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Explaining the Tri-Cell Model and Atmospheric Circulations
The equatorial region experiences the greatest amount of insolation causing
the air temperatures to be consistently high. This warm air rises, through
convection, causing low pressure at ground level (Equatorial Low). As the
warm air rises it travels north (and southward) and will sink back down to
earth as it cools. This sinking air causes pressure to build at ground level –
hence the Sub-Tropical High. It is at these latitudes that deserts form.
Fig 1
At ground level, the air moves away from the Sub-Tropical High due to the
pressure gradient. Some of this will return to the Equatorial region (filling the
low) to complete the cycle of the Hadley Cell. Some air will continue
northwards forming part of the Ferrell Cell.
Fig 2
Over the North Pole there is very little heating due to the angle of incidence,
short day light hours and the larger footprint size of the insolation. As a result
the air sinks resulting in high pressure at ground level (Polar High). Air will flow
away from the area of high pressure until it meets the warm air travelling
northwards in the Ferrell Cell. The meeting of these different air masses is
called the Polar Front with the warmer air is forced to rise above the colder polar air. The meeting of these two
different air masses is recognised as an area of Low Pressure but referred to as the Polar Front.
These sequences of air movements result in the Tri-Cell model – Hadley Cell, Ferrell Cell and the Polar Cell.
As the air moves from north to south, or vice versa, it appears to be deflected. This process is due to the Earth
rotating below the atmosphere and is called to the Coriolis Effect. In the
Fig 3
Northern Hemisphere the deflection appears to be towards the right and in the
Southern hemisphere to the left (Figure 3).
It is due to Coriolis that we refer to trade winds travelling in different directions:
1. Hadley Cell the air at ground level is travelling back towards the Low
pressure and will also be travelling East to West so is referred to as
North Easterlies.
2. Ferrell Cell the air is travelling south to north (H to L pressure) but is still deflected to the right so is moving
West to East, hence South Westerlies.
3. Polar Cell air moves from the Pole southwards and is deflected again to
the right (East to West) so is a North Easterly.
Fig 4
This is shown in Figure 4.
The Doldrums – An area of calm conditions either side of the equator where the
air is moving vertically (convection) rather than horizontally. Being in the
Doldrums is a metaphor for being dejected and unhappy as sailors hated this
area of the world as there were no winds to help them on their way.
Horse Latitudes – One theory is based around the fact that the very light winds
associated with the Sub-Tropical High caused the Spanish, who were transporting horses by ship to their colonies in
the West Indies and Americas, to stall for long periods of time. During this time they were likely to experience water
shortages, making it impossible for the crew to keep the horses alive, and they would throw the dead or dying
animals overboard.