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

7. © 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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8. © 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

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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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11. © 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

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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

14. © 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

15. © 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

17. © 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

18. © 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

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

20. © 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

21. © 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

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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

23. © 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

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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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26. © 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

27. © 2011 Pearson Education, Inc.
The General Circulation of the
Atmosphere
• The seven components of the general circulation
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Figure 5-26

28. © 2011 Pearson Education, Inc.
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

29. © 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

30. © 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

31. © 2011 Pearson Education, Inc.
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

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

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

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

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

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

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

39. © 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

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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