Chapter 5Chapter 5

Atmospheric Pressure and WindAtmospheric Pressure and Wind

Atmospheric PressureAtmospheric Pressure

DefinitionDefinition: The pressure exerted by the weight of : The pressure exerted by the weight of all the air(atmosphere) aboveall the air(atmosphere) above

Approximately 14.7 lbs/sq inApproximately 14.7 lbs/sq in Air pressure is exerted on us and on everything in Air pressure is exerted on us and on everything in

all directions at onceall directions at once Gas molecules freely move about – bounce Gas molecules freely move about – bounce

aroundaround They are contained in our atmosphere by the They are contained in our atmosphere by the

earth’s surface and by gravityearth’s surface and by gravity

Atmospheric ProcessesAtmospheric Processes

Pressure, Density, and Temperature – they Pressure, Density, and Temperature – they are all related – dependent on one anotherare all related – dependent on one another

Pressure and Pressure and DensityDensity

Mapping PressureMapping Pressure

InchesInches: Barometers express atm pressure : Barometers express atm pressure in inches (or millimeters) – an expression of in inches (or millimeters) – an expression of height of mercury in a barometerheight of mercury in a barometer– Example: 29.06 inExample: 29.06 in

MillibarsMillibars: express atm pressure in terms of : express atm pressure in terms of force per surface areaforce per surface area– Example: 1013.25 millibarsExample: 1013.25 millibars

average sea level pressure = 1013.25 millibars average sea level pressure = 1013.25 millibars

Terms Mapping PressureTerms Mapping Pressure

Highs: areas of higher pressure relative to Highs: areas of higher pressure relative to surrounding air massessurrounding air masses

Lows: areas of lower pressure relative to Lows: areas of lower pressure relative to surrounding air massessurrounding air masses

Ridges: elongated areas of relatively high Ridges: elongated areas of relatively high pressurepressure

Troughs: elongated areas of relatively lower Troughs: elongated areas of relatively lower pressurepressure

Mapping with IsobarsMapping with Isobars

Rotation High/Low Pressure Rotation High/Low Pressure SystemsSystems

High = Clockwise Low = Counterclockwise

Pressure GradientPressure Gradient Drawing isolines demonstrates or shows Drawing isolines demonstrates or shows

how quickly the pressure is rising or fallinghow quickly the pressure is rising or falling This is called the pressure gradientThis is called the pressure gradient The closer the isolines the steeper the slope The closer the isolines the steeper the slope

and the faster the rate of pressure changeand the faster the rate of pressure change Movement along the pressure gradient Movement along the pressure gradient

generates movement of air masses i.e., generates movement of air masses i.e., windwind

WindsWinds

Remember: Remember: InsolationInsolation is the ultimate cause is the ultimate cause of windof wind– Unequal heating of earth’s surfaceUnequal heating of earth’s surface– Temp gradients generate pressure gradientsTemp gradients generate pressure gradients– Pressure gradient sets winds in motionPressure gradient sets winds in motion

The Nature of WindThe Nature of Wind

Direction of MovementDirection of Movement– Pressure GradientPressure Gradient– Coriolis EffectCoriolis Effect– FrictionFriction

Cyclones and AnticyclonesCyclones and Anticyclones Wind SpeedWind Speed

Pressure Pressure GradientGradient

FrictionFriction

Cyclones and AnticyclonesCyclones and Anticyclones

Cyclones and AnticyclonesCyclones and Anticyclones

Wind SpeedWind SpeedDetermined by the Pressure GradientDetermined by the Pressure Gradient

Farther apart = slower

Closer together = faster

General Circulation of the General Circulation of the AtmosphereAtmosphere

Hadley Cells Hadley Cells Components of General CirculationComponents of General Circulation Jet StreamsJet Streams

Hadley CellsHadley Cellsoccur only in occur only in tropicaltropical latitudes latitudes

Equator

CyclonicSystem

General Circulation of AtmosphereGeneral Circulation of Atmosphere

ITCZ =Inter-tropical convergence zone

Easterlies

Westerlies

Easterlies

Easterlies

Westerlies

Easterlies

0°

0°N

0°S

Components of General Components of General CirculationCirculation

Subtropical HighsSubtropical Highs Trade WindsTrade Winds Intertropical Convergence ZoneIntertropical Convergence Zone The WesterliesThe Westerlies Polar HighsPolar Highs Polar EasterliesPolar Easterlies Subpolar LowsSubpolar Lows

Trade WindsTrade Winds 2525°N to 25°S°N to 25°S Easterlies – named for the direction they Easterlies – named for the direction they

come fromcome from Most dominant windsMost dominant winds

WesterliesWesterlies 3030° to 60° N & S° to 60° N & S Can be influenced by surfaceCan be influenced by surface

Jet StreamJet Stream Polar to temperate latitudesPolar to temperate latitudes Rossby wave influenceRossby wave influence

Global Atmospheric CirculationGlobal Atmospheric Circulation

Jet StreamsJet Streams

The term "jet stream" is often used to refer to the rivers of wind high in the atmosphere - above about 20,000 feet - that steer storms. They also help determine locations of areas of high and low air pressure at the Earth's surface.

The Jet Stream and Rossby The Jet Stream and Rossby WavesWaves

Modifications of General Modifications of General CirculationCirculation

Seasonal Variations in LatitudeSeasonal Variations in Latitude MonsoonsMonsoons

Seasonal Variations in Latitude - MonsoonsSeasonal Variations in Latitude - Monsoons

Seasonal variations are less likely to affect the equatorial region and the poles

South Asian MonsoonSouth Asian Monsoon

Seasonal Pressure and Precipitation PatternsSeasonal Pressure and Precipitation Patterns

Localized Wind SystemsLocalized Wind Systems

Sea and Land BreezesSea and Land Breezes Valley and Mountain BreezesValley and Mountain Breezes Katabatic WindsKatabatic Winds Foehn/Chinook WindsFoehn/Chinook Winds

Sea and Land BreezesSea and Land Breezes

Valley BreezesValley Breezes

Hot a

ir rises

Cold

air sin

ks

Mountain BreezesMountain Breezes

Katabatic - Chinook – Foehn – Santa AnaKatabatic - Chinook – Foehn – Santa AnaDownslope WindsDownslope Winds

Katabatic: powerful, simple spilling of air downslope – air drainage, most common in Greenland and Antarctica, most famous is the “mistral” in southern FranceChinook: Rocky MountainsFoehn: AlpsSanta Anas: California, high speed, high temp, and very dry