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AS GEOGRAPHY - ATMOSPHERE AND WEATHER - ATMOSPHERIC CIRCULATION MODEL

AS GEOGRAPHY - ATMOSPHERE AND WEATHER - ATMOSPHERIC CIRCULATION MODEL. Hadley Cells, Ferrel Cells, Polar Cells.

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AS GEOGRAPHY - ATMOSPHERE AND WEATHER - ATMOSPHERIC CIRCULATION MODEL

  1. 1. ASGEOGRAPHY ATMOSPHEREANDWEATHER HADLEYFERRELPOLARCELLS
  2. 2. HADLEYCELL The Hadley cell, named after George Hadley, is a global scale tropical atmospheric circulation that features air rising near the Equator, flowing poleward at a height of 10 to 15 km above the earth's surface, descending in the subtropics, and then returning equatorward near the surface.
  3. 3. WHATITCREATES? This circulation creates the trade winds, tropical rain-belts and hurricanes, subtropical deserts and the jet streams. Hadley cell are the low-altitude circulation that have air sinking at roughly zero to 30 degree latitude.
  4. 4. DRIVINGFORCE The major driving force of atmospheric circulation is the uneven distribution of solar heating across the Earth, which is greatest near the equator and least at the poles. The atmospheric circulation transports energy polewards, thus reducing the resulting equator-to-pole temperature gradient. The mechanisms by which this is accomplished differ in tropical and extratropical latitudes.
  5. 5. HADLEYCELLSMECHANISM Hadley cells exist on either side of the equator. Each cell encircles the globe latitudinally and acts to transport energy from the equator to about the 30th latitude. The circulation expose the following: • Warm, moist air converging near the equator causes heavy precipitation. This releases latent heat, driving strong rising motions. • This air rises to the tropopause, 10–15km above sea level, where the air is no longer buoyant. • Unable to continue rising, this sub-stratospheric air is instead forced poleward by the continual rise of air below. • As air moves poleward, it both cools and gains a strong eastward component due to the Coriolis effect and the conservation of angular momentum. The resulting winds form the subtropical jet streams. • At this latitude, the cool, dry, high altitude air begins to sink. As it sinks, it warms adiabatically, decreasing its relative humidity. • Near the surface, a frictional return flow completes the loop, absorbing moisture along the way. The Coriolis effect gives this flow a westward component, creating the trade winds.
  6. 6. FERRELCELLS Part of the air rising at 60° latitude diverges at high altitude toward the poles and creates the polar cell. The rest moves toward the equator where it collides at 30° latitude with the high-level air of the Hadley cell. There it subsides and strengthens the high pressure ridges beneath. A large part of the energy that drives the Ferrel cell is provided by the polar and Hadley cells circulating on either side and that drag the Ferrel cell with it. The Ferrel cell, theorized by William Ferrel is a secondary circulation feature, whose existence depends upon the Hadley and polar cells on either side of it. It might be thought of as an eddy created by the Hadley and polar cells.
  7. 7. WESTERLIES Ferrel’s model was the first to account for the westerly winds between latitudes 35° and 60° in both hemispheres. The Ferrel cell, however, is still not a good representation of reality because it requires that the upper-level mid-latitude winds flow westward; actually the eastward-flowing surface winds become stronger with height and reach their maximum velocities around the 10-km level in the jet streams.
  8. 8. WESTERLIESWINDS The air of the Ferrel cell that descends at 30° latitude returns poleward at the ground level, and as it does so it deviates toward the east. In the upper atmosphere of the Ferrel cell, the air moving toward the equator deviates toward the west. Both of those deviations, as in the case of the Hadley and polar cells, are driven by conservation of angular momentum. As a result, just as the easterly Trade Winds are found below the Hadley cell, the Westerlies are found beneath the Ferrel cell.
  9. 9. ACTINGASHEATPUMPS The Ferrel cell is weak, because It has neither a strong source of heat nor a strong sink, so the airflow and temperatures within it are variable. The Ferrel system acts as a heat pump.
  10. 10. WHEREARESITUATED? This cell shares its southern, descending side with the Hadley cell to its south. Its northern rising limb is shared with the Polar cell located between 50 degrees N to 60 degrees N and the North Pole, where cold air descends. There are three mirror image circulation cells in the Southern Hemisphere.
  11. 11. HADLEYCELLSREVISION The largest cells extend from the equator to between 30 and 40 degrees north and south, and are named Hadley cells, after English meteorologist George Hadley. Within the Hadley cells, the trade winds blow towards the equator, then ascend near the equator as a broken line of thunderstorms, which forms the Inter-Tropical-Convergence Zone (ITCZ). From the tops of these storms, the air flows towards higher latitudes, where it sinks to produce high-pressure regions over the subtropical oceans and the world's hot deserts, such as the Sahara desert in North Africa.
  12. 12. FERRELCELLSREVISION In the middle cells, which are known as the Ferrel cells, air converges at low altitudes to ascend along the boundaries between cool polar air and the warm subtropical air that generally occurs between 60 and 70 degrees north and south. The circulation within the Ferrel cell is complicated by a return flow of air at high altitudes towards the tropics, where it joins sinking air from Hadley cell. The Ferrel cell moves in the opposite direction to the two other cells (Hadley cell and Polar cell) and acts rather like a gear. In this cell the surface wind would flow from a southerly direction in the northern hemisphere. However, the spin of the Earth induces an apparent motion to the right in the northern hemisphere and left in the southern hemisphere. This deflection is caused by the Coriolis effect and leads to the prevailing westerly and south-westerly winds often experienced over the UK.
  13. 13. POLARCELLSREVISION The smallest and weakest cells are the Polar cells, which extend from between 60 and 70 degrees north and south, to the poles. Air in these cells sinks over the highest latitudes and flows out towards the lower latitudes at the surface.

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