The global movement of air, known as atmospheric circulation, is the massive, large-scale engine that transfers heat from the warmer equatorial regions toward the colder poles. This distribution of thermal energy is accomplished through a system of distinct, vertical loops of moving air that span the globe. Understanding these circulation patterns is foundational to comprehending why certain regions experience specific climate zones and how weather systems travel across the Earth. These organized cells of circulation determine the stability and variability of global weather.
Defining the Ferrel Cell
The Ferrel Cell is the middle segment of the three-cell model of atmospheric circulation, located in the mid-latitudes of both the Northern and Southern Hemispheres. It is situated between the tropical circulation cell and the polar circulation cell, roughly spanning the area from 30 to 60 degrees latitude.
The air movement within the Ferrel Cell features air sinking around the 30-degree latitude mark and rising again near 60 degrees latitude. At the Earth’s surface, the air flows poleward, moving from the subtropical high-pressure zone toward the subpolar low-pressure zone. In the upper atmosphere, the circulation is reversed, with air flowing back toward the equator to complete the loop.
The Mechanism of Indirect Circulation
Unlike the other two major atmospheric cells, the Ferrel Cell is not primarily driven by temperature differences in a direct thermal circulation. The Hadley Cell near the equator and the Polar Cell are considered thermally direct because they are driven by warm air rising and cold air sinking. The Ferrel Cell, however, is mechanically driven by the momentum and heat exchanges created by its two stronger neighbors, making it a thermally indirect circulation.
The cell acts as an eddy between the Hadley Cell to its south and the Polar Cell to its north, forcing its counter-rotating motion. Air descending from the Hadley Cell around 30 degrees latitude pushes the Ferrel Cell air poleward along the surface. Meanwhile, cold air sinking from the Polar Cell meets the rising Ferrel Cell air around 60 degrees latitude, which helps pull the upper-level air back toward the equator.
This indirect forcing means the Ferrel Cell is weaker and more variable than the other cells. The circulation pattern goes against the natural thermal gradient, as the surface air flows from a warmer region (30 degrees) to a colder one (60 degrees).
Impact on Global Weather and Climate
The Ferrel Cell’s movement is responsible for the prevailing surface winds in the mid-latitudes, known as the Westerlies, which blow from west to east. As the surface air flows poleward from 30 degrees, the Coriolis effect deflects it eastward, creating these consistent winds that carry weather systems across continents like North America and Europe.
The most significant weather consequences occur at its northern boundary, approximately 60 degrees latitude, known as the Polar Front. This boundary is where the warm, moist air flowing poleward within the Ferrel Cell meets the much colder air flowing equatorward from the Polar Cell. The convergence of these two air masses causes the warmer air to rise rapidly, creating a persistent low-pressure zone.
This unstable boundary is the birthplace of mid-latitude cyclones, or large storm systems, which bring much of the variable, stormy weather experienced in temperate regions. The Polar Front Jet Stream, a fast-flowing ribbon of air high in the atmosphere, is situated near this boundary and guides the path and intensity of these traveling storm systems.