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|"... as idle as a painted ship upon a painted ocean"|
|Day after day, day after day,
We stuck, nor breath nor motion;
As idle as a painted ship
Upon a painted ocean.
Water, water, everywhere,
And all the boards did shrink;
Water, water, everywhere,
Nor any drop to drink.
The Rime of the Ancient Mariner by Samuel Taylor Coleridge
|Thinking point: Where on the earth's surface would a sailing ship become becalmed?|
|There is a surplus of energy at the
Equator and a deficit at the Poles (and in the upper atmosphere).
Theoretically, surplus energy should be transferred to areas of
deficiency by a single convection cell.
This would be the case for a non-rotating Earth as first suggested by Halley in 1686 and expanded by Hadley in 1735.
Ferrel suggested three cells in 1856 and Rossby developed this further in 1941.
Modern technology including radiosonde readings, satellite images, and computer modelling all have advanced our knowledge but the tricellular model still forms the basis of our understanding of the general circulation of the atmosphere.
|Tricellular Model Activity:|
|Your teacher will describe a diagram of the tricellular model to you and you must try and draw this exactly as described - but without seeing the finished diagram yourself.|
|Tricellular Model Activity: cut out labels and add to a diagram. Your teacher will explain.|
|[Issue each group with an outline diagram and a cut up set of labels. Can they sort the labels into a logical sequence. Can they match the labels and diagram perhaps add label numbers to a point on the diagram.] CUT THE NUMBERS OFF THE PRINT OUT.|
|Answer Labels and diagram|
|Start diagram A3|
|However, since the earth rotates, the
axis is tilted, and there is more land mass in the northern hemisphere
than in the southern hemisphere, the actual global pattern is much more
complicated - than suggested by the unicellular model.
Instead of one large circulation between the poles and the equator, there are three circulations...
|Tricellular Model for northern hemisphere|
|Between each of these circulation cells are bands of high and low pressure at the surface. The high pressure band is located about 30° N/S latitude and at each pole. Low pressure bands are found at the equator and 50°-60° N/S.|
|Tricellular Model showing atmospheric circulation on curving earth|
|The Hadley Cell (Tropical circulation)|
|The meeting of the trade winds in the equatorial region forms the Inter-tropical Convergence Zone (ITCZ). The trade winds pick up latent heat across the warm tropical oceans, and are forced to rise by violent convection currents. The unstable warm moist air is rapidly cooled adiabatically to produce towering cumulonimbus clouds, frequent thunderstorms and low pressure characteristics of the equatorial climate. It is these upward currents that form the 'powerhouse of the general global circulation' and which turn latent heat into sensible heat and later into potential energy. At ground level , the ITCZ experiences only very gentle variable winds known as the doldrums.|
|As rising air cools to the temperature of the surrounding environment air, uplift ceases and it begins to move away from the Equator. Further cooling, increasing density, and diversion by the Coriolis force cause the air to slow down and to subside forming the descending limb of the Hadley Cell. In looking at the northern hemisphere (the southern is a mirror image) it can be seen that the air subsides at about 30*N to create the subtropical high pressure belt (formerly known as the Horse Latitudes) with its clear skies and dry stable conditions. On reaching the Earth's surface, the cell is completed as some of the air is returned to the Equator as the north east trade winds.|
|The Ferrel cell (Mid-Latitude cell)|
|The remaining air is diverted polewards, forming warm south westerlies which collect moisture when they cross sea areas. These warm winds meet cold Arctic air at the polar front about 60*N and are uplifted to form an area of low pressure and the rising limb of the Ferrel and Polar Cells. The resultant unstable conditions produce the heavy cyclonic rainfall associated with mid-latitude depressions.|
|Depressions are another method by which surplus heat is transferred. While some of this rising air eventually returns to the tropics, some travels polewards....|
|... where, having lost its heat, it descends to form a stable area of high pressure. Air returning to the polar front does so as the cold easterlies|
|Coming Soon to a computer screen near you .... animated gif of the Tricellular Model in action see the next page!|