Air Pressure and WindAir Pressure and Wind

Atmospheric pressure Atmospheric pressure

Force exerted by the weight of the air Force exerted by the weight of the air aboveabove

Decreases with increasing altitudeDecreases with increasing altitude Units of measurement Units of measurement

MillibarMillibar (mb) – standard sea level pressure is (mb) – standard sea level pressure is 1013.2 mb 1013.2 mb

Inches of mercuryInches of mercury – standard sea level – standard sea level pressure is 29.92 inches of mercury pressure is 29.92 inches of mercury

WindWind

Horizontal movement of air Horizontal movement of air Out of areas of high pressure Out of areas of high pressure Into areas of low pressureInto areas of low pressure

Controls of windControls of wind Pressure gradient force Pressure gradient force

• IsobarsIsobars – lines of equal air pressure – lines of equal air pressure • Pressure gradientPressure gradient – pressure change over – pressure change over

distance distance

WindWind

Controls of windControls of wind Coriolis effect Coriolis effect FrictionFriction

WindWind

Upper air winds Upper air winds Generally blow parallel to isobars – called Generally blow parallel to isobars – called

geostrophic windsgeostrophic winds Jet streamJet stream

• "River" of air "River" of air • High altitudeHigh altitude• High velocity (120-240) kilometers per hour High velocity (120-240) kilometers per hour

Rotating Air BodiesRotating Air Bodies

Low Pressure Zone Low Pressure Zone FormationFormation Warm air risesWarm air rises Creates a low Creates a low

pressure zonepressure zone At the Earth’s surface, At the Earth’s surface,

air “feeds” the low air “feeds” the low pressure zone, moves pressure zone, moves counterclockwisecounterclockwise

High Pressure Zone High Pressure Zone FormationFormation Cool air sinks Cool air sinks Creates a high Creates a high

pressure zonepressure zone At the Earth’s surface, At the Earth’s surface,

winds blow clockwisewinds blow clockwise

Rotating Air BodiesRotating Air Bodies

Bends in the polar jet create troughs and Bends in the polar jet create troughs and ridgesridges

Forms cyclones and anticyclonesForms cyclones and anticyclones

Rotating Air BodiesRotating Air Bodies

CyclonesCyclones Low pressure zone in polar Low pressure zone in polar

jet troughjet trough Winds at surface flow Winds at surface flow

counterclockwise towards counterclockwise towards the corethe core

Air is updrafted and cooledAir is updrafted and cooled Forms clouds, rain and Forms clouds, rain and

upper level outflow of airupper level outflow of air

Rotating Air BodiesRotating Air Bodies

AnticyclonesAnticyclones High pressure zone at High pressure zone at

ridge of polar jetridge of polar jet Air converges in upper Air converges in upper

atmosphereatmosphere Descends towards the Descends towards the

groundground Flows outward at surfaceFlows outward at surface Dry, windy conditionsDry, windy conditions

Cyclones and AnticyclonesCyclones and Anticyclones

Cyclonic and anticyclonic winds Cyclonic and anticyclonic winds in the Northern Hemispherein the Northern Hemisphere

General atmospheric circulationGeneral atmospheric circulation

Idealized global Idealized global circulation circulation Equatorial low Equatorial low

pressure zone pressure zone • Rising airRising air• Abundant precipitationAbundant precipitation

General atmospheric circulation General atmospheric circulation

Idealized global Idealized global circulation circulation Subtropical high Subtropical high

pressure zone pressure zone • Subsiding, stable, dry airSubsiding, stable, dry air• Location of great desertsLocation of great deserts• Air traveling equatorward Air traveling equatorward

from the subtropical high from the subtropical high produces the trade winds produces the trade winds

• Air traveling poleward Air traveling poleward from the subtropical high from the subtropical high produces the westerly produces the westerly winds winds

General atmospheric circulation General atmospheric circulation

Idealized global Idealized global circulation circulation Subpolar low Subpolar low

pressure zone pressure zone • Warm and cool winds Warm and cool winds

interact interact • Polar front – an area of Polar front – an area of

storms storms

General atmospheric circulation General atmospheric circulation

Idealized global Idealized global circulation circulation Polar high pressure Polar high pressure

zone zone • Cold, subsiding air Cold, subsiding air • Air spreads Air spreads

equatorward and equatorward and produces polar easterly produces polar easterly winds winds

• Polar easterlies collide Polar easterlies collide with the westerlies with the westerlies along the polar frontalong the polar front

General atmospheric circulation General atmospheric circulation

Influence of continents Influence of continents Seasonal temperature differences disrupt theSeasonal temperature differences disrupt the

• Global pressure patterns Global pressure patterns • Global wind patterns Global wind patterns

Influence is most obvious in the Northern Influence is most obvious in the Northern Hemisphere Hemisphere

General atmospheric circulation General atmospheric circulation

Influence of continents Influence of continents MonsoonMonsoon

• Seasonal change in wind direction Seasonal change in wind direction • Occur over continents Occur over continents

During warm months During warm months • Air flows onto land Air flows onto land • Warm, moist air from the ocean Warm, moist air from the ocean

