-circulation in the atmosphere

7/25/2019 -Circulation in the Atmosphere

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Solar Radiation - initial source of energy to theEarth. It can be absorbed, reflected andreradiated. The redistribution of this energycontrols the structure and dynamics of theAtmosphere and Oceans.

Circulation of the Atmosphere


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The Atmosphere Is Composed Mainly ofNitrogen, Oxygen, and Water Vapor

What are some properties of the atmosphere?

The lower atmosphere is a fairly homogeneous mixture of gases.Water vapor occupies up to 4% of the volume of the atmosphere.The density of air is influenced by temperature and water content.

Ascending air cools as it expands. Cooler

air can hold less water, so water vaporcondenses into tiny droplets - clouds.Descending air warms as it compresses the droplets (clouds) evaporate.


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GreenhouseGases

Gases: permanent andvariable

Permanent =present inconstant relative% of total volume Variable =

concentrationchanges with timeand location

Suspendedmicroscopic particles

Water droplets


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The Atmosphere Moves in Response to UnevenSolar Heating and Earths Rotation

An estimate of the heat budget for Earth. On an average day, about half ofthe solar energy arriving at the upper atmosphere is absorbed at Earthssurface. Light (short-wave) energy absorbed at the surface is converted intoheat. Heat leaves Earth as infrared (long-wave) radiation. Since input equals

output over long periods of time, the heat budget is balanced.


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Heat budget

Solar input must balance solar output

Temperature increases/decreases if input isgreater/less than output

Average Earth Temperature is 16

o

C Solar Energy is reradiated from the surface as a longwave.

Surface of Earth (including oceans) is heated from

above Atmosphere is heated from below


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O3absorbsUV Solar

Radiation,heatsAtm.

Vertical (thermal) structure of theatmosphere

Troposphere: lowest layer 0-12 km,temperature decreases with altitude Tropopause: minimum temperaturezone between the troposphere andstratosphere Stratosphere: layer above

tropopause 12-50 km, temperatureincreases with altitude Mesosphere: layer abovestratosphere 50-90 km, temperaturedecreases with height Thermosphere: layer above

mesosphere >90 km, extends out tospace

The Atmosphere

Density of air depends on temperature, water vapor and altitudeTemperature decrease = density increase

Water vapor increase = density decreaseAltitude increase = density decrease


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The Solar Heating of Earth Varies withLatitude

How solar energy input varies

with latitude.

Equal amounts of sunlight arespread over a greater surfacearea near the poles than in thetropics.

Ice near the poles reflectsmuch of the energy thatreaches the surface there.

The atmosphere reflects, scatters and absorbs solar radiation. At highlatitudes solar radiation travels a longer path through atmosphere.


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The Solar Heating of Earth Varies with Latitude

Earth as a whole is in thermal

equilibrium, but different latitudes arenot.

The average annual incoming solar radiation(red line) absorbed by Earth and the averageannual infrared radiation (blue line) emittedby Earth. Polar latitudes lose more heat to

space than they gain, and tropical latitudesgain more heat than they lose. The amount ofradiation received equal the amount lost atabout 38N and S. The area of heat gained(orange area) equals the area of heat lost(blue areas) so Earths total heat budget isbalanced.

What factors govern the global circulation of air?

Uneven solar heating

The Coriolis effect


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Warm equatorial water flows to higher latitudes

Cool Polar water flow to lower latitudes

Re-distribution of heat

Heat gained at Equatorial latitudes

Heat lost at higher latitudes Winds and ocean currents redistribute heat around

the EarthOceans do not boil away near the equator or freeze solid near

the poles because heat is transferred by winds and oceancurrents from equatorial to polar regions.


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The Seasons: solar heating varies with seasons

During the Northern Hemisphere winter, the Southern Hemisphere istilted toward the sun and the Northern Hemisphere receives less light and

heat. During the Northern Hemisphere summer, the situation is reversed.

* Earth revolves around theSun (365 days)

* Earth rotates about its ownaxis (1 day)

* angle (tilt) that axis ofrotation makes with planethat contains trajectoryaround the Sun is 23 andit remains that way (sameorientation) as the Earth

revolves around Sun


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Winter (Northern Hemisphere

tilts away from sun)Spring (sun aims

directly at

equator)

Summer

(Northern

Hemisphere tilts

toward sun) Fall(sun aims

directly at

equator)

To

Polaris

The tilt (23 inclination) causes the seasons


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Earths Uneven Solar Heating Results in Large-Scale Thermal Cell type of Atmospheric Circulation

A convection current formsin a room when air flowsfrom a hot radiator to a coldwindow and back.

