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Class #13: Monday, September 27 Chapter 7 Global Winds Class #13 Monday, September 27, 2010 1 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 Class #13 Monday, September 27, 2010 3 Class #13 Monday, September 27, 2010 4 Fig. 7-4a, p. 191 Class #13 Monday, September 27, 2010 5 Fig. 7-4b, p. 191 Class #13 Monday, September 27, 2010 6 Fig. 7-5a, p. 192 Class #13 Monday, September 27, 2010 7 Fig. 7-5b, p. 192 Class #13 Monday, September 27, 2010 8 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 #13 Monday, September 27, 2010 9 Class #13 Monday, September 27, 2010 10 Class #13 Monday, September 27, 2010 11 Class #13 Monday, September 27, 2010 12 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 #13 Monday, September 27, 2010 13 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 #13 Monday, September 27, 2010 14 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 #13 Monday, September 27, 2010 15 Class #13 Monday, September 27, 2010 16 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 #13 Monday, September 27, 2010 17 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 #13 Monday, September 27, 2010 18 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. Class #13 Monday, September 27, 2010 19 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 #13 Monday, September 27, 2010 20 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 #13 Monday, September 27, 2010 21 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 #13 Monday, September 27, 2010 22 Fig. 7-12a, p. 197 Fig. 7-12a (1), p. 197 Fig. 7-12a (2), p. 197 Fig. 7-12b, p. 198 Fig. 7-12b (1), p. 198 Fig. 7-12b (2), p. 198 Fig. 7-12b (2), p. 198 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 #13 Monday, September 27, 2010 30 Class #13 Monday, September 27, 2010 31 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. Class #13 Monday, September 27, 2010 32 Class #13 Monday, September 27, 2010 33 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. Class #13 Monday, September 27, 2010 34 Class #13 Monday, September 27, 2010 35 Class #13 Monday, September 27, 2010 36 Class #13 Monday, September 27, 2010 37 Class #13 Monday, September 27, 2010 38 Class #13 Monday, September 27, 2010 39 Class #13 Monday, September 27, 2010 40 Class #13 Monday, September 27, 2010 41 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. Class #13 Monday, September 27, 2010 42 Class #13 Monday, September 27, 2010 43 Class #13 Monday, September 27, 2010 44 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. Class #13 Monday, September 27, 2010 45 Class #13 Monday, September 27, 2010 46 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. Class #13 Monday, September 27, 2010 47 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 #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. Class #13 Monday, September 27, 2010 49 Class #13 Monday, September 27, 2010 50 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 #13 Monday, September 27, 2010 51 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 #13 Monday, September 27, 2010 52 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. Class #13 Monday, September 27, 2010 53 Class #13 Monday, September 27, 2010 54 Class #13 Monday, September 27, 2010 55 Class #13 Monday, September 27, 2010 56 Class #13 Monday, September 27, 2010 57 Class #13 Monday, September 27, 2010 58 Class #13 Monday, September 27, 2010 59 Class #13 Monday, September 27, 2010 60 Class #13 Monday, September 27, 2010 61 Other monsoons • Africa, North America, and Australia have monsoon-like wind patterns, particularly in the warm season. Class #13 Monday, September 27, 2010 62 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. Class #13 Monday, September 27, 2010 63 Class #13 Monday, September 27, 2010 64 Class #13 Monday, September 27, 2010 65 Class #13 Monday, September 27, 2010 66 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. Class #13 Monday, September 27, 2010 67 Class #13 Monday, September 27, 2010 68 Class #13 Monday, September 27, 2010 69 Class #13 Monday, September 27, 2010 70 Class #13 Monday, September 27, 2010 71