Chapter 1 of 8

All 8 chapters
  1. 1The air and its layers, readunderstood
  2. 2Weather and air pressure, readunderstood
  3. 3Winds, monsoons and storms, readunderstood
  4. 4Climate zones, readunderstood
  5. 5Air pollution, readunderstood
  6. 6The greenhouse effect, readunderstood
  7. 7Climate change, readunderstood
  8. 8Responding to climate change, readunderstood

S4.14.1 Chapter 1

The air and its layers

Mei climbs a high mountain and finds it hard to breathe. What is the air made of, and why is there less of it up there?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

  • You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Give the rough share of each gas in dry air
  • Name the layers of the atmosphere in order
  • Explain how the ozone layer protects life from ultraviolet

The atmosphere is the layer of gases that wraps round the Earth. We live at the bottom of it. Gravity holds it in place, as it holds the sea. Air looks like nothing, but it is a mixture of several gases.

Scientists give the make-up of dry air. That means air with its water vapour left out, because the amount of water vapour changes from place to place. It can be almost 0 % over a desert and up to about 4 % in hot, damp air.

Here is dry air as a hundred squares. One square is one percent, so each gas gets as many squares as its percent.

A square of one hundred small squares in ten rows of ten. One square stands for 1 %. Dotted squares are nitrogen: 78 squares. Hatched squares are oxygen: 21 squares. One black square stands for all the other gases together. Labels: nitrogen 78 %, oxygen 21 %, others 1 %.nitrogen 78 %oxygen 21 %others 1 %
Dry air. The one black square is about 0.9 % argon, 0.04 % carbon dioxide and traces of other gases.

Nitrogen is about 78 % of dry air. Oxygen is about 21 %. Argon, a gas that does not react with anything, is about 0.9 %. Carbon dioxide is only about 0.04 %. It is so small that it does not fill even one square.

Scientists often count a very small share in parts per million, or ppm. One percent is 10 000 ppm. In 2023 the air held about 420 ppm of carbon dioxide. That means 420 molecules in every million. A small share, but later chapters show that it matters a great deal.

Worked example

Out of 20 000 molecules of dry air, about how many are oxygen? Use 21 %.

  1. A share of 21 % means 21 in every 100.
  2. 20 000 molecules is 200 lots of 100.
  3. So 200 × 21 = 4 200 molecules.

Answer: 4 200 molecules

Worked example

Carbon dioxide is 420 ppm. Write that as a percent.

  1. 420 ppm means 420 parts in every 1 000 000.
  2. A percent is parts in every 100. Count the hundreds in a million: 1 000 000 ÷ 100 = 10 000.
  3. So divide by 10 000: 420 ÷ 10 000 = 0.042 %.

Answer: 0.042 %

The air is thickest at the ground, because the air above is pressing down. As you climb, there is less air above you, so the air is thinner and the air pressure is lower. The picture shows how fast it falls.

A graph with height in km along the bottom, from 0 to 12 in steps of 2, and air pressure in hPa up the side, from 0 to 1 200 in steps of 200. Seven dots are joined by straight lines. The line starts high at the left and falls steeply at first, then more gently. Labels: height in km, air pressure in hPa.246810122004006008001 0001 2000height in kmair pressure in hPa
Air pressure falls with height. By about 5.5 km, half of the air at sea level is below you. The share of oxygen is still 21 %. There are just fewer molecules in each breath.

This is why Mei finds it hard to breathe. At the top of a high mountain each breath holds fewer oxygen molecules, even though the air is still 21 % oxygen.

The atmosphere has four main layers, one above the other. Look at how the temperature changes as you go up. It falls, then rises, then falls again, and then rises. The turns mark the edges of the layers. The lines are straightened, to show the pattern.

