Chapter 5: Atmospheric Pressure and Wind
I.
II.
The Impact of Pressure and Wind on the Landscape
Pressure has an intimate relationship with wind: spatial variations in
pressure are responsible for air movements
The Nature of Atmospheric Pressure
A. Intro
1. atmospheric pressure: force exerted by the gas molecules on some
area of Earth’s surface or on any other body
a. pressure is omnidirectional
2. atmospheric pressure at sea level = 14.7 pounds/in2 (1 kg/cm2)
a. atmospheric pressure decreases with increasing altitude
B. Factors Influencing Atmospheric Pressure
1. pressure of a gas is proportional to its density and temperature
2. Density and Pressure
a. density: mass of matter in a unit volume
b. density of gas varies with location: a gas expands as far the
environmental pressure will allow
1) pressure a gas exerts is proportional to its density
2) the denser the gas, the greater the pressure it exerts
c. atmospheric pressure is directly proportional to the air density at that
altitude
1) lower altitudes: atmospheric gas molecules are packed more
densely because of stronger gravitational pull  more molecular
collisions  higher pressure
2) higher altitudes: less dense air  decrease in pressure
3. Temperature and Pressure
a. heated air  increase molecular agitation  increase speed 
greater collision force  higher pressure
b. if other conditions (in particular volume) remain the same
1) an increase in gas temperature  increase in pressure
2) a decrease in temperature  decrease in pressure
c. when air is heated and no control kept on volume, it expands 
decreased density
1) warm weather associated with low atmospheric pressure
2) cool weather associated with high atmospheric pressure
4. Dynamic Influences on Air Pressure
a. intricate relationship between atmospheric pressure, air density, air
temperature, and air movement
b. generalizations about surface pressure:
1) strongly descending air often produces high pressure at the
surface – a dynamic high
2) very cold surface conditions often produce high pressure at
the surface – thermal high
3) strongly rising air often produces low pressure at the surface –
dynamic low
Chapter 5: Atmospheric Pressure and Wind – p. 2 of 11
III.
4) very warm surface conditions often produce low pressure at
the surface – a thermal low
C. Mapping Pressure with Isobars
1. barometer: instrument used to measure atmospheric pressure
2. millibars (mb): unit of measure of atmospheric pressure expressed as
force per surface area
a. average sea level pressure = 1013.25 mb (29.92 in)
3. isobar: isolines of equal pressure
4. pressure is relative
a. pressure is higher or lower than that of surrounding areas
b. roughly circular or oval isobars on weather map indicate an area of
high or low pressure
c. a ridge of high pressure: separates 2 isobars of low pressure; a
trough of low pressure: separates 2 isobars of high pressure
5. mapped air pressure readings usually adjusted to represent
pressures at sea level
6. pressure gradient: horizontal rate of pressure change
a. relative closeness of isobars indicates the pressure gradient
b. closely spaced isobars indicate steep pressure gradient
c. direct influence on wind speed
The Nature of Wind
A. Intro
1. atmosphere is always in motion
2. wind: horizontal air movement
3. updrafts and downdrafts/ascents and subsidences refer to vertical air motion
B. Direction of Movement
1. intro
a. insolation is ultimate cause of wind
1) uneven heating of different parts of Earth’s surface 
temperature gradients  pressure gradients  wind
2) wind is nature’s attempt to even out the uneven distribution of air pressure
b. air flows from area of high pressure to low pressure
1) because of Earth’s rotation and friction
c. direction of wind movement determined by interaction of:
1) pressure gradient
2) Coriolis effect
3) friction
2. Pressure Gradient
a. air moves from areas of high pressure toward areas of low pressure
b. pressure gradient force acts at right angles to the isobars in the
direction of the lower pressure
