Ferrel cell

The three-cell model divides each hemisphere's large-scale atmospheric circulation into Hadley, Ferrel and Polar cells. The Ferrel cell occupies the mid-latitudes, roughly between the subtropical and subpolar circulation zones; the boundaries shift with season and are not fixed latitude lines. The model is useful for summarizing average winds and vertical motion, but actual air movement is shaped by changing weather systems, land and ocean contrasts, Earth's rotation and seasonal shifts in solar heating.

In the simplified Ferrel-cell diagram, air near the surface moves poleward and is deflected eastward by Earth's rotation, producing the mid-latitude westerlies. Air is shown returning equatorward at higher levels. This direction is opposite to the simplified Hadley-cell circulation. NOAA describes the Ferrel cell as a mid-latitude circulation and associates it with westerly winds; the Met Office describes the cell as a useful way to connect the prevailing surface winds to the larger circulation pattern. These descriptions refer to an averaged model, not a uniform wind field at every location or time.

The Ferrel cell differs from the Hadley cell, in which tropical heating drives a thermally direct overturning pattern. Mid-latitude circulation is strongly influenced by travelling weather systems and eddies, which transport heat and momentum. Consequently, the Ferrel cell is often described as thermally indirect: its mean rising and sinking motions do not simply follow the warm-air-rises, cold-air-sinks rule. Encyclopaedia Britannica notes that the upper-level westward flow in the original idealized model is not representative of the actual strengthening eastward winds and jet streams. The term therefore describes a statistical mean pattern, rather than a single, persistent ring of air.

The circulation model is named for American meteorologist William Ferrel, who proposed it in 1856. The mid-latitude westerlies and the circulation that helps represent them matter for the transport of heat and moisture and for the development and movement of storms. In regional weather, the interaction of westerly flow, fronts and jet streams can help steer unsettled systems, although the cell alone cannot predict a particular area's weather.