Back to main Back to A-Level Back to Weather
The Tri-cellular model Get the plan
 

Our atmospheric system is incredibly complex, but we do have models that can help us to explain it.  Generally, air moves from high to low pressure areas on the globe.  On a non rotating globe with no other factors, this would mean that air molecules would move from the area with the greatest amount of energy, the Tropics (also the hottest area) to both of the Poles, the areas with the least energy and coldest areas.  Cold air would move in the opposite direction.  This occurs as the atmosphere seeks to balance out the uneven distribution of energy received from the sun. See some great explanations of this here. http://www.bom.gov.au/lam/Students_Teachers/pressure.shtml

The Earth's heat budget
Diagram 1

Unfortunately, our Earth is far more complex than this, and in reality there is a tri-cellular model of atmospheric circulation that is itself IMPERFECT!  The tri-cellular model is a 2 dimensional model that give us a general understanding of how our atmosphere functions.  It is a global scale model that is based entirely upon the fact that there are recognisable insolation differences between the Equator and the Poles. The insolation budget of our planet determine that because of the tilt of the earth and the way that it orbits around the sun, the Poles receive an overall deficit of insolation over a year and there is a surplus at the equator (see diagram 1).  This puts or whole atmospheric system out of balance and the tri-cellular model of atmospheric circulation tries to equalise those differences.  The model can be seen in diagram 2. 

Tri cell model animation

Diagram 2 - Gif animation

It starts in the Doldrums, an area of intense low pressure found at the equator where the intense heating (be convection) of the earth’s surface forces air to rise through the Troposphere.  This area is known as the Inter Tropical Convergence zone (ITCZ).  As this air rises it cools and condenses forming a belt of clouds.  Some of this air migrates northwards in the upper Troposphere to equalise out the temperature and insolation differences of our globe.  As this air migrates north it cools relative to the air around it, becomes denser and sinks to the Earth’s surface at around 30°N and S of the Equator, creating a band of high pressure.  Some of this air migrates (because of Pressure gradient force) back to the low pressure area at the equator to complete the first cell of the system, the Hadley cell.  Some of the air continues towards the poles to continue equalising the temperature differences.  When this air reaches 60°N and S it reaches cold polar air that is migrating south.  This is our second convergence zone where 2 surface air streams meet.  This causes the warmer, less dense tropical air to rise through the atmosphere again creating an area of low surface pressure. It is this zone where we find the mid-latitude weather systems that blight British weather.  Some of this air migrates back towards the Equator where it eventually sinks at 30°N and S to form the middle cell of the model, the Ferrell cell.  The rest of the air migrates to the pole, where it cools and sinks creating high pressure in the Polar Regions and completing a weak polar cell.  Near the Tropopause at 30°N and S and 60 °N and S we find the high speed jet stream winds.

 

This model has many applications and limitations.  The model fails to accommodate other major transfers of energy, such as the El Nino and La Nina models of circulation from West to east or Vice Versa across The Pacific Ocean.  It also fails to acknowledge the presence and impact of Geomorphological features such as the Himalaya which complexly disrupt the movement of jet streams and surface level winds within the Hadley cell on a yearly basis.    However, it does offer people a starting point for understanding atmospheric circulation, ad does allow for some level of prediction of the weather that affects billions of people around the globe.  The understanding of high level jet streams within the model is also of use to pilots and balloon enthusiasts!
 
The second major way that heat is redistributed around our planet is by oceanic circulation or ocean currents.  These are hugely important and our understanding of then is increasing with time.  The globes ocean currents are interlinked into a global system, which is commonly known as the Thermohaline conveyor.  The word can be broken down, “Thermo” relates to temperature, whilst “haline” relates to salinity differences.  Basically, warm less salty water travels at the surface of our oceans driven by surface winds that blow over the top of those oceans.  This water cools as it travels north and south from the Equator and increases in salinity as the salt is left behind during evaporation of the warm water.  This water, now cooler and salt laden, sinks and returns to the equator as another method of balancing out the Earth’s heat budget.  This mechanism is hugely important for the people of Western Europe, as a warm ocean current called the Gulf Stream brings warm ocean waters which warm Western Europe well beyond what it should be given its latitude.  Consider this, Newcastle in the UK is on a similar latitude as MOSCOW – but the Gulf Stream and the moderating impact of the seas and oceans around the British Isles massively impact the temperatures.  In the past scientists think that this Thermohaline conveyor has shut down, for example at the end of the last ice age when an ice dammed lake in North America broke and flooded the North Atlantic with cold fresh water, shutting off the current.  This was the pretence for the disaster movie “The Day After Tomorrow”.