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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