Ch05

Chapter 5: Atmospheric Pressure and Wind

McKnight’s Physical Geography: A Landscape Appreciation,

Tenth Edition, Hess

© 2011 Pearson Education, Inc.

Atmospheric Pressure and Wind

• The Impact of Pressure and Wind on the Landscape

• The Nature of Atmospheric Pressure• The Nature of Wind• Vertical Variations in Pressure and Wind• The General Circulation of the

Atmosphere• Modifications of the General Circulation

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Atmospheric Pressure and Wind

• Localized Wind Systems• El Niño-Southern Oscillation• Other Multiyear Atmospheric and Oceanic

Cycles

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The Impact of Pressure and Wind on the Landscape

• Atmospheric pressure indirectly affects the landscape

• Changes manifest primarily by changes in wind and temperature

• Wind has a visible component to its activity• Severe storm winds can drastically affect the

landscape

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The Nature of Atmospheric Pressure

• Gas molecules continuously in motion

• Force exerted by gas molecules is called atmospheric pressure

• Force exerted on every surface the gas touches

• Pressure is approximately 14 lbs per square inch

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Figure 5-1



• Factors influencing atmospheric pressure– Density—at higher

density, particles are closer and collide more frequently, increasing pressure

– Temperature—warmer particles move faster and collide more frequently, increasing pressure

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Figure 5-3



• Dynamic influences on air pressure– Strongly descending air, a dynamic high– Very cold surface conditions, a thermal high– Strongly ascending air, a dynamic low– Very warm surface conditions, a thermal low

• Dynamic influences work in tandem with influences from density to affect air pressure

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• Mapping pressure with isobars– Pressure measured with

a barometer– Typical units are

millibars or inches of mercury

– Contour pressure values reduced to sea level

– Shows highs and lows, ridges and troughs

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Figure 5-4


The Nature of Wind

• Origination of wind– Uneven heating of

Earth’s surface creates temperature and pressure gradients

– Direction of wind results from pressure gradient

– Winds blow from high pressure to low pressure

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Figure 5-5


The Nature of Wind

• Forces which govern the wind– Pressure gradient force

• Characterized by wind moving from high to low pressure, always

• Winds blow at right angles to isobars

– Coriolis force• Turns wind to the right in the Northern Hemisphere, left in

Southern Hemisphere• Only affects wind direction, not speed, though faster winds

turn more

– Friction• Wind is slowed by Earth’s surface due to friction, does not

affect upper levels

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The Nature of Wind

• Force balances– Geostrophic balance

• Balance between pressure gradient force and Coriolis

• Winds blow parallel to isobars

– Frictional balance• Winds blow slightly towards

low pressure and slightly away from high pressure

• Winds slowed by friction weaken Coriolis, so pressure gradient force is stronger and turns the winds

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Figure 5-6


The Nature of Wind

• Anticyclones and cyclones

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Figure 5-8


The Nature of Wind

• Vertical motions– Surface convergence and

low pressure indicate rising motion

– Surface divergence and high pressure indicate sinking motion

– Rising motion results in clouds and storms

– Sinking motion results in sunny skies

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Figure 5-9


The Nature of Wind

• Wind speed– Tight pressure gradients

(isobars close together) indicate faster wind speeds

– Wind speeds are gentle on average

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Figure 5-11Figure 5-10


Vertical Variations in Pressure and Wind

• Atmospheric pressure decreases rapidly with height

• Atmospheric surface pressure centers lean with height

• Winds aloft are much faster than at the surface

• Jet streams

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The General Circulation of the Atmosphere

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• Atmosphere is in constant motion• Major semipermanent conditions of wind and

pressure—general circulation• Principal mechanism for longitudinal and

latitudinal heat transfer• Second only to insolation as a determination

for global climate



• Simple example: A non-rotating Earth– Strong solar heating at equator– Little heating at poles– Thermal low pressure forms

over equator– Thermal high forms over poles– Ascending air over equator– Descending air over poles– Winds blow equatorward at

surface, poleward aloft

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Figure 5-12



• Observed general circulation– Addition of Earth’s rotation

increases complexity of circulation

– One semipermanent convective cell near the equator

– Three latitudinal wind belts per hemisphere

– Hadley cells

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Figure 5-14



• Seasonal differences in the general circulation

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Figure 5-15



• Components of the general circulation– Subtropical highs

• Persistent zones of high pressure near 30° latitude in both hemispheres

• Result from descending air in Hadley cells

• Subsidence is common over these regions

• Regions of world’s major deserts

• No wind, horse latitudes

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Figure 5-16



– Trade winds• Diverge from subtropical

highs• Exist between 25°N and

25°S latitude• Easterly winds:

southeasterly in Southern Hemisphere, northeasterly in Northern Hemisphere

• Most reliable of winds• “Winds of commerce”

