Chapter 5: Atmospheric
Pressure and Wind
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
Figure 5-2
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
Figure 5-1
The Nature of Atmospheric
Pressure
• Factors influencing atmospheric pressure
– Density
• At higher density, particles are closer &
collide more frequently, increasing pressure
– Temperature
• Warmer particles
move faster &
collide more
frequently,
increasing
pressure
Figure 4-15
Figure 5-3
The Nature of Atmospheric
Pressure
Figure 5-4
• Measurement of Pressure
– Barometer – device to measure
atmospheric pressure
– Average Sea Level Pressure
• 1013.25 MILLIBARS
• 29.92 INCHES OF HG
(MERCURY)
• 14.7 LBS / SQ. INCH
• 760 MM OF HG
– No matter which unit of measurement is used, pressure ALWAYS
decreases with altitude
• Mapping pressure with isobars
– Isobar: line joining points of equal pressure
– Pressure gradient: horizontal rate of pressure change
• Directly affects wind speed
The Nature of Wind
Figure 5-5
• Wind: horizontal air movement
– In theory, why does air move?
• Energy imbalances on globe
– Why does air actually move?
• Differences in horizontal
atmospheric pressure
• Origination of wind
– Uneven heating of Earth’s surface
creates temperature & pressure
gradients
– Direction of wind results from
pressure gradient
– Winds blow from high pressure to
low pressure
• Winds are named for the
direction they blow from!!!
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 N Hemisphere, left in
S Hemisphere
• Only affects wind direction,
not speed, but faster winds
turn more
– Friction
• Wind is slowed by Earth’s
surface due to friction,
does not affect upper levels
Figure 5-7
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
Figure 5-6
The Nature of Wind
• Anticyclones and cyclones
Figure 5-8
The Nature of Wind
• Cyclone: low-pressure cell
– Air converges & ascends
– Surface convergence & low pressure indicate rising motion
– Rising motion results in clouds and storms
• Anticyclone: highpressure cell
– Air descends & diverges
– Surface divergence &
high pressure indicate
sinking motion
– Sinking motion results
in sunny skies
Figure 5-9
The Nature of Wind
• Wind speed
– Determined by pressure
gradients
– Tight pressure gradients
(isobars close together)
indicate faster wind speeds
– Wind speeds are gentle on
average
Figure 5-10
Figure 5-11
The Nature of Wind
12
Vertical
Variations in
Pressure &
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
The General Circulation of the
Atmosphere
• Atmosphere is in constant
motion
• Major semi-permanent
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
The General Circulation of the
Atmosphere
• Simple example: Non-rotating Earth
– Strong solar heating
at equator
• Low pressure
forms over equator
• Ascending air over equator
– Little heating at poles
• High pressure forms
over poles
• Descending air over poles
– Winds blow equatorward
at surface, poleward aloft
Figure 5-12
The General Circulation of the
Atmosphere
• If Earth didn’t rotate
• Because Earth rotates
16
The General
Circulation of
the Atmosphere
• Observed general
circulation
– Earth’s rotation increases
complexity of circulation
– Hadley cells
• Complete vertical
circulation cells
where warm air
rises, cools,
moves poleward,
& subsides
• Around 30° N/S
Figure 5-13
Figure 5-14
The General Circulation of the
Atmosphere
• 7 Surface Components
1. Intertropical convergence
zone (ITCZ) (0°)
2. Trade winds (easterlies)
(0° - 30° N/S)
3. Subtropical highs (30° N/S)
4. Westerlies (30° - 60° N/S)
5. Subpolar low (polar front)
(60° N/S)
6. Polar easterlies
(60° - 90° N/S)
7. Polar highs (90° N/S)
The General Circulation of the
Atmosphere
• Components of the general circulation
– Subtropical highs (STH’s)
Figure 5-16
• Persistent zones
of high pressure
near 30° N/S
• Result from
descending air in
Hadley cells
• Regions of
world’s major
deserts
• No wind
• AKA horse latitudes
• Source of 2 of the 3 major wind systems: trade winds & westerlies
The General Circulation of the
Atmosphere
• Winds are named for the
direction they blow from!
