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
19.1 Understanding Air Pressure
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
19.1 Understanding 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
19.1 Understanding Air Pressure
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.
Factors Affecting Wind
19.1 Understanding Air Pressure
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.
Factors Affecting Wind
19.1 Understanding Air Pressure
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
19.1 Understanding Air Pressure
Coriolis Effect
• The Coriolis effect describes howEarth’s rotation affects movingobjects. In the Northern
Factors Affecting Wind
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
Factors Affecting Wind
19.1 Understanding Air Pressure
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.
19.2 Pressure Centers and Winds
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.
19.2 Pressure Centers and Winds
Cyclonic and Anticyclonic Winds
• 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.
Cyclonic and Anticyclonic Winds
Highs and Lows
19.2 Pressure Centers and Winds
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
19.2 Pressure Centers and Winds
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
19.2 Pressure Centers and 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
19.2 Pressure Centers and 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
19.2 Pressure Centers and Winds
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
19.2 Pressure Centers and Winds
Rotating Earth Model
• 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.
19.2 Pressure Centers and Winds
Rotating Earth Model
• 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
19.2 Pressure Centers and 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
19.2 Pressure Centers and 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.
19.3 Regional Wind Systems
• 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
19.3 Regional Wind Systems
• 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
19.3 Regional Wind Systems
• 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
19.3 Regional Wind Systems
• An anemometer is an
Wind Speed
• An anemometer is aninstrument thatresembles a cup and iscommonly used tomeasure wind speed.
19.3 Regional Wind Systems
• 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.
19.3 Regional Wind Systems
• 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
19.3 Regional Wind Systems
Global precipitation can be Global precipitation can beexplained if knowledge ofglobal winds and pressuresystems are applied.