Fundamentals of
Physical Geography 1e
Chapter 4:
Atmospheric Pressure, Winds, and Circulation
Petersen
Sack
Gabler
Representations of Earth: San
Francisco
Bay Region
and
Airport
The swirling
circulation
patterns
in Earth’s
atmosphere are created by changes in pressure
and winds.
Pressure
• Atmospheric
Pressure
– Variations in
pressure create
atmospheric
circulation
(including
wind).
Variations in Atmospheric Pressure
• Atmospheric Pressure
– Standard Sea level
pressure is:
•1013.2 millibars (mb)
•29.92 inches of mercury
– When air pressure
increases, what
happens to the
mercury in the tube?
Variations in Atmospheric Pressure
• Air Pressure and
Altitude
– Air Pressure decreases with
increasing elevation
(altitude).
– Mount Everest (29,028 feet)
has only 1/3 of the pressure
at sea level.
By approximately how much
does density drop between
0 and 100 km?
Variations in Atmospheric Pressure
• Cells of High and Low
Pressure
– Low (Cyclone) = L
•Air is ascending (rising)
•Low pressure
•Convergence at surface
– High (anticyclone) = H
•Air is descending
(subsidence)
•High pressure
•Divergence at surface
CYCLONE
ANTICYCLONE
Low pressure
(converging air)
High pressure
(diverging air)
Stepped Art
Fig. 4-3, p. 78
Variations in Atmospheric Pressure
• Horizontal Pressure Variations
– Determined by thermal (temp) or dynamic
(motion of atmosphere) conditions.
– Thermal
•Warm/hot air is less dense and wants to rise. This
creates low pressure near the equator.
•Cold air is more dense and wants to sink, creating
high pressure, near the poles.
– Dynamic:
•High pressure in the subtropics.
•Low pressure in the subpolar regions (e.g. 40-60o N
and S)
Variations in Atmospheric Pressure
• Mapping Pressure Distribution
– Adjust to sea level pressure
– Isobars: lines of equal pressure
– Pressure Gradient
•Strong pressure gradient (isobars close together
causes stronger winds
•Weak pressure gradient (isobars farther apart)
causes weaker winds
Wind
• Pressure Gradients and
Wind
– Wind
•Horizontal movement of air
due to pressure differences.
•High to Low
•Corrects radiational
imbalances between N and S
pole
Where on this figure would
winds be the strongest?
Wind
• Wind Terminology
– Windward
– Leeward
– Winds are named for
where they come from
•Wind from NE is called NE
wind
– Prevailing winds
How might vegetation differ on the
windward and leeward sides of an
island?
Windward
Leeward
Stepped Art
Fig. 4-5, p. 79
Wind
• The Coriolis Effect and
Wind
– Coriolis Effect
•Apparent deflection of
the wind
•N. hem: wind is deflected
to the right
•S. hem: wind is deflected
to the left.
If no Coriolis effect exists at the
equator, where would the
maximum Coriolis effect be
located?
Wind
• The Coriolis Effect and
Wind
– Surface Wind
•Pressure Gradient Force
(PGF)
•Friction force
•Coriolis effect
•Wind crosses isobars
– Geostrophic Wind
•PGF
•Coriolis force
•Wind parallel to isobars
Wind
• Cyclones, Anticyclones,
and Wind Direction
– Anticyclone (H) – wind
moves away from center in
a clockwise spiral in N. hem.
– Wind goes form high to
low pressure
– Cyclone (L) – wind moves
towards center in a
counterclockwise spiral in
N. hem
Wind
What do you think might happen to the diverging air
of an anticyclone if there is a cyclone nearby?
Global Pressure and Wind Systems
• A Model of Global
Pressure
– Equator low (trough)
– Subtropical High -30oN
and S
– Subpolar low (L)
– Polar high (H)
– This idealized pressure
pattern is affected by
landmasses and
topography
Global Pressure and Wind Systems
• A Model of Global Pressure
Global Pressure and Wind Systems
• Seasonal Variations in
the Pressure pattern
– January
•Shift southward in due
to location of sun’s
direct rays.
•Icelandic Low
•Aleutian low
Global Pressure and Wind Systems
• Seasonal Variations
in the Pressure
Pattern
– July
•Shift northward due
to location of sun’s
direct rays.
•Bermuda/Azores
High
•Pacific High
Global Pressure and Wind Systems
– What is the
difference
between the
Jan. and July
average sealevel pressure at
your location?