– Trade winds (easterlies)
• Diverge from equatorward side of STH’s
• About 30°N to 30°S
– SE in S Hemisphere,
– NE in N Hemisphere
• Blow east to west
• Most reliable/consistent
of wind systems
• AKA Winds of commerce
Figure 5-17
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
Figure 5-20
The General
Circulation of
the Atmosphere
– Intertropical Convergence
Zone (ITCZ)
• Zone of rising air from surface
heating & convection
• Zone of low pressure &
convergence toward which
trade winds blow
• Near equator
• Constant rising motion &
storminess in this region
• Position seasonally shifts
(more over land than water)
• AKA Doldrums
Figure 5-21
The General Circulation of the
Atmosphere
– Westerlies
• Form on poleward sides of STH’s
• Wind system of the midlatitudes
– Blow west to east
• 2 cores of high winds – jet streams
– Subtropical Jet
– Polar Jet
• Rossby waves
Figure 5-22
Figure 5-24
The General Circulation of the
Atmosphere
Figure 5-27
– Polar highs
• Develop over poles
due to extensive cold
conditions
• Anticyclonic winds
• Strong subsidence
• Arctic desert
– Polar easterlies
• Regions north of 60°N
& south of 60°S
• Winds blow east to
west
• Cold & dry
The General Circulation of the
Atmosphere
– Polar front
• Low pressure around 60° N/S
• Air mass conflict between warm westerlies & cold polar easterlies
• Rising motion =
clouds &
precipitation
– Very strong winds
over oceans
• Polar jet stream
position typically
coincident with
the polar front
• AKA subpolar low
Figure 5-25
The General Circulation of the
Atmosphere
• 7 components of the general circulation
Figure 5-26
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
• Blow from SW in N Hemisphere, & from NW in S Hemisphere
Figure 5-28
Modifications of the General
Circulation
• Seasonal modifications
– 7 general circulation components shift seasonally
– Components shift northward during Northern Hemisphere summer
– Components shift southward during Southern Hemisphere summer
Figure 5-29
Modifications of the General
Circulation
• Monsoons
– Seasonal wind shift
– Most significant disturbance of
general circulation
• Offshore: wind movement from land to water
• Onshore: wind movement from water to land
– Seasonal Winds
• Wet summer
• Dry winter
– Develop due to:
• Shift in position
of ITCZ
• Unequal heating
of land & water
Figure 5-30
Figure 5-31
Modifications of the General
Circulation
• Major monsoon
systems
Figure 5-32
Localized Wind Systems
Figure 5-34
• Convectional circulation from
differential heating of land &
water
• Sea breezes
– Water heats more slowly than
land during the day
– Lower pressure over land, higher
pressure over sea
– Wind blows from sea to land
• Land breezes
– At night, land cools faster
– Higher pressure over land, lower
pressure over sea
– Wind blows from land to sea
Localized Wind Systems
• Convectional circulation from
differential heating of higher vs.
lower elevations
• Valley breeze
– Mountain top during the day heats
faster than valley, creating lower
pressure at mountain top
– Upslope winds out of valley
• Mountain breeze
– Mountain top cools faster at night,
creating higher spressure at
mountain top
– Winds blow from mountain to valley,
downslope
Figure 5-35
Localized Wind Systems
• Katabatic winds
– Cold winds that originate from cold upland areas
– Descend quickly down mountain, can be destructive
• Chinook winds
– Warm downslope winds
– H pressure on windward side, L pressure on leeward side
– Warms adiabatically as it moves down leeward slope of Rocky
Mountains
Figure 5-36
Localized
Wind
Systems
• Santa Ana winds
– Hot, dry, high speed winds
– Occur in S. California from Oct to
Mar
– Spread wildfires in dry seasons
El Niño-Southern Oscillation
• Warming of waters in the eastern equatorial Pacific
• Associated with numerous changes in weather patterns
worldwide
• Typically occurs every 3 to 7 years for about 18 months
Figure 5-37
• El Niño
– Reference to the
Christ Child
• 1st signs usually
appear around
Christmas
El Niño-Southern Oscillation
• Circulation patterns
– Normal
– El Nino
– Upwelling: winds
blowing offshore on
W side of continents
pushes water
westward & brings
up cold, nutrient-rich
water from below
Figure 5-38
Upwelling
• Phytoplankton
shows where
upwelling
occurs
38
El Niño-Southern Oscillation
Figure 5-40
• Patterns associated
with El Niño
– Teleconnections
– Weather effects
• La Niña—opposite of
El Niño
– Weather effects
– Circulation is similar to
“normal”
• Causes of El Niño
– Which changes first,
the atmosphere or
the ocean?
El Niño-Southern Oscillation
– Sea Levels & Sea Surface Temperatures during normal, El Niño &
La Niña years (http://www.pmel.noaa.gov/tao/)
Figure 5-39
Figure 5-C
Figure 5-D
Figure 5-E
El Niño-Southern Oscillation
– Forecasting El Niño
• Array of 70 buoys –
Galapagos Islands to
New Guinea
• Forecasts?
Figure 5-B
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
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
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
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