Earth’s weather is fundamentally driven by the temperature different between the equator and the poles. The equator is warmer because it faces the sun more directly compared to high latitude regions, so the solar insolation it receives is stronger. In an attempt to achieve equilibrium between these two regions, heat travels from the warmer equator to the cooler polar regions. Warm air rises at the equator and flows north or south to the poles, while cooler air from the poles flows equatorward along the Earth’s surface.
Unfortunately, this fairly simple circulation model is interrupted by the fact that the Earth rotates. As the Earth spins, the Coriolis force acts on the atmosphere. Because the Coriolis force is strong at the poles and negligible at the equator, the flow of air between the equator and the poles is broken up into three distinct circulations in each hemisphere, known as cells. The Hadley cell is between the equator and the subtropics, the Ferrel cell is in the midlatitudes, and the polar cell is the most poleward cell.
Hadley cell
Hadley cell circulation is driven by convection. Air rises at the equator to the tropopause and then flows north or south (depending on the hemisphere) to a latitude of about 30° where it descends and flows back towards the equator. This circulation drives a number of weather phenomena. Firstly, the rising air near the equator creates a region of instability, known as the intertropical convergence zone, which is characterized by low winds and extensive rainfall from squalls and thunderstorms. Due to the unreliable winds in this region, it is also known as the doldrums. While rising air creates instability, falling air creates stability. Consequently, the area of subsidence where the Hadley cell circulation returns to the surface (approximately 30° N or S) is characterized by low winds, high pressure, clear skies, and very little rainfall. This region is known as the subtropical ridge or the horse latitudes. On land, the majority of the world’s arid regions are located under the subsiding air of Hadley cell circulations.
Once air in the Hadley circulation has subsided at the horse latitudes, it returns to the equator along the Earth’s surface, creating a reliable, consistent wind field known as the trade winds. Characteristically, the trade winds flow from east to west because they are deflected westwards in both hemispheres by the Coriolis force.
Polar cell
Like the Hadley cell, the polar cell is driven by convection. At approximately 60°, relatively warm and humid air rises to the tropopause then flows poleward where it descends and flows equatorwards at the surface. These surface winds are deflected by the Coriolis force so that they flow in an easterly direction. Because surface winds in the Polar cell have travelled from the poles, they are usually very cold.
Ferrel cell
The Ferrel cell lies in the midlatitudes between the Hadley and Polar cells. Circulation in the Ferrel cell behaves differently. It is driven by convergence between poleward travelling subtropical air and equatorward travelling polar air. These two flows converge at a latitude of approximately 60°, creating the uplift that drives both the Polar and Ferrel cells. Because surface air in this region flows towards the poles, it is deflected to the east by the Coriolis force, and results in the strong and consistent westerlies typical of the midlatitudes.
Because the Ferrel cell circulation can be considered as a zone of transition, weather in the region is frequently unsettled. In particular, the zone up convergence and uplift between the Polar and Ferrel cells frequently produces low pressure systems which can in turn develop into deep extratropical storms. As these systems develop and are steered by the winds aloft, they can interrupt the prevailing surface winds. Consequently, westerly winds in this region are not as consistent as the easterly trade winds of the subtropics.
Jet streams
Fast flowing narrow bands of very strong westerly winds occur at the tropopause between the Hadley and Ferrel cells and Ferrel and Polar cells. These are known as the jet streams. The subtropical jet is at the boundary of the Hadley and Ferrel cells, while the polar jet is at the boundary of the Ferell and Polar cells.
Seasonal movement
The positions of the Hadley, Ferrel, and Polar cells change seasonally. For example, the intertropical convergence zone will shift north of the equator during the northern hemisphere summer and south of the equator during the southern hemisphere summer. Likewise, the subtropical ridges of the northern and southern hemisphere shift northwards during the northern hemisphere summer, and southwards during the northern hemisphere winter. Consequently, midlatitude regions experience hot and dry summers due to the position of the subtropical ridge. The same regions experience cold, wet, and stormy winters as the subtropical ridge moves equatorwards, to be replaced by the unsettled air masses of the Ferrel cell.
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