Air warms, expands,becomes less dense, andrises over the radiator. Aircools, contracts, becomesmore dense, and falls nearthe cold glass window.

A convection cell is driven by density differences


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The Coriolis Effect Deflects the Path ofMoving Objects

Sketch of the thought experiment in thetext, showing that Buffalo travels ashorter path on the rotating Earth eachday then Quito does.

A continuation of the thought experiment.A look at Earth from above the North Poleshows that Buffalo and Quito move atdifferent velocities.


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The Coriolis Effect Deflects the Pathof Moving Objects

As observed from space,cannonball 1 (shot northward)and cannonball 2 (shotsouthward) move as we mightexpect; that is, they travelstraight away from the cannonsand fall to Earth.

Observed from the ground,however, cannonball 1 veersslightly east and cannonball 2veers slightly west of their

intended targets.The effect depends on theobservers frame of reference.


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The Coriolis Effect Influences the Movement ofAir in Atmospheric Circulation Cells

Global air circulation as described in the six-cell circulation model. Air rises atthe equator and falls at the poles, but instead of one great circuit in eachhemisphere from equator to pole, there are three in each hemisphere. Note the

influence of the Coriolis effect on wind direction. The circulation show here isidea that is, a long-term average of wind flow.


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Wind bands Three convectioncells in each hemisphere

Trade winds = NE (30N to

0) and SE (30S to 0)Westerlies = 60N to30N and 60S to 30S

Polar easterlies = 90Nto 60N and 90S to

60SLow pressure at 0, 60N,and 60S

Low pressure, ascending air,clouds, increased

precipitationHigh pressure at 30N, 30S,90N, and 90SHigh pressure, descendingair, clear skies, low

precipitation


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The Coriolis Effect Influences the Movement ofAir in Atmospheric Circulation Cells

A large circuit of air is called an atmospheric circulation cell.

Three cells exist in each hemisphere.

Hadley cells are tropical cells found on each side of the equator.

Ferrel cells are found at the mid-latitudes.Polar cells are found near the poles.

What are some of the wind patterns found between and withincells?

Doldrums are calm equatorial areas where two Hadley cells

convergeHorse latitudes are areas between Hadley and Ferrel cells.Trade winds are surface winds of Hadley cells.Westerlies are surface winds of Ferrel cells.


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THE THREE CELL MODEL

FORMATION OF THE HADLEY CELL (1)

SOLAR ENERGY

Insolation in tropicalareas causes warm airto rise and spreadpolewards, carryingheat energy.


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FORMATION OF THE HADLEY CELL (2)

SOLAR ENERGY

Air cools and begins to fall at

about 30N and 30S of Equator.Cooled air returns to the Equator.

This circulation of air iscaused by solar heating.

It is called the HADLEY CELL.

Heat energy is transferredfrom the Equator to sub-

tropical latitudes.


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FORMATION OF THE POLAR CELL (1)

Intensely cold,

dense air sinks atthe poles, thenblows as surfacewinds towards theEquator.


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FORMATION OF THE POLAR CELL (2)

This circular motion iscalled the POLAR CELL.

At about 60N and 60 S,the cold polar air is warmedin contact with the earthssurface.

This warmed air rises andreturns polewards,carrying heat energy.


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FORMATION OF THE FERREL CELL (1)

The Hadley Cell is drivenby differences in heatenergy at the Equator.

As the air in the HadleyCell falls at about 30Nand 30S, it pulls the airbeside it down as well,due to friction



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The Polar Cell is driven by

differences in heat energy.Cold polar air falls andspreads towards the Equator.

As the air in the Polar Cellrises at about 60N and60S, it pulls the air

beside it up as well, due tofriction.


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The Polar Cell causes anuplift at about 60N andS.

Unlike the Hadley and PolarCells, the Ferrel Cell is not

driven by differences in heatenergy.

The Ferrel Cell is caused byfriction where air is incontact with the other twocells.

The Hadley Cell drags airdown at about 30N and S.


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THE THREE CELLS TOGETHER

Ferrel Cell

Polar Cell

Hadley Cell

Polar Cell

Ferrel Cell

Hadley Cell


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THE TRANSFER OF HEAT ENERGY FROM

EQUATORIAL TO POLAR AREAS

Where air carrying energy from theEquator in the Hadley Cell comes into

contact with air in the Ferrel Cell,there is a transfer of heat energyinto the Ferrel Cell.