Draw it first

How the temperature changes with height

  1. A graph with temperature in °C along the bottom, from −100 to 20, and height in km up the side, from 0 to 90. Step 1: from 15 °C at the ground the line goes up and to the left, falling to −56 °C at 11 km, labelled falls. Step 2: the line then goes up and to the right, rising to −3 °C at 47 km, labelled rises. Step 3: the line then goes up and to the left again, falling to −86 °C at 86 km, labelled falls. Labels: temperature in °C, height in km, falls, rises, falls.falls−80−402060800temperature in °Cheight in km

    1 In the lowest layer the air gets colder as you go up. At about 11 km it is about −56 °C.

  2. A graph with temperature in °C along the bottom, from −100 to 20, and height in km up the side, from 0 to 90. Step 1: from 15 °C at the ground the line goes up and to the left, falling to −56 °C at 11 km, labelled falls. Step 2: the line then goes up and to the right, rising to −3 °C at 47 km, labelled rises. Step 3: the line then goes up and to the left again, falling to −86 °C at 86 km, labelled falls. Labels: temperature in °C, height in km, falls, rises, falls.fallsrises−80−402060800temperature in °Cheight in km

    2 In the next layer the air gets warmer as you go up. A gas called ozone takes in ultraviolet from the Sun there. At about 50 km it is near 0 °C.

  3. A graph with temperature in °C along the bottom, from −100 to 20, and height in km up the side, from 0 to 90. Step 1: from 15 °C at the ground the line goes up and to the left, falling to −56 °C at 11 km, labelled falls. Step 2: the line then goes up and to the right, rising to −3 °C at 47 km, labelled rises. Step 3: the line then goes up and to the left again, falling to −86 °C at 86 km, labelled falls. Labels: temperature in °C, height in km, falls, rises, falls.fallsrisesfalls−80−402060800temperature in °Cheight in km

    3 In the third layer the air gets colder again, to about −86 °C at 86 km. Above that it warms again.

Now we can name the layers. The troposphere is the lowest layer, from the ground to about 12 km. All weather happens here, and about three quarters of all the air is here. It is thinner over the poles, about 8 km, and thicker over the tropics, about 18 km.

Above it is the stratosphere, to about 50 km. It holds the ozone layer. Next comes the mesosphere, to about 85 km, the coldest layer. Most meteors burn up there. Above is the thermosphere, from about 85 km out to about 600 km. The air there is so thin that it hardly feels warm, even though its temperature is high. The edges between layers are not sharp, and the heights change a little with place and season.

A tall column on a height scale in km from 0 to 100 in steps of 10. The scale is the same all the way up. From the bottom: the troposphere, dotted, from 0 to 12 km. The stratosphere, plain, from 12 to 50 km. Inside the stratosphere a cross hatched band, the ozone layer, from 15 to 35 km. The mesosphere, hatched, from 50 to 85 km. The thermosphere, plain, from 85 km, with an arrow up off the top to show it goes on to about 600 km. Labels: km, troposphere 0 to 12 km, ozone layer 15 to 35 km, stratosphere 12 to 50 km, mesosphere 50 to 85 km, thermosphere 85 to 600 km.0102030405060708090100kmtroposphere0 to 12 kmozone layer15 to 35 kmstratosphere12 to 50 kmmesosphere50 to 85 kmthermosphere85 to 600 km
The layers on one scale. The troposphere, where we live and where the weather is, is the thin bottom slice.
A table with the columns Height and What is there. Row 1: 8.8 km, highest peak. Row 2: 10 to 12 km, passenger jets. Row 3: 20 to 25 km, ozone, middle. Row 4: 80 to 100 km, meteors burn. Row 5: 400 km, space station.HeightWhat is there8.8 kmhighest peak10 to 12 kmpassenger jets20 to 25 kmozone, middle80 to 100 kmmeteors burn400 kmspace station
Things at different heights. A space station circles in the thermosphere.

The sunlight that reaches the Earth has more than light in it. It also has ultraviolet light, or UV. It is a kind of wave just beyond violet light, and we cannot see it. UV has more energy than visible light. It can burn skin, damage the eyes and harm the cells of plants and tiny sea creatures.

Experts split ultraviolet into three kinds. UV-A has the least energy. UV-B has more. UV-C has the most and is the most harmful. A little UV is useful, because skin uses it to make vitamin D. But too much is harmful.

In the stratosphere there is a thin band of ozone, a gas whose molecule is three oxygen atoms joined, O₃. It is spread through the air between about 15 and 35 km. The ozone layer takes in nearly all UV-C and most UV-B. Most UV-A and all of the visible light go through.

Four arrows come down from the top of the picture, labelled UV-C, UV-B, UV-A and light. A cross hatched band across the picture is labelled ozone layer. The UV-C arrow stops at the top of the band. The UV-B arrow stops at the band too, and a short dashed arrow under the band shows the little that gets through to the ground. The UV-A arrow and the light arrow go straight through the band to the ground, which is hatched at the bottom and labelled ground. Labels: UV-C, UV-B, UV-A, light, ozone layer, ground.UV-CUV-BUV-Alightozone layerground
A solid arrow is most or all of it going on. A dashed arrow is a little. The ozone layer stops UV-C, stops most UV-B, and lets most UV-A and nearly all the visible light through.

