A2 Geography
What is the Tri-cellular model for atmospheric circulation?
The Tri-cellular model shows how energy redistributed across the globe and ensures there is not a surplus at the equator and deficit at the Poles. This surplus or deficit would be the result of differential heating of the surface of the Earth by the sun. As suggested by the name, the model is made of three air masses known as cells which redistribute heat energy and control movement of air. There a three cells in each hemisphere: the Hadley cell, the Ferrel cell and the Polar cell.
Convection Cells
Convection cells are self-contained zones in which warmer air in the centre is pushed upwards and is balanced by the downward motion of cooler air on the edge. UV radiation from the sun is absorbed which heats the ground before it is re-emitted with a longer wave length. As a result of this the temperature rises causing the air to expand and rise upwards due to convection; generating low pressure. Low pressure conditions are usually associated with cloud formation and precipitation. This is because as air cool when it ascends meaning that it can hold less water vapour which in turn condenses to produce water droplets forming clouds. When the temperature decreases, the air contracts causing it to become denser and sinks, generating high pressure (associated with dry conditions).
Originally, it was suggested that only two cells where present (The Hadley cells) on in the North and the other in the South Hemisphere. Heat would have been transferred from Equator as the Warm tropical rises to the Polar region at a much higher altitude. Air would then return to the equator as it cooled due to the low temperatures and high altitudes. This theory was later dismissed/improved by Hadley (who initially proposed it) in 1735 as it did not take into account the rotation of the Earth.

Coriolis Force
The Earth rotates in an Eastward direction which has a significant impact on atmospheric circulation. The Earth has its greatest circumference at the equator and therefore the Eastward motion of the Earth’s surface is greatest at this point. On the contrary, the poles have the smallest circumference and therefore the Eastward motion is minimal if not at all (velocity is 0 here). As air moves from high pressure at the Equator to low pressure towards the poles in the Northern hemisphere, it is deflected to the right (east). Whereas, in the Southern Hemisphere the air is deflected to the left (west). The strength of the Coriolis Effect increases as the wind move further away from the equator as the earth beneath the wind will slow down as the proximity from the poles decreases. The greater the distance from the equator the more the wind turns to the right or left.

The Three Cells
As described in the atmospheric heat budget intense insolation occurs at the Equator resulting in the formation of convection currents, leading to the formation of the Hadley cell, the Ferrel Cell and the Polar Cell. Convection currents are the transfer of heat by the mass movement of heated particles into an area of cooler fluid. In this case the air represents the fluid .The Hadley cell is the closest to the equator, then the Ferrel cell and then of course the Polar cell. This pattern is the same in both the Northern and Southern Hemisphere.
The Hadley Cells
The Hadley cells are the closest of the cells to the equator and produce both the tropical easterlies and the trade wind. In these cells the air at or near the equator rises due to the increase in temperature and move towards the poles above the Earth’s surface. However, this air does not reach the Polar Regions as it returns to the Earth surface in the subtropical regions, flowing back towards the equator.
But why does this occur? The radiation from the sun heats the air at the equator. This warm air then rises as it expands, creating a band of low pressure (as described previously). The rising air will eventually reach the troposphere at 10-15 km above the Earth’s surface and move towards the north and south poles. Around 30 degrees North and 30 degrees South the colder air at higher altitudes begins to sink or subside, back down to the Earth’s surface, moving towards the equator to complete the Hadley Cell. This produces the northeast trade winds in the Northern Hemisphere and the southeast trades in the Southern Hemisphere. The Coriolis Effect has an impact on the direction of flow on these winds. Finally, as the air descends at 30 degrees latitude it warms and any residual moisture will evaporate resulting in high pressures at the surface and clear skies usually prevail. Hence why many deserts are located in these regions.
Inter-Tropical Convergence Zone
In Equatorial regions, usually 5 degrees north and 5 degrees south, the north east trade winds and the southeast trade winds converge in a low pressure zone (typical of the Equatorial climate) known as the Inter-Tropical Convergence Zone. As the trade winds cross warm tropical oceans they pick up latent heat ( the heat needed to convert a liquid to a vapour), and along with solar radiation from the sun causes the trade winds to rise due to convection currents. The warm moist air is cooled adiabatically due to the fact as the air rises to higher altitudes the pressure decreases and the temperature therefore decreases also. The cooling of the moist air results in the formation of cumulo-nimbus clouds and thunderstorms along with low pressures. The Intertropical Convergence Zone has been called the doldrums by sailors due to the lack of horizontal air movement (the air simply rises with convection).

The Ferrel Cells
At slightly higher latitudes, from 30° latitude to 60° latitude, a new cell begins known as the Ferrel Cell. Therefore the Ferrel cell is further from the equator in comparison to the Hadley cell. Some of the air sinking at 30° latitude continues traveling northward toward the poles and the Coriolis force bends it to the right in the Northern Hemisphere instead of returning to the Earth’s surface and moving towards the equator. As a result the Ferrel cell is responsible for the production of the prevailing westerly winds at the surface within these latitudes. The travelling air is still warm but at 60° latitude cold air from the poles approaches it and converges. As the two masses of air converge at the surface, the low surface pressure at 60° latitude causes air to rise and form clouds. Some of the rising warm air returns to 30° latitude to complete the Ferrel Cell.

The Polar front and the Polar Cells
As described before the two air masses at 60° latitude do not merge well and as a result form the polar front. The polar front is the boundary between warm tropical air masses and the colder polar air moving from the north. The Polar Front Jetstream is located above the polar front and flows generally from west to east. The PFJS is strongest in the winter because of the greater temperature difference (greater gradient) in winter than in summer. Waves along this front can pull the boundary north or south, resulting in local warm and cold fronts which affect the weather at particular locations. Many countries within Europe including the UK often experience unsettled weather, which comes from travelling areas of low pressure which form when moist air rises along the polar front. Regularly weather systems which hold rain and unsettled condition move across the Atlantic. The weather systems are guided and directed by the jet streams, so its position is important for UK weather.
At latitudes greater than 60°, the polar cell circulates cold air from the polar region towards the equator. The air from the poles rises at 60° latitude where the polar cell and Ferrel cell meet, and some of this air returns to the poles completing the polar cell completing both the ascending and descending limbs. Because the wind flows from high to low pressure and taking into account the effects of the Coriolis force, the winds above 60° latitude are prevailing easterlies.

Finally, Global Circulation is determined by the combined effect of the Hadley Cells, Ferrel Cells and Polar cells in both the Northern and Southern Hemisphere and the results of the Coriolis Effect. The overall effect of Tri Cellular model and Coriolis Effect on Global circulation is the transfer of energy from the tropics towards the poles in a conveyor belt motion.