Why is there a band of desert running right across North Africa, the Middle East and Australia at roughly the same latitude? Not coincidence. Air rises at the equator, drops its rain, and comes back down around 30 degrees bone dry. The tricellular model explains that, and once you have it, the world biome map stops looking random.
📘 What you need to know
Global atmospheric circulation moves solar heat energy from the equator towards the poles.
Wind is air moving from high pressure to low pressure.
Insolation is greatest at the equator because of the Earth’s curvature and tilt, so heating is uneven.
Uneven heating creates pressure cells in which warm air rises and cool air sinks.
Each hemisphere has three cells: Hadley, Ferrel and Polar.
Rising air brings rainfall; sinking air brings dry conditions.
Ocean currents move warm water poleward and cold water equatorward, moderating coastal climates.
Why the tropics get more energy
Sunlight arrives at the equator almost straight on, so its energy is concentrated into a small area. At high latitudes the same beam hits at a shallow angle and spreads over a much larger area, so each square metre receives less. That is the whole reason the poles are cold.
Uneven heating creates pressure differences, and air always flows from higher pressure to lower pressure. That flow is what we call wind.
The core rule
Warm air rises → low pressure → cloud and rain
Cool air sinks → high pressure → clear and dry
The tricellular model
Only the northern hemisphere is shown; the southern hemisphere mirrors it. Follow one loop all the way round and you can see where the air is rising and where it is coming back down.
The Hadley cell
The largest cell, running from the equator to between 30 and 40 degrees north and south. Trade winds blow from the tropics towards the equator. Where they meet, hot air rises and forms thunderstorms — the tropical rainstorms that keep rainforests wet. From the top of those storms the air flows poleward, cools, and sinks over the subtropics. That descending air is dry and cloudless, and it is warmed as it comes down. The result is the world’s belt of hot deserts.
The Ferrel cell
The middle cell, from the edge of the Hadley cell to roughly 60 to 70 degrees. It is the awkward one because it turns in the opposite direction to its neighbours, like a cog driven by the two cells either side. Air joins the sinking air of the Hadley cell, travels at low level to mid-latitudes, and rises where it meets the cold air of the Polar cell. That meeting is why mid-latitude weather is so changeable.
The Polar cell
The smallest and weakest cell, running from the edge of the Ferrel cell to the poles. Cold air sinks, creating high pressure over the highest latitudes. That cold air flows out towards lower latitudes at the surface, warms slightly, and rises to return at altitude.
Do not try to memorise the cells as three separate diagrams. Learn one rule — rising air is wet, sinking air is dry — then read the biome straight off the latitude. Rainforest at 0, desert at 30, temperate forest at 60, tundra at the pole.
How this shapes biomes
Latitude
What the air is doing
Climate
Biome
0 degrees
Rising (Hadley cell)
Hot, very wet
Tropical rainforest, savanna nearby
30 degrees
Sinking (Hadley cell)
Hot, very dry
Hot desert
60 degrees
Rising (Ferrel and Polar cells meet)
Moderate, unsettled
Temperate forest and grassland
90 degrees
Sinking (Polar cell)
Cold, dry
Polar desert and tundra
The model is useful for three things: understanding the global distribution of biomes, understanding their ecological characteristics, and predicting how biomes will shift as the climate changes.
Ocean currents
The oceans do a similar job, and they matter because water stores heat far better than air. Solar radiation is absorbed mainly in the top layer of the ocean, warming it and storing thermal energy — which makes oceans vast heat reservoirs.
Surface currents, driven by winds and the Earth’s rotation, then redistribute that heat:
Warm water moves from the equator towards the poles.
Cold water moves from the poles towards the equator.
The effect is to moderate temperature extremes. Warm currents bring milder conditions to coastal regions; cold currents cool them. This is why coastal climates are gentler than inland ones at the same latitude.
Currents matter for life too. Oceanic heat transport affects patterns of ocean productivity, where marine species are found, and overall marine biodiversity — not just the temperature on shore.
WORKED EXAMPLE
Explain, using the tricellular model, why the world’s major hot deserts are found at around 30 degrees north and south.
Step 1: Start at the equatorIntense insolation heats the surface, so warm air rises and forms rainStep 2: Follow the air polewardit has already lost its moisture as rainStep 3: What happens at 30 degreesThe dry air cools, sinks and creates high pressure; descending air warms and does not form cloudSinking dry air at 30 degrees gives clear skies and very low rainfall
WORKED EXAMPLE
Two cities sit at the same latitude. One is on a western coast washed by a warm current; the other is deep inside the continent. Suggest how their climates differ and explain why.
Step 1: The coastal cityThe ocean stores heat and releases it slowlymilder winters, cooler summers, more rainfallStep 2: The inland cityNo large water body nearby to buffer the temperaturehotter summers, colder winters, drierThe coast has a smaller annual temperature rangeThis is the maritime versus continental contrast
💡 Exam tip
Name the cells: Hadley, Ferrel, Polar. Generic answers about “air moving around” score poorly.
Always link pressure to weather: low pressure and rising air means rain, high pressure and sinking air means dry.
Mention insolation and the angle of incoming sunlight when explaining why heating is uneven.
For ocean currents, use the word moderate — currents reduce temperature extremes.
Finish by naming the biome. Linking circulation to a named biome is what turns a description into an explanation.
⚠ Common mix-up
Saying the equator is closer to the Sun. The difference is the angle of the sunlight, not the distance.
Getting the Ferrel cell direction wrong. It turns the opposite way to the other two, driven by them.
Assuming sinking air is cold at the surface. Descending air warms as it comes down, which is why deserts are hot.
Confusing pressure with temperature. High pressure is about sinking air, not necessarily cold air.
Thinking currents only affect the sea. They strongly affect the climate of the land they flow past.
Up next: Classifying the World’s Climates (HL) — tropical, temperate and polar, their subtypes, and why the predicted biome does not always appear.
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