This document summarizes key concepts about atmospheric pressure and global wind patterns from a geography textbook chapter. It defines important terms like isobars, high and low pressure systems, and describes the major wind systems - including the trade winds, westerlies, and polar easterlies - that make up the general circulation of the atmosphere. Specific wind patterns like the Hadley cell and subtropical high are explained in terms of atmospheric pressure gradients and the Coriolis effect.

1. Title Photo Page “ Who has seen the wind? Neither you nor I but when the trees bow down their heads, the wind is passing by.” —Christina Rossetti (Brainyquote.com)

2. Chapter Five Vocabulary pressure gradient (p. 109) ridge (p. 109) Rossby waves (p. 122) Santa Ana winds (p. 131) sea breeze (p. 128) Southern Oscillation (p. 133) subpolar lows (p. 124) subtropical high (STH) (p. 116) teleconnection (p. 136) thermal high (p. 109) thermal low (p. 109) thermocline (p. 134) trade winds (p. 117) trough (p. 109) valley breeze (p. 130) Walker Circulation (p. 133) westerlies (p. 121) wind (p. 110) horse latitudes (p. 117) intertropical convergence zone (ITCZ) (p. 121) isobar (p. 109) jet stream (p. 122) katabatic winds (p. 130) land breeze (p. 128) La Niña (p. 134) low (p. 109) millibar (p. 109) monsoon (p. 126) mountain breeze (p. 130) offshore flow (p. 126) onshore flow (p. 126) polar easterlies (p. 124) polar front (p. 124) polar high (p. 124) anticyclone (p. 112) antitrade winds (p. 125) atmospheric pressure (p. 107) barometer (p. 109) chinook (p. 131) cyclone (p. 112) doldrums (p. 121) dynamic high (p. 109) dynamic low (p. 109) El Niño (p. 131) ENSO (El Niño–Southern Oscillation) (p. 133) foehn (p. 131) friction layer (p. 111) geostrophic wind (p. 111) Hadley cells (p. 115) high (p. 109)

3. The Impact of Pressure and Wind on the Landscape Humans not as sensitive to air pressure as they are to other three climate elements (heat, air movement, and humidity). Air pressure acts and responds to other three climate elements, but most intimately with wind. Spatial variations in pressure create air movements.

4. The Nature of Atmospheric Pressure Pressure—the force a gas exerts on some specified area of the container walls. Atmospheric pressure—the force exerted by gas molecules in the atmosphere. Affects Earth’s surface as well as any other body on Earth.

5. The Nature of Atmospheric Pressure Omnidirectional force—exerted equally in all directions. Force drops with increasing altitude because actual number of gas molecules also drops.

6. Factors Influencing Atmospheric Pressure The pressure of a gas is proportional to its density and temperature. This cause-and-effect relationship between these three variables is explained by the ideal gas law. Variations in any one—pressure, density, and temperature of atmosphere—affect the other two. Relationship is very complex, so difficult to make exact predictions of how change in one changes the others.

7. Density and Pressure Density is the mass of matter in a unit of volume. Density of gas changes easily because gas expands as far as the environmental pressure will allow. The denser the gas, the greater the pressure it exerts.

8. Temperature and Pressure If air is heated the molecules become more agitated and they exert greater pressure. An increase in temperature equals an increase in pressure and a decrease in temperature equals a decrease in pressure. If a mass of air is not confined, it will expand when heated and may actually lead to a decrease in pressure as temperature increases.

9. Dynamic Influences on Air Pressure Surface air pressure may also be influenced by dynamic factors such as the movement of air. i.e., upper atmosphere convergence can cause higher pressure at the surface.

10. Dynamic Influences on Air Pressure Generalizations regarding high and low pressure at the surface: Strongly descending air is associated with high pressure at the surface—a dynamic high. Very cold surface conditions are often associated with high pressure at the surface—a thermal high. String rising air is often associated with low pressure at the surface—a dynamic low. Very warm surface conditions are often associated with relatively low pressure at the surface—a thermal low.

11. Mapping Pressure with Isobars Barometer—instrument for measuring atmospheric pressure. Millibar—an “absolute” measure of pressure, consisting of one-thousandth part of a bar, or 1000 dynes per square centimeter; equals 0.0147 pound per square inch. Average sea-level pressure is 1013.25 millibars. Isobar—a line joining points of equal atmospheric pressure. “ High” and “low” pressures are relative conditions, with the distinction depending on the pressure of the adjoining areas.

