Climate is not a list of facts about places. It is the result of a few physical processes shifting heat and water around the planet, over and over. Once you can see how the Sun heats the Earth unevenly, and what the air does about it, the rest of this topic stops being memorising and starts being explaining.
📚 What you need to know
Weather is what the air is doing over hours or days. Climate is the average of that weather over a long period, usually 30 years or more.
The Sun heats the equator far more strongly than the poles. That difference is the engine behind everything else in this page.
The atmosphere moves that surplus heat away from the equator through circulation cells (Hadley, Ferrel and Polar).
Convection, evaporation, condensation and precipitation carry energy as well as water — the water cycle is also a heat transport system.
The greenhouse effect is natural and necessary. Without it the surface would average roughly −18 °C instead of about 15 °C.
The climate of any region is mostly set by its seasonal pattern of temperature and rainfall, and that pattern decides which biome grows there.
Start with the real cause: uneven heating
Every single process in this topic traces back to one thing. Sunlight arrives at the Earth in near-parallel beams, but the Earth is a sphere. Near the equator a beam hits the surface almost face-on, so its energy is delivered to a small patch of ground. Near the poles the same beam hits at a shallow angle and is smeared over a much bigger patch. Same energy, bigger area, so less energy per square metre.
There is a second effect on top of that. A slanted beam has to travel through more atmosphere before it lands, so more of it is scattered or absorbed on the way down. Both effects push in the same direction: the tropics get a lot, the poles get very little.
The Sun is not shining harder on the tropics. The tropics are simply catching the light at a better angle, and that alone sets up the temperature gradient that drives the whole climate system.
Examiners like the phrase “energy per unit area”. If you write “the equator is closer to the Sun” you will lose the mark — the difference in distance is tiny and irrelevant. It is the angle that matters.
The atmosphere moves the surplus heat
You now have a planet with too much heat at the equator and not enough at the poles. Nature does not leave that alone. Warm air near the equator expands, becomes less dense and rises. As it rises it cools, its water vapour condenses, and it dumps enormous rain. That rising air has to go somewhere, so it spills polewards high in the atmosphere, cools further, and sinks back down at about 30° north and south.
That single loop is the Hadley cell. Two more loops sit behind it in each hemisphere — the Ferrel cell (about 30° to 60°) and the Polar cell (60° to 90°). The Earth’s rotation bends the moving air sideways, which is why surface winds blow at an angle rather than straight north or south.
The Sahara, the Arabian, the Kalahari and the Australian deserts all sit near 30°. They are dry because they sit under the sinking arm of the Hadley cell, not because they happen to be far from the sea.
Link it to biomes. If a question asks why a biome is where it is, the circulation cells are usually half the answer. Rainforest at the equator, desert at 30°, temperate forest and grassland near 50°–60°, tundra and ice at the pole.
Water does a lot of the heavy lifting
Air is not the only thing carrying energy. When water evaporates it absorbs a large amount of heat and stores it invisibly as latent heat. That water vapour can then travel thousands of kilometres before it condenses into cloud, and when it condenses it releases all of that heat again, somewhere completely different. The water cycle is therefore also a global heat pipeline.
Process
What is physically happening
What you see in the climate
Solar radiation
Energy from the Sun reaches the surface, unevenly because of the Earth’s curved shape and its tilt
Hot tropics, cold poles, and seasons as the tilt changes which hemisphere leans towards the Sun
Convection
Warm air is less dense, so it rises; cooler air sinks to replace it
Afternoon thunderstorms, tropical cyclones, and the vertical arms of the circulation cells
Atmospheric circulation
Rising and sinking air joined up into looping cells, bent sideways by the Earth’s spin
Trade winds, westerlies, jet streams, and the belts of wet and dry climate
Evaporation
Liquid water absorbs heat and becomes vapour, taking the energy with it
Cooling of oceans and land surfaces; humidity feeding into storm systems
Condensation
Vapour cools, turns back to droplets or ice crystals, and releases its stored heat
Cloud formation; clouds then reflect sunlight away and also trap outgoing heat
Precipitation
Droplets or crystals grow heavy enough to fall out of the cloud
Rain, snow, sleet or hail, and the rainfall pattern that defines a region’s climate
Greenhouse effect
Gases in the atmosphere absorb outgoing infrared and re-emit some of it downwards
A surface roughly 33 °C warmer than it would otherwise be — warm enough for liquid water
The natural greenhouse effect
Sunlight arrives mostly as short-wave radiation. Short waves slide straight through the atmosphere without much being absorbed. The ground soaks that energy up, warms, and then radiates its own heat back out — but a warm surface radiates in the infrared, which is long-wave. Greenhouse gases such as carbon dioxide, methane and water vapour are good at absorbing long-wave radiation and poor at absorbing short-wave. So energy finds it easy to get in and harder to get out.
