This chapter discusses atmospheric pressure and wind. It describes how pressure and wind impact the landscape and explains the nature of atmospheric pressure and wind patterns. The key components of the general circulation are outlined, including Hadley cells, trade winds, the ITCZ, westerlies, and polar cells. Seasonal variations and localized wind systems are also covered, along with phenomena like monsoons, El Niño, and other multi-year cycles.

1. Chapter 5: Atmospheric
Pressure and Wind
McKnight’s Physical Geography:
A Landscape Appreciation,
Tenth Edition, Hess

2. © 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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4. © 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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5. © 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

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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
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Figure 5-3

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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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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

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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

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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

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The Nature of Wind
• Anticyclones and cyclones
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Figure 5-8

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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

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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

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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

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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

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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

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The General Circulation of the
Atmosphere
• Seasonal differences in
the general circulation
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Figure 5-15

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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

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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

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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

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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

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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

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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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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

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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

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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

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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

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Modifications of the General
Circulation
• Major monsoon
systems
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Figure 5-32

32. © 2011 Pearson Education, Inc.
Modifications of the General
Circulation
• Minor monsoon
systems
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Figure 5-33

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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

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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

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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

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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

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El Niño-Southern Oscillation
• Circulation patterns—Walker circulation
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Figure 5-38

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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

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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

40. © 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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41. © 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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42. © 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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43. © 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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