The cool air is dense and when it reaches a high pressure zone it sinks to the ground. The air is sucked back toward the low pressure at the equator. This describes the convection cells north and south of the equator. If the Earth did not rotate, there would be one convection cell in the northern hemisphere and one in the southern with the rising air at the equator and the sinking air at each pole.
But because the planet does rotate, the situation is more complicated. Air rises at the equator, but as it moves toward the pole at the top of the troposphere, it deflects to the right. Remember that it just appears to deflect to the right because the ground beneath it moves. At about 30oN latitude, the air from the equator meets air flowing toward the equator from the higher latitudes. This air is cool because it has come from higher latitudes. Both batches of air descend, creating a high pressure zone.
Once on the ground, the air returns to the equator. This convection cell is called the Hadley Cell and is found between 0 degrees and 30 degrees N. There are two more convection cells in the Northern Hemisphere. The Ferrell cell is between 30oN and 50o to 60oN.
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.
For the height to increase, the stratosphere would also have to become less stable. If CO 2 concentrations increased and if stratospheric ozone concentrations decreased, the stratosphere would cool substantially, and this change would destabilize the stratosphere.
As a result of the alterations to tropospheric and stratospheric stability, the tropopause height would increase. Farrell estimates the height would have doubled under Cretaceous conditions, and as a result, the Rossby number would have doubled. This change would have allowed the Hadley Cells to extend to the poles and would have made equable climates more likely. Hadley cells could extend all the way to the poles. While each of these alterations to the atmosphere would extend the Hadley Cells, Farrell found that a combination of the two effects was necessary to make his model's results agree with proxy data from equable climates.
He graphed the atmosphere's potential temperature versus latitude at different tropopause height and friction values. The results reveal that as tropopause height and friction increase, the EPTD decreases. This value agrees with Cretaceous climate reconstructions.
As a result, Farrell's theory seems to be a reasonable explanation for equable climates. Farrell, The main problem is that Farrell does not provide any explanation for why angular momentum sinks would have become stronger during the Cretaceous and the Eocene. He provides a few examples of potential momentum sinks: "small scale diffusion However, he does not explain why any of these sinks would have become stronger during the Eocene and, thus, would have prevented angular momentum from being conserved.
This lack of information in the argument makes the theory harder to accept, and until this portion of the argument is explored in greater depth, Farrell's theory cannot be accepted as the correct explanation of equable climates. This qualitative explanation supplies the fundamental ideas of Farrell's theory, but to fully understand it, a quantitative approach is necessary.
To see the mathematical approach used as the fundamental basis for this theory, click here. Hadley Cells. That air would then move toward the poles where it would become very cold and sink, then return to the equator above right. One large area of high pressure would be at each of the poles with a large belt of low pressure around the equator. 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.
Instead of one large circulation between the poles and the equator, there are three circulations Between each of these circulation cells are bands of high and low pressure at the surface.
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