|The Tri-cellular model|
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
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.
Diagram 2 - Gif animation
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
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
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
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,