There is a similar transferof heat energy from the

Ferrel Cell to the Polar Cell.

In this way, heat energy istransferred from the Equator,where there is a surplus ofenergy, to the poles wherethere is a deficit.

SOLAR ENERGY


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THE CORRESPONDING MOVEMENT OF COLDER AIR

In the Polar cell cold air from polarregions flows to mid-latitudes as

polar easterly winds

In the Ferrel Cell there is amovement of cold air athigh altitude.

In the Hadley Cell, coolerair moves from the sub-tropics to the Equator.


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ASSOCIATED PRESSURE BELTS

Rising air at the equator causes theequatorial belt of low pressure

Descending air at about 30N and30S causes the sub-tropical beltof high pressure

Rising air at about 60N and60S causes a mid-latitudebelt of low pressure

Descending air at the polescauses the polar highpressure areas

Mid latitude low pressure


Equatorial low pressure

Sub-tropical high pressure


Polar high pressure

Polar high pressure


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ASSOCIATED SURFACE WIND PATTERNS

Winds always blow from high pressure to

low pressure.They are deflected because of theCoriolis Force which come about becauseof the rotation of the earth.

Winds in Northern Hemisphereare deflected to the right.

Winds in the southern hemisphereare deflected to the left.

These wind belts shiftseasonally. (See next section) Mid latitude low pressure



Polar high pressure



Polar high pressure


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POSITION OF THE THREE CELLS IN DECEMBER

Polar Cell

Hadley

Cell

FerrelCell

Polar Cell

FerrelCell

Hadley

Cell

SUN OVERHEAD 23S

The sun is overhead at theTropic of Capricorn, 23Sof the Equator.

The cells shiftsouthwards as the

heat equator is in thesouthern hemisphere.


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IN THE NORTHERN HEMISPHERE:

The winds that blow to the equatoriallow pressure belt are called the NorthEast Trade Winds

The line along which they converge

(meet) is called the INTER-TROPICAL CONVERGENCE ZONE.

This is often abbreviated to ITCZ

THE INTER-TROPICAL CONVERGENCE ZONE

IN THE SOUTHERN HEMISPHERE:

The winds that blow to theequatorial low pressure belt arecalled the South East Trade Winds

Inter-Tropical Convergence Zone


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IN THE NORTHERN HEMISPHERE

OVER WEST AFRICA

The sub-tropical high pressure beltdevelops over the Sahara so is hot anddry.

This is known as continental Tropical(cT) air.

THE INTER-TROPICAL CONVERGENCE ZONE

Inter-Tropical Convergence ZoneIN THE SOUTHERN HEMISPHERE

OVER WEST AFRICA

The sub-tropical high pressure belt

develops over the Atlantic so is warmand moist.

This is known at maritime Tropical (mT)air.


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In December, the zone of maximuminsolation (solar energy) is south ofthe Equator. This means that the wind

belts shift southwards.

This means that winds blow out of thesub-tropical high pressure area over theSahara, and take dry air from thecontinental Tropical (cT) air mass acrossmost of West Africa. This causes a dryseason.

THE ITCZ IN DECEMBER

SOLAR ENERGY

Moist air from the maritimeTropical(mT) air mass from the Atlantic cannotreach far inland, where there is a dryseason.


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THE ITCZ IN JUNE

By contrast, in June, the zone ofmaximum insolation is well to the north

of the Equator. This means that thewind belts shift northwards.

The winds blow out of the sub-tropical highpressure area over the Sahara, now only affect

the northern part of sub-Saharan Africa.

SOLAR ENERGY

Moist maritime Tropical air from theAtlantic now reaches far inland, wherethere is a rainy season. These winds flow

northwards to the ITCZ to replace airthat has become unstable and risen.


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Monsoons Are Wind Patterns That Change withthe Seasons

Monsoons are patterns of wind circulation that change with the season.Areas with monsoons generally have dry winters and wet summers.

Sea breeze is cool air from over the water moving toward land. Seabreezes occur after sunrise.

Land breezes occur after sunset when air warmed by the land blowstoward the water.


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In the Northern Hemisphere

Air flows clockwise around high pressure systemsAir flows counterclockwise around low pressure systems

M A Wi d P tt Th t Ch


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Monsoons Are Wind Patterns That Changewith the Seasons

A monsoon is a pattern of windcirculation that changes with theseason. Locations where monsoons occurtypically have wet summers and drywinters.