3. The Coriolis Effect
a. results from Earth’s rotation on its axis
b. appears to deflect flowing objects to the right in the northern
hemisphere and to the left in the southern hemisphere
c. upper atmosphere geostrophic wind
Chapter 5: Atmospheric Pressure and Wind – p. 3 of 11
1) combined effect of pressure gradient and Coriolis effect
2) wind moves parallel to the isobars
3) upper atmosphere only where there is no friction
4. Friction
a. frictional drag of Earth’s surface slows wind and reduces
Coriolis effect
b.  angle of wind flow across isobars neither parallel nor
perpendicular
c. frictional influence greatest near Earth’s surface; angle of wind
across isobars closest to 90o at low altitudes
d. impact of friction extends to only about 1,000 m/3,300 ft
e. higher atmosphere most winds are geostrophic
C. Cyclones and Anticyclones
1. High-Pressure Circulation Patterns (Fig 5-8)
a. anticyclone: high-pressure center
b. 4 anti-cyclonic wind circulation patterns:
1) northern hemisphere upper atmosphere: clockwise flow parallel to
the isobars
2) northern hemisphere lower atmosphere (surface friction layer):
divergent clockwise flow; air spirals out away from center of
anticyclone
3) southern hemisphere upper atmosphere: counterclockwise flow
parallel to the isobars
4) southern hemisphere lower atmosphere (surface friction layer):
air diverges in a counterclockwise direction
2. Low-Pressure Circulation Patterns (Fig 5-8)
a. cyclones: low pressure centers
b. 4 cyclonic wind circulation patterns:
1) northern hemisphere upper atmosphere: counterclockwise flow
parallel to the isobars
2) northern hemisphere lower atmosphere (surface friction layer):
converging counterclockwise flow
3) southern hemisphere upper atmosphere: clockwise flow parallel to
the isobars
4) southern hemisphere lower atmosphere (surface friction layer):
air converges in a clockwise spiral
3. vertical component of air movement associated with pressure centers
a. air descends in anticyclones
1) air sinks in center of high
2) diverges near ground surface
b. air rises in cyclones
1) air converges horizontally into cyclone
2) then rises
D. Wind Speed
1. wind speed is determined primarily by the pressure gradient
a. steep gradient = strong winds
Chapter 5: Atmospheric Pressure and Wind – p. 4 of 11
IV.
V.
b. gentle gradient = weak winds
c. spacing of isobars indicates steepness of pressure gradient
2. variations
a. surface winds are generally relatively gentle
b. annual average North American wind speed = 6-12 knots
c. Cape Dennison, Antarctica = windiest place on Earth: annual
average wind speed = 38 knots
d. most persistent winds: coastal and high mountain areas
E. Renewable Energy: Wind Power
1. typical commercial turbines:
a. 50m/165 ft high
b. blades 30m/100 ft long
c. full generating capacity at wind speeds of 45 mph
d. 1.5 megawatts capacity = supply a couple of thousand households
2. best physical geography locations
a. higher elevation (= higher wind speed)
b. minimal vegetation (trees create friction)
3. economic viability of wind generation
a. less expensive than fuel oil and gas-generated electricity
b. more expensive than coal
4. advantages
a. wind is free
b. not dependent on fossil fuels
c. no greenhouse gas emissions
5. disadvantages
a. intermittency of wind
b. NIMBY (not in my back yard)
6. US goals
a. 20% wind-generated power by 2030
b. geographic challenge:
1) best wind resources on Great Plains
2) greatest population density on coasts
Vertical Variations in Pressure and Wind
A. pressure variations
1. atmospheric pressure decreases rapidly with height
2. pressure change is most rapid at lower altitudes
B. wind variations
1. wind speed usually increases with height
2. wind moves faster above the friction layer
3. strongest tropospheric winds
a. intermediate levels: jet streams
b. violent storms at Earth’s surface
The General Circulation of the Atmosphere
A. intro
1. determinants of world climates
a. global pattern of insolation is primary determinant
Chapter 5: Atmospheric Pressure and Wind – p. 5 of 11