Winter months Winter months • Air flows off the land Air flows off the land • Dry, continental airDry, continental air

Local winds Local winds

Produced from temperature differencesProduced from temperature differences Small scale winds Small scale winds TypesTypes

Land and sea breezesLand and sea breezes Mountain and valley breezesMountain and valley breezes Chinook and Santa Ana winds Chinook and Santa Ana winds

Illustration of a sea breeze Illustration of a sea breeze and a land breezeand a land breeze

The Santa Ana WindsThe Santa Ana Winds

Wind measurement Wind measurement

Two basic measurements Two basic measurements DirectionDirection

• Winds are labeled from where they originate Winds are labeled from where they originate • Direction indicated by eitherDirection indicated by either

Compass points (N, NE, etc.) Compass points (N, NE, etc.) Scale of 0º to 360ºScale of 0º to 360º

• Prevailing wind comes more often from one Prevailing wind comes more often from one direction direction

SpeedSpeed • often measured with a cup anemometeroften measured with a cup anemometer

Wind measurement Wind measurement

Changes in wind direction Changes in wind direction Associated with locations of Associated with locations of

• CyclonesCyclones• AnticyclonesAnticyclones

Often bring changes in Often bring changes in • Temperature Temperature • Moisture conditions Moisture conditions

El NiñoEl Niño

Normal conditions in the tropical Normal conditions in the tropical Pacific OceanPacific Ocean

Surface winds move from east to westSurface winds move from east to west From high pressure in S. America to low From high pressure in S. America to low

pressure in Australiapressure in Australia Drags water westwardDrags water westward Warm water pools in the western PacificWarm water pools in the western Pacific

Normal conditionsNormal conditions

Every 3 – 8 years, system Every 3 – 8 years, system reversesreverses

Called the Southern OscillationCalled the Southern Oscillation Trade winds weaken or reverseTrade winds weaken or reverse Warm water migrates from Australia to S. Warm water migrates from Australia to S.

AmericaAmerica Arrives in time for Christmas – Arrives in time for Christmas – Corriente Corriente

del Niñodel Niño

El NiñoEl Niño

What is El Niño?What is El Niño?

Basically, it's a giant puddle (or pod) of Basically, it's a giant puddle (or pod) of heated water that sloshes across the heated water that sloshes across the Pacific OceanPacific Ocean

Similar to an icebergSimilar to an iceberg Bulge on the surfaceBulge on the surface Most of “pod” beneath the surfaceMost of “pod” beneath the surface Due to difference in densityDue to difference in density

NationalNational Geographic’s Geographic’s Model Model

ENSO - El Niño-Southern ENSO - El Niño-Southern OscillationOscillation

Typically lasts 1 yearTypically lasts 1 year May last up to 3May last up to 3 In multi-year events, first year not as In multi-year events, first year not as

affectedaffected Affects both hemispheresAffects both hemispheres

Recognizing an El NiñoRecognizing an El Niño

Sea Surface Temperatures (SST)Sea Surface Temperatures (SST) Normal: 6-8° C warmer in the western Normal: 6-8° C warmer in the western

tropical Pacific than in the eastern tropical tropical Pacific than in the eastern tropical PacificPacific

Check SST to see if in “normal” rangeCheck SST to see if in “normal” range

La NiñaLa Niña

Return to “normal” conditions from an El Niño Return to “normal” conditions from an El Niño strongstrong

Produces:Produces: Strong currentsStrong currents Powerful upwellingPowerful upwelling Chilly and stormy conditions along S. American coastChilly and stormy conditions along S. American coast

Eastern Pacific cools rapidly, Western Pacific Eastern Pacific cools rapidly, Western Pacific warms rapidlywarms rapidly

Renewed Trade Wind activity spreads the cooler Renewed Trade Wind activity spreads the cooler eastern Pacific waters westwardeastern Pacific waters westward

Global distribution of Global distribution of precipitation precipitation

Relatively complex pattern Relatively complex pattern Related to global wind and pressure Related to global wind and pressure

patterns patterns High pressure regions High pressure regions

• Subsiding airSubsiding air• Divergent windsDivergent winds• Dry conditions Dry conditions • e.g., Sahara and Kalahari deserts e.g., Sahara and Kalahari deserts

Global distribution of Global distribution of precipitation precipitation

Related to global wind and pressure Related to global wind and pressure patterns patterns Low pressure regions Low pressure regions

• Ascending air Ascending air • Converging winds Converging winds • Ample precipitation Ample precipitation • e.g., Amazon and Congo basins e.g., Amazon and Congo basins

Average annual precipitation Average annual precipitation in millimetersin millimeters

Global distribution of Global distribution of precipitation precipitation

Related to distribution of land and water Related to distribution of land and water Large landmasses in the middle latitudes Large landmasses in the middle latitudes

often have less precipitation toward their often have less precipitation toward their centerscenters

Mountain barriers also alter precipitation Mountain barriers also alter precipitation patterns patterns • Windward slopes receive abundant rainfall from Windward slopes receive abundant rainfall from

orographic lifting orographic lifting • Leeward slopes are usually deficient in moisture Leeward slopes are usually deficient in moisture

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