

SeaSea--level pressurelevel pressure

Station pressure observations are normallyStation pressure observations are normallyadjusted to a level of mean seaadjusted to a level of mean sea--levellevel thethelevel representing the average surface of thelevel representing the average surface of theocean.ocean.


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Near the Earths surface, atmosphericNear the Earths surface, atmosphericpressure decreases on the average bypressure decreases on the average by

about 10 mb for every 100 m increase inabout 10 mb for every 100 m increase inelevation (about 1 in. of Mercury for eachelevation (about 1 in. of Mercury for each10001000--ft rise).ft rise).


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IsobarsIsobars

Lines connecting points of equal pressure.Lines connecting points of equal pressure.

Isobars are drawn as solid dark lines atIsobars are drawn as solid dark lines atintervals of 4 mb, with 1000 mb being theintervals of 4 mb, with 1000 mb being thebase value.base value.

Surface map (seaSurface map (sea--level pressure chart)level pressure chart)

Surface weather mapSurface weather map When weather data are plotted on the map.When weather data are plotted on the map.


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SeaSea--level pressure chartlevel pressure chart


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Surface ChartSurface Chart


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Surface Map (seaSurface Map (sea--level pressurelevel pressure

chart)chart) Shows areas of high and low pressure andShows areas of high and low pressure and

arrows that indicate wind directionarrows that indicate wind direction thethe

direction from which the wind is blowing.direction from which the wind is blowing. The Hs on the map indicate the centers ofThe Hs on the map indicate the centers of

high pressure, which are also calledhigh pressure, which are also called

anticyclones.anticyclones. The Ls represent centers of low pressure,The Ls represent centers of low pressure,

also known as depressions, midalso known as depressions, mid--latitudelatitudecyclones, or extra tropical cyclones.cyclones, or extra tropical cyclones.


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UpperUpper--Air ChartAir Chart


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UpperUpper--Air ChartAir Chart

The upperThe upper--air map is a constant pressureair map is a constant pressurechart because it is constructed to showchart because it is constructed to show

height variations along a constantheight variations along a constantpressure (isobaric) surface, which is whypressure (isobaric) surface, which is whythese maps are also known as isobaricthese maps are also known as isobaric

maps.maps.


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Why the Wind blows?Why the Wind blows?

Forces that affect the horizontalForces that affect the horizontalmovement of air:movement of air:

Pressure Gradient Force (PGF)Pressure Gradient Force (PGF)

Coriolis Force (CF)Coriolis Force (CF)

Centripetal ForceCentripetal Force

Frictional ForceFrictional Force


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Pressure Gradient Force (PGF)Pressure Gradient Force (PGF)


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PGFPGF

When differences in horizontal air pressureWhen differences in horizontal air pressureexist there is a net force acting on the air.exist there is a net force acting on the air.

PGF is directed from higher toward lowerPGF is directed from higher toward lowerpressure at right angles to the isobarpressure at right angles to the isobar

Steep pressure gradient correspond to strongSteep pressure gradient correspond to strong

pressure gradient forces and vice versa.pressure gradient forces and vice versa. The PGF is the force that causes the windThe PGF is the force that causes the wind

to blow.to blow.


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Coriolis ForceCoriolis Force


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CFCF

Describes an apparent force that is due toDescribes an apparent force that is due tothe rotation of the Earth.the rotation of the Earth.

The CF causes the wind to deflect to theThe CF causes the wind to deflect to theright of its intended path in the Northernright of its intended path in the NorthernHemisphere (NH) and to the left of itsHemisphere (NH) and to the left of its

intended path in the Southern Hemisphereintended path in the Southern Hemisphere(SH).(SH).


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Centripetal ForceCentripetal Force

The inwardThe inward--directed force and results fromdirected force and results fromthe imbalance between the Coriolis forcethe imbalance between the Coriolis force

and the pressure gradient force.and the pressure gradient force.


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Frictional ForceFrictional Force

Winds on a surface weather map do notWinds on a surface weather map do notblow exactly parallel to the isobars;blow exactly parallel to the isobars;

instead, they cross the isobars, movinginstead, they cross the isobars, movingfrom higher to lower pressure.from higher to lower pressure.

The frictional drag of the ground slows theThe frictional drag of the ground slows the

wind down.wind down. The atmospheric layer that is influenced byThe atmospheric layer that is influenced by

friction usually extends upward to an altitudefriction usually extends upward to an altitudenear 1000 m or 3000 ft above the surface.near 1000 m or 3000 ft above the surface.


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Wind DirectionWind Direction

13. air pressure and winds

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