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Figure 5-17



– Trade winds (cont.)• Heavily laden with

moisture• Do not produce rain

unless forced to rise• If they rise, they

produce tremendous precipitation and storm conditions

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Figure 5-20



– Intertropical Convergence Zone (ITCZ)

• Region of convergence of the trade winds

• Constant rising motion and storminess in this region

• Position seasonally shifts (more over land than water)

• Doldrums

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Figure 5-21



– Westerlies• Form on poleward sides

of subtropical highs• Wind system of the

midlatitudes• Two cores of high winds

– jet streams• Rossby waves

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Figure 5-22

Figure 5-24



– Polar highs• Thermal highs that develop over poles due to

extensive cold conditions• Winds are anticyclonic; strong subsidence• Arctic desert

– Polar easterlies• Regions north of 60°N and south of 60°S• Winds blow easterly• Cold and dry

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– Polar front• Low pressure area between

polar high and westerlies• Air mass conflict between

warm westerlies and cold polar easterlies

• Rising motion and precipitation

• Polar jet stream position typically coincident with the polar front

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Figure 5-25



• The seven components of the general circulation

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Figure 5-26



• Vertical wind patterns of the general circulation– Most dramatic

differences in surface and aloft winds is in tropics

– Antitrade winds

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Figure 5-28


Modifications of the General Circulation

• Seasonal modifications– Seven general

circulation components shift seasonally

– Components shift northward during Northern Hemisphere summer

– Components shift southward during Southern Hemisphere summer

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Figure 5-29



• Monsoons– Seasonal wind shift of up to

180°– Winds onshore during

summer– Winds offshore during

winter– Develop due to shifts in

positions of ITCZ and unequal heating of land and water

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Figure 5-30



• Major monsoon systems

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Figure 5-32



• Minor monsoon systems

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Figure 5-33


Localized Wind Systems

• Sea breezes– Water heats more slowly than

land during the day– Thermal low over land,

thermal high over sea– Wind blows from sea to land

• Land breezes– At night, land cools faster– Thermal high over land,

thermal low over sea– Wind blows from land to sea

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Figure 5-34



• Valley breeze– Mountain top during the day

heats faster than valley, creating a thermal low at mountain top

– Upslope winds out of valley• Mountain breeze

– Mountain top cools faster at night, creating thermal high at mountain top

– Winds blow from mountain to valley, downslope

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Figure 5-35



• Katabatic winds– Cold winds that originate from

cold upland areas, bora winds– Winds descend quickly down

mountain, can be destructive• Foehn/Chinook winds

– High pressure on windward side of mountain, low pressure on leeward side

– Warm downslope winds– Santa Ana winds

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Figure 5-36


El Niño-Southern Oscillation

• Warming of waters in the eastern equatorial Pacific

• Associated with numerous changes in weather patterns worldwide

• Typically occurs on time scales of 3 to 7 years for about 18 months

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Figure 5-37



• Circulation patterns—Walker circulation

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Figure 5-38



• Patterns associated with El Niño

• ENSO—Southern oscillation

• La Niña—opposite of El Niño

• Causes of El Niño– Atmosphere changes first

or ocean changes first?– Weather effects of El Niño

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Figure 5-40


Other Multiyear Atmospheric and Oceanic Cycles

• Pacific decadal oscillation (PDO)

• North Atlantic Oscillation (NAO) and Arctic Oscillation (AO)

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Figure 5-41


Summary

• Atmospheric pressure and wind affect the geographic landscape in several ways

• Atmospheric pressure is the force exerted by air molecules on all objects the air is in contact with

• Pressure is influenced by temperature, density, and dynamic• Isobars show areas of high pressure and low pressure• Vertical and horizontal atmospheric motions are called wind• Wind is affected by many forces

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Summary

• Geostrophic balance represents a balance between the Coriolis force and the pressure gradient force

• Friction slows the wind and turns it towards lower pressure• Wind patterns around high and low pressure systems are

anticyclonic and cyclonic, respectively• Areas of divergence at the surface are associated with

sinking motion, convergence at the surface with rising motion• Close isobar spacing indicates faster winds

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Summary

• Winds increase rapidly with height, pressure decreases rapidly with height

• The global atmospheric circulation is called the general circulation• There are seven components to the general circulation• Each component has associated weather conditions• Seasonal modifications to the general circulation exist, including

monsoons• Localized wind systems affect wind direction locally on diurnal time

scales

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Summary

• El Niño is a warming of eastern equatorial Pacific water and subsequent switching of the high and low air pressure patterns

• El Niño is associated with varied weather patterns in different locations globally

• Other examples of teleconnections include the PDO and the NAO/AO.

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Ch05

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