12. Mapping Pressure with Isobars Fig. 5-4

13. Mapping Pressure with Isobars Ridge—an elongated area of relatively high pressure. Trough—an elongated area of relatively low pressure. Pressure gradient—the horizontal rate of pressure change, representing the “steepness” of the pressure slope; has a direct effect on the speed of wind. Wind Speed Determined by the pressure gradient Closer spacing of isobars, steeper the pressure gradient, faster the wind blows

15. The Nature of Wind Wind—horizontal movements of air; involve more area than do vertical motions. Updrafts and downdrafts—small-scale vertical motions. Ascents and subsidences—large-scale vertical motions.

16. Direction of Movement Depends on the interaction of three factors: pressure gradient friction Coriolis effect (Earth’s rotation)

17. Direction of Movement Coriolis effect—the apparent deflection of free moving objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, in response to the rotation of Earth. Geostrophic wind—a wind that moves parallel to the isobars as a result of the balance between the pressure gradient force and the Coriolis effect.

18. Fig. 5-7: Friction slows the wind and weakens the Coriolis Force

19. Cyclones and Anticyclones Cyclone—low-pressure cell. Anticyclone—high-pressure cell.

21. Cyclones and Anticyclones Eight circulation patterns are possible because of the interaction of the pressure gradient, Coriolis effect, and friction. Four involve anticyclones. Four involve cyclones. Each is dependent on the cell’s location (hemisphere and altitude [whether surface-layer or upper air]).

22. Cyclones and Anticyclones Vertical Movement Within Cyclones and Anti cyclones Anticyclone pattern is upper air sinking down into the center of the high and diverging near the surface. Cyclonic pattern is converging air at the surface and then rising,

23. Wind Speed Is determined by pressure gradient. The steeper its slope, the faster the wind. Most persistent winds are usually in coastal areas or high mountains.

24. Vertical Variations in Pressure and Wind Atmospheric pressure usually decreases rapidly with height. Wind speed usually increases with height; winds tend to move faster above friction layer.

25. The General Circulation of the Atmosphere Rotation of Earth and its variable surfaces is key in creating a complex circulation pattern for atmosphere. Only the tropical regions have a complete vertical cell. Hadley cell—complete vertical circulation cells in which warm air rises to elevations of about 50,000 feet (15 km), where it cools and moves poleward, then subsides. The cell’s air rises at the equator and descends at about 30° of latitude (either north or south, depending on cell). There are two. Idealized Pattern -Uniform surface -No Earth rotation

26. The General Circulation of the Atmosphere At midlatitudes and high latitudes, vertical cells do not exist or are weakly and sporadically developed. The general circulation of the atmosphere has seven surface components: Polar high Polar easterlies Subpolar low Westerlies Subtropical high Trade winds Intertropical convergence zone

27. The seven components are a focus of this presentation: The order of presentation is: Fig. 5-27 1. 2. 3. 4. 5. 6 7

28. Subtropical Highs Subtropical latitudes serve as the “source’ of the major surface winds of the planet. Subtropical highs— (STHs) large semipermanent high-pressure (anticyclone) cells centered at about 30° latitude over the oceans; have average diameters of 3,200 kilometers (2,000 miles) and are usually elongated east–west. Develop from the descending air of the Hadley cells.

29. Subtropical Highs Horse latitudes—areas in the subtropical highs characterized by warm, tropical sunshine and an absence of wind; created because weather within an STH is nearly always clear, warm, and calm. STHs also coincide with most of the world’s major deserts. STHs serve as source for two of the world’s three major surface systems: Trade winds Westerlies

30. Trade Winds Trade winds—the major wind system of the tropics, issuing from the equatorward sides of the subtropical highs and diverging toward the west and toward the equator. Most reliable of all winds, being extremely consistent in both direction and speed. Winds are named for the direction they blow from. Trade winds’ origin depends on which hemisphere they are in.

31. Trade Winds In Northern Hemisphere, originate in northeast, so are sometimes called northeast trades. In Southern Hemisphere, originate in southeast, so are sometimes called southeast trades. Warming, drying winds capable of holding enormous amounts of moisture. Do not release moisture unless forced by a topographic barrier or pressure disturbance. Pass over low-lying islands, which thus are desert islands. Windward slopes in trade winds, as in Hawaii, are some of the wettest places on Earth.

32. Tropical coastal areas are typically breezy Fig. 5-20

33. Trades do not produce rain unless forced to rise. Fig. 5-21

34. Intertropical Convergence Zone Intertropical convergence zone— (ITCZ) a belt of calm air where northeast trades and southeast trades converge, generally in the vicinity of the equator. Also called equatorial front, intertropical front, and doldrums. Front—a zone of discontinuity between unlike air masses.

