Class #15: Friday, October 2

1. Class #15: Friday, October 2 Chapter 7 Global Winds Class #15 Friday, October 2

2. 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. Class #15 Friday, October 2

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6. 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 Class #15 Friday, October 2

7. 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 Class #15 Friday, October 2

8. 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 Class #15 Friday, October 2

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10. 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. Class #15 Friday, October 2

11. 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. Class #15 Friday, October 2

12. 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). Class #15 Friday, October 2

13. 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. Class #15 Friday, October 2

14. 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 Class #15 Friday, October 2

15. 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. Class #15 Friday, October 2