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Why Does the Wind Blow?

Why Does the Wind Blow?. Prof. John Toohey-Morales, CCM St. Thomas University Miami Gardens, Florida. Pressure Differences. Wind: Air molecules move in an attempt to equalize imbalances in air pressure Air pressure, air density, and air temperature are all interrelated

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Why Does the Wind Blow?

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  1. Why Does the Wind Blow? Prof. John Toohey-Morales, CCM St. Thomas University Miami Gardens, Florida

  2. Pressure Differences • Wind: Air molecules move in an attempt to equalize imbalances in air pressure • Air pressure, air density, and air temperature are all interrelated • If one variable changes, then the other two change as well • Colder more dense air compacted into a “shorter column” can exert the same surface pressure as a “taller column” of warm less dense air

  3. Go to Visualization Different Temperature,Different Pressure • Heating or cooling of a “column” of air can establish horizontal variations in pressure that can cause the air to move • Warm air aloft is normally associated to HIGH atmospheric pressure • Cold air aloft is normally associated to LOW atmospheric pressure

  4. Different Temperature,Different Pressure (question) • The surface pressures at the bases of warm and cold columns of air are equal. Air pressure in the warm column of air will with increasing height than in the cold column. a. decrease, more rapidly b. decrease, more slowly c. increase, more rapidly d. increase, more slowly

  5. Different Temperature,Different Pressure (question) • The surface pressures at the bases of warm and cold columns of air are equal. Air pressure in the warm column of air will with increasing height than in the cold column. a. decrease, more rapidly b. decrease, more slowly c. increase, more rapidly d. increase, more slowly

  6. Measuring Atmospheric Pressure • Barometers are used to measure the pressure force of air over a given area

  7. Measuring Atmospheric Pressure (question) • Sea-level pressure values generally fall in the range a. 750 to 950 mb. b. 500 to 1500 mb. c. 100 to 1000 mb. d. 950 to 1050 mb.

  8. Measuring Atmospheric Pressure (question) • Sea-level pressure values generally fall in the range a. 750 to 950 mb. b. 500 to 1500 mb. c. 100 to 1000 mb. d. 950 to 1050 mb.

  9. Pressure Readings • When reading the station pressure, altitude corrections to sea level must be made to make the data useful • Comparing the corrected sea level pressure values across a map allows us to see pressure patterns by drawing isobars

  10. Pressure Readings (question) • Lines connecting points of equal pressure are called a. isobars. b. millibars. c. contours. d. isotherms.

  11. Pressure Readings (question) • Lines connecting points of equal pressure are called a. isobars. b. millibars. c. contours. d. isotherms.

  12. Surface Pressure Charts • H represents areas of high pressure, or anticyclones • L represents areas of low pressure, or cyclones • Solid lines are isobars • Wind blows from High toward Low pressure systems

  13. Upper Air Charts • Keeping the pressure constant makes these isobaric charts (500 mb chart) • Maps depict height contours (like a topo-graphic map), or lines of equal elevation above sea level • Lines (contours) illustrate pressure just like isobars • Low height equals low pressure • High height equals high pressure

  14. Upper Air Charts (continued) • Ridges show where air is warmer (elongated highs) • Troughs show where air is colder (elongated lows) • Winds flow parallel to the contour lines

  15. Upper Level Charts (question) • The contour lines drawn on a 500 mb chart are lines of constant a. pressure. b. altitude. c. density. d. wind direction.

  16. Upper Level Charts (question) • The contour lines drawn on a 500 mb chart are lines of constant a. pressure. b. altitude. c. density. d. wind direction.

  17. Upper Level Charts (question) • Warm air aloft is associated with constant pressure surfaces that are found at ____ altitude than normal and ____ than normal atmospheric pressure aloft. a. higher, higher b. higher, lower c. lower, higher d. lower, lower

  18. Upper Level Charts (question) • Warm air aloft is associated with constant pressure surfaces that are found at ____ altitude than normal and ____ than normal atmospheric pressure aloft. a. higher, higher b. higher, lower c. lower, higher d. lower, lower

  19. Setting the Air in Motion • Pressure Gradient Force (PGF) is determined by the amount of pressure change over a given distance • The PGF is directed from higher to lower pressure at right angles to the isobars

  20. Setting the Air in Motion (cont.) • The magnitude of the PGF is directly related to the pressure gradient • Closely spaced isobars indicate a strong pressure gradient

  21. Setting the Air in Motion (question) • The pressure gradient force is directed from higher pressure toward lower pressure a. only at the equator. b. at all places on earth except for the equator. c. only in the Northern Hemisphere. d. only in the Southern Hemisphere. e. at all places on earth.

