Atmospheric Pressure and Wind

1. Atmospheric Pressure and Wind GEOG 1112- Lecture 10

2. Chapter 5: Atmospheric Pressure and Wind McKnight’s Physical Geography: A Landscape Appreciation, Tenth Edition, Hess

3. Atmospheric Pressure and Wind • The Impact of Pressure and Wind on the Landscape • The Nature of Atmospheric Pressure • The Nature of Wind • Vertical Variations in Pressure and Wind • Localized Wind Systems

4. 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

5. The Nature of Atmospheric Pressure • Gas molecules continuously in motion • Force exerted by gas molecules is called atmospheric pressure • Force exerted on every surface the gas touches • Pressure is approximately 14 lbs per square inch Figure 5-1

6. 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 Figure 5-3

7. Atmospheric Pressure • Atmospheric pressure: pressure exerted by the atmosphere because of the force of gravity acting upon the overlying column of air • Measured with a barometer • Average pressure at sea level can be expressed in different ways: • 1 kilogram/cm2=15 pounds/in2=101,320 Pa (pascals)=76 cm Hg (mercury)=29.92 in Hg=1013.2 mb (millibars) • Atmospheric pressure at a single location varies slightly from day to day Barometer: instrument that measures atmospheric pressure

8. Pressure • Weight of atmosphere above • Barometer: instrument for measuring atmospheric pressure • Evangelista Torricelli (1608-1647) devised a mercury tube barometer in 1643

9. Standard Sea Level Pressure • 76 cm (29.92 in) rise of mercury in a vacuum tube • 1013.2 mb • Millibar: unit of measurement for atmospheric pressure • One millibar equals a force of 1000 dynes per square centimeter

10. Gas Laws dealing with pressure • Boyle’s Law (1600): At a constant temperature, the volume of a given mass varies inversely with the pressure • P1 * V1 = P2 * V2 • If volume is reduced by half, pressure doubles • Kinetic theory: molecules collide with the container wall twice as often • Increase in pressure = increase in density (mass/volume)

11. Gas Laws dealing with pressure • Charles’ Law (1787): At a constant pressure, the volume of a given mass is directly proportional to the absolute temperature • V1/V2 = T1/T2 • Gas expands when heated

12. Gas Laws dealing with pressure • Ideal Gas Law • PV = nRT • Pressure, Volume, number of moles, constant, temperature • When the volume is kept constant, the pressure of a gas is directly proportional to its absolute temperature • When the temperature is kept constant, the pressure of a gas is proportional to its density and inversely proportional to its volume • When the pressure is kept constant, the absolute temperature of a gas is proportional to its volume and inversely proportional to its density

13. Atmospheric Pressure • Air Pressure and Altitude • Atmospheric pressure decreases with altitude • Vertical Variations in Pressure • Mt. Everest is 33% of sea level pressure • Pressure lapse in the lower troposphere • -0.1063” / 100 foot altitude • -11.81 mb / 100 m

14. Horizontal Pressure Distribution • Horizontal Variations in Pressure • Thermal: heating expands air • Dynamic: changes hourly, daily, annually • Isobar: line drawn on a map to connect all points with the same atmospheric pressure • Pressure Gradient: rate of change of atmospheric pressure horizontally with distance, measured along a line perpendicular to the isobars on a map of pressure distribution

15. The Nature of Atmospheric Pressure • Dynamic influences on air pressure • 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

16. The Nature of Atmospheric 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 Figure 5-4

17. Pressure Gradient • Pressure gradient: rate of change of atmospheric pressure horizontally with distance, measured along a line perpendicular to the isobars on a map of pressure distribution • Slope of isobars shows force

18. Wind

19. Wind • Air in motion from areas of higher pressure to areas of lower pressure; movement is generally horizontal, relative to the ground surface • Winds are identified by the direction from which the wind comes • Wind direction tracked with a wind vane • Wind speed measured with an anemometer

