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Physical GeographyPhysical Geographyby Alan Arbogastby Alan Arbogast

Chapter 6Chapter 6

Atmospheric Pressure, Wind, & Global Circulation

Lawrence McGlinnDepartment of GeographyState University of New York - New Paltz

From reading chapter 6, why is it not very windy today?

a) Because air moves from low pressure to high pressureb) because there is a weak pressure gradientc) because there is a strong pressure gradientd) because there is the absence of a pressure gradient

Answer: B

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Atmospheric Pressure, Wind,Atmospheric Pressure, Wind, & Global Circulation & Global Circulation

• Atmospheric Pressure

• Atmospheric Pressure Systems

• The Direction of Air Flow

• Global Pressure & Atmospheric Circulation

• Local Wind Systems

• Oceanic Circulation

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Air PressureAir Pressure

• Weight of air exerting pressure on surface of Earth

• Air pressure decreases with altitude

• Warm air is lighter than cool air, so it tends to rise – cool air sinks

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Air Pressure & AltitudeAir Pressure & Altitude

Lower Density,Lower Pressure

Higher Density,Higher Pressure

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Measuring Air PressureMeasuring Air Pressure

Barometer measures air pressure in millibars or inches

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Atmospheric Pressure SystemsAtmospheric Pressure Systems

High-pressure system- also called anticyclone- circulating body of air- descending air-clockwise circulation in

northern hemisphere

Low-pressure system- also called cyclone- circulating body of air- rising air- counterclockwise circulation in

northern hemisphere

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Atmospheric Pressure MapAtmospheric Pressure MapIsobars – lines of equal air pressure on a map

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Wind DirectionWind Direction

Winds are namedaccording to thedirection from which they are blowing

"Wen Xiu - The Last Imperial Consort of the Qing Dynasty in China." Http://www.cultural-china.com/. Web. 06 Feb. 2012.

In the Northern Hemisphere, descending air spirals in a _____ direction, and upon reaching the surface it _____.

a) clockwise; convergesb) counterclockwise; convergesc) clockwise; divergesd) counterclockwise; divergese) vertical; rains

Answer: C

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Direction of Air FlowDirection of Air Flow

• Unequal heating of land surfaces

• Pressure gradient force

• Coriolis force

• Frictional forces

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Unequal HeatingUnequal Heating

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

Fluctuations in the Pressure Gradient

High pressure to low pressure; perpendicular to isobars

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Coriolis ForceCoriolis Force•Due to Earth's rotation

•Function of latitude

•Pulls wind to right in northern hemisphere

•Pulls wind to left in southern hemisphere

•Strongest at poles

•Weakest at equator Coriolis Force

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Dynamic ConvectionDynamic Convection

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Frictional ForcesFrictional Forces• Near surface, friction (f) works against

pressure gradient force (pgf), so resulting wind direction is between pressure gradient and coriolis force (cf)

pgf

f

cf – northern hemisphere

WIND

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Three Forces CombinedThree Forces Combined

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Global Pressure & Global Pressure & Atmospheric CirculationAtmospheric Circulation

• Unequal heating of tropics and poles

• Global circulation on non-rotating, uniform Earth

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Global Pressure &Global Pressure &Atmospheric CirculationAtmospheric Circulation

• Rotation and variation on Earth’s surface complicates atmospheric circulation

• Latitudinal belts of high or low pressure and/or winds develop

• Convection loops develop over tropics and over mid-latitudes

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Global Circulation ModelGlobal Circulation Model

Global Atmospheric Circulation

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Tropical CirculationTropical Circulation

• Intertropical Convergence Zone - ITCZ

• Zone of low pressure around equator

• Warmed by direct sunlight

• Also called equatorial trough

• Converging surface air called Trade Winds - from NE or SE

• Trades winds form Tropical Easterlies

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Tropical CirculationTropical Circulation

• Subtropical High Pressure Systems (STH)

• Air rising from ITCZ sinks at 25-30 degrees N and S

• Warm, dry air leads to deserts

• This convection loop called Hadley Cell

• Diverging air on equatorial side of STHs drives trade winds

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Midlatitude CirculationMidlatitude Circulation

