IB Environmental systems and societies 2.4C the Tricellular Mode of Atmospheric Circulation Notes

The Tricellular Model: How Uneven Heating Drives Global Circulation

1. The tricellular model divides each hemisphere into three circulation cells: the Hadley, Ferrel, and Polar cells.
2. Together they redistribute the Sun's heat from the equator toward the poles, driving global wind belts and the distribution of biomes.

Latitude and Insolation Set It Off

1. Solar energy reaches the Earth unevenly across latitudes.
2. At the equator the Sun's rays strike almost vertically, concentrating energy on a small area.
3. Near the poles the rays spread over a larger area and pass through more atmosphere, so it is cooler.
4. These heating differences create zones of rising and sinking air, the foundation of the tricellular model.

The Three Atmospheric Cells

The Hadley Cell (0–30°)

1. Intense heating at the equator makes warm, moist air rise at the Intertropical Convergence Zone (ITCZ).
2. As it rises it cools and condenses, giving heavy rain and the tropical rainforest belt.
3. The dry air moves poleward and descends near 30°, forming subtropical high-pressure belts.
4. This descending, dry, stable air produces arid climates and deserts.

The Ferrel Cell (30–60°)

1. It lies between the Hadley and Polar cells and turns in the opposite direction, like a cog between them.
2. Warm subtropical air meets cold polar air near 60°, forming a low-pressure zone called the polar front.
3. The rising warm air brings frequent storms and variable weather, with temperate forests and grasslands below.

The Polar Cell (60–90°)

1. Cold, dense air sinks at the poles, creating high pressure.
2. Surface polar easterlies flow toward 60°, where they meet warmer air and rise again.
3. Low precipitation and cold temperatures produce tundra and polar desert.