Chapter 6


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Chapter 6

  1. 1. +Air Pressure and WindsChapter 6 © 2011 Pearson Education, Inc.
  2. 2. +6.1 Wind and Air Pressure © 2011 Pearson Education, Inc.
  3. 3. + Wind and Air Pressure Wind is the result of horizontal differences in atmospheric pressure. Atmospheric (air) pressure is the force exerted against a surface by continuous collision of gas molecules.  Atmosphere is without walls but the force of gravity prevents its escape.
  4. 4. + Wind and Air Pressure The air pressure at sea level is about 1 kg/cm2. Specifically, a column of air is 1 sq inch in cross section, measured from sea level to the top of the atmosphere would weigh 14.7 lbs.© 2011 Pearson Education, Inc.
  5. 5. +6.2 Measuring Air Pressure © 2011 Pearson Education, Inc.
  6. 6. + Measuring Air Pressure  Air pressure is measured in millibars. 1 millibar (mb) = 100 newtons/m2  1013.25 mb = sea level pressure  The mercury barometer is the standard pressure- measuring instrument.  Air pressure pushes on a pool of mercury, which forces the mercury up a tube.  High pressure: The mercury goes further up the tube.  Low pressure: The mercury stays lower.
  7. 7. + Measuring Air Pressure Fair weather = high pressure Rain = low pressure© 2011 Pearson Education, Inc.
  8. 8. + Measuring Air Pressure
  9. 9. +© 2011 Pearson Education, Inc.
  10. 10. +6.3 Pressure Changes with Altitude © 2011 Pearson Education, Inc.
  11. 11. + Pressure Changes with Altitude Air pressure is defined as the force exerted against a surface by continuous collision of gas molecules.  Air pressure decreases with altitude.  Because of the decrease of pressure above you  Air pressure increases with depth.  Because of the increase of pressure above you
  12. 12. + Pressure Changes with Altitude Air pressure decreases about 10mb per 100m increase in elevation Air pressure is reduced by approximately ½ for each 5 km increase in altitude.© 2011 Pearson Education, Inc.
  13. 13. +6.4 Why does Air Pressure Vary? © 2011 Pearson Education, Inc.
  14. 14. + Why Does Air Pressure Vary? Recall that variations in air pressure causes the wind to blow, which in turn causes changes in temperature and humidity Water vapor affects air pressure.  The amount of water vapor reduces the density of air.  Cold dry air has higher pressure.  Warm, dry air has higher pressure than equally warm, moist air.
  15. 15. + Why Does Air Pressure Vary? The movement of air can also cause variations in air pressure.  Convergence occurs when a net airflow into a region causing pressure to rise.  Divergence results when there is a net outflow of air from a region and surface pressure drops.
  16. 16. + Why Does Air Pressure Vary? Temperature influences air pressure.
  17. 17. +6.5 Factors Affecting Wind © 2011 Pearson Education, Inc.
  18. 18. + Factors Affecting Wind Pressure gradient force Coriolis force Friction
  19. 19. + Factors Affecting Wind- Pressure Gradient Force The force that generates winds results from horizontal pressure differences. Greater differences the greater the wind blows Pressure gradient force  Spacing of the isobars indicate the amount of pressure change occurring over a given distance.
  20. 20. + Factors Affecting Wind- Pressure Gradient Force  Isobars are lines connecting places of equal pressure.  If isobars are close together a steep gradient and high winds result.  If isobars are far apart, the result is a low gradient and lower wind speeds.© 2011 Pearson Education, Inc.
  21. 21. + Factors Affecting Wind- Pressure Gradient Force How temperature differences generate wind Sea Breeze figure 6-11 page 169 A- Temp and pressure don’t vary horizontally
  22. 22. + Factors Affecting Wind- Pressure Gradient Force B- as the sun rises the earth begins to warm. Air above the land warms and expands creating a high pressure area (less dense air column) and the air aloft begins to flow away from the land© 2011 Pearson Education, Inc.
  23. 23. + Factors Affecting Wind- Pressure Gradient Force C- The mass transfer of air aloft creates a surface high pressure area over the land. The surface circulation that develops from this redistribution of mass is from the sea towards the land = sea breeze.© 2011 Pearson Education, Inc.
  24. 24. + Factors Affecting Wind- Pressure Gradient Force Isobars on a surface chart:  Low-pressure systems (L) that occur in the middle latitudes are called midlatitude cyclones.  Produce stormy weather  High-pressure systems (H) are called anticyclones, which tend to be associated with clearing conditions.
  25. 25. + Factors Affecting Wind- Coriolis Force TheCoriolis force cannot generate wind; it modifies airflow. The Coriolis force causes all free flowing objects including wind:  In the Northern Hemisphere, a deviation to the right is due to the Earth’s rotation.  In the Southern Hemisphere, the deviation is to the left.
