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Lecture 12 Winds Continued

Lecture 12 Winds Continued. What is Wind?. Wind is nature’s attempt at balancing inequalities in pressure FACT: Unequal heating of the Earth’s surface generates these inequalities. FACT: Solar radiation is the ultimate source of energy for Wind. What is Wind?.

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Lecture 12 Winds Continued

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  1. Lecture 12Winds Continued

  2. What is Wind? Wind is nature’s attempt at balancing inequalities in pressure FACT: Unequal heating of the Earth’s surface generates these inequalities. FACT: Solar radiation is the ultimate source of energy for Wind

  3. What is Wind? Windis the result of horizontal differences in air pressure! Air flows from areas of HIGH pressure to areas of LOW pressure HIGH LOW

  4. Factors Affecting Wind If the Earth did NOT rotate and if there was NO friction wind would flow in a straight line from High to Low pressure Main forces that affect wind YOU NEED TO MEMORIZE THESE!!! Pressure Gradient Force Coriolis Force Centripetal force Friction

  5. HPD? • Unfortunately HPD doesn’t stand for “Honolulu Police Department”…. • HPD = Horizontal Pressure Differences

  6. Pressure Gradient Force Horizontal Pressure Differences (HPD) Winds flow from High pressure to Low pressure if only affected by HPD Higher P at surface Lower P at surface 500 mb 500 mb 700 mb 700 mb Sea Breeze 1000 mb 1000 mb WARM Nighttime COOL

  7. PGF – Change over Horizontal Difference STRONGER when isobars are closer together Same CHANGE in Pressure (ΔP) When given Pressure Heights, the PGF points from regions of High Pressure to regions of Low Pressure ΔP ΔP T = 20 T = 30 T = 20 T = 30 LARGE DISTANCE SMALL DISTANCE

  8. ISOBARS & PGF If all we had was the PGF, wind would act like a Ball rolling down a slope… rolling at 90 Degrees to the slope! 100 m 500 m 200 m 300 m 400 m 400 m 500 m 300 m 200 m 100 m The STEAPER the SLOPE the FASTER the ball will roll!!! 100 m 500 m 300 m

  9. ISOBARS & PGF - More Examples Winds if we ONLY knew the PGF. WIND IS SLOW WIND IS FAST If the isobars are further or closer together… 1004 mb 992 mb 996 mb 1008 mb 1000 mb 1012 mb 1004 mb 1008 mb 1016 mb 1012 mb 1016 mb For a conical hill, the PGF points in all direction 1020 mb 1020 mb PGF PGF Change in P over large distance: SMALL PGF Change in P over small distance: LARGE PGF

  10. Pressure Gradient Force Summary: Change in P over large distance = small PGF Change in P over small distance = large PGF PGF is at right angles to isobars Causes wind to START MOVING However… two forces cause wind speed and direction to be different than predicted by the PGF Coriolis (rotation of the Earth) Friction

  11. ISOBARS – Add in the PGF! L L H H

  12. Surface Charts NOTE: Wind blows across the Isobars WIND SPEED

  13. Coriolis Force Results from the rotation of the Earth Causes the PGF to cross isobars NOT at right angles. • Winds curve to the RIGHT in the Northern Hemisphere • Winds curve to the LEFT in the Southern Hemisphere

  14. Coriolis Force - Example

  15. On a non-rotating Earth, the rocket would travel straight to it’s target. Coriolis Force - Example • Earth rotates 15 deg per hour…. • Even though the rock travels in STRAIGHT line, when we plot it’s path on the surface it follows a path that CURVES to the RIGHT!

  16. Coriolis Force – Visualization

  17. Coriolis Force – Summary Always Deflects a moving body (wind) to the right in the Northern Hemisphere Only affects wind direction, not speed Is affected by wind speed (the stronger the wind, the greater the deflecting force) Is strongest at the poles and nonexistent at the equator… latitude dependent These two determine the MAGNITUDE of the Coriolis Force

  18. Winds Aloft and Geostrophic Flow Where friction doesn’t play a role!! When only the PGF and Coriolis Forces (Fc) affect an air parcel 1000 mb 1004 mb 1008mb 1012 mb 1016 mb 1020 mb PGF Fc Fc Fc Fc WIND Direction of MOTION!

  19. Winds Aloft and Geostrophic Flow An air parcel is at equilibrium only if PGF acts in the opposite direction to the Coriolis force (no net force). Therefore in Geostrophic Flow, winds run parallel to isobars in a straight path PGF Direction of MOTION! Coriolis, Fc WIND 900 mb 904 mb 908 mb 912 mb

  20. Curved Flow and Gradient Wind Gradient Wind – winds that follow curved paths around high and low pressure cells. Speed of the wind depends on how close the isobars are L H PGF Coriolis Wind

  21. ISOBARS – PGF + Coriolis L L H H Wind Flows PARALLEL to Isobars!!! c

  22. Upper Air Charts NOTE: Wind blows PARALLEL to the Isobars These aren’t Pressure lines. They are meters above sea level

  23. Friction Applied to wind within ~1.5 km of the surface Friction ALWAYSacts in the direction OPPOSITE the direction of motion!!!! Friction affects air at the surface more than air aloft.

  24. Adding in Friction to Coriolis and PGF Geostrophic Flow and Friction Causes parcel to slow down Coriolis decreases in strength Friction cases wind to lean towards the direction of the PGF PGF Direction of MOTION! Friction Coriolis, Fc

  25. Adding in Friction to Coriolis and PGF The addition of friction causes the wind to lean toward the PGF force (or in the direction of the low pressure) in both hemispheres. Because the Coriolis Force pulls wind to the right in the NH and to the left in the SH we see opposite wind directions when comparing the NH to the SH.

  26. Surface Winds - Friction + Coriolis + PGF

  27. ISOBARS – PGF + Coriolis + Friction! L L H H Wind CROSSES the Isobars!!! c

  28. Lows in Different Hemispheres Northern Hemisphere Southern Hemisphere

  29. Remember: Airflow and Pressure Movement of air can cause variations in pressure Net flow of air into a region = CONVERGENCE Net flow of air out of a region = DIVERGENCE

  30. How Winds Generate Vertical Air Motion

  31. Factors that Promote Vertical Airflow Friction – can cause convergence and divergence When air moved from the smooth ocean to the “rough” land, the wind slows down Results convergence as air “pile up” upstream (like on a highway with construction). When air goes from land to ocean you see divergence and subsidence

  32. Factors that Promote Vertical Airflow Mountains – hinder the flow of air As air passes over it is compressed vertically, causing divergence aloft After going over, onto the lee side, air experiences vertical expansion… causing horizontal convergence.

  33. Observing Wind Prevailing Winds Name given to the wind direction most often observed during a give time period Can affect the climate of a region Helpful for city planning (think dumps)

  34. Observing Wind Wind Rose Diagram that represents the percentage of time the wind blows from different directions. Example: September in Honolulu

  35. Example Wind Roses

  36. Observing Wind Wind Vane Wind Direction • Anemometer • Wind Speed • Wind Profiler • Vertical profile of speed and direction • Aerovane • Wind Direction and Speed

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