chapter 19 air pressure and wind.ppt - jkaser.com...microsoft powerpoint - chapter 19 air pressure...

Chapter Air Pressure andWind

Air Pressure Defined

19.1 Understanding Air Pressure

Air pressure is the pressureexerted by the weight of air.

Air Pressure Defined


Air pressure is exerted in alldirections — down, up, andsideways. The air pressuredirections — down, up, andsideways. The air pressurepushing down on an object exactlybalances the air pressure pushingup on the object.

Measuring Air Pressure


A barometer is a device used formeasuring air pressure.measuring air pressure.

When air pressure increases, themercury in the tube rises. When airpressure decreases, so does theheight of the mercury column.

A MercuryBarometer


Wind is the result of horizontaldifferences in air pressure. Airflows from areas of higherpressure to areas of lower

Factors Affecting Wind

pressure to areas of lowerpressure.

The unequal heating of Earth’ssurface generates pressuredifferences. Solar radiation is theultimate energy source for mostwind.



Three factors combine to controlwind: pressure differences, theCoriolis effect, and friction.Coriolis effect, and friction.

Pressure Differences• A pressure gradient is the amount

of pressure change occurring over agiven distance.



Closely spaced isobars—lines on amap that connect places of equalmap that connect places of equalair pressure—indicate a steeppressure gradient and high winds.

Widely spaced isobars indicate aweak pressure gradient and lightwinds.

Isobars


Coriolis Effect

• The Coriolis effect describes howEarth’s rotation affects movingobjects. In the Northern


objects. In the NorthernHemisphere, all free-movingobjects or fluids, including thewind, are deflected to the right oftheir path of motion. In theSouthern Hemisphere, they aredeflected to the left.

The Coriolis Effect



Friction

• Friction acts to slow air movement,• Friction acts to slow air movement,which changes wind direction.

• Jet streams are fast-moving riversof air that travel between 120 and240 kilometers per hour in a west-to-east direction.

Effect of Friction

19.2 Pressure Centers and Winds

Cyclones are centers of lowpressure.

Anticyclones are centers of highpressure.

Highs and Lows

In cyclones, the pressuredecreases from the outer isobarstoward the center. In anticyclones,just the opposite is the case—thevalues of the isobars increasefrom the outside toward the center.

pressure.


Cyclonic and Anticyclonic Winds

• When the pressure gradient andthe Coriolis effect are applied to

Highs and Lows

the Coriolis effect are applied topressure centers in the NorthernHemisphere, winds blowcounterclockwise around a low.Around a high, they blowclockwise.



• In either hemisphere, friction

Highs and Lows

• In either hemisphere, frictioncauses a net flow of air inwardaround a cyclone and a net flow ofair outward around an anticyclone.

Highs and Lows


Weather and Air Pressure

• Rising air is associated with cloud• Rising air is associated with cloudformation and precipitation,whereas sinking air produces clearskies.

Highs and Lows


Weather Forecasting

• Weather reports emphasize the• Weather reports emphasize thelocations and possible paths ofcyclones and anticyclones.

• Low-pressure centers can producebad weather in any season.

Airflow Patterns, Surface and Aloft

Global Winds


The atmosphere balances thesedifferences by acting as a giantdifferences by acting as a giantheat-transfer system. This systemmoves warm air toward highlatitudes and cool air toward theequator.

Global Winds


Non-Rotating Earth Model

• On a hypothetical non-rotating• On a hypothetical non-rotatingplanet with a smooth surface ofeither all land or all water, two largethermally produced cells wouldform.

Circulation on a Non-Rotating Earth


Rotating Earth Model

• If the effect of rotation were addedto the global circulation model, thetwo-cell convection system would

Global Winds

two-cell convection system wouldbreak down into smaller cells.

• Trade winds are two belts of windsthat blow almost constantly fromeasterly directions and are locatedon the north and south sides of thesubtropical highs.

Global Winds



• Westerlies are the dominant west-• Westerlies are the dominant west-to-east motion of the atmospherethat characterizes the regions onthe poleward side of the subtropicalhighs.



• Polar easterlies are winds that blowfrom the polar high toward thesubpolar low. These winds are not

Global Winds

subpolar low. These winds are notconstant like the trade winds.

• A polar front is a stormy frontalzone separating cold air masses ofpolar origin from warm air massesof tropical origin.

Circulation on a Rotating Earth

Global Winds


Influence of Continents• The only truly continuous pressure

belt is the subpolar low in thebelt is the subpolar low in theSouthern Hemisphere. In theNorthern Hemisphere, where landmasses break up the oceansurface, large seasonaltemperature differences disrupt thepressure pattern.

Global Winds


Influence of Continents

• Monsoons are the seasonal• Monsoons are the seasonalreversal of wind directionassociated with large continents,especially Asia. In winter, the windblows from land to sea. In summer,the wind blows from sea to land.

Surface Pressure

Local Winds

19.3 Regional Wind Systems

The local winds are caused eitherby topographic effects or byby topographic effects or byvariations in surface composition—land and water—in theimmediate area.


• In coastal areas during the warmsummer months, the land surfaceis heated more intensely during the

Land and Sea Breezes

Local Winds

is heated more intensely during thedaylight hours than an adjacentbody of water is heated. As a result,the air above the land surfaceheats, expands, and rises, creatingan area of lower pressure. At nightthe reverse takes place.

Sea and Land Breezes


• In mountainous regions duringdaylight hours, the air along the slopesof the mountains is heated more

Valley and Mountain Breezes

Local Winds

of the mountains is heated moreintensely than the air at the sameelevation over the valley floor. Becausethis warmer air on the mountain slopesis less dense, it glides up along theslope and generates a valley breeze.After sunset the pattern may reverse.

Valley and Mountain Breezes

How Wind Is Measured


• The prevailing wind is the windthat blows more often from one

Wind Direction

• The prevailing wind is the windthat blows more often from onedirection than from any other.

• In the United States, the westerliesconsistently move weather fromwest to east across the continent.

How Wind Is Measured


• An anemometer is an

Wind Speed

• An anemometer is aninstrument thatresembles a cup and iscommonly used tomeasure wind speed.


• At irregular intervals of three to seven

• El Niño is the name given to theperiodic warming of the ocean thatoccurs in the central and eastern Pacific.

El Niño and La Nina

• At irregular intervals of three to sevenyears, these warm countercurrentsbecome unusually strong and replacenormally cold offshore waters with warmequatorial waters.

• A major El Niño episode can causeextreme weather in many parts of theworld.


• Researchers have come torecognize that when surfacetemperatures in the eastern

La Niña Spanish for the little girl

El Niño and La Niña

recognize that when surfacetemperatures in the easternPacific are colder than average, aLa Niña event is triggered that hasa distinctive set of weatherpatterns.

Global Distribution of Precipitation


Global precipitation can be Global precipitation can beexplained if knowledge ofglobal winds and pressuresystems are applied.

chapter 19 air pressure and wind.ppt - jkaser.com...microsoft powerpoint - chapter 19 air pressure...

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