1. Class #13 Monday, September 27, 2010 Class #13: Monday, September 27 Chapter 7 Global Winds 1

2. What a description of global winds should explain Seasonal patterns of precipitation around the world Seasonal patterns of cloudiness around the world The relationships between average wind patterns and pressure patterns and upward and downward air motions Class #13 Monday, September 27, 2010 2

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4. Fig. 7-4a, p. 191 Class #13 Monday, September 27, 2010 4

5. Fig. 7-4b, p. 191 Class #13 Monday, September 27, 2010 5

6. Fig. 7-5a, p. 192 Class #13 Monday, September 27, 2010 6

7. Fig. 7-5b, p. 192 Class #13 Monday, September 27, 2010 7

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9. 9 The surface winds over Earth Are very complicated because of the changing seasons, differences between land and water, and differences in latitude. Can be simplified using a conceptual model. Have been described using a 3-cell model with no land and no seasons. Only temperature differences from equator to pole are included.

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13. Class #13 Monday, September 27, 2010 13 The 3-cell conceptual model of the general circulation Has 3 wind belts in each hemisphere NH and SH –Polar easterlies –Prevailing or mid-latitude westerlies –Trade winds Trade winds blow equatorward –Northeasterly trade winds in NH –Southeasterly trade winds in SH

14. Class #13 Monday, September 27, 2010 14 Surface pressure in the 3-cell model High at both poles, called Polar Highs High in the subtropics, about 30ºN and 30ºS, called Subtropical Highs Low near the equator, called the Equatorial Low, or the Intertropical Convergence Zone (ITCZ) Generally light winds at the Polar and Subtropical Highs, and in the ITCZ

15. Class #13 Monday, September 27, 2010 15 Average vertical motions in the 3-cell model Downward at the poles where surface pressure is high and the troposphere has low temperatures over ice Downward at the subtropical highs Upward in the ITCZ Upward at about 60°N and S near the polar front

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17. Class #13 Monday, September 27, 2010 17 Thermal circulations in the 3-cell model The Hadley cells have their rising branch in the ITCZ and their sinking branch in the subtropics. The Hadley cells cover half of the surface area of Earth. The polar cells have a rising branch near the polar front and sinking at the pole.

18. Class #13 Monday, September 27, 2010 18 The 3-cell model’s circulation in middle latitudes Is thermally indirect, because the air nearer the pole is rising, and the air nearer the equator is sinking. Is an average based on smaller wind patterns in extratropical cyclones, in which the warmer air does rise, and the colder air sinks. Has the motions required by the polar and Hadley cells.

19. Class #13 Monday, September 27, 2010 19 More on the thermal circulation The thermal circulation begins aloft. In diagrams of the thermal circulation, “H” and “L” refer to the horizontal pressure gradient, not to the vertical pressure gradient. The thermal circulation comes about because hydrostatic balance requires that the warmer air column expands compared to the cooler air column.

20. Class #13 Monday, September 27, 2010 20 Consequences of Earth’s rotation from west to east The trade winds in the NH do not blow from the north, but are deflected to the right in the NH, so blow from the northeast. If Earth rotated much more slowly, there would be only the Hadley cell. If Earth rotated much more quickly, there would be more wind belts (like on Jupiter).

21. Class #13 Monday, September 27, 2010 21 More consequences of Earth’s rotation If it were not for the Midlatitude westerlies, Earth’s speed of rotation would slow. Easterlies alone would everywhere act to slow the rotation. The polar easterlies blow from the pole and curve, blowing from the northeast in the NH and from the southeast in the SH. The westerlies blow away from the equator and curve in both hemispheres, that is, they blow from the southwest in the NH, and from the northwest in the NH.

22. Class #13 Monday, September 27, 2010 22 Complications of the real Earth Earth has seasons –The ITCZ (sometimes called the thermal equator) shifts latitude with the seasons. –The ITCZ shifts north of the equator in NH summer, and south of the equator in SH summer (NH winter) Earth has large land masses –Continents and oceans set up thermal circulations

23. Fig. 7-12a, p. 197

24. Fig. 7-12a (1), p. 197

25. Fig. 7-12a (2), p. 197

26. Fig. 7-12b, p. 198

27. Fig. 7-12b (1), p. 198

28. Fig. 7-12b (2), p. 198

30. Class #13 Monday, September 27, 2010 30 Observed surface pressures Vary with the seasons, requiring both a January and a July depiction Are on average high in the sub-tropics (near 30°) and near the pole Are on average low in the ITCZ and along the polar front (near 60°) In summer are high over the oceans and low over the continents (thermal lows). In winter are high over the continents and low over the oceans.

