Why Deserts Form Where They Do: The Hadley Cell
Part of Hot Deserts — GCSE Geography
This deep dive covers Why Deserts Form Where They Do: The Hadley Cell within Hot Deserts for GCSE Geography. Revise Hot Deserts in The Living World for GCSE Geography with 0 exam-style questions and 22 flashcards. This topic shows up very often in GCSE exams, so students should be able to explain it clearly, not just recognise the term. It is section 2 of 14 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
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🌍 Why Deserts Form Where They Do: The Hadley Cell
Hot deserts do not occur randomly. Look at a world map and you will notice them clustered in two bands: roughly 20°–30° north of the equator (Sahara, Arabian Desert, Thar) and 20°–30° south (Atacama, Namib, Australian Outback). This is not coincidence — it is the direct result of atmospheric circulation driven by the way the Earth's atmosphere responds to the intense solar heating at the equator.
The equator receives the sun's rays at a near-vertical angle all year round, heating the land surface intensely. This causes the air above the equator to warm rapidly.
Warm air is less dense than cool air, so it rises in a great column above the equatorial zone. As air rises, it expands and cools. Cool air cannot hold as much water vapour as warm air — so the moisture condenses into clouds and falls as torrential rainfall. This is why equatorial zones (like the Congo and Amazon basins) receive 2,000–3,000mm of rain per year and support tropical rainforests.
After losing its moisture at the equator, the now-dry air continues to rise and then spreads out at high altitude, travelling north and south away from the equatorial zone in the upper atmosphere.
At roughly 20°–30° north and south of the equator, this dry upper-atmosphere air sinks back down to the surface. This is the Hadley Cell circulation completing its loop.
As air descends, it is compressed by the weight of the atmosphere above it and warms up. Warm air can hold more water vapour — so instead of releasing moisture, this descending air actively absorbs any moisture it encounters from the ground and vegetation below.
The descending air creates stable, high-pressure zones at the surface. High pressure suppresses cloud formation and prevents rainfall. The result: hot deserts at precisely 20°–30° latitude. This pattern is so reliable that geographers can predict where deserts will form simply from a globe and basic knowledge of atmospheric physics.
The World's Major Hot Deserts
| Desert | Location | Area | Key Fact |
|---|---|---|---|
| Sahara | North Africa | 9.2 million km² | Largest hot desert; 3.5mm average annual rain in central zones |
| Arabian Desert | Middle East | 2.3 million km² | Includes the world's largest continuous sand mass (Rub' al Khali) |
| Thar Desert | India/Pakistan | ~200,000 km² | Most densely populated hot desert: 83 million people |
| Atacama | Chile/Peru coast | 105,000 km² | Driest place on Earth: some areas receive <3mm per year; rain-shadow + cold Humboldt Current |
| Australian Outback | Central Australia | ~1.5 million km² | Includes Gibson, Great Victoria and Simpson Deserts |
Note that the Atacama demonstrates that Hadley Cell subsidence is not the only cause of deserts. The Atacama sits in the rain shadow of the Andes mountains and is cooled by the Humboldt Current (cold water upwelling from the Pacific reduces evaporation). Multiple mechanisms can produce arid conditions, but Hadley Cell subsidence explains the majority of the world's hot deserts.