Between the great tropical Hadley cell and the cold polar cell, sandwiched in the mid-latitudes, sits a third atmospheric circulation: the Ferrel cell. The Ferrel cell extends approximately from 30 to 60 degrees latitude in each hemisphere, and it is fundamentally different in character from the other two cells. The Hadley and polar cells are direct, thermally driven circulations — warm air rises, cool air sinks, the cycle is driven by temperature contrasts. The Ferrel cell is an indirect or mechanically driven circulation — it has rising motion at its cool boundary (the polar front) and descending motion at its warmer boundary (the subtropical high). The Ferrel cell exists not because it would form naturally on its own, but because it is sandwiched between and driven by the surrounding direct cells. Despite its indirect, more complex character, the Ferrel cell's consequences are highly visible: the prevailing westerlies, the storm tracks, and most of the day-to-day weather variability experienced by mid-latitude populations.
Where It Sits
The Ferrel cell occupies the latitude band from approximately 30 degrees to approximately 60 degrees in each hemisphere. The exact boundaries vary with season and year. In the Northern Hemisphere, this covers most of the contiguous United States, Europe, central Asia, China, Japan, and the southern half of Canada. In the Southern Hemisphere, it covers southern South America, southern Africa, southern Australia, New Zealand, and the Southern Ocean. The Ferrel cell sits between the Hadley cell's descending branch at the subtropical high (around 30 degrees) and the polar cell's region of descending air over the poles. The boundaries with the other cells are not sharp; they are dynamic features that move with weather systems and seasons.
The Naming
The Ferrel cell is named after the American meteorologist William Ferrel, who in 1856 published a paper called "Essay on the Winds and the Currents of the Ocean" that included the first treatment of the three-cell model of atmospheric circulation. Ferrel was working on extensions to the work of George Hadley (after whom the Hadley cell is named) and was trying to reconcile Hadley's simple one-cell model with observations that suggested a more complex circulation. Ferrel's contribution was recognising that a thermally driven cell could not extend from the equator to the pole in a single loop, given the effects of Earth's rotation. He proposed the intermediate "Ferrel cell" structure that emerged from the interaction between the tropical Hadley cell and a cold polar cell. The basic three-cell model has been refined and extended substantially since Ferrel's 1856 paper, but the basic structure he proposed has been retained.
The Indirect Circulation
The Hadley cell is "thermally direct" — warm air rises near the equator (where the surface is warmest) and cool air sinks in the subtropics (where the surface is somewhat cooler). The polar cell is similar — relatively warm air at the polar front rises and cool air over the poles sinks. The Ferrel cell, however, is "thermally indirect" — warm air sinks at the subtropical high (where the surface is relatively warm) and cool air rises at the polar front (where the surface is relatively cool). This is the opposite of what would happen if the Ferrel cell were independently driven by temperature contrasts. The Ferrel cell exists despite its thermal disadvantage because it is mechanically coupled to the surrounding direct cells. The Hadley cell's descending motion at 30 degrees and the polar cell's rising motion at 60 degrees create a "gap" in the circulation that the Ferrel cell fills, more or less reluctantly.
The Westerlies
The most visible surface signature of the Ferrel cell is the prevailing westerly winds at mid-latitudes. The Ferrel cell's lower branch flows from the descending air around 30 degrees latitude toward the rising air around 60 degrees latitude — that is, poleward. The Coriolis effect deflects this poleward motion to the east (in the Northern Hemisphere; to the east also in the Southern Hemisphere, where the Coriolis deflection is to the left of motion but the motion direction is southward, producing the same result). The result is the prevailing westerly winds (winds from the west) experienced across the mid-latitudes. These westerlies are the dominant climatological winds in places like the US Pacific Northwest, the British Isles, France, Germany, central Asia, and the Southern Ocean. The "roaring forties" and "furious fifties" of the Southern Hemisphere — referring to the strong westerly winds at 40-50 degrees south — are the most extreme expression of the Ferrel cell's westerlies, undisturbed by continents.
