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Copyright © 2013 Pearson Education, Inc.

The Atmosphere:

An Introduction to

Meteorology, 12th

Lutgens • Tarbuck

Lectures by:

Heather Gallacher,Cleveland State University

Chapter 7: Circulation of the Atmosphere



Scales of Atmospheric Motion

� Small- and large-scale circulation:

� Microscale

� Mesoscale

� Macroscale



� Microscale winds:

� The circulation is small and chaotic.

� They can last from seconds to minutes.

� They can be simple gusts, downdrafts, and small vortices,

such as dust devils.

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� Mesoscale winds:

� They can last from minutes to hours.

� They are usually less than 100 km across.

� Some mesoscale winds (thunderstorms and tornadoes) also

have a strong vertical component.



� Macroscale winds:

� These winds are the largest wind patterns.

� These planetary-scale patterns can remain unchanged for

weeks at a time.

� Smaller macroscale circulation is called synoptic scale.

� These wind systems are about 1000 km in diameter.

� Smaller macroscale systems are tropical storms and

hurricanes.



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� Structure of wind patterns:

� Global winds are a composite of motion on all scales.

� Hurricanes appear as a large cloud moving slowly across

the ocean.

� The large cloud contains many mesoscale thunderstorms.

� The thunderstorms consist of numerous microscale bursts.


Local Winds

� Land and sea breezes


Local Winds

� Mountain and valley breezes

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Local Winds

� Chinook (foehn) winds

� Chinooks are warm dry winds that sometimes move down

slopes of mountains.

� These winds can bring on drastic changes in temperature.

� The winds will melt snow cover rapidly.

� The Native American word chinook means snow-eater.

� Similar winds are called foehns in Europe.


Local Winds

� Katabatic (fall) winds:

� These winds originate when cold, dense air begins to

move.

� Better known katabatic winds have local names, such as

mistral, which blows from the French alps to the

Mediterranean.

� Country breezes:

� These breezes are mesoscale winds.

� They are caused by the uneven heating of urban and

country areas.

� This results in the flow from country to urban areas.


Global Circulation

� Single-cell circulation

model

� Hadley model

� Hadley proposed that the

contrast in temperatures

between the poles and

the equator creates a

large convection cell in

both the Northern and

Southern hemispheres.

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Global Circulation

� Three-cell model


Global Circulation

� A three-cell circulation model was proposed in the

1920s.� Warm air rises at the equator (Hadley cell).

� As the flow moves poleward, it begins to cool and sinks at

20°–35° latitude.

� Trade winds meet at the equator, in a region with a weak

pressure gradient, called the doldrums.

� The westerly circulation of surface winds (prevailing

westerlies) between 30°–60° latitude is called the Ferrel

cell.

� Circulation (at 60°–90°) within a polar cell produces

polar easterlies; surface flows that move toward the

equator.


Pressure Zones Drive Winds

� Idealized zonal pressure belts:

� The equatorial low is an intertropical convergence zone

(ITCZ).

� Subtropical highs (STH) are high-pressure zones in the

belts about 20°–35° latitude on either side of the equator.

� Polar highs near the Earth’s poles are where the polar

easterlies originate.

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� Semi permanent pressure systems: The real world

� January pressure and wind patterns



� Semi permanent pressure systems: The real world

� July pressure and wind pattern

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Monsoons

� Monsoon refers to a seasonal reversal of winds.

� The Asian monsoon, which affects India and its

surrounding areas, China, Korea, and Japan.

� The monsoon is driven by pressure differences.

� The North American monsoon occurs in the southwestern

U.S. and northwestern Mexico.

� This monsoon is driven by the extreme temperatures, which

generate a low-pressure center over Arizona and results in a

circulation pattern that brings moist air from the Gulf of

California and from the Gulf of Mexico, to a lesser degree.


Monsoons


Monsoons

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The Westerlies

� Why Westerlies?


The Westerlies

� Waves in the westerlies:

� Westerlies flow in wavy paths that have long wavelengths.

� The longest wave patterns are known as Rossby waves, which

usually consist of 4–6 waves that encircle the globe.

� Rossby waves can have a large impact on weather.


Jet Streams

� Jet streams:

� Embedded in westerlies

� Widths vary from less than 100 km to more than 500 km.

� Speeds can attain 100–400 kph.

� Polar and subtropical

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Jet Streams

� The polar jet stream is

the most prevalent.

� It occurs along a major

frontal zone, the polar

front.

� The jet stream moves

faster in winter.

� During the winter,

occasionally it moves

north–south.


Jet Streams

� The subtropical jet stream is a semipermanent jet

stream over the subtropics.

� It is a west-to-east current, centered at 25° N and S.

� It is mainly a winter phenomenon.

� The subtropical jet stream is slower than the polar.


Jet Streams

� Jet streams and Earth’s heat budget

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Global Winds and Ocean Currents

� The Coriolis force deflects surface currents

poleward, which form nearly circular patterns of

ocean currents called gyres.

� The Gulf stream is strengthened by westerly winds

and continues northeastward.



� Importance of ocean currents:

� Ocean currents have an important on climate, which helps

maintain the Earth’s heat balance.

� Cold currents offshore result in a dry climate.

� Warm offshore current produce a warm moist climate.



� Ocean currents and upwelling:

� Upwelling, a wind-induced vertical movement, is the

rising of cold water from deeper layers to replace warmer

surface water.

� It occurs where winds blow parallel to the coast toward the

equator.

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El Niño and La Niña and

the Southern Ocean



the Southern Ocean

� El Niño is a gradual warming of eastern Pacific

waters in December or January.

� La Niña is the opposite of El Niño and refers to

colder-than-normal ocean temperatures.



the Southern Ocean

� Impact of El Niño:

� It is noted for its potentially catastrophic impact on

weather and economies of Chile, Peru, Australia, and

other countries.

� Arid areas can receive a lot of precipitation.

� A change in surface water temperature can kill fish.

� El Niño has been recognized as part of the global

atmospheric circulation pattern.

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the Southern Ocean

� Impact of La Niña:

� La Niña is also an important atmospheric phenomenon.

� In the western Pacific, wetter than normal conditions

occur.

� There are also more frequent hurricanes in Atlantic.

� Southern oscillation:

� This is the seesaw pattern of atmospheric pressure

between the eastern and western Pacific.

� Winds are the link between pressure changes and the

ocean warming and cooling associated with El Niño and

La Niña.


Global Distribution of Precipitation

� Zonal Distribution of

precipitation


Global Distribution of Precipitation

� Distribution of precipitation over the continents

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End Chapter 7

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