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Understanding the Hadley Cell | The Single Cell Circulation

Heat is transferred from the equatorial to the polar regions by a system of wind circulation that is independent of the longitudes, called axisymmetric circulation (symmetrical to the Earth’s axis), and is commonly referred to as the Hadley Cell of single cell circulation. In the Hadley Cell, the warm air at the equatorial regions rises and moves towards the polar regions and then sinks at the higher latitudes due to the colder temperatures of these regions.

This air then begins moving back towards the equator at a lower altitude due to surface wind patterns to complete a cycle of airflow called the Hadley Cell of single cell circulation.

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George Hadley’s Contributions

The Earth’s rotation contributes to the direction of the mean surface winds on Earth, and George Hadley, a British lawyer and an amateur meteorologist, proposed in 1735 that a global system of circulation is necessary to attribute for this. Hadley said that differential heating around the equator causes warm air to rise up and amalgamate upwards into a mass.

This mass of air then begins to flow towards the Earth’s polar regions. Hadley believed that this system of steady and cyclic circulation was symmetrical to the Earth’s axis of rotation.

Differential cooling at the Earth’s polar regions then causes the cooled air to sink downwards at the higher latitudes. Hadley said that this air joins the surface westerlies (moving west to east) at the higher latitudes. Friction from the Earth’s surface then causes these winds to be deflected towards the Earth’s equatorial regions.

These winds, once they reach the equatorial regions, participate in the return flow that closes the cycle of circulation. The mechanism of this circulation is named the Hadley Cell after George Hadley’s identification of these air circulation patterns.

Theoretical Amendments

The nineteenth century witnessed the establishment of an observation that the surface westerlies also have a component of air circulation that sends air towards the polar regions. As such Hadley’s theories were amended by Thomson and then Ferrel in the late nineteenth century. They suggested the preponderance of secondary cells in the midlatitudes that represented thermally indirect meridional circulation.

These indirect circulation cells, later called Ferrel Cells, embedded in the Hadley Cell circulation that due to the Coriolis effect were deflected towards the polar regions. This, Thomson and Ferrel said, acted to balance the drag between the Hadley Cell and the surface westerlies

Hadley’s and his successors’ suggestion that single cell circulation was axisymmetric was challenged principally by Defant and then  Jeffreys in the twentieth century. The thermally direct meridional circulation of the Hadley Cell was found to extend to just about 30 degrees latitude vertically in each hemisphere.

Also the Ferrel Cells could not be understood in terms of axisymmetric circulations. The new formulation that arose suggested that the mean meridional surface wind is diverted towards the polar regions by the westerlies and towards the equatorial regions by the easterlies.

Three circulation cells in each hemisphere were suggested in place of a single Hadley Cell going directly from the polar regions to the equatorial regions (T. Schneider, 2006). These circulation cells include the Hadley Cell, the Ferrel Cell and the Polar Cell.

Here it was established that the Hadley Cell circulation in the troposphere did not extend fully until the polar regions but that tropospheric air circulation consisted of three distinct cells of air circulation.

Fig: The Hadley Cell along with the Ferrel Cell and the Polar Cell Source: Adam P. Showman

Present Understanding of Hadley Cell

Even in the present understanding of the Hadley Cell, the air circulation is not understood to extend from the Earth’s equatorial regions till the Earth’s polar regions. The angular momentum achieved by the equatorial airflow due to the Earth’s rotation limits the actual Hadley Cell airflows towards the polar regions at about 30 degrees latitude.

The present understanding of the Hadley Cell thus includes the amendments made in differentiating the Hadley Cell, the Ferrel Cell and the Polar Cell.

Air circulation in the Hadley Cell however, transports warm air from the Earth’s equatorial regions to the higher latitudes through the process of advection, which involves the transfer of heat due to the horizontal flow of warm air in the troposphere. This results in a lack of latitudinal temperature contrast along a particular cell cycle, in this case the Hadley Cell to be more or less constant.

The Hadley Cell greatly influences the climate, the structure of wind patterns, and temperature contrast between latitudes.

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Through the processes of advection and baroclinic instabilities, the Hadley Cell initiates the process of the transfer of heat from the Earth’s equatorial regions gradually towards the Earth’s polar regions. Beyond about 30 degrees latitude north and south polewards of the equator, the transfer of heat beyond the relatively constant temperature zone is performed due to baroclinic instabilities (A.P. Showman, 2009) as the whirling rotational nature of wind flows in these regions.

These draw currents of warm air from the Hadley Cell and distribute them to the regions in the higher latitudes in the Earth’s atmosphere. These also act to push colder air from the higher latitudes towards the equatorial regions thus contributing to displacing the warmer air at the surface in Earth’s equatorial regions.  The Hadley Cell is responsible for a great amount of heating in the atmosphere of Earth, thus influencing climatic patterns to a great extent.

The Hadley Cell is responsible for the formation of clouds and plentiful rainfall in the Earth’s equatorial regions. This phenomenon is principally responsible for the presence of tropical rainforests in the equatorial regions. Upon reaching about 30 degrees latitude however, the air circulation of the Hadley Cell generally consists of dry air, leading to arid climates in regions of the Earth falling into these coordinates.

The Hadley Cell exerts an immense influence on cloudiness and mean temperatures in regions between about 30 degrees latitudes on both sides of the equator. The regions falling at about 30 degrees north and south of the equator also witness whirlwinds, cyclones and other such weather anomalies due to baroclinic instabilities.

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