• Westerlies blow from poleward side of STHs

• Polar Front separates cold air poleward from warm air equatorward

• Polar Jet Stream - high altitude wind driven by temp gradient – defines polar front

• Rossby Waves - undulations in polar front – N-S heat exchange

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Polar CirculationPolar Circulation

• Cold, descending air over or nearly over the poles

• Forms the Polar High

• Polar high drives polar easterlies

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Seasonal Changes in CirculationSeasonal Changes in Circulation

• ITCZ migrates with subsolar point

• Trade winds, STHs, westerlies, all follow the ITCZ north & south with seasons

Band of high air pressure, calm winds, and clear skies that exist at about 25-30 degrees N and S latitude is...a) Intertropical Convergence Zone b) Subtropical High Pressure Systemc) Hadley Cell d) Polar Front e) Polar High

Answer: B

In general, the surface winds underneath the northen hemisphere Hadley Cell are...a) Northerlies b) Easterlies c) Westerlies d) Northeasterlies e) Southeasterlies

Answer:

What is one major cause of deserts around 30 degrees latitude?

a) Subtropical high pressure Systems b) This system can be directly underneath the sun c) This area is under the ascending branch of the Hadley Cell d) Mid-latitude low pressure systems e) Intertropical Convergence Sun

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Seasonal Changes in CirculationSeasonal Changes in Circulation

• Monsoon

• Seasonal shift of prevailing wind direction due to land/water contrasts

• Happens throughout subtropical regions

• Most significant in South & SE Asia

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South Asian MonsoonSouth Asian MonsoonWinter Monsoon•Cold air over Asia

•Sinking air/High pressure

•Cool, dry NE winds

•ITCZ far south

Summer Monsoon•Warm air over Asia

•Rising air/Low pressure

•Warm, moist wind – rain

•ITCZ swings north

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Local Wind SystemsLocal Wind SystemsSea Breeze•Sun-heated air over land rises – Low pressure•Cooler air over water sinks – High pressure

Land Breeze•Night air over land cools fast – High pressure•Air over water stays warmer – Low pressure

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Local Wind SystemsLocal Wind Systems

•Valley breeze - during day, low pressure develops on mtn slopes – upslope wind

•Mountain breeze - at night mtn air cools, creating downslope wind

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Local Wind SystemsLocal Wind Systems• Chinook Wind - high pressure on windward

side of mtn range, low pressure on leeward side – strong, hot, dry wind on leeward side

Upslope air flow that develops when mountain slopes heat up due to re-radiation and conduction over the course of the day:

a) sea breeze b) Land breeze c) valley breeze d) Mountain breeze e) Katabatic winds

Answer: C

What is the difference between the concepts used to describe the Hadley Cell circulation and those of land/sea breezes?

a) Hadley cell has friction involved, land/sea breezes do notb) Hadley Cell has Coriolis Force involved, land/sea breezes do notc) Hadley Cell has differential heating involved, land/sea breezes do notd) Hadley Cell is no different from land/sea breezes

Answer:

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Oceanic CirculationOceanic Circulation

• Ocean currents driven by friction from wind

• Gyres form under high pressure areas in oceans – steered by landmasses

• Gulf Stream runs along western edge of STH in N. Atlantic – warm water far north

• Warm, Caribbean water saltier and cooler as it flows north – Thermohaline Circulation

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Global Oceanic CirculationGlobal Oceanic Circulation

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Oceanic CirculationOceanic Circulation• Thermohaline Circulation - warm water in gulf

stream cools and evaporates as it heads north making it heavier – downwelling, then a long trip to the Pacific where upwelling occurs

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El NiEl Niñoño

• Reversal of normal water flow in the Pacific

• Occurs every 3-8 years

• Underlying cause unknown

• Long-reaching climatic effects

• Called El Niño by Peruvian fishermen in honor of Christ (occurs near Christmas)

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El NiEl NiñoñoNormal El Niño

El Niño