  22. Setting the Air in Motion (question) • The pressure gradient force is directed from higher pressure toward lower pressure a. only at the equator. b. at all places on earth except for the equator. c. only in the Northern Hemisphere. d. only in the Southern Hemisphere. e. at all places on earth.

  23. Coriolis Force • An apparent force due to the rotation of Earth • All free moving objects seem to deflect from a straight line because Earth rotates under them

  24. Coriolis Force (continued) • The stronger the wind, the greater the deflection produced • Coriolis acts only at right angles to the wind, deflecting to the right in the northern hemisphere and to the left in the southern hemisphere • It affects wind direction but not its speed • Coriolis is ZERO at the Equator

  25. Coriolis Force (question) • Which of the following combinations produces the strongest Coriolis force? a. fast winds and low latitude b. fast winds and high latitude c. slow winds and low latitude d. slow winds and high latitude

  26. Coriolis Force (question) • Which of the following combinations produces the strongest Coriolis force? a. fast winds and low latitude b. fast winds and high latitude c. slow winds and low latitude d. slow winds and high latitude

  27. Coriolis Force (question) • The Coriolis force is the force that causes the wind to blow. TRUE or FALSE

  28. Coriolis Force (question) • The Coriolis force is the force that causes the wind to blow. TRUE or FALSE

  29. Geostrophic Wind • When Coriolis balances the PGF, the wind blows in a straight path parallel to the isobars • Low pressure is always to the left and high pressure to the right • Speed of the geostrophic flow is directly proportional to the pressure gradient force (PGF)

  30. Gradient Wind Aloft • Wind blows parallel to curved contours above the level of frictional influence • Meandering north to south (or S to N) winds are known as meridional • Winds blowing west to east (or E to W) are called zonal

  31. Gradient Wind Aloft (question) • Suppose that the winds aloft in the Northern Hemisphere are geostrophic and blowing from the north. Low pressure is located to the a. north. b. south. c. east. d. west.

  32. Gradient Wind Aloft (question) • Suppose that the winds aloft in the Northern Hemisphere are geostrophic and blowing from the north. Low pressure is located to the a. north. b. south. c. east. d. west.

  33. Surface Winds • Due to friction they do NOT blow parallel to the isobars, but across at a 30° angle into low pressure and out of high pressure • Friction reduces the wind speed, which reduces the Coriolis force so that it no longer balances PGF • Wind rotates counterclockwise or cyclonically around low pressure • Wind rotates clockwise or anticyclonically around high pressure in the northern hemisphere (backwards in the southern hemisphere)

  34. Surface Winds (question) • The wind around a surface low pressure center in the Southern Hemisphere blows a. counterclockwise and outward from the center. b. counterclockwise and inward toward the center. c. clockwise and outward from the center. d. clockwise and inward toward the center.

  35. Surface Winds (question) • The wind around a surface low pressure center in the Southern Hemisphere blows a. counterclockwise and outward from the center. b. counterclockwise and inward toward the center. c. clockwise and outward from the center. d. clockwise and inward toward the center.

  36. Vertical Air Motion • As air moves into low pressure it converges, has nowhere to go but up, rises, cools, and leads to clouds and precipitation • Aloft air diverges, eventually sinking to generate highs

  37. Measuring Winds • As the wind blows on an object it exerts a force which increases as the square of the wind velocity • Wind direction is given as the direction from which the wind is blowing

  38. Prevailing Winds • Prevailing wind direction is the one most often observed, which can affect climate as well as city planning decisions • Seattle or Portland are cloudy & rainy thanks to the prevailing westerlies off the Pacific • Prevailing winds can impact… • Airports, homes, office buildings • Factories, dumps, sewage treatment plants

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