The atmosphere is a thin skin of gas between us and space, and almost all of it is two gases that do very little to the climate. The gases that matter most for warming, weather and life make up less than one part in a hundred. That imbalance is the whole point of this page.
📚 What you need to know
Dry air is about 78% nitrogen and 21% oxygen. The remaining 1% is mostly argon, with about 0.04% carbon dioxide and traces of methane, ozone and nitrous oxide.
Water vapour is left out of those figures because it varies — from almost nothing over a desert to around 4% in warm humid air.
Gases are redistributed by wind, convection, diffusion, turbulence and jet streams.
The atmosphere is divided into layers by how temperature changes with height. The two that matter for living systems are the troposphere (0–10 km) and the stratosphere (10–50 km).
The troposphere holds the weather and most of the water vapour and carbon dioxide. The stratosphere holds the ozone layer, which absorbs harmful UV.
The atmosphere is a system: storages of gas, flows driven by circulation, natural and human inputs, and outputs back to the other spheres.
The composition of dry air
The atmosphere is the outer limit of the biosphere — the boundary between Earth and space, and the reason life is possible here. Its composition has been remarkably steady for a long time, which is exactly why the recent change in one tiny component matters so much.
Every percentage you are asked to quote is for dry air. That is why the four figures add up to 100% with no room left for water.
Gas
Roughly how much
What it does
Nitrogen
78%
Largely unreactive in the air; enters living systems through nitrogen fixation, so it matters to life but not to climate
Oxygen
21%
Used in aerobic respiration and in combustion; produced by photosynthesis
Argon
0.93%
An inert gas — it takes part in no reactions and simply makes up the volume
Carbon dioxide
about 0.04%
The raw material for photosynthesis and a major greenhouse gas, despite the tiny share
Water vapour
0–4%, variable
Drives weather, clouds and precipitation, and is the most abundant greenhouse gas
Trace gases
tiny
Methane, ozone and nitrous oxide have effects on climate and atmospheric chemistry far larger than their concentration suggests
If a percentage looks too small to matter, ask what the gas does rather than how much of it there is. Carbon dioxide is 1 molecule in 2,500 and it still controls the temperature of the planet.
How gases get moved around
Gases do not stay where they are released. If they did, cities would sit under permanent domes of their own pollution. Five processes keep the atmosphere mixed:
Wind — the main mover, caused by differences in air pressure between places.
Convection — warm air rises and cool air sinks, giving vertical mixing.
Diffusion — gases spread from where they are concentrated to where they are not.
Turbulence — irregular flow stirred up by mountains, buildings and rough ground.
Jet streams — fast, narrow ribbons of air high in the atmosphere that carry material a long way, quickly.
Why this matters for pollution: mixing dilutes a pollutant, which usually reduces harm near the source — but it also carries the problem across borders. Acid deposition and long-range dust transport are both consequences of good mixing.
The layers, and why they are layers
The atmosphere is stratified into layers, and the boundaries are set by one thing: how temperature changes with height. Not composition, not pressure. Temperature.
The kink at about 50 km is the warmest point of the stratosphere. It sits above the ozone layer because that is where the UV is absorbed first.
Troposphere (surface to about 10 km)
The lowest layer, and the one you live in. Temperature falls with height.
Contains almost all the weather: clouds, precipitation, winds and the vertical mixing of gases.
Holds the highest concentrations of water vapour, carbon dioxide and other important trace gases.
Chemical reactions here between pollutants, greenhouse gases and particles control air quality and climate.
Stratosphere (about 10 to 50 km)
Sits above the troposphere, and temperature rises with height — the opposite trend.
Contains the ozone layer, which absorbs most of the Sun’s harmful ultraviolet radiation.
That absorption is what heats the layer, and it is what protects living things at the surface from UV damage.
Stable and calm, with little vertical mixing, which is why anything that gets up here stays for a long time.