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32. Class #13 Monday, September 27, 2010 32 The thermal circulation The sea breeze is a thermal circulation. A thermal circulation has both horizontal and vertical air motions. The horizontal pressure gradient force is most important in a thermal circulation. Upward air motions occur in the warmer air column of the circulation; downward air motions occur in the cooler air column.

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34. Class #13 Monday, September 27, 2010 34 The sea breeze Is a daytime circulation. Depends on differential heating at the surface between land and water. Has the warmer air column over the land, which absorbs more incoming solar radiation. Has the cooler air column over the water, which absorbs less radiation.

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42. Class #13 Monday, September 27, 2010 42 The sea breeze and the land breeze As solar heating diminishes in the late afternoon, the sea breeze weakens. At night, differential cooling occurs. The cooler air column is over land, where radiational cooling is more rapid than over the water. The warmer air column is over the water. The land breeze develops at night.

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45. Class #13 Monday, September 27, 2010 45 Scales of motion in the atmosphere Describe the size and lifetime of wind patterns in the atmosphere. Determine which forces are most important to forming the wind patterns. Are largest when the lifetimes are longest. Are smaller when the lifetime is shorter. Have a variety of names and definitions.

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47. Class #13 Monday, September 27, 2010 47 More on scales of motion The horizontal pressure gradient force is important for all scales of motion. The Coriolis Force is important for the planetary scale, the synoptic scale, and for the larger mesoscale wind patterns. The vertical pressure gradient force is important for small mesoscale winds.

48. Class #13 Monday, September 27, 2010 48 The surface winds over Earth Are very complicated because of the changing seasons, differences between land and water, and differences in latitude. Can be simplified using a conceptual model. Have been described using a 3-cell model with no land and no seasons. Only temperature differences from equator to pole are included.

49. Class #13 Monday, September 27, 2010 49 The surface winds over Earth Are very complicated because of the changing seasons, differences between land and water, and differences in latitude. Can be simplified using a conceptual model. Have been described using a 3-cell model with no land and no seasons. Only temperature differences from equator to pole are included.

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51. Class #13 Monday, September 27, 2010 51 Complications of the real Earth Earth has seasons –The ITCZ (sometimes called the thermal equator) shifts latitude with the seasons. –The ITCZ shifts north of the equator in NH summer, and south of the equator in SH summer (NH winter) Earth has large land masses –Continents and oceans set up thermal circulations

52. Class #13 Monday, September 27, 2010 52 Observed surface pressures Vary with the seasons, requiring both a January and a July depiction Are on average high in the sub-tropics (near 30°) and near the pole Are on average low in the ITCZ and along the polar front (near 60°) In summer are high over the oceans and low over the continents (thermal lows). In winter are high over the continents and low over the oceans.

53. Class #13 Monday, September 27, 2010 53 Seasonal shifts The ITCZ, the subtropical highs, and the polar front all shift southward in NH winter and northward in NH summer. Seasonal shifts are most intense over Asia, which has the largest continental air mass. The summer monsoon is wet, with low pressure over land; the winter monsoon is dry, with high pressure over land.

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62. Class #13 Monday, September 27, 2010 62 Other monsoons Africa, North America, and Australia have monsoon-like wind patterns, particularly in the warm season.

63. Class #13 Monday, September 27, 2010 63 Winds and pressures (heights) well above the surface Pressures and heights are on average high in the tropics and decrease to lows close to the poles. Upper-level (500mb and above) winds are generally easterlies (blowing east to west) in the tropics and westerlies (blowing west to east) in higher latitudes.

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67. Class #13 Monday, September 27, 2010 67 Jet Streams Jet streams are regions of especially high wind speed in the atmosphere. In the upper-level westerlies, there can be two jet streams, the Polar front jet stream, above the polar front, and the Subtropical jet stream above the subtropical highs. Sometimes these jet streams merge into one.

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