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Class #12: Friday, September 25

Class #12: Friday, September 25. Chapter 6 Forces and Force Balances. The horizontal pressure gradient force. Is always directed from high to low pressure Is stronger where the density is less—higher in the troposphere When stronger, causes stronger winds

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Class #12: Friday, September 25

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  1. Class #12: Friday, September 25 Chapter 6 Forces and Force Balances Class #12 Friday, September 25

  2. The horizontal pressure gradient force • Is always directed from high to low pressure • Is stronger where the density is less—higher in the troposphere • When stronger, causes stronger winds • Is always important in horizontal winds • Is not generally in the same direction the wind blows, because other forces can act Class #12 Friday, September 25

  3. The HPGF always acts from high to low pressure Class #12 Friday, September 25

  4. The Coriolis Force • Deflects the wind to the right in the NH • Deflects the wind to the left in the SH • Is strongest at the poles • Is zero at the equator • Is stronger for stronger winds • Is weaker for weaker winds • Is zero for calm. It cannot start a wind Class #12 Friday, September 25

  5. The earth rotates fastest at the equator Class #12 Friday, September 25

  6. Coriolis force deflects the wind Class #12 Friday, September 25

  7. The Coriolis force is stronger: towards the poles, stronger winds Class #12 Friday, September 25

  8. The Friction Force • Acts in the direction opposite to the direction the wind is blowing. • Acts to slow down the wind. • Is most important at Earth’s surface • Gets stronger when the winds are stronger. • Is not important above the boundary layer (the lowest 1 km in the atmosphere). . Class #12 Friday, September 25

  9. Class #12 Friday, September 25

  10. Force Balances with the Coriolis force Class #12 Friday, September 25

  11. Why Force-Balances are important • Force-balances simplify Newton’s second law of motion by limiting the number of forces. • Force-balances describe winds that come close to describing the observed winds. • Even though the forces are balanced, the wind need not be calm. Class #12 Friday, September 25

  12. Class #12 Friday, September 25

  13. Hydrostatic Balance • Gravity (downward) balances the Vertical Pressure Gradient Force (upward). • Does not apply inside cumulus clouds, because buoyancy is important there. • Does apply generally in the atmosphere. • Limits vertical motions to be much weaker than horizontal winds. Class #12 Friday, September 25

  14. Class #12 Friday, September 25

  15. More on Hydrostatic Balance • The pressure gradient force is stronger when the air is less dense. • The density of air is less when the air Temperature is higher. • Pressure decreases upward less rapidly when the air has a higher temperature • Hydrostatic balance helps explain the sea breeze and other thermal circulations. Class #12 Friday, September 25

  16. Pressure levels on weather maps • The atmosphere is very close to hydrostatic balance. • This means that the height of a particular pressure level is roughly equivalent to the pressure at a related height level. • An altimeter is a barometer with a height scale. • Upper-level weather maps are labeled in m. Class #12 Friday, September 25

  17. Class #12 Friday, September 25

  18. Geostrophic balance • Is a balance between the horizontal pressure gradient force and the Coriolis force. • Ignores the friction force. • Has isobars that are straight lines. • Does not mean that the wind is calm. • Has a wind called the geostrophic wind. • Is not a feature of everyday life. Class #12 Friday, September 25

  19. Class #12 Friday, September 25

  20. The Geostrophic Wind • Is a straight horizontal wind (no curves) • Is parallel to the isobars • In the NH has low pressure on the left • In the SH has low pressure on the right. • In the NH the wind blows clockwise around high pressure centers and counterclockwise around low pressure centers. Class #12 Friday, September 25

  21. Class #12 Friday, September 25

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