b. general circulation pattern of Earth’s atmosphere is principal
mechanism for longitudinal and latitudinal heat transfer
2. hypothetical non-rotating Earth of uniform surface (Fig 5-12)
a. insolational heating at equatorial region  girdle of low pressure
around globe
b. radiational cooling at poles  cap of high pressure at poles
c. surface winds would flow from poles to equator
d. convection cell: air rising at equator, descending at poles
3. because Earth does rotate and has an extremely varied surface, only
the tropical regions have a complete vertical convective circulation cell
4. Hadley cells (Fig 5-13)
a. vertical equatorial circulation cells
b. gigantic convection system:
1) warm air rises at the equator
 low pressure at surface: converging, rising air
2) rising air cools, moves poleward, descends at ~ 30oN and S
 high pressure at surface: descending, diverging air
3) pressure gradient: air flows from high to low pressure
a) descending air flows from 30oN/S  equator
b) descending air flows from 30oN/S  poleward
5. 7 surface components of basic pattern of circulation from equator to
pole; replicated north and south of equator: (Fig 5-14)
a. intertropical convergence zone (ITCZ)
b. trade winds
c. subtropical high
d. westerlies
e. polar front (subpolar low)
f. polar easterlies
g. polar high
6. subtropical latitudes of the 5 major ocean basins serve as source of
major surface winds
B. Subtropical Highs
1. large semi-permanent high-pressure cells (anticyclones) centered
over each ocean basin at ~30o N/S
a. develop from descending air of Hadley cells
b. 2 general high pressure ridges extend around globe at these latitudes
c. broken up over continents, especially in summer
2. general subsidence of air from higher altitudes
a.  clear, warm, calm weather
b. coincide with most of the world’s major deserts
c. horse latitudes: becalmed sailing ships dumped horses overboard
3. anticyclonic air circulation pattern
a. divergent
b. clockwise in northern hemisphere; counterclockwise in southern
4. source of 2 of the 3 world’s major wind systems:
a. trade winds - equatorward
Chapter 5: Atmospheric Pressure and Wind – p. 6 of 11
b. westerlies - poleward
C. Trade Winds
1. major wind system of the tropics: between latitude 25oN and 25oS
2. easterlies (blow from east to west)
a. northeast trades: northern hemisphere
b. southeast trades: southern hemisphere
3. most reliable of all winds – used by sailing ships/commerce
4. trade winds and moisture
a. originate as warm, drying winds
b. evaporate enormous quantities of moisture over tropical oceans
c. low-lying islands in trade wind zone: often desert
d. some of wettest places on Earth are windward slopes in the
trade winds – Hawaii
D. Intertropical Convergence Zone (ITCZ)
1. northeast and southeast trades converge at equator
2. position shifts seasonally
a. follows high sun
b. shift greater over land than sea
3. globe-girdling zone of low pressure
a. feeble and erratic winds
1) doldrums – sailing ships often becalmed
b. rising, unstable air  thunderstorms
1) pumps enormous amount of sensible and latent heat into upper
troposphere
2)  poleward transfer of heat
c. appears as well-defined, relatively narrow cloud band over
oceans
E. The Westerlies
1. great wind system of midlatitudes: between 30o and 60o N/S
2. west winds on poleward side of subtropical highs
3. surface westerlies less consistent and persistent than the trades
a. surface friction
b. topographic barriers
c. migratory pressure systems
4. Jet Streams
a. persistent westerly geostrophic winds aloft
b. 2 jet streams: core of high-speed winds; 60 knots minimum speed
1) polar jet
a) follows meandering path located along the polar front
b) 30,000-40,000 feet above surface
c) located along area of greatest horizontal pressure gradient
(1) cold polar air to north
(2) warm tropical air to south
5. Rossby Waves
a. large north-south undulations of the upper air westerlies produce
meandering jet stream path
Chapter 5: Atmospheric Pressure and Wind – p. 7 of 11
VI.
