Every desert on Earth sits at roughly the same latitude. Every rainforest belt sits at another. That is not luck — it is the result of the atmosphere moving heat away from the equator in three giant loops, helped along by the oceans doing the same job in water. Once you can draw those loops, biome distribution stops being a list to memorise and becomes something you can work out.
📚 What you need to know
Insolation is strongest at the equator and weakest at the poles, because of the Earth’s curvature and tilt.
Uneven heating creates pressure differences, and air moving from high to low pressure is wind.
Warm air rises (low pressure at the surface); cool air sinks (high pressure at the surface).
Each hemisphere has three cells: Hadley (largest), Ferrel (middle, turns the opposite way) and Polar (smallest and weakest).
This is the tricellular model, and it explains the belts of rainforest, desert, temperate forest and tundra.
Oceans are enormous heat reservoirs; surface currents driven by wind and the Earth’s rotation carry warm water polewards and cold water back.
Together they moderate temperature extremes and shape both regional climates and marine ecosystems.
Start with uneven heating
The Sun sends out roughly parallel rays. The Earth is a sphere, so those rays cannot possibly strike every part of it in the same way.
At the equator the rays hit almost head-on, so a beam of a given width lands on a small patch of ground. Near the poles the same beam arrives at a shallow angle and smears out across a much larger patch — the same energy, thinly spread. It also has more atmosphere to travel through, so more of it is absorbed or reflected on the way.
The two thick arcs are lit by exactly one beam each. The red one is nearly twice as long, so every square metre inside it receives roughly half as much energy.
Say it in the exam like this: “Insolation is greater at the equator because the rays strike at a higher angle, concentrating the same energy on a smaller surface area.” That single sentence sets up almost every explanation on this page.
From heating to wind
Air behaves the way air always does: heat it and it expands, becomes less dense and rises. Cool it and it becomes denser and sinks. Do this over a whole planet and you get pressure cells.
🧩 One convection cell, step by step
The Sun heats the surface. The air above it warms, expands and starts to rise.
Surface pressure falls where the air is leaving, because there is less air pressing down. This is a low.
The rising air adds pressure aloft, so it spreads out sideways high in the atmosphere.
Up there it cools, becomes denser and begins to sink somewhere else.
Where it lands, surface pressure rises. This is a high.
Air rushes from the high to the low along the surface to even things out — that flow is wind. The bigger the pressure difference, the stronger the wind.
Wind is not the cause of anything here — it is the symptom. The cause is uneven heating; pressure difference is the mechanism; wind is the result. Write your answers in that order and they read like an explanation instead of a list.
The tricellular model
If the Earth did not rotate, there would be one huge loop per hemisphere: rise at the equator, sink at the pole. Rotation breaks that into three cells each side, and those three cells are what the syllabus calls the tricellular model.
Follow the arrows round each box. Hadley and Polar turn the same way; Ferrel is dragged round backwards between them, like a cog caught between two other cogs.
The Hadley cell
The largest and strongest cell, running from the equator to about 30–40° north and south.
Trade winds blow from the subtropics towards the equator, where they meet.
Intense insolation heats that air; it rises and cools, water vapour condenses, and you get towering thunderstorms and daily tropical downpours.
At altitude the air flows polewards, cools further and sinks over the subtropics at about 30°.
Sinking air warms and dries as it descends, so it brings clear skies and almost no rain. This is where the world’s hot deserts are.
The Ferrel cell
The middle cell, roughly 30° to 60–70°. It is the awkward one, because it turns in the opposite direction to its neighbours — driven by them rather than by direct heating.
Air joins the sinking limb of the Hadley cell and travels polewards near the surface as the westerlies.
At around 60° it meets cold polar air and is forced upwards along that boundary.
Warm and cold air masses colliding is exactly why mid-latitudes get so much unsettled, changeable weather.
The Polar cell
The smallest and weakest cell, from about 60° to the poles.
Very cold, dense air sinks over the poles, creating permanent high pressure.
It flows out towards lower latitudes at the surface as the polar easterlies.
It warms slightly, rises at about 60°, and returns to the pole at altitude.
Cold air holds little moisture, so the poles are technically deserts by rainfall.
Why this decides where biomes sit
Rising air means clouds and rain. Sinking air means clear, dry skies. That single rule, applied at four latitudes, gives you the global pattern of biomes.
