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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
2
© 2011 Pearson Education, Inc.
Atmospheric Pressure and Wind
• Localized Wind Systems• El Niño-Southern Oscillation• Other Multiyear Atmospheric and Oceanic
Cycles
3
© 2011 Pearson Education, Inc.
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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© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
The Nature of Atmospheric Pressure
• 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
6
Figure 5-3
© 2011 Pearson Education, Inc.
The Nature of Atmospheric Pressure
• 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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© 2011 Pearson Education, Inc.
The Nature of Atmospheric Pressure
• 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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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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© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
The Nature of Wind
• Anticyclones and cyclones
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Figure 5-8
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
The General Circulation of the Atmosphere
• 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
© 2011 Pearson Education, Inc.
The General Circulation of the Atmosphere
• 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
© 2011 Pearson Education, Inc.
The General Circulation of the Atmosphere
• Seasonal differences in the general circulation
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Figure 5-15
© 2011 Pearson Education, Inc.
The General Circulation of the Atmosphere
• 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
© 2011 Pearson Education, Inc.
The General Circulation of the Atmosphere
– 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
© 2011 Pearson Education, Inc.
The General Circulation of the Atmosphere
– 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
© 2011 Pearson Education, Inc.
The General Circulation of the Atmosphere
– 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
© 2011 Pearson Education, Inc.
The General Circulation of the Atmosphere
– 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
© 2011 Pearson Education, Inc.
The General Circulation of the Atmosphere
– 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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© 2011 Pearson Education, Inc.
The General Circulation of the Atmosphere
– 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
© 2011 Pearson Education, Inc.
The General Circulation of the Atmosphere
• The seven components of the general circulation
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Figure 5-26
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The General Circulation of the Atmosphere
• 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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Modifications of the General Circulation
• 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
© 2011 Pearson Education, Inc.
Modifications of the General Circulation
• Major monsoon systems
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Figure 5-32
© 2011 Pearson Education, Inc.
Modifications of the General Circulation
• Minor monsoon systems
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Figure 5-33
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Localized Wind Systems
• 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
© 2011 Pearson Education, Inc.
Localized Wind Systems
• 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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
El Niño-Southern Oscillation
• Circulation patterns—Walker circulation
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Figure 5-38
© 2011 Pearson Education, Inc.
El Niño-Southern Oscillation
• 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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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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© 2011 Pearson Education, Inc.
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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© 2011 Pearson Education, Inc.
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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© 2011 Pearson Education, Inc.
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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