1) result in meridional flow: redistributes cold air toward equator,
warm air toward pole
2) bring severe weather changes to midlatitudes
b. subtropical jet stream
1) high in troposphere, just below tropopause
2) located over poleward margin of subsiding air of the subtropical high
3) less influence on surface weather because less temperature
contrast associated with it
6. midlatitudes experience greatest short-run variability of weather of any
part of Earth
F. Polar Highs
1. high pressure cells situated over both polar regions
2. anticyclonic: descending air that diverges horizontally at the surface
a. clockwise in northern hemisphere
b. counterclockwise in southern hemisphere
c. source of polar easterlies
G. Polar Easterlies
1. 3rd broad-scale global wind system
2. between polar highs and 60o N/S
3. cold, dry, variable and generally from east to west
H. Polar Front
1. zone of low pressure at ~ 50o – 60o N/S
2. polar front:
a. zone of conflict between cold polar easterlies and warmer westerlies
b. subpolar lows: semi-permanent zones of low pressure
3. hemispheric differences of subpolar lows
a. southern hemisphere: virtually continuous over cold seas surrounding
Antarctica
b. northern hemisphere: interrupted by continents; more prominent in
winter than summer
4. rising air, cloudiness, precipitation, unsettled/stormy weather
5. conflict zone of polar front is source of many of the migratory storms
that travel with the westerlies
I. Vertical Patterns of the General Circulation
1. biggest surface/upper level difference over tropics: antitrade winds
result from Coriolis effect at top of Hadley cell
Modifications of the General Circulation
A. Seasonal Variations in Location
1. 7 surface components shift latitudinally with the changing seasons
and resultant surface heating
a. in Northern Hemisphere summer, all components are displaced
northward; southward in December
b. greatest displacement in low latitudes
c. ITCZ can range from 25oN (Jul) to 20oS (Jan) (Fig 5-29)
2. impact of shift: significant effect in midlatitudes and their fringes
B. Monsoons
Chapter 5: Atmospheric Pressure and Wind – p. 8 of 11
1. most significant disturbance of the pattern of general circulation
2. seasonal reversal of winds
a. onshore (sea to land) flow in summer
b. offshore (land to sea) flow in winter
3. results in seasonal precipitation regime
a. heavy summer rains – result of moist maritime onshore flow
b. winter dry season – continental air dominates
4. cause
a. thermally induced pressure differences
b. unusually large latitudinal migrations of trade winds associated with
the large seasonal shifts of the ITCZ over southeastern Eurasia
5. human impact:
a. more than half the world’s population inhabits monsoon regions
b. region where the majority of the populations depends on agriculture
for livelihood
6. regions impacted by monsoons
a. major monsoonal systems
1) South Asia
2) East Asia
b. minor monsoonal systems
1) Australia
2) West Africa
c. regions with monsoonal tendencies
1) Central America
2) southeastern United States
VII. Localized Wind Systems
A. Sea and Land Breezes
1. daytime sea breeze (onshore flow from sea to land)
2. nighttime land breeze (flow from land to sea)
3. convectional circulation pattern develops as result of differential
heating of land and water surfaces
a. day: land heats more rapidly than water creating low pressure over
land
b. night: land cools off more quickly than water creating high pressure
over land
B. Valley and Mountain Breezes
1. daytime valley breeze: slopes warm up faster  low pressure 
upslope flow of cooler valley air
a. rising air  cloud formation over peaks
b.  afternoon showers common in high country
c. prominent in summer
2. night mountain breeze: mountain slopes lose heat rapidly through
radiation
a. high pressure  downslope air movement
b. more prominent in winter
c. winter: cold air drainage even in areas of gentle slope
Chapter 5: Atmospheric Pressure and Wind – p. 9 of 11
C. Katabatic Winds
1. winds originate in cold upland areas and cascade toward lower
elevations under influence of gravity
2. common in Greenland and Antarctica: flow off high, cold ice sheets
3. specialized katabatic winds that are channeled through narrow valleys
and can attain high, destructive speeds:
a. mistral: France’s Rhône Valley
