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Lecture11 oct16-bb
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Lecture11 oct16-bb


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  • 1. 1
  • 2. Geostrophic Wind when the isobars are straight, parallel lines, and the only two forces acting on a parcel are the PGF and the CF, then the wind is called geostrophic wind L 475 mb • PGF and CF are equal in magnitude and opposite in direction • Geostrophic winds are always parallel to the isobars PGF CF Wind 500 mb H 500 mb Surface 2
  • 3. Strength of Geostrophic Wind geostrophic conditions are that PGF and CF are equal in magnitude and opposite in direction P G F  C F  2  V g sin  Vg  PGF L 475 mb PGF CF 2  sin  Wind 500 mb H strength of geostrophic wind is determined by PGF not by CF, why? 3
  • 4. Vg  PGF 2  sin  The Coriolis force can only change a parcel's direction, it CAN NOT affect its speed. 4
  • 5. Centripetal force and gradient wind 5
  • 6. Gradient Wind counter lines are relatively straight Counter lines are curvature Geostrophic Wind 6
  • 7. Gradient Wind Centripetal force if you attach a string to a ball and swing it in a circular manner, then the force required to keep the ball moving in a circular path is called centripetal force magnitude of the centripetal force is centripital force  V 2 r V=speed of a parcel (object) r= distance from the center direction of the centripetal force is directed inward, towards the axis of 7 rotation V
  • 8. recall geostrophic wind L 475 mb PGF Wind CF 500 mb H 8
  • 9. Gradient Wind Centripetal force Gradient wind is produced by the balance between PGF, CF and centripetal force PGF PGF PGF PGF 9
  • 10. Free troposphere and Boundary layer 10
  • 11. above 850 mb level, the flow is either geostrophic wind or gradient wind(free troposphere) near the surface about 1-2 km (boundary layer), we must include the effect of friction and therefore, the flow is no longer geostrophic or gradient balance 11
  • 12. Q: As a result, which force becomes smaller, the PGF or the CF? the atmosphere above the boundary layer is called free troposphere friction has a significant impact on wind in boundary layer (1-2 km) 12
  • 13. Four forces in the atmosphere 1. Pressure gradient force 2. Coriolis force  Phigh  Plow distance  2  V sin( ) 3. Centripetal force  V 2 r 4. Friction   kV which force is not caused by wind? which two forces do not affect wind speed? Coriolis force and centripetal force 13 PGF
  • 14. at the center of a surface low, the air converges, and then must rise H L at the center of a surface high, the air diverges, and must come from aloft due to sinking motion 14
  • 15. Why temperature is the most important to understand weather?
  • 16. What is temperature definition? Why earth’s radiation is infrared while solar radiation is short-wave? Wien’s Law  m ax  3000 T (K ) ( m)
  • 17. The earth’s radiation energy is determined by what? Stefan-Boltzmann Law E  T 4
  • 18. The capacity of air for holding water vapor is determined by what?
  • 19. Saturation vapor pressure (mb) Temperature determines the capacity of holding water vapor in the air 120 110 100 90 80 70 60 50 40 30 20 10 0 * H 2O P This curve indicates how much water vapor can be held in the air at a given temperature 17.27 T PH 2 O (T )  0.6108 e T  273.3 * T=44 oC T=10 oC T=30 oC 0 10 20 30 40 Temperature (oC) 50
  • 20. Thermal circulation is generated by pressure gradient Pressure gradient is produced by different heating
  • 21. Rule Cool surface  air sinking  high surface pressure Warm surface air rising  low surface pressure Wind is blowing from high pressure to low pressure Where is clear sky? Cool High P Warm Low P
  • 22. Scales of Atmospheric Motions Time and space scale of atmospheric motions Typical size Global scale 5000 km Synoptic scale 2000 km Mesoscale 20 km Microscale 2m Typical life span 23
  • 23. Sea breeze
  • 24. isobars 7 AM 820 mb 860 mb 900 mb 65oF 65oF
  • 25. Sea breeze isobars 820 mb 860 mb Cold Warm 780 mb 900 mb 90oF 70oF
  • 26. isobars 820 mb 860 mb 900 mb 90oF 70oF Cold Warm 780 mb