Climate is not a list of facts about places. It is the outcome of a handful of physical processes acting on a spinning, tilted, unevenly heated planet. Learn the processes and you can explain why deserts sit where they do, why the tropics are wet, and why any change to the energy balance changes everything downstream.
📚 What you need to know
Weather is short term, days to weeks. Climate is the long-term average, typically over 30 years.
Climate is produced by solar radiation, atmospheric circulation, convection, evaporation, condensation and cloud formation, precipitation and the greenhouse effect.
The Sun heats the Earth unevenly: energy is concentrated at the equator and spread thin near the poles.
That temperature difference drives the tricellular model — Hadley, Ferrel and Polar cells — which moves heat and moisture from equator towards poles.
Where air rises you get cloud and rain; where air sinks you get dry conditions. This is why rainforests sit near the equator and deserts near 30°.
The main things that define a region’s climate are seasonal patterns of temperature and precipitation.
Weather is what you get, climate is what you expect
A cold week in a warming decade is not evidence against climate change, and one hot day is not evidence for it. Weather is the day-to-day state of the atmosphere, shaped by temperature, humidity and air pressure right now. Climate is the average of that weather over decades, and it is what determines the vegetation, agriculture and biodiversity a region can support.
The line to remember
climate is what you expect — weather is what you get
Why the Sun does not heat us evenly
This is the root cause of every circulation pattern you will study. The Earth is a sphere, so sunlight strikes the equator almost head-on but hits high latitudes at a shallow angle. The same amount of energy is spread over a much larger area, and it also passes through more atmosphere on the way in, so more is absorbed or scattered before it arrives.
Nothing about the Sun changes across the diagram. Only the angle changes — and that is enough to set up the entire global climate system.
The tricellular model
Warm air at the equator rises. As it rises it cools, water vapour condenses, clouds form and heavy rain falls — which is why equatorial regions are wet. The now-dry air moves polewards high in the troposphere, cools further, and sinks at around 30° latitude. Sinking air warms as it descends, so it holds its moisture instead of releasing it, and the result is the belt of great deserts.
That loop is the Hadley cell. Two more cells — the Ferrel and Polar cells — complete the pattern in each hemisphere, and together they carry heat from the equator towards the poles.
Rainforest at 0°, desert at 30°, temperate rain at 60°, polar desert at 90°. The biome map is really a map of where air rises and sinks.
If you can say “air rises, cools, condenses, rains” and “air sinks, warms, holds its moisture”, you can explain the position of nearly every major biome on Earth. Two sentences, enormous payoff.
The processes, one at a time
Process
What happens
Why it matters for climate
Solar radiation
Energy from the Sun reaches the surface, varying with latitude, tilt and time of day
The energy source for everything else; the uneven heating creates the temperature gradients that drive circulation
Atmospheric circulation
Air moves in large cells, shaped by solar heating and the Earth’s rotation
Transports heat and moisture between latitudes and sets global wind belts
Convection currents
Warm air rises, cool air sinks
Produces vertical mixing, and with it thunderstorms and tropical cyclones
Evaporation
Liquid water absorbs heat and becomes vapour, which rises
Moves both water and energy into the atmosphere; the start of the water cycle
Condensation and cloud formation
Rising vapour cools and condenses into droplets or ice crystals
Makes clouds, which reflect incoming sunlight but also absorb outgoing infrared
Precipitation
Droplets or crystals grow heavy and fall as rain, snow, sleet or hail
Determines water availability, and so vegetation, agriculture and biome type
Greenhouse effect
Gases absorb outgoing infrared and re-emit it in all directions
Keeps the surface warm enough for life; strengthening it raises average temperatures and disturbs every process above
Clouds cut both ways. A bright cloud top reflects sunlight straight back to space, which cools. The same cloud absorbs infrared rising from the surface, which warms. Which effect wins depends on the cloud’s height and thickness — and it is one of the hardest things to model in climate science. Mentioning this shows real understanding.
Worked examples
WORKED EXAMPLE
Explain why most of the world’s large deserts are found close to 30° north and south. [4]
Step 1: start at the equator
Intense solar radiation heats the surface, air rises, cools and its water vapour condenses, so heavy rain falls near the equator.
Step 2: follow the air
That air is now dry. It travels polewards high in the troposphere and sinks at about 30° latitude.
Step 3: what sinking does
Sinking air is compressed and warms, so it can hold more moisture rather than releasing it. Cloud formation is suppressed.
Step 4: the outcomeVery little precipitation falls at these latitudes, year after year, so arid conditions and desert biomes develop.Rising air at 0° = rain; sinking air at 30° = desert
WORKED EXAMPLE
A newspaper reports the coldest March day in 40 years and asks whether global warming has stopped. Comment on this claim. [3]
Name the error
The report confuses weather with climate. One cold day is a short-term atmospheric condition.
Give the definition
Climate is the average of weather over a long period, usually about 30 years, so single events cannot show a climate trend.
Explain what evidence would countLong temperature records show fluctuations within an overall upward trend. Individual cold records still occur; they are just becoming less frequent than hot ones.A weather event cannot disprove a climate trend
💡 Exam tip
Define weather and climate with a timescale in each definition. Without the timescale, the definitions are worth little.
For circulation questions, always start from uneven heating. It is the first marking point almost every time.
Use the chain “rises, cools, condenses, rains” — examiners are looking for the sequence, not the vocabulary alone.
Name the three cells: Hadley, Ferrel and Polar. Knowing which sits where is quick, easy credit.
Remember that the greenhouse effect belongs on this list of processes. It interacts with all the others rather than sitting apart from them.
⚠ Common mix-up
Confusing weather and climate. The most commonly lost mark in the whole topic.
Thinking the equator is hot because it is closer to the Sun. The difference in distance is negligible; it is the angle of the beam that matters.
Saying rising air warms. Rising air expands and cools; sinking air compresses and warms. Getting this backwards ruins the desert explanation.
Assuming clouds only cool. They reflect sunlight but also trap infrared, so the net effect depends on the cloud.
Treating seasons as a distance effect. Seasons come from the Earth’s tilt changing the angle and day length, not from the orbit’s shape.
Up next: What Is Causing Climate Change — the evidence from ice cores and tree rings, why the recent rise is different from every natural cycle before it, and the feedback loops making it worse.
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