Cambridge Primary Science · Stage 4 · Earth and Space
A blanket of gases wrapped all the way around the Earth. It is where every cloud forms, every bird flies and every breath you take comes from.
Start here
Hold out your hand. It looks like there is nothing there — but there is. Air is all around you, and air is a real material. It takes up space, it has mass, and you can feel it push against you when you run or when the wind blows.
The atmosphere is the huge layer of air that wraps around the whole Earth. Gravity holds it in place, the same way gravity holds you on the ground. Without gravity, the air would simply drift away into space.
The atmosphere is not the same all the way up. As you climb higher the air gets thinner — the gas particles spread further apart, so there is less air to breathe. Scientists divide the atmosphere into five layers, and each layer behaves in its own way.
Figure 1
Read this chart from the bottom up — that is the direction you would travel. Point at any layer to light it up.
Not to scale — the vertical scale is broken between layersabout 600 km → 10,000 km · the outer edge
The last, thinnest layer. The air here is so spread out that particles almost never bump into each other, and some drift off into space for ever. There is no wall and no line — the atmosphere simply fades away.
85 km → 600 km · over 1,500 °C — but you would still feel freezing
The hottest layer by temperature, because the few particles up here absorb strong energy from the Sun. But there are so few of them that they could hardly warm you at all. This is where the beautiful auroras glow.
50 km → 85 km · the coldest place on Earth, about −90 °C
This layer is Earth's shield. Millions of small rocks from space race towards us every day, and nearly all of them burn up here because of the friction with the air. We see the streaks of light and call them shooting stars.
12 km → 50 km · gets warmer as you go up
Dry, calm and almost cloud-free. It holds the ozone layer (about 15–35 km up), a thin band of gas that soaks up the Sun's harmful ultraviolet rays before they reach us. Weather balloons climb up into this layer, then burst.
ground level → about 12 km · gets colder as you go up
You live here, and so does all of the weather. Rain, snow, wind, thunder and nearly every cloud happen inside this bottom layer. It is the thinnest layer of the five, yet it holds about three quarters of all the air in the atmosphere, because gravity squashes the air down towards the ground.
Figure 1 has to stretch the layers so you can read them. Here is the same troposphere drawn at its true size against the Earth — every cloud, every storm and every aeroplane that has ever flown fits inside that thin orange line.
Try this size comparison. If the Earth shrank to the size of a football, the whole troposphere would be a skin about 0.2 mm thick — about two sheets of paper. That is all the air we have.
Figure 2
Air is not one gas — it is a mixture. Imagine you could catch exactly 100 tiny pieces of dry air in a jar. Count what you would find.
1 square = 1 piece in every 100
| Gas | Share of the air | In 100 pieces |
|---|---|---|
| Nitrogen | 78.08% | 78 pieces |
| Oxygen | 20.95% | 21 pieces |
| Argon | 0.93% | 1 piece |
| Carbon dioxide | 0.04% | far less than 1 |
| Neon, helium, methane and others | traces | far less than 1 |
Where is the water? Real air also carries water vapour — water as an invisible gas. It changes from almost none on a dry day to about 4 pieces in 100 in a hot, damp place, so scientists measure it separately from the gases above.
Why it matters
01
Every animal on Earth takes oxygen out of the air, and every plant takes carbon dioxide. No atmosphere, no life.
02
Gases in the air trap some of the Sun's heat. Without them Earth's average temperature would be about −18 °C instead of a comfortable 15 °C.
03
The ozone layer high in the stratosphere absorbs the Sun's harmful ultraviolet rays — the ones that would burn skin and damage plants.
04
Rocks from space hit the air at enormous speed and burn away in the mesosphere long before they can reach the ground.
05
The water cycle runs inside the troposphere. Water evaporates, rises, cools, condenses into clouds and falls again as rain or snow.
06
Sound needs a material to travel through, and air is that material. In space there is almost no air — so space is completely silent.
Thinking and working scientifically
Predict first, then test. Before you start each investigation, write down what you think will happen and why. Comparing your prediction with what really happens is what scientists do.
You need: a clear plastic cup · a paper towel · a deep bowl of water
What happens: the paper stays dry. Air is trapped inside the cup, and the water cannot get in because the air is already taking up that space. When you tip the cup, you see the air escape as bubbles — and only then does the water rush in and soak the paper.
You need: 2 identical balloons · a ruler or stick · string · a pencil
What happens: the side with the inflated balloon dips down. The only thing you added was air — so air has mass. It is a real material, just like water or wood, even though you cannot see it.
Check what you know
Nitrogen — about 78%, or roughly 4 pieces in every 5. Many people guess oxygen, but oxygen is only about 21%.
The troposphere — the bottom layer, from the ground up to about 12 km. It holds about three quarters of all the air and nearly all the water vapour.
Coldest: the mesosphere, about −90 °C. Hottest: the thermosphere, over 1,500 °C — but the air there is so thin that an astronaut would still feel freezing cold.
They are small rocks from space, called meteors, burning up as they rub against the air in the mesosphere. The atmosphere destroys them before they can reach the ground.
It absorbs the Sun's harmful ultraviolet rays. It sits in the stratosphere, roughly 15–35 km above the ground.
No. The air just gets thinner and thinner and fades away. Scientists agreed to call 100 km the edge of space so that everyone measures the same way — but there is no wall there, and a little air carries on for thousands of kilometres.
Keep going
A spoken lesson you answer with your pen. Draw the air around the Earth, colour a jar of 100 pieces, mark the ozone layer and write your answers — then save the sheet for your teacher.
Start the lesson → Four gamesStack the five layers, build air from 100 pieces, burn up meteors in the mesosphere, and fly a weather balloon to 35 km.
Play →Word bank
For the teacher
By the end of the lesson, learners can:
Curriculum fit: Earth and Space, with Thinking and Working Scientifically covered through predicting, observing and explaining in the two investigations. Objective reference codes differ between framework versions — map the objectives above to the codes in your current Cambridge Primary Science framework before you plan the lesson.
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