35. Intertropical Convergence Zone ITCZ zone’s thunderstorms provide the updrafts where all the rising air in of the tropics ascends. Often appears as a narrow band of clouds over oceans, but it is less distinct over continents.

36. The Westerlies Westerlies—the great wind system of the midlatitudes that flows basically from west to east around the world in the latitudinal zone between about 30° and 60˚ both north and south of the equator.

37. Jet Streams Two cores of high-speed winds at high altitudes in the westerlies: Polar front jet stream Subtropical front jet stream

38. Fig. 5-26: Relative position of two jet streams

39. Rossby Waves Rossby waves—sweeping north–south undulations that westerlies frequently develop in upper air. Rossby waves and the migratory pressure systems and storms associated with westerly flow make midlatitudes have more short-run variability of weather than any other place on Earth.

40. Polar Highs Polar high—a high-pressure cell situated over either polar region. Because it forms over an extensive, high-elevation, very cold continent, Antarctic high differs greatly from Arctic high. Antarctic high is strong, persistent, and almost a permanent feature, while Arctic high is much less pronounced and more transitory.

41. Polar Easterlies Polar easterlies—a global wind system that occupies most of the area between the polar highs and about 60° of latitude. The winds move generally from east to west and are typically cold and dry.

42. Polar Front Polar front is sometimes clearly visible by the semi-permanent zones of low pressure called the subpolar low A zone of low pressure that is situated at about 50° to 60° of latitude in both Northern and Southern hemispheres and which often contains the polar front. Characteristics vary in either hemisphere because the continents interrupt Northern subpolar system, while Southern is virtually continuous over the oceans. Polar front—the meeting ground of the polar easterlies’ cold winds and the westerlies’ warm winds.

43. Vertical Patterns of the General Circulation Winds in upper elevations of troposphere are different from surface winds. Most dramatic difference occurs between surface trade winds and the upper-elevation antitrade winds. Antitrade winds—tropical upper air winds that blow toward the northeast in the Northern Hemisphere and toward the southeast in the Southern Hemisphere.

44. Modifications of the General Circulation The general circulation varies because of many factors, but the two principal modifications are seasonal variations in location and monsoons.

45. Seasonal Variations in Location The seven surface components of the general circulation shift latitudinally with the changing seasons. Affect weather only minimally in equator and polar regions, but significantly alter weather in midlatitudes and their fringes. Mumbai (Bombay) is slowly limping back to life after days of being under water.

46. Monsoons Monsoon—a seasonal reversal of winds; a general onshore movement in summer and a general offshore flow in winter, with a very distinctive seasonal precipitation regime. Most significant disturbance to the pattern of general circulation. Offshore flow—wind movement from land to water. Onshore flow—wind movement from water to land.

47. Monsoons Control the climates of regions with more than half of the world’s population. Origin of monsoons is still not understood, though there is increasing evidence that it is associated with upper-air phenomena, particularly jet stream behavior. Monsoons have an essential impact—their failure or even late arrival of monsoonal moisture inevitably causes widespread starvation and economic disaster.

48. Monsoons The most significant disturbance of the general circulation Seasonal Winds Wet summer Dry winter Fig. 5-32

49. Principal Monsoon Areas Fig. 5-31

50. Two Major Monsoon Systems Fig. 5-33

51. Two Minor Monsoon Systems Fig. 5-34

52. Localized Wind Systems Lesser winds have a considerable effect on weather and climate on a localized scale.

53. Sea and Land Breezes Cycle of sea breezes and land breezes is a common local wind system along tropical coastlines and somewhat in summer in midlatitude coastal areas. Essentially a convectional circulation caused by differential heating of land and water surfaces. Land breeze—local wind blowing from land to water, usually at night (and normally considerably weaker flow than that of sea breeze). Sea breeze—local wind blowing from sea toward the land, usually during the day.

54. Valley and Mountain Breezes Daily cycle of airflow occurs with valley and mountain breezes. Convectional circulation caused by differential heating of higher versus lower elevations. Mountain air cools quickly at night, allowing cooler air to drain down the slope in the evening. Conversely, valley air heats more rapidly during the day, allowing warm air to move upslope during the day. Valley breeze—an upslope flow, during day. Mountain breeze—a downslope flow, during night. Air drainage—the sliding of cold air downslope to collect in the lowest spots, usually at night; a modified form of mountain breeze common in winter.

