1. © 2014 Pearson Education, Inc. Chapter 2 Lecture McKnight's Physical Geography 11e Lectures Chapter 5 Atmospheric Pressure and Wind © 2014 Pearson Education, Inc. Andrew Mercer Mississippi State University

2. © 2014 Pearson Education, Inc. Learning Goals of This Chapter Explain the development of high pressure cells near Earth's surface. Explain the development of low pressure cells near Earth's surface. Identify the forces that act on near-surface winds. Identify the forces that act on upper-air winds. Describe the significant aspects of the Coriolis effect as it acts on winds. Explain the differences in the forces acting on winds near the surface and in the upper air.

3. © 2014 Pearson Education, Inc. Learning Goals of This Chapter Explain the differences in wind patterns between wind near the surface and in the upper air. Compare and contrast the forces acting on the winds near the surface and those in the upper air. Describe the vertical air movement within a cyclone. Describe the vertical air movement within an anticyclone. Describe the near surface winds associated with an anticyclone in the Northern Hemisphere. Describe the near surface winds associated with an anticyclone in the Southern Hemisphere.

4. © 2014 Pearson Education, Inc. Learning Goals of This Chapter Describe the near surface winds associated with a cyclone in the Northern Hemisphere. Describe the near surface winds associated with a cyclone in the Southern Hemisphere. Describe the upper-air winds associated with an anticyclone in the Northern Hemisphere. Describe the upper-air winds associated with an anticyclone in the Southern Hemisphere. Describe the upper-air winds associated with a cyclone in the Northern Hemisphere. Describe the upper-air winds associated with a cyclone in the Southern Hemisphere.

5. © 2014 Pearson Education, Inc. Learning Goals of This Chapter Explain the circulation and resulting pressures associated with the Hadley cells. Explain the origin and wind patterns within the Hadley cells. Describe the formation and characteristics of the intertropical convergence zone. Explain the relationship of the subtropical highs to the westerlies in both the Northern and Southern Hemispheres. Explain the relationships of the polar front, the polar easterlies, and the polar highs.

6. © 2014 Pearson Education, Inc. Learning Goals of This Chapter Explain the relationships of the polar front, the Rossby waves, and the jet streams. Identify the locations and names of the high pressure belts. Identify the locations and names of the low pressure belts. Explain the seasonal shift of the global wind and pressure systems. Define monsoon. Explain the development of monsoons and where they occur.

7. © 2014 Pearson Education, Inc. Learning Goals of This Chapter Explain the development of land breezes. Explain the development of sea breezes. Explain the development of valley breezes. Explain the development of mountain breezes. Explain the development of katabatic winds. Explain the development of foehns. Describe the changes in the ocean during El Niño. Describe the changes in the atmosphere and the impacts on humans during El Niño. Differentiate between an El Niño and La Niña.

8. © 2014 Pearson Education, Inc. Learning Goals of This Chapter Define teleconnection and propose an example. Describe the impact of the Pacific Decadal Oscillation on regional weather conditions. Describe the impact of the North Atlantic Oscillation on regional weather conditions. Describe the impact of the Arctic Oscillation on regional weather conditions.

9. © 2014 Pearson Education, Inc. Atmospheric Pressure and Wind The Nature of Atmospheric Pressure The Nature of Wind Cyclones and Anticyclones The General Circulation of the Atmosphere Modifications of the General Circulation Localized Wind Systems El Niño-Southern Oscillation Other Multiyear Atmospheric and Oceanic Cycles

10. © 2014 Pearson Education, Inc. The Impact of Pressure and Wind on the Landscape Atmospheric pressure indirectly affects the landscape. Changes manifest primarily by changes in wind and temperature. Wind has a visible component to its activity. Severe storm winds can drastically affect the landscape.

11. © 2014 Pearson Education, Inc. The Nature of Atmospheric Pressure Gas molecules are continuously in motion. Force exerted by gas molecules is called atmospheric pressure. Force is exerted on every surface the gas touches. Pressure is approximately 14 pounds per square inch.

12. © 2014 Pearson Education, Inc. The Nature of Atmospheric Pressure Factors influencing atmospheric pressure –Density – at higher density, particles are closer and collide more frequently, increasing pressure –Temperature – warmer particles move faster and collide more frequently, increasing pressure –Remember ideal gas law! P = ρRT

13. © 2014 Pearson Education, Inc. The Nature of Atmospheric Pressure Dynamic and thermodynamic influences on air pressure are as follows: –Strongly descending air, a dynamic high –Very cold surface conditions, a thermal high –Strongly ascending air, a dynamic low –Very warm surface conditions, a thermal low Dynamic influences work in tandem with influences from density to affect air pressure. All of these influences come back to the original influence of temperature and are related through the ideal gas law.