How does the ozone layer form? Sunlight splits an oxygen molecule into two oxygen atoms. Each atom can join a whole oxygen molecule to make ozone. Ozone takes in UV, and in doing so it splits again, and the pieces can rejoin. This cycle goes on all the time, and each turn of it takes in UV.

See it three ways

As a picture

A table with the columns Step and What happens. Row 1: 1, UV splits O₂. Row 2: 2, O + O₂ makes O₃. Row 3: 3, O₃ takes in UV.StepWhat happens1UV splits O₂2O + O₂ makes O₃3O₃ takes in UV

In words

UV splits an oxygen molecule into two atoms. An atom joins another oxygen molecule to make ozone. Ozone takes in UV and splits. Each time UV is taken in, so less reaches the ground.

In symbols

O₂ + UV → O + O. O + O₂ → O₃. O₃ + UV → O₂ + O.

In the 1970s scientists showed that some gases that people made, called CFCs, could destroy ozone. CFCs were used in fridges and spray cans. In 1985 a very thin patch, the ozone hole, was found over the south pole in the southern spring. It is a patch where there is much less ozone, not a real hole. In 1987 countries signed an agreement to cut CFCs, and a few years later they agreed to stop making them. The ozone layer is now slowly recovering. The United Nations reports that it should be back to its 1980 level about 2040 for most of the world.

Try it

A model of the atmosphere on a strip of string

You need: a long piece of string, about 7 m, a ruler or a tape measure, a pen or small bits of tape, a long room, a yard or a corridor.

Be safe: Keep the string off the floor of a room where people walk, so no one trips. Work in a room or yard with room to move.

  1. Let 1 cm on the string stand for 10 km. Lay the string out straight.
  2. Mark the string every 10 cm, starting at one end. The first mark is for 100 km, the edge of space.
  3. Mark where the stratosphere starts, at 1.2 cm from the end. Mark where the mesosphere starts, at 5 cm.
  4. Now measure how far you need to go to show the Earth’s radius, 6 371 km. Walk to the end of the string.

What you will see: The layer that holds the weather is only about 1 cm on your string. Even the first 100 km is only 10 cm. The Earth’s radius is over 6 metres on the same scale. Next to the Earth, the atmosphere is a very thin skin.

If you cannot try it, this is what happens.
A table with the columns Height and On the string. Row 1: 12 km, 1.2 cm. Row 2: 100 km, 10 cm. Row 3: 6 371 km, 637.1 cm, which is a little over 6 m.HeightOn the string12 km1.2 cm100 km10 cm6 371 km637.1 cm

A wrong idea many people have

Air is nearly all oxygen.

Why it feels right: Oxygen is the gas we need to breathe, and fire needs it, so it seems it must be the main gas.

The test that breaks it: Look at the hundred squares. Oxygen has 21 squares. Nitrogen has 78. If you breathe in 100 molecules of air, about 78 are nitrogen. Your body breathes most of that nitrogen straight out again.

What is true: Nitrogen is the main gas in air, about 78 %. Oxygen is about 21 %.

A wrong idea many people have

The higher you go, the closer you are to the Sun, so the warmer it gets.

Why it feels right: We think of the Sun as the source of heat, so being nearer to it seems to mean being warmer.

The test that breaks it: Look at the temperature graph. In the lowest layer, which is where mountains are, the air gets about 6.5 °C colder with every km. The ground absorbs sunlight and warms the air above it. The air higher up is far from that warm ground. The Sun is about 150 million km away, so 10 km makes no difference to the Sun’s distance.

What is true: In the troposphere the air gets colder as you go up. Higher, in the stratosphere, it warms again, but for a different reason: ozone takes in ultraviolet.

A wrong idea many people have

Ozone is good wherever it is.

Why it feels right: We hear that the ozone layer protects us, so ozone seems always to be helpful.

The test that breaks it: Ozone high in the stratosphere takes in UV. Ozone near the ground, made when sunlight acts on fumes from vehicles and factories, hurts the lungs and plants. It is a pollutant there.