b. bora: Adriatic region
c. taku: southeastern Alaska
D. Foehn/Chinook Winds
1. downslope winds
2. originates when a steep pressure gradient develops
a. high pressure on windward side of a mountain
b. low pressure on leeward side
3. dry, warming wind on arrival at the base of the mountain range
4. foehn: Alps
5. chinook: Rocky Mountains - the “snow-eater” (Fig 5-36)
6. Santa Anas – California
a. persistent cell of high pressure over interior of western US flows
toward coast
b. high speed, high temperature, extreme dryness
c. wildfire threat
d. late summer/early fall
VIII. El Niño-Southern Oscillation (ENSO)
A. El Niño: episodic atmospheric and oceanic phenomenon of the
equatorial Pacific
1. particularly prominent along the west coast of South America
2. major event 1982-83 impacts;
a. droughts, floods, and destructive cyclones in parts of the Pacific
b. ocean temperature 14oF above normal in equatorial Pacific
c. massive die-offs of fish, seabirds and coral
d. cost 1500 human deaths and $9 billion
3. strong El Niño 1997-1998
a. cost: 2,100 human deaths; $30 billion
b. U.S. impacts:
1) severe blizzards in Midwest
2) devastating tornadoes in southeast
3) heavy rainfall in California
B. Normal Pattern
1. wind and pressure patterns off west coast of South America are
dominated by subtropical high and trade winds that flow from east to
west across the Pacific
2. airflow drags warm surface water westward  upwelling of cold
nutrient-rich ocean water into the Peru current and elevated sea level
in Indonesia
Chapter 5: Atmospheric Pressure and Wind – p. 10 of 11
IX.
3. persistent low pressure and convective thunderstorms develop in ITCZ
around Indonesia and northern Australia, producing high annual rainfall
4. Walker Circulation: rising air at ITCZ in western Pacific flows eastward
in upper atmosphere antitrade winds, eventually subsiding along the
eastern side of the Pacific Ocean
C. El Niño Pattern
1. Southern Oscillation (ENSO: El Niño-Southern Oscillation): reversal of
pressure in which high pressure develops over Australia and low
pressure to the east near Tahiti
2. Kelvin waves, slow moving bulges of warm water, travel east along the
equator to the coast of South America → warm water pools and rising
sea level
3. upwelling no longer brings cold water to surface
4. high pressure and southeast trade winds weaken or reverse
5. impacts:
a. drought in northern Australia and Indonesia
b. failed or weakened South Asian monsoon
c. shift in subtropical jet stream over the eastern Pacific brings
increased winter storms in California and Arizona
D. La Niña
1. opposite of El Niño: unusually cold temperatures off South America
coast and strong trade winds
2. southwest US drier than usual
E. Causes of ENSO
1. unclear whether the change in ocean temperature or the change in
pressure and wind comes first: which is the trigger?
2. effects not completely predictable
F. Teleconnections
1. clear coupling between atmospheric and oceanic circulations
2. teleconnections: coupling of weather and oceanic events in one part of
the world with those in another
3. El Niño occurs on average every 2-7 years
4. becoming more frequent and progressively warmer: 1997-97 event
strongest El Niño in last 200 years – speculation that global warming is
impacting it
G. People and the Environment: Forecasting El Niño
1. 70 instrument buoys anchored in tropical Pacific Ocean since 1985 to
monitor sea surface temperatures and wind direction
2. helped successfully predict 1997-98 El Niño
3. recently expanded to monitor Indian and Atlantic oceans
Other Multi-Year Atmospheric and Oceanic Cycles
A. Pacific Decadal Oscillation
1. long term pattern of abrupt sea surface temperature change between
the northern/west tropical and eastern tropical Pacific Ocean
2. occurs every 20-30 years
Chapter 5: Atmospheric Pressure and Wind – p. 11 of 11
3. influences location of jet stream (and storm tracks across US) and
intensity of El Niño events
B. The North Atlantic Oscillation and the Arctic Oscillation
1. NOA
a. irregular seesaw of pressure differences between Icelandic Low
and the subtropical (Azores) high
b. impacts winter storms in Europe, U.S. and Greenland
2. Arctic Oscillation alternates between warm and cold phases that are
closely associated with the NAO