14. © 2014 Pearson Education, Inc. The Nature of Pressure Mapping pressure with isobars –Pressure measured with a barometer –Typical units are millibars or inches of mercury –Contour pressure values reduced to sea level –Shows highs and lows, ridges and troughs –Mapping of highs and lows and the change in pressure with altitude

15. © 2014 Pearson Education, Inc. The Nature of Wind Origination of wind –Uneven heating of Earth’s surface creates temperature and pressure gradients –Direction of wind results from pressure gradient –Winds blow from high pressure to low pressure

16. © 2014 Pearson Education, Inc. The Nature of Wind Forces that govern the wind –Pressure gradient force Characterized by wind moving from high to low pressure, always Winds blow at right angles to isobars –Coriolis force Turns wind to the right in the Northern Hemisphere; left in Southern Hemisphere Only affects wind direction, not speed; though faster winds turn more –Friction Wind is slowed by Earth’s surface due to friction; does not affect upper levels

17. © 2014 Pearson Education, Inc. The Nature of Wind Force balances –Geostrophic balance Balance between pressure gradient force and Coriolis Winds blow parallel to isobars –Frictional balance Winds blow slightly towards low pressure and slightly away from high pressure Winds slowed by friction weaken Coriolis, so pressure gradient force is stronger and turns the winds

18. © 2014 Pearson Education, Inc. The Nature of Wind Wind speed –Tight pressure gradients (isobars close together) indicate faster wind speeds –Wind speeds are gentle on average

19. © 2014 Pearson Education, Inc. Cyclones and Anticyclones Wind directions around cyclones and anticyclones dictated by force balances described previously Differing Coriolis force causes winds to blow opposite in each hemisphere

20. © 2014 Pearson Education, Inc. Cyclones and Anticyclones Vertical motions –Surface convergence and low pressure indicate rising motion. –Surface divergence and high pressure indicate sinking motion. –Rising motion results in clouds and storms. –Sinking motion results in sunny skies.

21. © 2014 Pearson Education, Inc. The General Circulation of the Atmosphere Atmosphere – in constant motion Major semipermanent conditions of wind and pressure – general circulation Principal mechanism for longitudinal and latitudinal heat transfer Second only to insolation as a determination for global climate

22. © 2014 Pearson Education, Inc. The General Circulation of the Atmosphere Simple example – a nonrotating Earth –Strong solar heating at equator –Little heating at poles –Thermal low pressure forms over equator –Thermal high forms over poles –Ascending air over equator –Descending air over poles –Winds blow equatorward at surface; poleward aloft

23. © 2014 Pearson Education, Inc. The General Circulation of the Atmosphere Observed general circulation –Addition of Earth’s rotation increases complexity of circulation –One semipermanent convective cell near the equator –Three latitudinal wind belts per hemisphere –Hadley cells

24. © 2014 Pearson Education, Inc. The General Circulation of the Atmosphere Seasonal differences in the general circulation

25. © 2014 Pearson Education, Inc. The General Circulation of the Atmosphere Components of the general circulation –Subtropical highs Persistent zones of high pressure near 30° latitude in both hemispheres Result from descending air in Hadley cells Subsidence is common over these regions Regions of world’s major deserts No wind, horse latitudes

26. © 2014 Pearson Education, Inc. The General Circulation of the Atmosphere Components of the general circulation (cont.) –Trade winds Diverge from subtropical highs Exist between 25° N and 25° S latitude Easterly winds; south- easterly in Southern Hemisphere, northeasterly in Northern Hemisphere Most reliable of winds “Winds of commerce”

27. © 2014 Pearson Education, Inc. The General Circulation of the Atmosphere Components of the general circulation (cont.) –Trade winds (cont.) Heavily laden with moisture Do not produce rain unless forced to rise If they rise, they produce tremendous precipitation and storm conditions

28. © 2014 Pearson Education, Inc. The General Circulation of the Atmosphere Components of the general circulation (cont.) –Intertropical Convergence Zone (ITCZ) Region of convergence of the trade winds Constant rising motion and storminess in this region Position seasonally shifts (more over land than water) Doldrums

29. © 2014 Pearson Education, Inc. The General Circulation of the Atmosphere Components of the general circulation (cont.) –Westerlies Form on poleward sides of subtropical highs Wind system of the midlatitudes Two cores of high winds – jet streams Rossby waves (see figure)

