THE 3 CELL MODEL

Tropical areas receive more insolation (energy from the sun) than the polar regions. There is an energy surplus at the Tropics and an energy deficit at the Poles

In theory this would result in the Tropical regions getting warmer and the Polar regions getting colder… In reality, energy is transferred polewards… One of the ways in which this happens is by atmospheric circulation.

The Single Cell Model The single cell model was proposed in 1735 by George Hadley. It suggests that: Air at the Equator is heated up causing it to rise ,creating a belt of LP The warm air moves towards the poles where it sinks,creating a belt of HP Surface winds always blow from high to low pressure This completes the Hadley Cell. In this way, surplus energy from the Equator is transferred polewards.

The Three-Cell Model Later observations proved that Hadley cells exist but only extend as far as 30°N and 30°S of the Equator. This is because the air warm air… before it reaches the Poles. In reality there are actually three cells –the Hadley cell, Ferrel cell and polar cell

The Hadley Cell The Hadley cell extends from the equator to tropics Air at the equator is heated up. Warm air rises creating a zone of LP Warm air moves polewards but begins to sink The cold air sinks at 30 degrees creating a zone of HP The cold air then flows back towards the equator because surface winds always blow from HP to LP

The Polar Cell At the Poles, air is being cooled. Cold air is dense and therefore sinks The descending cold air moves towards the lower, warmer latitudes. As it moves towards the lower latitudes it becomes warmer and therefore rises at 60 degrees creating a zone of LP The rising warm air then moves polewards completing the Polar cell.

The Ferrel Cell The Ferrel cell lies in between the other two cells. The Hadley cells and Polar cells are said to thermally indirect cells because they are driven by different heating The Ferrel cell is a thermally cell because it is driven by friction between the other two cells

The Ferrel Cell As the air in the Hadley Cell descends at about 30ºN and 30ºS, it pulls the air beside it as well, due to friction As air in the Polar Cell rises at about 60ºN and 60ºS, it pulls the air beside it up as well. In this way warm air is transferred from the Hadley cell to the Polar cell via the Ferrel cell for cooling. And cold air is transferred from the Polar cell to the Hadley cell via the Ferrel cell for warming.

Global Wind Circulation Complete the diagram on the next slide by filling in the blanks in the text boxes to show the names of the pressure belts and associated winds. Draw arrows on the diagram to show the wind direction. Remember winds blow from high to low pressure and winds are deflected to the right in the northern hemisphere and to the left in the southern hemisphere due to the Coriolis Effect.

Sub-Polar low pressure Polar High Pressure Polar Easterlies Sub-Polar low pressure Mid Latitude… Sub-Tropical high pressure North East… Equatorial low pressure…