Evaluate the three-cell model (12 marks)

Evaluate the extent to which the three-cell model of atmospheric circulation explains global weather and climate patterns. [12 marks]

Level mark scheme:

Level	Marks	Descriptor
L1	1–4	Limited description of cells; no genuine evaluation; weak/no examples.
L2	5–8	Sound explanation of how the model accounts for some climate features; partial recognition of limitations; named examples used.
L3	9–12	Detailed evaluation; clear strengths AND weaknesses balanced; multiple named examples; recognition of seasonal ITCZ migration, monsoons, ENSO; justified conclusion on the model's usefulness.

Indicative content (for):

• Explains pressure belts → desert/rainforest distribution very effectively.
• Predicts prevailing wind directions (NE trades, westerlies, polar easterlies).
• Helps explain why UK weather is mild and wet (Polar Front, westerlies).

Indicative content (against / limitations):

• ITCZ migrates seasonally → monsoon climate of South Asia not explained by static model.
• Cells are idealised — the real Ferrel cell is weak/disorganised, dominated by Rossby waves and jet streams.
• Doesn't explain anomalies: ENSO (El Niño/La Niña) shifting Pacific rainfall, North Atlantic Oscillation, blocking highs.
• Local factors (topography, ocean currents, urban heat islands) override large-scale circulation.
• Climate change is shifting cell boundaries (Hadley cell expansion → Mediterranean drying).

Conclusion: The model is a strong first-order framework — useful for explaining major biome distributions and prevailing winds — but cannot explain regional anomalies, seasonal variability or climate change without additional concepts (jets, oscillations, ocean–atmosphere coupling).