55. Katabatic Winds Katabatic wind—a wind that originates in cold upland areas and cascades toward lower elevations under the influence of gravity. Air is cold and dense, and usually colder than the air it displaces in its downslope flow. Mistral—a cold, high-velocity wind that sometimes surges down France’s Rhone Valley, from the Alps to the Mediterranean Sea. Has considerable destructive power. Similar winds are called bora in Adriatic region and taku in Alaska.

56. Foehn/Chinook Winds Chinook—a localized downslope wind of relatively dry and warm air, which is further warmed adiabatically as it moves down the leeward slope of the Rocky Mountains. Called foehn when it occurs in Europe. Santa Ana Winds—similar to chinook/foehn. Have high speed, high temperature, and extreme dryness and prompt wildfires.

57. El Niño-Southern Oscillation El Niño—an anomalous oceanographic-weather phenomenon of the eastern equatorial Pacific, particularly along coast of South America. Occurs when southeastern trades abnormally slacken or reverse direction, which triggers a warm surface flow, which displaces the cold, nutrient-rich upwellings that usually prevail on the surface. Disrupts productive offshore fisheries.

58. El Niño-Southern Oscillation Was once believed to be a local phenomenon, but is now understood to be associated with changes in global pressure, wind, and precipitation. Occurs every few years around Christmas time (El Niño is Spanish for “the Christ child”).

59. El Niño-Southern Oscillation Even though archeological and paleoclimatological records have indicated past El Niño phenomenon, it was not until 1982–1983 that great attention was drawn to it. In 1982–1983, the El Niño event brought on a series of weather events called “the most disastrous in recorded history” (1500 human deaths, about $9 billion in damage, and vast ecological havoc):

60. El Niño-Southern Oscillation Crippling droughts (in Australia, India, Indonesia, the Philippines, Mexico, Central America, and southern Africa); Devastating floods (in western and southeastern United States, Cuba, and northwestern South America); Destructive tropical cyclones (in Tahiti and Hawaii); Die offs of fish, seabirds, and coral (from abnormally warm water over a 12,888-kilometer [8000-mile] stretch of equatorial Pacific).

61. El Niño-Southern Oscillation In 1997-1998 another strong El Niño cycle occurred. Property damage exceeded $30 billion and at least 2100 people died.

62. Forecasting El Niño Array of 70 buoys – Galapagos Is. to New Guinea Source: NOAA (http://www.pmel.noaa.gov/tao/) - Fig. 5-B

63. Cruise ship: Ocean Seeker (Ka’im-im-o-ana) Source: NOAA (http://www.pmel.noaa.gov/tao/proj_over/diagrams/buoy.html)

64. Other Multi-Year Atmospheric and Oceanic Cycles Pacific Decadal Oscillation (PDO) North Atlantic Oscillation (NAO) Arctic Oscillation (AO) Pacific Decadal Oscillation Fig. 5.42

65. Normal Pattern In order to understand El Niño phenomenon, it is critical to understand normal pressure, wind, and ocean current patterns in the Pacific. Dominance of subtropical high in the eastern Pacific causes westward movement of the trade winds toward low-pressure cell in the western Pacific. Trade winds create frictional drag on the Pacific Ocean and create westward moving warm equatorial current. The removal of surface water near the western coast of South America allows cold water to upwell. This phenomenon is known as the Walker Circulation. This is a closed convection cell, but in reality is probably more complex.

66. El Niño Pattern Every few years normal pressure pattern changes. High pressure develops over northern Australia, and low pressure develops to the east over Tahiti. This reversal is known as the Southern Oscillation, which is a large-scale fluctuation in sea-level atmospheric pressure that occasionally occurs in the eastern and western tropical Pacific; caused by differences in water temperature.

67. El Niño Pattern When El Niño and Southern Oscillation coincide (called ENSO), unusual atmospheric and oceanic conditions are more frequent and more intense than when either event occurs alone. Pressure reversal causes the trade winds to reverse direction, and this allows warm water from the western Pacific to “backwash” toward the eastern Pacific. For many months before the onset of El Niño, the trade winds pile up warm water in the western Pacific, and then a bulge of warm equatorial water about 25 cm high moves eastward in a series of bulges known as Kelvin waves. These waves can take 2 to 3 months to arrive off the coast of South America.

68. El Niño Pattern This causes the sea level to rise off the coast of South America as the warm water pools. The thermocline boundary between near-surface and cold deep ocean waters lowers. This impedes the upwelling of cold water off of the coast, and thus causes temperatures off the coast of South America to rise. This shift in the normal pressure pattern in the Pacific can cause increased precipitation in the deserts of Peru, droughts in northern Australia and Indonesia, decreased monsoon activity in South Asia, and more powerful winter storms in the southwestern United States.