The absorbed infrared is re-emitted in all directions, and roughly half of it heads back down. The surface therefore receives energy twice: once from the Sun, once from the atmosphere. That is the greenhouse effect, and it is entirely natural.
Stable climate condition
energy absorbed from the Sun = energy radiated back to space
Notice that nothing here is pollution. This is the ordinary state of the atmosphere and it is the reason the planet is habitable. The problem in the next page is that we have made step 4 stronger.
Weather is not climate
This trips people up in every exam session. Weather is short term — hours to a couple of weeks — and it is what you actually experience: today’s rain, this week’s cold snap. Climate is the long-run statistics of that weather for a place, normally averaged over 30 years, and it is what decides whether a region grows olives or spruce trees.
A cold winter therefore proves nothing about climate change, in the same way that one tall student proves nothing about the average height of a school. The useful sentence to memorise: climate is what you expect, weather is what you get.
WORKED EXAMPLE
Explain why the equator receives more solar energy per square metre than the poles. [3]
Mark point 1 — the shape
The Earth is curved, so sunlight strikes the equator almost at right angles but strikes polar regions at a shallow, glancing angle.
Mark point 2 — the consequenceA beam of a given width is spread over a much larger surface area at high latitudes.Mark point 3 — the extra bit that earns the third markSlanted beams also pass through a greater thickness of atmosphere, so more energy is reflected, scattered or absorbed before it lands.3 / 3Notice the answer never mentions distance from the Sun. It cannot — that is not the reason.
WORKED EXAMPLE
Outline how atmospheric circulation explains the location of hot deserts at around 30° north and south. [3]
Step 1 — start at the equator
Strong heating makes air rise. As it rises it cools, water vapour condenses and heavy rain falls, so the air loses most of its moisture.
Step 2 — follow the airThis now-dry air moves polewards high up, cools further and sinks back to the surface at about 30°.Step 3 — what sinking air doesSinking air is compressed and warms, which lowers its relative humidity, so cloud cannot form and rainfall is very low.3 / 3This chain — rise, rain out, travel, sink, dry — is worth learning as one sentence.
💡 Exam tip
Answer “why” questions as a chain: cause, then consequence, then result. One arrow per mark is a good rule of thumb.
Use the phrase energy per unit area when explaining unequal heating. It is the wording examiners look for.
If a question gives you a location, check its latitude first. Latitude usually tells you which circulation cell it sits under.
Remember that clouds do two opposite things: they reflect incoming sunlight (cooling) and trap outgoing infrared (warming). Saying so shows real understanding.
Be precise with short-wave in, long-wave out. Mixing these up will cost you the greenhouse effect marks every time.
Learn one line for weather versus climate and use it whenever the command word is “distinguish”.
⚠️ Common mix-up
“The equator is closer to the Sun.” It is, by a tiny amount, and it makes no meaningful difference. The angle of the incoming beam is the reason.
Saying the greenhouse effect is bad. The natural greenhouse effect keeps the planet liveable. It is the enhanced effect that causes the problem.
Describing greenhouse gases as “trapping” heat like a lid. They absorb infrared and re-emit it in all directions — some down, some up. Heat still escapes, just more slowly.
Confusing the greenhouse effect with ozone depletion. Different gases, different part of the spectrum, different problem entirely.
Using one weather event as evidence. A single heatwave or blizzard is weather. Climate claims need long records.
Forgetting the seasons come from tilt, not from the Earth’s distance from the Sun changing over the year.
Up next: What Is Causing Climate Change — how we know the recent warming is ours, and what the ice cores, tree rings and sediments actually tell us.
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