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How Does Air Move Around the Globe?

How Does Air Move Around the Globe?. Review of last lecture. Thickness of the atmosphere: less than 2% of Earth ’ s thickness Definition of temperature. 3 units. Four layers of the atmosphere, what separate them? Definition of pressure and its unit.

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How Does Air Move Around the Globe?

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  1. How Does Air Move Around the Globe?

  2. Review of last lecture • Thickness of the atmosphere: less than 2% of Earth’s thickness • Definition of temperature. 3 units. • Four layers of the atmosphere, what separate them? • Definition of pressure and its unit. • Definition of pressure gradient. Pressure gradient sets the air in motion. • Equation of state (Relationship between P, ρ, and T) • Vertical Pressure Distribution. How does pressure change with height? What is the hydrostatic equilibrium?

  3. Review of last lecture (cont.) • Know 3 Forces that affect wind speed /direction • Especially work on Coriolis force, as this is the hardest to understand. Which direction is air deflected to by Coriolis force? • What is the geostrophic balance? At which level is it valid? Difference between upper level and surface winds • Does cyclones correspond to high or low surface pressure? Is the air moving clockwise or counter-clockwise around them? How about anticyclones? • What are the troughs and ridges?

  4. The most common atmospheric circulation structure H L Cooling or No Heating Heating H L Imbalance of heating  Imbalance of temperature  Imbalance of pressure  Wind

  5. Introduction • Well-defined heating, temperature and pressure patterns exist across the globe • These define the general circulation of the planet • In describing wind motions: • Zonal winds (east-west): flow parallel to lines of latitude • Flowing eastward: Westerly wind • Flowing westward: Easterly wind • Meridional winds (north-south): flow parallel to lines of longitude • Flowing northward: Southerly wind • Flowing southward: Northerly wind

  6. Annual mean precipitation (heating) Extratropicalstormtrack Tropical rainfall Extratropicalstormtrack

  7. Primary Highs and Lows Equatorial low Subtropical high Subpolar low Polar high

  8. Three-cell model Zonal mean circulation Each hemisphere is divided into 3 distinct cells: Hadley Cell Ferrel Cell Polar Cell

  9. Vertical structure and mechanisms • Polar Cell (thermal): • Driven by heating at 50 degree latitude and cooling at the poles • Ferrel Cell (dynamical): Dynamical response to Hadley and polar cells • Hadley Cell (thermal): Heating in tropics  formssurface low and upper level high air converges equatorward at surface, rises, and diverges poleward aloft  descends in the subtropics Polar Hadley

  10. Zonal mean structure of temperature Two characteristics: • Horizontally uniform in the tropics • Steep gradient in the extratropics

  11. Vertical structure of zonal wind Two characteristics: • Westerly winds in the extratropical troposphere (caused by the Coriolis force). Explains why storms move eastward, flight times • Jet streams: local maximum of winds (caused by sharp pressure gradient across the boundary between warm tropical air and cold polar air)

  12. Caused by steep temperature gradients between cold and warm air masses Polar front - marks area of contact, steep pressure gradient  polar jet stream Low latitudes  subtropical jet stream Stronger in winter, affect daily weather patterns The Jet Streams

  13. Semipermanent Pressure Cells: NH winter South Atlantic high South Indian high South Pacific high Low pressure centers: weather and precipitation High pressure centers: warm, drought and desert

  14. Semipermanent Pressure Cells: NH summer South Pacific high South Atlantic high South Indian high

  15. Ocean surface currents – horizontal water motions Transfer energy and influence overlying atmosphere Surface currents result from frictional drag caused by wind - Ekman Spiral General circulation of the oceans • Water moves at a 45o angle (right) • in N.H. to prevailing wind direction • Due to influence of Coriolis effect • Greater angle at depth

  16. Global surface currents • Surface currents mainly driven by surface winds • North/ South Equatorial Currents pile water westward, create the Equatorial • Countercurrent • western ocean basins –warm poleward moving currents (example: Gulf Stream) • eastern basins –cold currents, directed equatorward

  17. Summary • Three precipitation (heating) belts. Primary high and lows • Three-cell model. Mechanism for each cell • Two characteristics of zonal mean temperature structure • Two characteristics of zonal mean wind structure. Why does westerly winds prevail in the extratropical troposphere? What cause the jet streams? • Semipermanent pressure cells. Low pressure is associated with clouds and precipitation. High pressure is associated with warm surface temperature, drought, and desert. • What drives the ocean surface currents? In the case of Ekman spiral, what is the direction of surface current relative to surface wind?

  18. Works cited Images • http://pulleysandgears.weebly.com/gears.html • http://visual.merriam-webster.com/earth/meteorology/meteorological-measuring-instruments/measure-wind-direction.php

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