69. La Niña La Niña—unusually cold temperatures in the eastern Equatorial Pacific. Both El Niño and La Niña are extreme cases of a naturally occurring climate cycle that involves large-scale changes in sea-surface temperatures across the eastern tropical Pacific. La Niña is not as predictable as El Niño.

70. Causes of ENSO There is no clear “trigger” of ENSO. It is not clear whether the changes in the ocean temperature or the changes in the pressure and wind occur first. The effects of ENSO are also not completely predictable.

71. Teleconnections Some generalizations, however, can be made (i.e., floods are more likely to occur in California during El Niño years). There is increasing recognition of El Niño connections with atmospheric and oceanic conditions outside of the Pacific. These connections are known as teleconnections.

72. Other Multi-Year Atmospheric and Oceanic Cycles Pacific Decadal Oscillation The North Atlantic Oscillation and the Arctic Oscillation

73. Pacific Decadal Oscillation The Pacific Decadal Oscillation (PDO) is a long-term pattern of sea-surface temperature change between the northern/west tropical and eastern tropical Pacific Ocean. About every 20 to 30 years the sea-surface temperatures in these zones abruptly shift. From the late 1940s to late 1970s, the northern/west tropical Pacific was relatively warm while the eastern tropical Pacific was relatively cool This is the PDO “negative” or “cool” phase. From the late 1970s through the mid-1990s, this pattern switched, with cooler sea surface temperatures in the northern/west tropical Pacific and warmer conditions in the east tropical Pacific. This is the PDO “positive” or “warm” phase. In the late 1990s the switch back to the negative phase was underway, but by the early 2000s it wasn’t clear if this negative phase was going to be short-lived. Although the PDO is not well understood, it seems to influence the location of the jet stream.

74. The North Atlantic Oscillation and the Arctic Oscillation In the North Atlantic Ocean basin, two related but somewhat irregular multi-year cycles of pressure, wind patterns, and temperature exist: The North Atlantic Oscillation—an irregular “seesaw” of pressure differences between two regional components of the general atmospheric circulation in the North Atlantic Ocean basin: the Icelandic Low and the subtropical high (the Azores High). “ Positive” phase of the NAO equals a greater pressure gradient between the Icelandic Low and the Azores High. During such a positive phase, winter storms tend to take a more northerly track across the Atlantic. Bring mild, wet winters to Europe and the eastern United States but colder, drier conditions in Greenland.

75. The North Atlantic Oscillation and the Arctic Oscillation During a “negative” phase of the NAO, both the Icelandic Low and the Azores High are weaker. Winter storms tend to bring higher than average precipitation to the Mediterranean and colder winters in northern Europe and the eastern United States, while Greenland experiences milder conditions. The Arctic Oscillation alternates between warm and cold phases that are closely associated with the NAO. During Arctic Oscillation “warm” phase (associated with the NAO positive phase), the polar high is weaker. Cold air masses don’t move as far south and sea-surface temperatures tend to be warmer in Arctic waters. During the Arctic Oscillation “cold” phase (associated with the NAO negative phase), the polar high is strengthened, bringing cold air masses farther south.

76. People and the Environment: Forecasting El Niño Since the powerful El Niño of 1982–1983 there have been international efforts to understand El Niño and teleconnections. Deployment of some 70 instrument buoys in the tropical eastern Pacific Ocean in the Tropical Atmosphere/Ocean Array (TAO/TRITON array) to monitor ocean and atmospheric conditions Especially sea-surface temperature and wind direction. By 1994, sufficient data had been gathered to begin to develop computer models to predict the onset of an El Niño event several months in advance. Analysis of data from this array successfully predicted the 1997–1998 El Niño event.

77. Basis for Global Wind System Heat Gain 90º S 90º N 30º N 30º S 60º S 60º N 0º S Heat Loss Heat Loss

78. Pressure Belts and Circulation Cells L Pressure Belts Equatorial Low (ITCZ*) *Intertropical Convergence Zone Also area of the doldrums **Horse latitudes 90º S 90º N 30º N 30º S 60º S 60º N 0º S L L H H H H Subtropical High (STH)** Subtropical High (STH)** Polar Front Zone Polar Front Zone Polar High Polar High 1 1 1 – Hadley cell 2 2 2 – Ferrel cell 3 3 3 – Polar cell

79. Global Winds L L L H H H H Global Winds Expected path (PGF) Coriolis effect 90º S 90º N 30º N 30º S 60º S 60º N 0º S NE Trades SE Trades Westerlies Westerlies Polar Easterlies Polar Easterlies