Weather and climate are not the same thing.

Climate is the long view of the same thing. It is the average weather of a place, taken over a long stretch of time — by international agreement, at least thirty years. One cold July does not change Britain's climate any more than one bad mood changes a person's character. Climate is the pattern you only see when you stand well back: how warm a place tends to be across the year, how much rain it tends to get, and when in the year that rain tends to fall.

It all starts with the sun, hitting unevenly.

The single biggest fact about climate is this: the Sun does not warm the Earth evenly. Because the Earth is a ball, sunlight arrives almost straight down at the equator — and at a low, glancing angle near the poles. The same beam of sunlight, near the poles, is smeared across a much wider patch of ground, so its warmth is shared more thinly. That is why it is hot at the equator and cold at the poles. How far you are from the equator — your latitude — is the first thing that sets a place's temperature.

That uneven heating is the engine behind almost everything else in this lesson. Hot air at the equator, cold air at the poles — and air, like water, will not sit still when one part is hot and another is cold. It moves. Warm air rises, cool air sinks, and the whole atmosphere sets up great looping currents to carry heat from where there is too much of it to where there is too little. Those loops are called circulation cells, and they are why the world's climate zones sit in tidy bands.

Latitude isn't the whole story.

If latitude were everything, every place on the same line of latitude would share a climate — and they plainly don't. London and the frozen middle of Canada sit at almost the same latitude, yet London almost never freezes hard. Four more things bend a place's climate away from what its latitude alone would give.

The Gulf Stream is Britain's secret. It is a vast, slow river of warm water inside the Atlantic Ocean, carrying heat from the Gulf of Mexico up past Florida and across to the western edge of Europe. By the time it reaches us it has cooled, but it is still warm enough to keep the British air far milder than our latitude has any right to be. Without it, London's winters would look more like Labrador's, on the same line of latitude in Canada — frozen solid for months.

How a cloud is born.

Rain has one recipe, and it is always the same: warm, moist air has to rise. Everything else follows from that. Warm air near the ground holds a great deal of invisible water as vapour — the more it carries, the more humid the day feels. When that air is pushed upwards, three things happen in order.

First, the rising air cools — the higher you go, the colder the air, so the parcel of air loses heat as it climbs. Second, cool air can hold less water vapour than warm air, so the vapour is forced to turn back into tiny droplets of liquid — this is condensation, and millions of those droplets together are what a cloud is. Third, if the droplets keep growing and clumping, they get too heavy for the air to hold up — and they fall as rain.

Three kinds of rain.

The recipe never changes — air must rise to make rain. But there are three different ways to push that warm moist air upward, and each gives its own kind of rainfall. Knowing the three apart is a piece of proper Year 9 geography.

The climate graph.

A climate graph (or climograph) puts a place's whole year of climate on one sheet. It carries two things at once, month by month across the bottom:

• Bars for monthly rainfall — read off the right-hand axis, in millimetres.
• A line for monthly average temperature — read off the left-hand axis, in °C.

Here is a climate graph for London. Read it the way a geographer would: look at the line first, then the bars.

Read the line first. It is wavy, not flat — London has real seasons, warm in July and August (about 19 °C), cold in January (about 5 °C). Now the bars: they are moderate and fairly even, around 40 to 70 mm in every single month, with a touch more in autumn. Rain all year, no real dry season, a clear warm summer and cool winter — that is the signature of a mild temperate climate, exactly what Britain has.

Show what you know.

Read a new place.

Below is a climate graph for a tropical rainforest city, close to the equator. Use the same three-step reading you learned a moment ago — line first, then bars — and answer the two questions underneath. Read off the graph; the numbers you need are all on it.

Explain why.

This is the kind of question that rewards full sentences and developed points — say something, then say why, then say so what. Aim for 100–150 words. No bullet points.

Follow one drop of water.

Write the journey of a single drop of water — from the moment the sun lifts it off the warm Atlantic, up into the rising air, into the cold where it joins a cloud, and down again as rain on a Cumbrian hillside. About 200 words. Let the drop notice the cold, the crowd of other droplets, the long fall.

Pressure contrasts that drive our weather.

Notice how differences between high and low pressure move air around — that movement is the weather.

The words from today.

Weather

The state of the air at one place over a short time — hours or days.

Climate

The average weather of a place over a long time — by convention, at least thirty years.

Latitude

How far north or south of the equator a place is, in degrees — the first thing that sets its temperature.

Condensation

Water vapour cooling back into liquid droplets — how clouds and rain begin.

Relief rainfall

Rain made when moist air is forced up over high ground, cools and condenses on the windward side.

Rain shadow

The dry, sheltered side of a range, where the air has already dropped its rain and sinks down warm and dry.

Gulf Stream

A warm ocean current carrying tropical heat across the Atlantic, keeping Britain mild for its latitude.