Fundamentals of Physical Geography 1e Chapter 4: Atmospheric Pressure, Winds, and Circulation Petersen Sack Gabler

Representations of Earth: San Francisco Bay Region and Airport Chapter cover photo (p. 75) 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). Chapter cover photo (p. 75)

Variations in 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? Figure 4.1

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? Figure 4.2

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 Figure 4.3

Low pressure (converging air) High pressure (diverging air) 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? Figure 4.4

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? Figure 4.5

Leeward Windward 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? Figure 4.6

Wind The Coriolis Effect and Wind Surface Wind Geostrophic Wind Pressure Gradient Force (PGF) Friction force Coriolis effect Wind crosses isobars Geostrophic Wind PGF Coriolis force Wind parallel to isobars Figure 4.7

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 Figure 4.8

Wind What do you think might happen to the diverging air of an anticyclone if there is a cyclone nearby? Figure 4.8

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 Figure 4.9

Global Pressure and Wind Systems A Model of Global Pressure Figure 4.9

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 Figure 4.10a

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 Figure 4.10b

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? Figure 4.10

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 Figure 4.11

Global Pressure and Wind Systems Polar Easterlies Westerlies Trade Winds (5o-25o) Northeast trades Southeast trades Figure 4.11

Global Pressure and Wind Systems Trade Winds (5o-25o) Northeast trades Southeast trades Tropical easterlies Figure 4.11

Global Pressure and Wind Systems The Intertropical Convergence Zone (ITCZ) Equatorial low Strong convergence, rising air, heavy rain, and calm winds Doldrums Figure 4.11

Global Pressure and Wind Systems Subtropical Highs Westerlies Polar Winds Polar Front Relatively warm air (westerlies) meets cold air (polar) Subpolar low Figure 4.11

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 Figure 4.12

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 Figure 4.13

Upper Air Winds and Jet Streams 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? Figure 4.14

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? Figure 4.15

Regional and Local Wind Systems Monsoon Winds Monsoon Seasonal shift of the winds Low pressure (summer) – wet High pressure (winter) – dry Cherapunji, India Figure 4.16

Regional and Local Wind Systems Chinook (Foehn) The term Chinook means “snow eater.” Can you offer an explanation for how this name came about? Santa Ana Katabatic (drainage) winds Figure 4.17

Regional and Local Wind Systems 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? Figure 4.18

Regional and Local Wind Systems Mountain breeze-valley breeze How might a green, shady valley floor and a bare, rocky mountain slope contribute to these changes? Figure 4.19

Ocean-Atmosphere Interactions Ocean Currents Gyres: major surface currents What influences the direction of these gyres? Figure 4.20

Ocean-Atmosphere Interactions Ocean Currents Warm Currents Gulf Stream Kuroshio Current Cold Currents California Current Labrador Current Upwelling Humboldt (Peru) Current Figure 4.21

Ocean-Atmosphere Interactions Ocean Currents How does this map of ocean currents help explain the mild winters in London, England? Figure 4.21

Ocean-Atmosphere Interactions El Niño Weak warm countercurrent that replaces cold coastal waters off the coast of Peru (equatorial Pacific) Figure 4.22

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 Figure 4.23

Ocean-Atmosphere Interactions El Niño and Global Weather Past few decades on average every 2.2 years 1982-83 Figure 4.23b

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 Figure 4.24a

Ocean-Atmosphere Interactions North Atlantic Oscillation (NAO) Negative NAO Weak Azores High and a weak Icelandic Low Eastern US may be colder and snowier Figure 4.24b

Fundamentals of Physical Geography 1e End of Chapter 4: Atmospheric Pressure, Winds, and Circulation Petersen Sack Gabler