Corel Photo of a Windsock  
Peralta Colleges, Physical Geography
Rita D. Haberlin, Instructor
ATMOSPHERIC CIRCULATION,
PRESSURE, AND WINDS STUDY GUIDE

CHIEF IDEAS:

  1. Winds perform important work throughout the global ecosystem. Their most important function in the atmosphere is to move excess heat from the tropics toward the poles and to move water vapor from above oceans to the land. Winds also drive waves and ocean currents and shape minor landforms.

  2.  
  3. A wind is a horizontal movement across a surface. Winds are created by air pressure differences due to unequal heating. Just as unequal heating in a British living room in winter sets up pressure differences that create drafts, so the sun’s unequal heating of the earth generates winds.

  4.  
  5. In general, air that is warmer than surrounding air is lighter and rises creating a low pressure at the earth's surface. Because cold air is denser than warm air, cold air sinks and builds up high pressure. (An important exception to this generalization is the dynamically-induced subtropical high pressure areas.)

  6.  
  7. Low pressure areas (cyclones) are zones of rising air and converging winds. Low pressure areas are often wet and stormy. High pressure areas (anticyclones) are zones of descending air currents and diverging winds with clear, fair weather.

  8.  
  9. Like winds, air pressure is invisible. Meteorologists show differences in air pressure by drawing isobars on maps. Isobars are lines joining places with the same air pressure. Isobars form roughly concentric circles around centers of relatively low and high pressure.

  10.  
  11. Winds always blow from high to low pressure. The greater the difference between the high and low pressure (pressure gradient) the greater the speed of the wind.

  12.  
  13. Due to the earth’s rotation, however, winds appear to be deflected to the right of the pressure gradient in the northern hemisphere and to the left in the southern hemisphere. This deflection is called the Coriolis effect. At the earth’s surface, winds cross isobars at about a 45 degree angle but in the upper atmosphere, the winds circulate parallel to the pressure cells.

  14.  
  15. Unequal heating of land and water in coastal areas leads to unbalanced pressures that drive air from high to low pressure. The difference in temperature between the land and sea produces land breezes at night and sea breezes during the afternoon. This entire mechanism is called a heat engine.

  16.  
  17. On a global scale, the atmospheric circulation system is driven by the solar heat engine with its unequal heating between polar and equatorial regions.

  18.  
  19. If the earth did not rotate, warm, rising air from the equatorial low would rise and move toward the poles at high altitudes, sink and then cold, dense air would return to the equator near the earth's surface.

  20.  
  21. The earth's rotation, however, causes rising air currents at the equator to be turned back toward the earth’s surface near latitude 30 degrees. This creates subtropical high-pressure cells (horse latitudes). Part of the subsiding air turns toward the equator creating trade winds (10 - 25 degrees latitude), and part turns poleward creating westerly winds (35 - 60 degrees latitude). Where the northeast and southeast trade winds meet they form the intertropical convergence zone (ITCZ). The ITCZ is a belt of low pressure, instability, and rains.

  22.  
  23. The westerly winds (westerlies) bring warm air from the tropics into contact with cold air from the polar easterlies. The polar easterlies blow out from the polar high. This low pressure zone of contact between the westerlies and polar easterlies is known as the polar front and is the birthplace of most mid-latitude cyclonic storms.

  24.  
  25. All of the earth’s planetary wind and pressure belts shift 5 degrees latitude north and south of the equator with the seasonal migration of the sun's vertical rays.

  26.  
  27. At upper levels along the polar front, wave-like undulations known as Rossby waves often develop where warm and cold air are in contact. Jet Streams develop in these upper air waves forming narrow zones of high velocity, pulse-like winds (200 - 250 mph).

  28.  
  29. Over large continental areas such as Eurasia, unequal heating of land and sea produces a seasonal reversal of winds called monsoons. In hot South Asia (India) in summer, hot air rises creating low pressure. Moisture-bearing winds from the ocean are then sucked in by the low pressure bringing torrential rains of the summer monsoon to the land. In winter, cold air sinks over Siberia producing the Asiatic high pressure with its dry, out-blowing winter monsoon winds.

  30.  
  31. Locally, special winds may develop due to pressure differences. In mountains, differences in heating and cooling give rise to mountain and valley winds. When air descends and warms by compression, hot, dry winds occur (e.g., Chinook, Santa Ana).

  32.  
  33. Atmospheric circulation and related oceanic circulation play an important role in transferring toxic substances, dust, and biological organisms far from their source of origin (12 - 25 days in mid-latitudes).

STUDY OBJECTIVES:

  1. Describe five functions of the wind in the global ecosystem.
  2. Define a wind. What is the fundamental cause of wind?
  3. Explain the relationship between wind and temperature.
  4. Describe the wind and weather characteristics of high and low pressure areas (cyclones and anticyclones).
  5. Define an isobar.
  6. Describe the relationship between pressure and winds. Winds blow from ___ to___. What is the pressure gradient?
  7. Give the cause of the Coriolis effect and explain how it influences the movement of winds in the Northern and Southern Hemispheres.
  8. Describe the effect of unequal heating of land and sea upon pressure and wind, on a daily basis with diagrams. What is a heat engine?
  9. What drives the atmospheric circulation system?
  10. Describe the global circulation system if the earth did not rotate.
  11. Describe the atmospheric circulation between the equator and the mid-latitudes and between the mid-latitudes and the poles.
  12. Explain the movement of the earth's pressure and wind belts with the seasons.
  13. What happens along the polar front? Define the jet stream.
  14. Describe with the help of diagrams the pressure and wind patterns associated with winter and summer monsoons over continents.
  15. List some local winds and how they develop.
  16. Explain how atmospheric and oceanic circulation systems connect the entire world by transferring pollutants far from their sources.

 

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