1. Class #17 Monday, February 16, 2009 1 Class #17: Monday, February 16 Surface pressure and winds Vertical motions Jet streams aloft

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3. 3 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

4. Class #17 Monday, February 16, 2009 4 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

5. Class #17 Monday, February 16, 2009 5 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.

6. Class #17 Monday, February 16, 2009 6 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.

7. Class #17 Monday, February 16, 2009 7 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).

8. Class #17 Monday, February 16, 2009 8 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.

9. Class #17 Monday, February 16, 2009 9 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

10. Class #17 Monday, February 16, 2009 10 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.

11. Class #17 Monday, February 16, 2009 11 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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20. Class #17 Monday, February 16, 2009 20 Other monsoons Africa, North America, and Australia have monsoon-like wind patterns, particularly in the warm season.

21. Class #17 Monday, February 16, 2009 21 Review of temperature and surface pressure patterns High at surface –Poles (low temperature) –Subtropical highs, especially over oceans –Land in winter, ocean in summer (low temp.) Low at surface (thermal lows have high T) –Subpolar low at about 60º with polar front –Intertropical Convergence Zone (ITCZ) –Land in summer, ocean in winter (high temp.)

22. Class #17 Monday, February 16, 2009 22 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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26. Class #17 Monday, February 16, 2009 26 Review of upper-level height (pressure) patterns and temperature High heights (pressure) in warm air columns with high temperature Low heights (pressure) in cold air columns with lower temperature Look for ridges over land in summer and oceans in winter Look for troughs over land in winter and oceans in summer

27. Class #17 Monday, February 16, 2009 27 Important reminder!!!! Pressure and height patterns aloft are generally opposite to those at the surface This opposition is necessary for thermal circulations, because the horizontal pressure gradient force must reverse aloft for a complete circulation High at surface/low aloft; Low at surface/high aloft

28. Class #17 Monday, February 16, 2009 28 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 polar highs. Sometimes these jet streams merge into one.

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33. Class #17 Monday, February 16, 2009 33 Patterns in the upper-level westerlies The pattern of upper-level westerlies has waves, with axes of high height (ridges and highs) and axes of low height (troughs and lows). These appear on day to day weather maps. These wavelike patterns help transport energy poleward (advection) to balance the energy budget of the Earth and atmosphere.

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