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Air pressure and atmospheric motion

Air pressure and atmospheric motion.

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Air pressure and atmospheric motion

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  1. Air pressure and atmospheric motion Explain the forces that generate winds Explain why there is well-developed westerly flow in the upper troposphere Differentiate between high and low pressure systems Compare and contrast surface and geostrophic winds Describe/read maps of constant pressure surfacesIdentify and discuss wind types and the forces that generate them Recognize surface and upper-level atmospheric maps and identify general patterns of windflow Explain monsoonal flow and land-sea breezes

  2. Air pressure and atmospheric motion Q: What makes the wind blow? A: Air pressure differences.

  3. Air pressure • Force exerted by molecules in atm due to gravity and temperature

  4. Air pressure

  5. Surface air pressure variability • Average sea level pressure 1013 mb

  6. Mercury barometer 1013 mb = 29.91 inches Aneroid barometer

  7. Pressure systems • Two types: high and low • Low: associated with clouds and instability. • High: associated with clear conditions and stability

  8. Low pressure systems • Cyclone • Converging rising air at surface • Diverging air aloft • Winds rotate counterclockwise in NH • Areas of “light” atmosphere; air is forced into these locations • Unstable surface conditions

  9. High pressure system • Anticyclone • Converging air aloft • Diverging sinking air at surface • Winds rotate clockwise in NH • Areas of “heavy” atmosphere; air is forced out of these locations • Stable surface conditions

  10. Is the location for these pressure systems the northern or the southern hemisphere?

  11. High and low pressure systems • Occur on a variety of spatial and temporal scales • Some pressure systems may be stationary for a long period of time, others may migrate rapidly around the planet • Some pressure systems are closed, others are more belt-like and open

  12. Low pressure systems • Types of low pressure systems: tornados, thunderstorms, hurricanes….

  13. Low pressure systems ….., midlatitude cyclones, the ITCZ, thermal lows

  14. Dust devil versus ITCZ

  15. Another secret of weather forecasting • The atmosphere is a heterogeneous collection of pressure systems in three dimensions. • Weather forecasting involves looking at surface conditions as well as upper level conditions (aloft, in the upper troposphere) • http://weather.unisys.com/

  16. “Deep” tropospheric phenomena • A midlatitude cyclone is a low pressure at the surface coupled to a low pressure aloft in the upper troposphere • A hurricane is a high pressure aloft and a low pressure at the surface

  17. Midlatitude cyclones • Strong, “deep” interaction between surface and upper levels • May travel large distances around the globe Midlatitude cyclone

  18. “Shallow” tropospheric phenomena • Thermal low (warm) • Thermal high (cold) • Weak interaction between surface and upper levels • May occur on a daily basis or persist over many months L H

  19. Pressure differences initiate…..

  20. …….winds • Named according to the direction they blow from • Winds can blow at different directions at different altitudes in the atm • Forces that act on winds: • PGF • CF • Surface friction

  21. 1. PGF: Pressure gradient force – winds blow from high to low Where are winds the fastest?

  22. 2. Coriolis Force (CF) • Apparent deflection of the winds due to rotation of the Earth • NH winds deflected right • SH winds deflected left

  23. Coriolis Force (CF) • CF is not a true force; it is an apparent force arising from the effect of the Earth’s rotation • Deflection is strongest at poles and zero at the Equator • CF acts perpendicular to the direction of motion • CF deflects to the right in the Northern Hemisphere

  24. Bart versus Australia

  25. Misconceptions about the Coriolis Force • The CF does not determine the rotation in a drain.

  26. 3. Surface friction (SF) • Topography (mnts, elevated plateaus) deflect winds

  27. Two major types of winds • Geostrophic (upper troposphere winds) • Influenced by PGF and CF only • Wind flow is parallel to isobars/geopotential heights • Geostrophic flow is westerly (west to east) in NH • Shown on geopotential height maps

  28. Geostrophic winds on geopotential height map

  29. Two major types of winds 2. Surface winds • Influenced by PGF, CF, and SF • Winds cross isobars

  30. Types of surface winds • Monsoonal flow • Creates extreme wet and dry seasons

  31. Location of thermal highs and lows associated with monsoonal flow Monsoon in India

  32. Winter Summer Weak monsoonal flow in southwestern US

  33. Types of surface winds • Land-sea breeze circulation • Many other different types of surface winds based on local physiography and arrangement of pressure cells. Many have unique names.

  34. Sea breeze blowing from ocean to land

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