Latitude
Air movement
Surface pressure
Climate
Biome
0° equator
Rising
Low
Hot and very wet
Tropical rainforest, then savanna at the edges
About 30°
Sinking
High
Hot and dry
Hot desert
About 60°
Rising
Low
Mild and changeable, moderate rain
Temperate forest and grassland
90° poles
Sinking
High
Very cold and dry
Tundra, polar desert and ice
The one-line version: deserts sit at 30° because that is where Hadley cell air comes back down, warming and drying as it sinks. If a question asks why the Sahara is dry, that sentence is the answer.
Ocean currents: the slower half of the system
The atmosphere is not the only thing moving heat around. Oceans absorb solar radiation in their surface layers and store it as thermal energy, and because water has a huge capacity to hold heat, they act as vast heat reservoirs.
Surface currents are driven by the winds you have just drawn, and bent by the Earth’s rotation. The result is a set of great loops called gyres, which carry warm water away from the equator and cold water back towards it.
This is why western Europe is far milder than Canada at the same latitude, and why the coast of Namibia is a desert cooled by a cold current running up from the south.
What the currents do for climate and ecosystems
They redistribute heat horizontally across the ocean surface, moving surplus energy out of the tropics.
They moderate temperature extremes, so coastal places have smaller ranges between summer and winter than inland ones.
A warm current brings milder, wetter conditions to the coast it touches; a cold current cools the coast and often keeps it dry.
They shape marine ecosystems by controlling nutrient supply, so they set patterns of ocean productivity, species distribution and marine biodiversity.
Cold currents are usually the productive ones. Cold water carries more dissolved oxygen and often brings nutrients up from the deep, which is why the world’s biggest fisheries sit next to cold currents, not warm ones.
Worked examples
EXAM Q1
Explain why hot deserts are found at about 30° north and south of the equator. [4]
Step 1: start at the equator
High insolation heats the surface, so air rises and rain falls over the tropics.
Step 2: follow the air polewards
It travels at altitude, cools, and sinks at about 30°, completing the Hadley cell.
Step 3: what sinking air does
Descending air is compressed and warms, so it holds its moisture instead of releasing it.
Step 4: the result
Little cloud, little rain, high pressure at the surface all year.
Sinking, warming, drying air at 30° gives hot desertthe marks are in the chain, so keep it in order: rises, moves, sinks, dries
EXAM Q2
Outline how the tricellular model helps explain the distribution of terrestrial biomes. [3]
Point 1: it sets rainfall
Rising limbs give high precipitation, sinking limbs give low precipitation.
Point 2: it sets temperature bands
Combined with insolation, this fixes the temperature of each latitude band.
Point 3: temperature plus rainfall equals biome
Rainforest at 0°, desert at 30°, temperate forest at 60°, tundra at the poles.
The cells set the climate, and the climate sets the biomeit also lets us predict biome shifts as the climate warms
EXAM Q3
Two coastal cities lie at the same latitude, but one is much milder in winter. Suggest why. [2]
Step 1: same latitude means same insolation
So the difference cannot come from the Sun angle — look at the ocean.
Step 2: name the mechanism
A warm surface current carries heat from the tropics past the milder city and releases it to the air.
The other city is likely cooled by a cold current, or is further from the sea“same latitude” in a question is a hint that the answer is currents or altitude
💡 Exam tip
Learn the cells in order with their latitudes: Hadley 0–30, Ferrel 30–60, Polar 60–90. Marks are often just for correct labelling.
Tie every climate statement to rising = wet, sinking = dry. It works at all four latitudes.
Use the word insolation rather than “sunlight”. It is the syllabus term and examiners look for it.
If you are asked to draw the model, label the pressure belts as well as the cells — they are separate marks.
For ocean current questions, always say which direction the heat is moving and what it does to the coast.
Watch the command word: outline wants brief points, explain wants the full cause-and-effect chain.
⚠ Common mix-up
Saying warm air rises “because it is light”. It rises because it expands, becomes less dense than the air around it, and floats up.
Mixing up the pressure. Rising air leaves low pressure at the surface; sinking air creates high pressure. Students routinely flip these.
Forgetting the Ferrel cell runs backwards. That is its whole personality, and it is a common one-mark question.
Thinking wind blows from low to high. It blows from high to low pressure.
Assuming the poles are wet because they are icy. They receive very little precipitation — the ice is old, not new.
Treating ocean currents as separate from wind. Surface currents are largely wind-driven, so the two systems are one story.
Describing only the Northern Hemisphere. The same three cells exist south of the equator, mirrored.
Up next: Zonation, Succession and Change in Ecosystems — from the global pattern down to how one patch of ground changes over time.
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