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Outline for Lecture 13

Outline for Lecture 13. Factors Affecting Wind. Winds Aloft and Geostrophic Flow. Curved Flow and the Gradient Wind. Surface winds. How Winds Generate Vertical Air Motion. 3/13/03. Formation of A Sea Breeze. No pressure gradient, no wind. (b) Unequal heating creates pressure

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Outline for Lecture 13

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  1. Outline for Lecture 13 Factors Affecting Wind Winds Aloft and Geostrophic Flow Curved Flow and the Gradient Wind Surface winds How Winds Generate Vertical Air Motion 3/13/03

  2. Formation of A Sea Breeze • No pressure gradient, • no wind. • (b) Unequal heating • creates pressure • differences aloft which • causes air to flow. • The transfer of air aloft • (from the land to the sea) • creates a surface high over • the sea which results in a • flow of air from sea to land • (a.k.a. a sea breeze). Returning air Sea Breeze

  3. Pressure Gradient Force steep pressure gradient (fast winds) shallow pressure gradient (slow winds) pressure p n n distance

  4. The Coriolis Force Named after the French Scientist Gaspard Gustave Coriolis free moving objects are deflected to the right of their path in the Northern Hemisphere (to the left of their path in the Southern Hemisphere) because of the Earth’s rotation. It depends on an object’s speed—higher speed means stronger Coriolis Force.

  5. The Coriolis Force Coriolis deflection of winds blowing eastward at different latitudes Coriolis force also increases with increasing Latitude… strong over poles, no effect over equator. Strong Middling Weak None

  6. Friction acts at the surface. winds at the surface aren’t as strong as those at higher altitudes Air is a little viscous, so the layer next to the surface is also affected, but not as much. Friction Altitude

  7. Winds Aloft and Geostrophic Flow Balance pressure gradient force Coriolis Force: Geostrophic Flow. • Geostrophic winds: • go in a straight line • go parallel to the isobars • have speeds proportional to the pressure gradient force.

  8. Geostrophic Flow • WHAT!?!? • pressure difference starts wind • wind gets going a little, starts being deflected by Coriolis force • wind goes faster in response to pressure difference, gets deflected more by Coriolis Force • Eventually, the two balance

  9. Newton’s Laws of Motion (condensed version) • An object in motion tends to stay in motion (unless acted upon by an external force) II. F = ma (that’s pretty much it.)

  10. Put down the pencils Let’s break down the forces

  11. Start with just a pressure gradient Then wind blows straight from High P to Low P And accelerates as it goes Just a Pressure Gradient View from top H L Pressure 1017 1015 1013 1011 1009 1007 1005 return

  12. Wind blows straight from High to Low, eventually evening out the pressure and stopping the wind. Just a Pressure Gradient H L Pressure 1014 1012 1010 1008

  13. The wind would blow at the same speed regardless of altitude. Just a Pressure Gradient H L Pressure 1013 1011 1009 1007 1005 return

  14. Then wind still blows straight from High P to Low P, but it doesn’t get moving as fast as soon, especially near the ground Now add Friction View from top H L Pressure 1017 1015 1013 1011 1009 1007 1005 back up

  15. The pressure difference also evens out eventually, though it might take a bit longer. Friction and Pressure Gradient H L Pressure 1014 1012 1010 1008

  16. Friction slows the wind at the ground—its effects decrease as you go up in the atmosphere. Friction and Pressure Gradient H L Pressure 1013 1011 1009 1007 1005

  17. Friction slows the wind at the ground—its effects decrease as you go up in the atmosphere. Friction and Pressure Gradient H L Pressure 1013 1011 1009 1007 1005 back up

  18. Forget friction. The wind starts out straight, but as soon as it starts building up speed, the Coriolis force turns it a bit to the right. Pressure Gradient and Coriolis H L The wind can’t accelerate any more over here because it’s going parallel to the isobars This is the Geostrophic Flow

  19. The Coriolis force limits the wind speed by redirecting it AND it prevents wind from blowing straight from H to L Pressure Gradient and Coriolis H L

  20. Since the wind never really reaches the low, the pressure difference is maintained, and the low never fills! Pressure Gradient and Coriolis H L

  21. Coriolis Force turns the wind some, friction slows the wind some, and the result is roughly a 30º angle between isobars and wind. Reality: Pressure Gradient, Coriolis Force, and Friction H L 30º

  22. The wind doesn’t blow straight from High to Low, but it does eventually get in there and even out the pressure difference, so H and L don’t last forever without a source of energy Reality: Pressure Gradient, Coriolis Force, and Friction H L 30º

  23. Friction slows the wind at the ground—its effects decrease as you go up in the atmosphere. Reality: Pressure Gradient, Coriolis Force, and Friction H L Pressure 1013 1011 1009 1007 1005

  24. Coriolis Force is turning the wind toward us in the right part of the picture. Reality: Pressure Gradient, Coriolis Force, and Friction H L Pressure 1013 1011 1009 1007 1005

  25. Since the Coriolis Force depends on wind speed, its effect decreases toward the ground where the wind speed is slower. Reality: Pressure Gradient, Coriolis Force, and Friction Coriolis H L Friction Pressure 1013 1011 1009 1007 1005

  26. Pencils are now allowed

  27. Winds Aloft and Geostrophic Flow Coriolis Force Pressure Gradient Wind flows from high to low pressure. Wind speed Coriolis Force Coriolis Force Latitude “Isobaric packing” P.G.F. Geostrophic winds are up high and go straight: only Coriolis and Pressure Gradient Forces are important. Friction is important down low: below about 1500 meters. How do the Coriolis and Pressure Gradient forces change?

  28. Winds Aloft and Geostrophic Flow Wind direction is directly linked to the prevailing pressure pattern. Dutch meteorologist Buys Ballott, 1857 Buys Ballott’s Law states: In the Northern Hemisphere if you stand with your back to the wind, lower pressure will be found to your left and higher pressure will be found to the right. Best when there are no frictional forces or topography involved .

  29. Curved Flow and the Gradient Wind Cyclonic Flow Anticyclonic Flow Actual flow around pressure systems are never this regular (because of small changes in the pressure field).

  30. Surface winds—high pressure(anticyclone) COLD DRY H WARM MOIST

  31. Surface winds—low pressure(cyclone) COLD DRY L WARM MOIST

  32. Upper Level Weather Chart 500mb chart Variations in height are analogous to variations in pressure. High height fields correspond to high pressure fields.

  33. Troughs and Ridges RIDGE RIDGE TROUGH

  34. Troughs and Ridges • An elongated region of low pressure (trough) or high pressure (ridge) • Tend to be quite common at higher altitudes • At the surface, a trough is usually a fairly weak feature

  35. How Winds Generate Vertical Air Motion Around a surface low pressure center, a net inward transport of air causes a shrinking of the area occupied by the mass. This is known as horizontal convergence.

  36. Airflow Associated with Cyclones and Anticyclones “Upper level support” is important in cyclone development

  37. Wind speeds and isobars STRONG WINDS Slack winds Slack winds The tighter they’re packed, the stronger the wind

  38. Factors that promote vertical airflow • Friction: • air flow from ocean to land (upward motion) • air flow from land to ocean (downward motion) • Mountain ranges

  39. Wind Measurement Wind roses provide a method of representing prevailing winds by indicating the percentage of time the wind blows from various directions

  40. Go Terps!ACC Tournament this Weekend

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