Monsoon patterns.During the monsoon circulations ofJanuary (a) and July (b), surface windsare deflected to the right in theNorthern Hemisphere and to the left inthe Southern Hemisphere. (c) Detail ofsummer Asian monsoon, showing location

of Cherrapunji, India, one f the worldswettest places. Rainfall amounts therecan exceed 10 meters (425 inches) per

year!

Sea Breezes and Land


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Sea Breezes and LandBreezes Arise fromUneven Surface Heating

The flow of air in coastal regionsduring stable weather conditions.

(a) In the afternoon, the land iswarmer than the ocean surface,and the warm air rising from theland is replaced by an onshoresea breeze.

(b) At night, as the land cools,the air over the ocean is now

warmer than the air over theland. The ocean air rises. Airflows offshore to replace it,generating an offshore flow (aland breezes).


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Storms Are Variations in Large-ScaleAtmospheric Circulation

Storms are regional atmospheric disturbances. Storms have high windsand most have precipitation.

Tropical cyclones occur in tropical regions. These storms can causemillions of dollars worth of damage and endanger life.

Extratropical cyclones occur in Ferrel cells, and are winter weatherdisturbances. These storms can also cause extensive damage.

Both types of storms are cyclones, or rotating masses of low-pressureair.


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Extratropical Cyclones Form between Two AirMasses

(a) The genesis and earlydevelopment of anextratropical cyclone inthe Northern Hemisphere

(b) How precipitation

develops in anextratropical cyclone.These relationshipsbetween two contrastingair masses areresponsible for nearly allthe storms generated in

the polar frontal zone andthus responsible for thehigh rainfall within thesebelts and the decreasedsalinities of surfacewaters below.


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Tropical Cyclones Form in One Air Mass

The internal structure of a mature tropical cyclone, or hurricane.(The vertical dimension is exaggerated in this model of ahurricane.)


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The dynamics of a tropical cyclone, showing the influence of the Corioliseffect. Note that the storm turns the wrong way (that is,counterclockwise) in the Northern Hemisphere, but for the right

reasons.


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The tracks of tropical cyclones. The breeding grounds of tropical cyclones areshown as orange-shaded areas. The storms follow curving paths: First theymove westward with the trade winds. Then they either die over land or turneastward until they lose power over the cooler ocean of mid-latitudes. Cyclonesare not spawned over the South Atlantic or the southeast Pacific because theirwaters are too chilly; nor in the still air - the doldrums - within a few degrees

of the equator.

Hurricanes


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Hurricanes

Easterly waves = pressure disturbancesresulting from variations in trade winds due tochanging surface water temperature

Critical sea surface temperature = 27o C Decrease in atmospheric pressure

Increases evaporation rate Increased wind speeds

Energy from ocean fuels hurricanes

Water vapor extracted from warm seasurface

Energy released during condensation


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Heat capacity (review)

Heat capacity of ocean is greater than that of land

Ocean currents, wind and waves can transfer heatdownward in summer and upward in winter

Larger seasonal variations of temperature over land Oceans control surface temperatures in Southern

Hemisphere

Land in the Northern Hemisphere influences the

range in surface temperature

Chapter 8 S mmar


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Chapter 8 - Summary

The interaction of ocean and atmosphere moderates surface

temperatures, shapes Earth's weather and climate, and creates most ofthe sea's waves and currents.

Different amounts of solar energy are absorbed at different latitudes,and this makes the tropics warmer than the polar regions.

Uneven solar heating causes convection currents to form in theatmosphere and leads to areas of different atmospheric pressures. Thedirection of air flow in these currents is influenced by the rotation ofEarth.

To observers on the surface, Earth's rotation causes moving air (or any

moving mass) in the Northern Hemisphere to curve to the rightof itsinitial path, and in the Southern Hemisphere to the left. This is known asthe Coriolis effect.

Chapter 8 Summary


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Chapter 8 - Summary

The atmosphere responds to uneven solar heating by flowing in three

great circulating cells over each hemisphere. The flow of air within thesecells is influenced by Earths rotation (Coriolis effect). Each hemispherehas three large atmospheric circulation cells: a Hadley cell, a Ferrel cell,and a polar cell (less pronounced over the South Pole).

Large storms are spinning areas of unstable air that develop between or

within air masses. Extratropical cyclones originate at the boundarybetween air masses.

Tropical cyclones, the most powerful of Earth's atmospheric storms,occur within a single humid air mass.

-circulation in the atmosphere

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