The tricellular model explains global atmospheric circulation as three circulation cells in each hemisphere that redistribute heat from the equator toward the poles. Unequal solar heating creates pressure differences, vertical air movement and prevailing winds.
Level: SL and HL | Subtopic 2.4: Climate and biomes
The Mechanism
At the equator, intense solar radiation heats the surface and the air above it. This air expands, becomes less dense and rises, producing a belt of low pressure called the Intertropical Convergence Zone (ITCZ). As the rising air cools, water vapour condenses, causing high precipitation typical of tropical rainforest biomes.
The air then moves poleward at high altitude. It cools and sinks near , creating subtropical high pressure and dry conditions associated with many desert biomes. Surface air returns toward the equator as the trade winds, completing the Hadley cell.
| Circulation cell | Approximate latitude and circulation | Main surface effect |
|---|---|---|
| Hadley cell | to ; thermally driven | Trade winds; wet equator and dry subtropics |
| Ferrel cell | to ; driven by the neighbouring cells | Prevailing westerlies; variable temperate weather |
| Polar cell | to ; cold air sinks at the poles | Polar easterlies; cold, dry polar conditions |
Near , warmer air from the Ferrel cell meets colder polar air and rises, producing low pressure and frequent precipitation. At the poles, very cold, dense air sinks and flows toward , completing the Polar cell.
Earth's rotation causes the Coriolis effect, deflecting winds to the right in the Northern Hemisphere and left in the Southern Hemisphere. This creates the trade winds, westerlies and polar easterlies rather than simple north-south airflow.
Exam Technique
For an explain question, link unequal heating to convection, pressure belts, the three cells, Coriolis deflection and precipitation patterns. A common misconception is that the model predicts exact weather; it is a simplified model of long-term global circulation and climate patterns.