What is true: Ozone high up is a shield. Ozone at ground level, where we breathe, is a pollutant. It is the same molecule, O₃, in a different place.

Key words

atmosphere
the layer of gases that surrounds the Earth.
air pressure
the push of the air on everything it touches. It is lower higher up.
troposphere
the lowest layer, from the ground to about 12 km. It holds the weather.
stratosphere
the layer above, to about 50 km. It holds the ozone layer.
mesosphere
the layer above that, to about 85 km. It is the coldest layer.
thermosphere
the top layer, from about 85 km out to about 600 km. The air there is very thin.
ultraviolet
(UV) light just beyond violet that we cannot see. Too much harms skin and eyes.
ozone
a gas with molecules of three oxygen atoms, O₃. The ozone layer takes in most UV.

Back to your guess

The answer is B: Air is mostly nitrogen, and the air is thinner up there, so each breath holds less oxygen.

About four fifths of the air is nitrogen, and about one fifth is oxygen. The air is thinner high up, so each breath holds fewer molecules of oxygen. The share of oxygen has not changed. The number in each breath has.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Which gas is the biggest part of dry air?

    Tap your answer.

    1. Not this one. Oxygen is the gas we use, so it feels like the main one. It is about 21 %, about a fifth. Go to S2.4.2. Try another.

    2. Not this one. Carbon dioxide is a tiny part of the air, about 0.04 %. It matters for the climate, but it is very little. Try another.

    3. Not this one. Argon is the third biggest gas, but it is only about 0.9 %. Try another.

    4. Yes, that is right. Dry air is about 78 % nitrogen, 21 % oxygen, 0.9 % argon and 0.04 % carbon dioxide.

    See the answer

    Nitrogen

    Dry air is about 78 % nitrogen, 21 % oxygen, 0.9 % argon and 0.04 % carbon dioxide.

    • If you chose A: Oxygen is the gas we use, so it feels like the main one. It is about 21 %, about a fifth. Go to S2.4.2.
    • If you chose B: Carbon dioxide is a tiny part of the air, about 0.04 %. It matters for the climate, but it is very little.
    • If you chose C: Argon is the third biggest gas, but it is only about 0.9 %.

  2. Carbon dioxide is about 0.04 % of dry air. Out of 10 000 molecules of dry air, about how many are carbon dioxide?

    Tap your answer.

    1. Yes, that is right. 0.04 % is 0.04 in every 100. Out of 10 000 molecules, that is 0.04 ÷ 100 × 10 000 = 4. It matches 420 ppm: 4.2 in every 10 000.

    2. Not this one. That would be 4 %. A share of 0.04 % is a hundred times smaller than 4 %. Try another.

    3. Not this one. 40 in 10 000 would be 0.4 %. The share here is ten times smaller. Try another.

    4. Not this one. 0.4 in 10 000 would be 0.004 %. Work it out as 0.04 ÷ 100 × 10 000. Try another.

    See the answer

    4

    0.04 % is 0.04 in every 100. Out of 10 000 molecules, that is 0.04 ÷ 100 × 10 000 = 4. It matches 420 ppm: 4.2 in every 10 000.

    • If you chose B: That would be 4 %. A share of 0.04 % is a hundred times smaller than 4 %.
    • If you chose C: 40 in 10 000 would be 0.4 %. The share here is ten times smaller.
    • If you chose D: 0.4 in 10 000 would be 0.004 %. Work it out as 0.04 ÷ 100 × 10 000.

  3. Which list gives the layers of the atmosphere in order, from the ground up?

    Tap your answer.

    1. Not this one. The mesosphere and stratosphere are the wrong way round. The stratosphere comes first, then the mesosphere. Try another.

    2. Not this one. The troposphere is the lowest layer, where we live. It comes first. Try another.

    3. Yes, that is right. From the ground up they are the troposphere, the stratosphere, the mesosphere and the thermosphere. Their tops are at about 12 km, 50 km and 85 km.

    4. Not this one. That is the order from the top down. Start from the ground, with the troposphere. Try another.

    See the answer

    troposphere, stratosphere, mesosphere, thermosphere

    From the ground up they are the troposphere, the stratosphere, the mesosphere and the thermosphere. Their tops are at about 12 km, 50 km and 85 km.

    • If you chose A: The mesosphere and stratosphere are the wrong way round. The stratosphere comes first, then the mesosphere.
    • If you chose B: The troposphere is the lowest layer, where we live. It comes first.
    • If you chose D: That is the order from the top down. Start from the ground, with the troposphere.