The Polar Front and the Jet Streams
The boundary between the Ferrel cell and the polar cell is the polar front — a transition zone where cold polar air meets warmer mid-latitude air. The polar front is one of the most weather-active zones in the atmosphere. The temperature contrast across it creates a jet stream — a fast-moving river of air at the tropopause that flows generally west to east. The polar jet stream sits at approximately the polar front's latitude and meanders north and south in complex patterns. The jet stream's position determines storm tracks — the corridor along which mid-latitude cyclones (low pressure systems) propagate eastward across continents and oceans. The boundary between the Hadley cell and Ferrel cell is the subtropical front, with the subtropical jet stream associated with it. The subtropical jet is generally weaker than the polar jet but still significant for tropical and subtropical weather.
Mid-Latitude Cyclones
The Ferrel cell's most prominent feature, in terms of human experience, is the parade of mid-latitude cyclones (extratropical cyclones) that move eastward across the continents and oceans. These low-pressure systems form along the polar front where temperature contrasts are strongest. They develop characteristic comma-shaped cloud patterns, with warm and cold fronts, and they carry weather — rain, wind, sometimes severe weather — across the mid-latitudes as they propagate. The week-to-week weather variability experienced by mid-latitude populations is essentially the passage of one cyclone after another. The detailed dynamics of these systems is the central subject of synoptic meteorology, and forecasting their motion and intensity is the central task of mid-latitude weather forecasting.
The Polar Front Theory
The Norwegian meteorologists Vilhelm Bjerknes and his colleagues developed the "polar front theory" in the early 20th century, which provided a unified picture of how mid-latitude cyclones form along the polar front and propagate as part of the larger Ferrel cell circulation. This theory was a major breakthrough in synoptic meteorology and remains the basis for understanding mid-latitude weather. The Bjerknes group recognised that the polar front was the boundary along which temperature contrasts in the mid-latitudes generated potential energy that was extracted by developing cyclones. The cyclones, in turn, mixed warm and cold air across the front, partially homogenising the temperature distribution and reducing the contrast. The Ferrel cell circulation is, in this sense, the time-averaged signature of the constant work being done by mid-latitude cyclones in transporting heat from the warm subtropics toward the cool polar regions.
Storm Tracks
Mid-latitude storms do not move randomly — they tend to follow particular corridors called storm tracks. The two most prominent are the North Atlantic storm track (extending from the eastern US northeastward across the Atlantic to the UK and Norway) and the North Pacific storm track (extending from East Asia northeastward across the Pacific to the Gulf of Alaska). These storm tracks are favoured by the geometry of the polar jet stream and the placement of mountain ranges and ocean currents. The storm tracks bring substantial precipitation to the regions they pass through; they also bring the severe weather (high winds, snowstorms) that periodically affects the mid-latitude populations. Southern Hemisphere storm tracks are more zonal (less constrained by continents) and form the "Southern Ocean storm belt" that contributes to the rough seas of the roaring forties.
Weather Variability
The Ferrel cell is responsible for much of the day-to-day weather variability experienced by mid-latitude residents. The Hadley cell's rising and descending branches produce relatively consistent climates (tropical wet and dry seasons; subtropical dry climates). The polar cell's cold sinking air produces consistently cold conditions. The Ferrel cell, by contrast, is the zone of greatest day-to-day weather variability — warm fronts followed by cold fronts followed by warm again, sometimes within a week. Sunny days alternate with cloudy and rainy ones. Temperatures swing 20°C or more on weekly timescales. This variability is a direct consequence of the constant traffic of mid-latitude cyclones characteristic of the Ferrel cell domain. The variability is one of the things that makes mid-latitude weather both interesting and challenging to forecast.
The Ferrel Cell and Climate Change
Climate change is affecting the Ferrel cell, though in subtler ways than the Hadley cell. The poleward expansion of the Hadley cell (well-documented in recent decades) is pushing the boundaries of the Ferrel cell poleward as well. This shifts the storm tracks poleward, with consequences for regional precipitation. The polar jet stream appears to be becoming more wavy under climate change — partly as a consequence of the rapidly warming Arctic reducing the temperature contrast across the polar front. The wavier jet stream produces more persistent weather patterns and contributes to extreme weather events (heat waves, cold spells, droughts, floods). The exact connections between Arctic warming, the jet stream, and mid-latitude weather are an active area of research, but the broad picture suggests that the Ferrel cell's normal circulation is being altered in ways that may have substantial impacts on mid-latitude weather patterns through the 21st century.