  26. 26. + Factors Affecting Wind- Coriolis Force The Coriolis force is a deflecting force.  It is always directed at right angles to the direction of airflow.  It controls only wind direction, not speed.  It is affected by wind speed (the stronger the wind, the greater the force).  The Coriolis force is strongest at the poles and is nonexistent at the equator.
  27. 27. + Factors Affecting Wind- Coriolis Force Coriolis force
  28. 28. + Factors Affecting Wind- FrictionRemember pressure gradient force is the primary driving force for winds. Air moves from high to low concentrations but wind speeds don’t increase indefinitely.Friction significantly influences airflow near Earth’s surface, but its effect is negligible at higher altitudes.
  29. 29. +6.6 Winds Aloft © 2011 Pearson Education, Inc.
  30. 30. + Winds Aloft  Geostrophic flow occurs when the pressure gradient force and the Coriolis force are balanced.  Thewind flows parallel to the isobars.  They flow in relatively straight paths.  Wind Direction is directly related to prevailing pressure patterns. So if we know direction we can establish a rough approximation of the pressure distribution.
  31. 31. +© 2011 Pearson Education, Inc.
  32. 32. + Winds Aloft Buys Ballot’s Law states that if you stand with the wind at your back, low pressure will be found on your left, high pressure on the right. The situation is reversed in the southern hemisphere.© 2011 Pearson Education, Inc.
  33. 33. + Winds AloftCurved airflow and the gradient windCurved airflow:  Winds around cells of high pressure or low pressure follow curved paths.
  34. 34. + Winds Aloft Gradient winds blow at a constant speed.  They are parallel to the curved isobars.  Centers of low pressure are called cyclonic; winds flow counter-clockwise in the Northern Hemisphere.  A trough is the result of isobars curving to form elongated regions of low pressure.  Centers of high pressure are anticyclonic; winds flow clockwise in the Southern Hemisphere.  A ridge is the result of isobars curving to form elongated regions of high pressure. Opposite is true in the Southern Hemisphere
  35. 35. + Winds Aloft Curved airflow and the gradient wind
  36. 36. + Winds Aloft In Low Pressure Centers- Figure A  The inward directed pressure gradient force is opposed by the outward directed Coriolis force. The pressure gradient force must exceed the coriolis force to overcome the air’s tendency to continue moving in a straight line. In High Pressure Centers- Figure B  Pressure gradient force and Coriolis force are not balanced. The imbalance provides a change in direction and generates the curved flow© 2011 Pearson Education, Inc.
  37. 37. +6.7 Surface Winds © 2011 Pearson Education, Inc.
  38. 38. + Surface Winds Surface winds travel at an angle across isobars, toward low pressure.
  39. 39. + Surface Winds Air flow crosses the isobars at varying angles depending on friction for terrain, but winds always move from high to low pressure. In a cyclone pressure decreases inward and friction causes a new flow towards its center In an anticyclone pressure decreases outward and friction causes a net flow away from its center© 2011 Pearson Education, Inc.
  40. 40. +6.8 How Winds Generate Vertical Air Motion © 2011 Pearson Education, Inc.
  41. 41. +How Winds Generate Vertical Air Motion Factors that promote vertical airflow: Friction  Increased friction causes a drop in wind speed resulting in a pileup of air upstream from the ocean to land.  Convergence can result in cloudy weather.  Decreased friction causes and increase in wind speed from the land to the ocean.  Subsidence and divergence results in clearing weather.
  42. 42. +How Winds Generate Vertical Air MotionVertical airflow is associated with cyclones and anticyclones.
  43. 43. + How Winds Generate Vertical Air Motion Cyclones  Horizontal convergence- net inward transport of air causes a shrinking of the area it occupies  This causes a pile up of air creating a dense column which increases pressure  TO compensate aloft, divergence (spreading out) occurs at an equal rate to inflow below  Remember rising air causes bad weather© 2011 Pearson Education, Inc.
  44. 44. + How Winds Generate Vertical Air Motion Anticyclones  Convergence (pulling in) happens aloft and general subsidence of the air column happens  This creates good, nice weather© 2011 Pearson Education, Inc.
  45. 45. +6.9 Wind Measurements © 2011 Pearson Education, Inc.
  46. 46. + Wind Measurement Wind direction is always measured from the direction it is coming.  A North wind blows north to south.  A East wind blows east to west. A prevailing wind consistently blows more often than not from one direction.
  47. 47. + Wind Measurement
  48. 48. + Wind Measurement A wind vane shows wind direction. Wind speed is often measured with a cup anemometer, which has a dial that shows wind speed. An aerovane is a combination of wind vane and anemometer.