1 Valley Breeze Example ©1997 Prentice-Hall, Inc..

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Valley Breeze Example

©1997 Prentice-Hall, Inc.

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Valley Breeze Clouds

J.M. Moran -- ©1997 Prentice-Hall, Inc.

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Heat Island -- Washington D.C.


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Sea Breeze Example

©1998 Wadsworth Publishing Company

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Hadley CellSmooth EarthNo Rotation

©1997 Prentice-Hall, Inc. ©1998 Prentice-Hall, Inc.

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Add Rotation• We now allow the

earth to rotate. As expected, air travelling southward from the north pole will be deflected to the right.

• Air travelling northward from the south pole will be deflected to the left.


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Three-Cell Conceptual Model


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Three-Cell Model

• By looking at the actual winds, even after averaging them over a long period of time, we find that we do not observe this type of motion.

• In the 1920’s a new conceptual model was devised that had three cells instead of the single Hadley cell.

• These three cells better represent the typical wind flow around the globe.

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Planetary Scale Circulations

• A very complicated pattern that we shall examine only in a very simple form.

• To begin, imagine the earth as a non-rotating sphere with uniform smooth surface characteristics.

• Assume that the sun heats the equatorial regions much more than the polar regions.

• In response to this, two huge convective cells develop.

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Horse Latitudes

• Around 30oN we see a region of subsiding (sinking) air.

• Sinking air is typically dry and free of substantial precipitation.

• Many of the major desert regions of the N.H. are found near 30o latitude.– Sahara– Middle East– SW United States

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Intertropical Convergence Zone

ITCZ -- Moves south in January


12ITCZ -- Moves north in July

Intertropical Convergence Zone


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Monsoon

• Wind systems that exhibit a pronounced seasonal reversal in direction.

• Best known monsoon is found in India and southeast Asia.

• Winter -- Flow is predominantly off the continent keeping the continent dry.

• Summer -- Flow is predominantly off the oceans keeping the continent wet.

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Monsoon -- Winter

Continents aredry -- dryseason.


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Monsoon -- Summer

Continents arewet -- rainy

season.


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Air Masses

• Air Mass

– A huge volume of air that is relatively uniform horizontally in temperature and water vapor concentration.

– Properties of an air mass are usually defined by the type of surface it develops over ---- the source region.

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Air Masses

• Source Regions

– Relatively flat

– Uniform Surface Compositions

• Oceans

• Great expanse of snow covered ground

• Air needs to be in contact with the source region for a long period of time to develop uniform characteristics.

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• Temperature– Cold Polar (P)– Warm Tropical (T)

• Moisture– Dry Continental (c)– Moist Maritime (m)

• Results in four basic air mass types.

Classification of Air Masses

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• Continental Tropical (cT)– Hot, Dry– Develops over the deserts of Mexico and

the Southwestern United States

• Maritime Tropical (mT)– Warm, Humid– Develops over the tropical and subtropical

oceans and the Gulf of Mexico

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• Continental Polar (cP)– Cold, Dry– Develops over the northern interior of

North America -- Central Canada

• Maritime Polar (mP)– Cold, Moist– Develops over the cold ocean waters of the

North Pacific and North Atlantic

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• Arctic Air [(A) or (cA)]– Very Cold and Usually Dry– Develops over the snow or ice covered

regions of continents– Similar to polar air masses but usually

develops in regions north of 60N where there is little or no insolation during the winter.

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mP

cT

mTmT mT

mPcP

Arctic

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Air Mass Modification

• Air masses do not remain over their source region.

• If the air moves over a region that is different from where it originated, the air mass will be modified, or changed, by the land that the air is travelling over.

• Changes include: warming, cooling, adding or reducing moisture

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Air Mass Modification

cP

WarmerLand

The cP air mass willbe warmed by thewarmer land that itpasses over.

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Air Mass Modification -- Changes in Stability

Cold air mass moves over a warmer surface.

cP

Warm, Moist

The lower layers arewarmed and maybecome unstable.

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0

1000

2000

3000

4000

5000

6000

7000

-10 0 10 20 30Temperature (C)

Altitude(m)

0

1000

2000

3000

4000

5000

6000

7000


Altitude(m)

Sounding Before Modification:Conditionally Unstable

After modification:Absolutely Unstable

Cold air mass moves over a warmer surface.

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Warm air mass moves over a colder surface.

mT

Warm, Moist

The lower layers arecooled and tend to

stablize.

Cold

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0

1000

2000

3000

4000

5000

6000

7000


Altitude(m)

0

1000

2000

3000

4000

5000

6000

7000


Altitude(m)

Sounding Before Modification:Conditionally Unstable

After modification:Absolutely stable

Warm air mass moves over a colder surface.

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Air Mass Modification -- Orographic Effects

mP

“Pacific”Air

Pacific Air is warm but not asdry as continental air.

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Fronts• Front

– A narrow transition zone between air masses of differing densities.

– The density differences usually arise from temperature differences.

– Density differences may be a result of humidity differences (summer).

• A front is the boundary or transition zone between different air masses.

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Fronts

• The transition zone is usually narrow (15 to 200 km in width).

• Fronts may be over 1000 km long.

• On maps, we draw the frontal boundary on the warm side of the transition zone.

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Idealized Cold Front

Isotherms

0C 5 10 15 20 25 30

100 km

Frontal Zone

44 75

COLDFRONT

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Idealized Cold Front

• Cold Front– A boundary that moves in such a way

that the colder (more dense) air advances and displaces the warmer (less dense) air.

– The largest temperature differences are normally associated with cold fronts.

1 Valley Breeze Example ©1997 Prentice-Hall, Inc..

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