30. © 2014 Pearson Education, Inc. The General Circulation of the Atmosphere Components of the general circulation (cont.) –Polar highs Thermal highs that develop over poles due to extensive cold conditions Winds are anticyclonic; strong subsidence Arctic desert –Polar easterlies Regions north of 60° N and south of 60° S Winds blow easterly Cold and dry

31. © 2014 Pearson Education, Inc. The General Circulation of the Atmosphere Components of the general circulation (cont.) –Polar front Low pressure area between polar easterlies and westerlies Air mass conflict between warm subtropics and cold polar air Rising motion and precipitation Polar jet stream position typically coincident with the polar front

32. © 2014 Pearson Education, Inc. The General Circulation of the Atmosphere The seven components of the general circulation

33. © 2014 Pearson Education, Inc. The General Circulation of the Atmosphere Vertical wind patterns of the general circulation –Most dramatic differences in surface and aloft winds is in tropics –Antitrade winds

34. © 2014 Pearson Education, Inc. Modifications to the General Circulation Seasonal modifications –Seven general circulation components shift seasonally. –Components shift northward during Northern Hemisphere summer. –Components shift southward during Southern Hemisphere summer.

35. © 2014 Pearson Education, Inc. Modifications to the General Circulation Monsoons –Seasonal wind shift of up to 180° –Winds onshore during summer –Winds offshore during winter –Develop due to shifts in positions of ITCZ and unequal heating of land and water

36. © 2014 Pearson Education, Inc. Modifications to the General Circulation Major monsoon systems Minor monsoon systems

37. © 2014 Pearson Education, Inc. Localized Wind Systems Sea breezes –Water heats more slowly than land during the day –Thermal low over land; thermal high over sea –Wind blows from sea to land Land breezes –At night, land cools faster –Thermal high over land; thermal low over sea –Wind blows from land to sea

38. © 2014 Pearson Education, Inc. Localized Wind Systems Valley breeze –Mountain top during the day heats faster than valley, creating a thermal low at mountain top –Upslope winds out of valley Mountain breeze –Mountain top cools faster at night, creating thermal high at mountain top –Winds blow from mountain to valley, downslope

39. © 2014 Pearson Education, Inc. El Niño-Southern Oscillation Warming of waters in the eastern equatorial Pacific Associated with numer- ous changes in weather patterns worldwide Typically occurs on time scales of 3 to 7 years for about 18 months

40. © 2014 Pearson Education, Inc. El Niño-Southern Oscillation Circulation patterns – Walker circulation (see figure)

41. © 2014 Pearson Education, Inc. El Niño-Southern Oscillation Patterns associated with El Niño ENSO - Southern Oscillation La Niña – opposite of El Niño Causes of El Niño –Atmosphere changes first or ocean changes first? –Weather effects of El Niño

42. © 2014 Pearson Education, Inc. Other Multiyear Atmospheric and Oceanic Cycles Pacific Secadal Oscillation (PDO – see figure) North Atlantic Oscillation (NAO) Arctic Oscillation (AO)

43. © 2014 Pearson Education, Inc. Summary Atmospheric pressure and wind affect the geographic landscape in several ways. Atmospheric pressure is the force exerted by air molecules on all objects the air is in contact with. Pressure is influenced by temperature, density, and dynamic. Isobars show areas of high pressure and low pressure. Vertical and horizontal atmospheric motions are called wind. Wind is affected by many forces.

44. © 2014 Pearson Education, Inc. Summary Geostrophic balance represents a balance between the Coriolis force and the pressure gradient force. Friction slows the wind and turns it toward lower pressure. Wind patterns around high and low pressure systems are anticyclonic and cyclonic, respectively. Areas of divergence at the surface are associated with sinking motion, while convergence at the surface with rising motion. Close isobar spacing indicates faster winds.

45. © 2014 Pearson Education, Inc. Summary Winds increase rapidly with height; pressure decreases rapidly with height. The global atmospheric circulation is called the general circulation. There are seven components to the general circulation. Each component has associated weather conditions. Seasonal modifications to the general circulation exist, including monsoons. Localized wind systems affect wind direction locally on diurnal time scales.

46. © 2014 Pearson Education, Inc. Summary El Niño is a warming of eastern equatorial Pacific water and subsequent switching of the high and low air pressure patterns. El Niño is associated with varied weather patterns in different locations globally. Other examples of teleconnections include the PDO and the NAO/AO.