– Why do they
vary?
Global Pressure and Wind Systems
• A Model of Atmospheric
Circulation
– Convergence and
Divergence
– Surface Winds (H  L)
– Differential heating, Earth’s
rotation, Coriolis force, and
atmospheric dynamics
Global Pressure and Wind Systems
• Polar Easterlies
• Westerlies
• Trade Winds (5o-25o)
– Northeast trades
– Southeast trades
Global Pressure and Wind Systems
• Trade Winds (5o-25o)
– Northeast trades
– Southeast trades
– Tropical easterlies
Global Pressure and Wind Systems
• The Intertropical
Convergence Zone
(ITCZ)
– Equatorial low
– Strong convergence,
rising air, heavy rain,
and calm winds
– Doldrums
Global Pressure and Wind Systems
• Subtropical Highs
• Westerlies
• Polar Winds
• Polar Front
– Relatively warm air
(westerlies) meets
cold air (polar)
– Subpolar low
Global Pressure and Wind Systems
• Latitudinal Migration
with the Seasons
– 5o-15o
•ITCZ and subtropical
high
– 30o-40o
•Subtropical high in
summer
•Wetter westerlies in
winter along polar front
Global Pressure and Wind Systems
• Longitudinal Variation
in Pressure and Wind
– Subtropical Highs
•West coasts
– Subsidence and
divergence
– Stable
– Relatively dry
•East coasts
– Unstable and moist
Upper Air Winds and Jet Streams
•Jet Stream
– Polar front
•Very strong, narrow
band of winds
embedded within the
upper air westerlies
– Subtropical
Which jet stream is most likely to
affect your home state?
Upper Air Winds and Jet Streams
•Rossby Waves
How are Rossby waves closely
associated with the
changeable weather of the
central and eastern United
States?
Regional and Local Wind Systems
•Monsoon Winds
– Monsoon
•Seasonal shift of the
winds
•Low pressure
(summer) – wet
•High pressure
(winter) – dry
•Cherapunji, India
Regional and Local Wind Systems
•Local Winds
– Chinook (Foehn)
•The term Chinook
means “snow eater.”
Can you offer an
explanation for how
this name came
about?
– Santa Ana
– Katabatic (drainage)
winds
Regional and Local Wind Systems
•Local Winds
– Land-Sea Breeze
•Diurnal (daily reversal of wind)
•Differential heating between land and water
•What is the impact on daytime coastal temperatures
of the land and sea breeze?
Regional and Local Wind Systems
•Local Winds
– Mountain breeze-valley breeze
•How might a green, shady valley floor and a bare,
rocky mountain slope contribute to these changes?
Ocean-Atmosphere Interactions
•Ocean Currents
– Gyres: major surface
currents
– What influences the
direction of these
gyres?
Ocean-Atmosphere Interactions
•Ocean Currents
– Warm Currents
•Gulf Stream
•Kuroshio Current
– Cold Currents
•California Current
•Labrador Current
– Upwelling
•California Current
•Humboldt (Peru)
Current
Ocean-Atmosphere Interactions
•Ocean Currents
– How does this map of ocean currents help
explain the mild winters in London, England?
Ocean-Atmosphere Interactions
•El Niño
– Weak warm countercurrent that replaces cold
coastal waters off the coast of Peru (equatorial
Pacific)
Ocean-Atmosphere Interactions
•El Niño Southern
Oscillation (ENSO)
– Easterly surface winds
weaken and retreat to
the eastern Pacific,
allowing central Pacific
to warm and the rain
area migrates
eastward.
•La Niña
– opposite of ENSO
Ocean-Atmosphere Interactions
•El Niño and
Global Weather
– Past few decades
on average every
2.2 years
– 1982-83
Ocean-Atmosphere Interactions
•North Atlantic
Oscillation (NAO)
– Relationship between
Azores High and
Icelandic Low
– Positive NAO
•Larger pressure difference
between Azores and
Icelandic
•Eastern US may be mild
and wet during winter
Ocean-Atmosphere Interactions
•North Atlantic
Oscillation (NAO)
– Negative NAO
•Weak Azores High and a
weak Icelandic Low
•Eastern US may be colder
and snowier
Fundamentals of
Physical Geography 1e
End of Chapter 4:
Atmospheric Pressure, Winds, and Circulation
Petersen
Sack
Gabler