Pressure and wind - PowerPoint PPT Presentation

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Pressure and wind

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  1. Pressure and wind Ch. 4

  2. Air Pressure • If you take a package of air and squeeze or compress it into a smaller area, that air will become higher in pressure Air will push against sides of package

  3. Barometers • Aneroid • Mercury

  4. Air Pressure • Measured in • millibars (mb) • Force per sq. meter • kilopascals (kPa) 1kPa=10mb • Normal sea-level pressure is 1013.2mb, 29.92 inches/Hg, 14.7 psi

  5. ‘Sea level pressure’ vs. ‘Station pressure’ Kent: mean SLP=1013mb mean station P=974mb Pressure corrections for altitude

  6. Wind • Wind is the general horizontal motion of air from one location to another based on differences in air pressure • Air speed is measured by an anemometer

  7. Moving the atmosphere • Primary forces: • Pressure gradient force (PGF) • Gravity (g) • Coriolis • Friction

  8. Gets things started PGF = DP/Dx Pressure gradient force

  9. Pressure gradient

  10. Gravity (g) • Why doesn’t the atmosphere collect into a small layer on the ground? • Hydrostatic equilibrium: DP / Dz = -rg • What does this say about how pressure changes vertically?

  11. The Coriolis Effect • When wind moves from zones of high pressure to zones of low pressure it veers due to earth rotation • To the right in the northern hemisphere • To the left in the southern hemisphere • http://www.classzone.com/books/earth_science/terc/content/visualizations/es1904/es1904page01.cfm?chapter_no=visualization

  12. Remembering the direction of bend in the coriolis effect Bends to the right Greatest at poles Least at equator Increases with increasing speed Bends to the left

  13. Friction • Slows down wind speed • In what part of the atmosphere is this most significant?

  14. Balance of forces in upper atmosphere Pressure Gradient + Coriolis = GEOSTROPHIC FLOW

  15. Upper-level heights and temperature

  16. Balance of forces near surface • Pressure Gradient + Coriolis + Friction= • SURFACE FLOW

  17. Upper air vs. surface H L Pressure gradient