What the atmosphere is made of
Nitrogen and oxygen dominate; carbon dioxide is tiny but vital; water vapour is variable.
The atmosphere is the layer of gases held around the Earth by gravity. It is a mixture, and Cambridge expects you to state its major components and have a feel for their proportions.
| Gas | Approximate proportion (by volume) | Why it matters |
|---|---|---|
| Nitrogen (N2) | ~78% | The most abundant gas; fairly unreactive; part of the nitrogen cycle and essential to proteins. |
| Oxygen (O2) | ~21% | Needed for aerobic respiration and combustion; made by photosynthesis. |
| Argon (Ar) | ~0.9% | An inert (unreactive) noble gas; the most common of the trace gases. |
| Carbon dioxide (CO2) | ~0.04% | Tiny in amount but vital — the raw material for photosynthesis and a key greenhouse gas. |
| Water vapour (H2O) | Variable (roughly 0-4%) | Amount changes with place and weather; drives clouds, rainfall and the greenhouse effect. |
Two gases do almost all the work by volume. Nitrogen (~78%) and oxygen (~21%) together make up about 99% of the dry air. Everything else — argon, carbon dioxide and the other trace gases — shares the remaining ~1%.
Why carbon dioxide deserves special attention. Although CO2 is only about 0.04% of the air, it has effects far out of proportion to its amount: it is the gas plants use to make food in photosynthesis, and it is one of the gases responsible for the greenhouse effect. A small change in this tiny figure can change the planet's climate.
Water vapour is the variable one. Unlike the other gases, the amount of water vapour changes from place to place and hour to hour — high over a warm ocean, almost zero over a cold desert. That is why it is usually quoted separately rather than given a single fixed percentage.
- Major components: nitrogen, oxygen, carbon dioxide, argon, water vapour.
- Nitrogen ~78%, oxygen ~21%, argon ~0.9%, carbon dioxide ~0.04%.
- Nitrogen + oxygen ≈ 99% of dry air.
- CO₂ is tiny (~0.04%) but vital for photosynthesis and the greenhouse effect.
- Water vapour is variable (it depends on place and weather).