fundamentals of physical geography 1e petersen sack gabler chapter 4: atmospheric pressure, winds,...
TRANSCRIPT
Fundamentals of Physical Geography 1e
Petersen
Sack
Gabler
Chapter 4: Atmospheric Pressure, Winds, and Circulation
Representations of Earth: San Francisco Bay Region and AirportThe 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
Fig. 4-3, p. 78
Stepped Art
Low pressure(converging air)
CYCLONE ANTICYCLONE
High pressure(diverging air)
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)
• 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
Variations in Atmospheric Pressure
• 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 windsHow might vegetation differ on
the windward and leeward sides of an island?
Wind
Fig. 4-5, p. 79
Stepped Art
LeewardWindward
• 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?
Wind
• 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 sea-level 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
Global Pressure and Wind Systems
• 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?
Upper Air Winds and Jet Streams
• 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?
Regional and Local Wind Systems
• 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
Ocean-Atmosphere Interactions
Fundamentals of Physical Geography 1e
Petersen
Sack
Gabler
End of Chapter 4: Atmospheric Pressure, Winds, and
Circulation