Above these sit the mesosphere and thermosphere. You are unlikely to be asked about them in detail, but knowing they exist — and that each is named for another reversal in temperature — shows you understand the rule.
Differential heating
The Sun does not heat the Earth evenly. Sunlight strikes the equator almost head-on, so the energy is concentrated, and the surface and air there are warm. Nearer the poles the same beam is spread over a larger area and passes through more atmosphere, so those regions are cooler.
That temperature difference is the engine of the whole atmosphere. It drives the tricellular model of atmospheric circulation, which redistributes heat from equator to poles and sets the pattern of rainfall, deserts and biomes you met in the climate and biomes topic.
The atmosphere as a system
Treat it like any other system in this course and the marks come easily.
System part
In the atmosphere
Storages
The gases themselves, held in different concentrations that change over time through natural and human activity — including greenhouse gases such as carbon dioxide and methane
Flows
Constant movement of gases and particles, driven by air currents, weather systems and global circulation
Natural inputs
Volcanic gases, gases released by plants and other organisms, dust blown from deserts, sea spray
Human inputs
Greenhouse gases from burning fossil fuels and livestock, air pollutants from industry, aerosols from combustion
Outputs
Gases removed by photosynthesis and respiration; pollutants and aerosols washed out by precipitation or settling as dry deposition
The atmosphere also exchanges continuously with the other spheres: with the biosphere through photosynthesis and respiration, with the hydrosphere through evaporation and gases dissolving into the oceans, and with the lithosphere through weathering, volcanic activity and dust. Those exchanges are what shape climate patterns and weather events.
Worked examples
WORKED EXAMPLE
Explain why the troposphere is the layer most affected by human air pollution. [3]
Step 1: where the pollution is released
Emissions come from the surface — vehicles, industry, farming — so they enter the lowest layer first.
Step 2: what the layer does with them
The troposphere has strong convection and turbulence, so pollutants are mixed through it rather than staying put.
Step 3: why it mattersIt also holds most of the water vapour and trace gases, so reactions between pollutants, greenhouse gases and particles happen here — affecting air quality, weather and climate.Released at the surface, mixed within the layer, reacting where we live
WORKED EXAMPLE
A student writes: “Carbon dioxide is only 0.04% of the atmosphere, so it cannot be important.” Evaluate this statement. [3]
What is correct
The figure is right — carbon dioxide really is a tiny fraction of dry air, far less than nitrogen or oxygen.
Why the conclusion is wrong
Importance depends on what a gas does, not how much there is. Carbon dioxide absorbs infrared radiation, so it drives the greenhouse effect, and it is the raw material for photosynthesis.
The comparison that proves itNitrogen makes up 78% of the air and does almost nothing to the climate, because it does not absorb infrared radiation.Concentration and importance are not the same thing
💡 Exam tip
Learn the four figures cold: 78, 21, 0.93, 0.04. They are quick marks and they anchor longer answers.
Say the percentages are for dry air and that water vapour is variable. It is the detail that separates a strong answer.
When describing layers, always name the temperature trend, not just the height range.
Link the ozone layer to the stratosphere and to UV absorption in one sentence — the two facts are usually marked together.
If asked about the atmosphere as a system, use the words storage, flow, input and output explicitly.
⚠ Common mix-up
The ozone layer is not the greenhouse effect. Ozone in the stratosphere absorbs UV; greenhouse gases in the troposphere absorb infrared. Different gases, different radiation, different layer.
Layers are defined by temperature, not composition. The air is well mixed far higher than the troposphere.
Nitrogen and oxygen are not greenhouse gases. Most of the atmosphere plays no part in warming at all.
Water vapour is not a trace gas. It can be several per cent of the air, and it is the most abundant greenhouse gas.
“The atmosphere is thick.” Most of its mass sits in the bottom 10 km — thinner, relative to the Earth, than the skin of an apple.
Up next: How the Greenhouse Effect Works — the energy path in and out, why the wavelength change is the whole trick, and the difference between the natural effect and the enhanced one.
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