  4. In which layer is the ozone layer?

    Tap your answer.

    1. Not this one. Ozone near the ground is only a trace, and it is a pollutant there. The ozone layer is higher up. Try another.

    2. Yes, that is right. The ozone layer lies in the stratosphere, between about 15 and 35 km up.

    3. Not this one. The mesosphere is above the ozone layer. It is the coldest layer. Try another.

    4. Not this one. The thermosphere is far above, from about 85 km. The ozone layer is much lower, at about 15 to 35 km. Try another.

    See the answer

    The stratosphere

    The ozone layer lies in the stratosphere, between about 15 and 35 km up.

    • If you chose A: Ozone near the ground is only a trace, and it is a pollutant there. The ozone layer is higher up.
    • If you chose C: The mesosphere is above the ozone layer. It is the coldest layer.
    • If you chose D: The thermosphere is far above, from about 85 km. The ozone layer is much lower, at about 15 to 35 km.

  5. What does the ozone layer do for life on the ground?

    Tap your answer.

    1. Not this one. That is the job of greenhouse gases, which a later chapter explains. The ozone layer takes in ultraviolet. Try another.

    2. Not this one. Ozone does not make the air we breathe. Oxygen comes from plants and tiny sea plants. Try another.

    3. Not this one. Visible light goes through the ozone layer. If it did not, the ground would be dark. Go to S4.10.7. Try another.

    4. Yes, that is right. The ozone layer takes in nearly all the UV-C and most of the UV-B. This protects skin, eyes, plants and sea life. Visible light goes through.

    See the answer

    It takes in nearly all UV-C and most UV-B from the Sun.

    The ozone layer takes in nearly all the UV-C and most of the UV-B. This protects skin, eyes, plants and sea life. Visible light goes through.

    • If you chose A: That is the job of greenhouse gases, which a later chapter explains. The ozone layer takes in ultraviolet.
    • If you chose B: Ozone does not make the air we breathe. Oxygen comes from plants and tiny sea plants.
    • If you chose C: Visible light goes through the ozone layer. If it did not, the ground would be dark. Go to S4.10.7.

  6. Why is it hard to breathe at the top of a very high mountain?

    Tap your answer.

    1. Not this one. The share is still about 21 %, up to about 80 km. It is the number of molecules in each breath that falls. Go to S3.7.8. Try another.

    2. Not this one. The shares of the gases stay the same as you climb. Nothing replaces the oxygen. Try another.

    3. Yes, that is right. The air pressure falls as you climb. Each breath holds fewer molecules, and so fewer oxygen molecules, even though the share of oxygen is the same.

    4. Not this one. Cold air can be breathed. Cold makes breathing feel sharp, but it does not make it hard to get oxygen. Try another.

    See the answer

    The air is thinner, so each breath has fewer oxygen molecules.

    The air pressure falls as you climb. Each breath holds fewer molecules, and so fewer oxygen molecules, even though the share of oxygen is the same.

    • If you chose A: The share is still about 21 %, up to about 80 km. It is the number of molecules in each breath that falls. Go to S3.7.8.
    • If you chose B: The shares of the gases stay the same as you climb. Nothing replaces the oxygen.
    • If you chose D: Cold air can be breathed. Cold makes breathing feel sharp, but it does not make it hard to get oxygen.

  7. Of 5 000 molecules of dry air, about how many are nitrogen? Use 78 %.

    See the answer

    3 900 molecules

    78 % is 78 in every 100. 5 000 is 50 lots of 100. So 50 × 78 = 3 900 molecules.

  8. On your string model, 1 cm stands for 10 km. The thermosphere starts at 85 km. How far from the ground end of the string is that, in cm?

    See the answer

    8.5 cm

    Each 10 km is 1 cm, so 85 km is 85 ÷ 10 = 8.5 cm.

Keep it

Copy this into your exercise book.

Dry air: 78 % nitrogen, 21 % oxygen, 0.9 % argon, 0.04 % carbon dioxide. Layers: troposphere (weather), stratosphere (ozone), mesosphere, thermosphere. Ozone takes in UV.

Draw: A tall column with four layers and their heights: 12, 50, 85 km. Mark the ozone layer inside the second one.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (1.0 MB). It opens on any phone.