20. 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 Figure 5-5

21. The Nature of Wind • Forces which 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

22. Local Wind Patterns Pressure Gradients Wind is caused by differences in atmospheric pressure from one place to another Air tends to move from regions of high pressure to regions of low pressure Isobars: lines on a map drawn through all points having the same atmospheric pressure Pressure gradient: change of atmospheric pressure measured along a line at right angles to the isobars

23. The Coriolis Effect and Winds • Effect of Earth’s rotation on horizontally moving bodies, such as wind and ocean currents • Stronger away from equator • Stronger when pressure gradient is high • Deflection to the right in the Northern Hemisphere and to the left in the Southern Hemisphere

24. Coriolis Effect

25. 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 Figure 5-6

26. The Nature of Wind • Wind speed • Tight pressure gradients (isobars close together) indicate faster wind speeds • Wind speeds are gentle on average Figure 5-10 Figure 5-11

27. Wind Terminology • Windward • Leeward • Prevailing Wind: direction from which the wind for a particular location blows during the greatest proportion of the time

28. Local Wind Patterns Pressure Gradients develop because of unequal heating in the atmosphere Convection loop: circuit of moving fluid, such as air or water, created by unequal heating of the fluid

29. Local Winds Land Breeze-Sea Breeze Mountain Breeze-Valley Breeze Drainage Winds Foehn Winds Monsoon Winds Subglobal Surface Wind Systems

30. Land Breeze-Sea Breeze • Onshore in Day • Offshore at Night

31. 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 Figure 5-34

32. Upslope in Day Downslope at Night Mountain Breeze-Valley Breeze

33. 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 Figure 5-35

34. Localized Wind Systems • Katabatic winds • Cold winds that originate from cold upland areas, bora winds • Winds descend quickly down mountain, can be destructive • Foehn/Chinook winds • High pressure on windward side of mountain, low pressure on leeward side • Warm downslope winds • Santa Ana winds Figure 5-36

35. Katabatic Winds • Downslope flow of cold, dense air that has accumulated in a high mountain valley or over an elevated plateau or ice cap • Drainage off of high cold plateaus • Local names • Bora (Adriatic) • Minstral (France) • Taku (Alaska

36. Foehn Winds • Warm, dry, downslope wind on lee side of a mountain range, caused by adiabatic heating of descending air • Local names • European Foehn • Rocky Mountains Chinook • Southern California and the Santa Ana wind

37. Monsoon Winds • Monsoon: seasonal wind that reverses direction during the year in response to a reversal of pressure over a large land mass • Out of Asia in Winter • Dry and Cold • Into Asia in Summer • Wet and Moist • Orographic Lifting

38. The Nature of Wind • 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 Figure 5-9

39. Basic Pressure Systems • Cyclones • Surface Low Pressure • Convergent circulation • Uplift • Cloudy • Anticyclones • Surface High Pressure • Divergent circulation • Subsidence • Clear

40. The Nature of Wind • Anticyclones and cyclones Figure 5-8

41. Cyclones • Converging winds • Air uplifting = Cloudy • Pressure decreases toward center • Steepness of pressure gradient determines winds • Average diameter = 1000 km (600 mi)

42. Anticyclones • Diverging winds • Air subsiding = Clear • Pressure increases toward center • Steepness of pressure gradient determines winds • Average diameter = 1500 km (900 mi)

43. Cyclones and Anticyclones

44. Friction and Wind • Surface winds are affected by friction • Cross isobars to the right in north and to the left in south

45. Cyclones, Anticyclones and Winds • Rotation in north and south hemisphere • Convergent- toward center, air rises, creates surface low • Divergent- away from center, air sinks, surface high

46. Geostrophic winds • Upper-level winds in which the Coriolis effect and pressure gradient are balanced, resulting in a wind flowing parallel to the isobars

47. Upper-level winds

48. Vertical Variations in Pressure and Wind • Atmospheric pressure decreases rapidly with height • Atmospheric surface pressure centers lean with height • Winds aloft are much faster than at the surface • Jet streams

49. 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

50. Summary • Geostrophic balance represents a balance between the Coriolis force and the pressure gradient force • Friction slows the wind and turns it towards 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, convergence at the surface with rising motion • Close isobar spacing indicates faster winds