Editing Atmospheric circulation (section)

=== Ferrel cell ===
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, which drag the air of the Ferrel cell with it.<ref name="The Climate System: General Circulation and Climate Zones">{{cite web | author = Yochanan Kushnir | title = The Climate System: General Circulation and Climate Zones | date = 2000 | url = http://eesc.columbia.edu/courses/ees/climate/lectures/gen_circ/index.html | archive-url = https://web.archive.org/web/20040822022845/http://eesc.columbia.edu/courses/ees/climate/lectures/gen_circ/index.html | url-status = dead | archive-date = 2004-08-22 | access-date = 13 March 2012 }}</ref> 
The Ferrel cell, theorized by [[William Ferrel]] (1817–1891), is, therefore, 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 (fluid dynamics)|eddy]] created by the Hadley and polar cells.

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.

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. For this reason, the mid-latitudes are sometimes known as the '''"zone of mixing."'''  The Hadley and polar cells are truly closed loops, the Ferrel cell is not, and the telling point is in the Westerlies, which are more formally known as "the Prevailing Westerlies." The easterly Trade Winds and the polar easterlies have nothing over which to prevail, as their parent circulation cells are strong enough and face few obstacles either in the form of massive terrain features or high pressure zones. The weaker Westerlies of the Ferrel cell, however, can be disrupted. The local passage of a cold front may change that in a matter of minutes, and frequently does. As a result, at the surface, winds can vary abruptly in direction. But the winds above the surface, where they are less disrupted by terrain, are essentially westerly. A low pressure zone at 60° latitude that moves toward the equator, or a high pressure zone at 30° latitude that moves poleward, will accelerate the Westerlies of the Ferrel cell. A strong high, moving polewards may bring westerly winds for days.

The Ferrel system acts as a [[heat pump]] with a [[coefficient of performance]] of 12.1, consuming kinetic energy from the Hadley and polar systems at an approximate rate of 275 terawatts.<ref name="RERLHadley" />