The Atmosphere: Structure & Composition
The gases that make up the atmosphere and its five layers — from the weather-bearing troposphere to the exosphere — with their key features.
Key Takeaways
- The atmosphere is mostly nitrogen (78%) and oxygen (21%).
- It has five layers: troposphere, stratosphere, mesosphere, thermosphere and exosphere.
- All weather occurs in the troposphere; the ozone layer lies in the stratosphere.
Core concept
The atmosphere is the envelope of gases held to the Earth by gravity. It sustains life (oxygen), shields us (ozone), moderates temperature (the greenhouse effect), and is the theatre of all weather and climate.
Static foundation — composition
By volume, dry air is about 78% nitrogen and 21% oxygen, with argon, tiny amounts of carbon dioxide (~0.04%), plus variable water vapour, ozone and dust. Though small in quantity, CO₂, water vapour and ozone have effects far beyond their share.
The Five Layers of the Atmosphere
The Layers at a Glance
| Layer | Altitude | Key feature |
|---|---|---|
| Troposphere | 0–12 km | All weather; temperature falls with height |
| Stratosphere | 12–50 km | Ozone layer absorbs UV; stable, ideal for jet aircraft |
| Mesosphere | 50–80 km | Coldest layer; meteors burn up here |
| Thermosphere | 80–700 km | Ionosphere reflects radio waves; auroras; temperature rises |
| Exosphere | 700 km + | Outermost; merges into outer space |
Why the ozone layer matters
The ozone (O₃) layer in the stratosphere absorbs harmful ultraviolet (UV) radiation. Its depletion by CFCs led to the Montreal Protocol (1987) — a rare global success. The tropopause and stratopause are the boundaries where the temperature trend reverses.
Prelims trap zones
- Weather is confined to the troposphere; the ozone layer is in the stratosphere — do not confuse the two.
- Temperature rises with height in the stratosphere and thermosphere (an inversion of the normal pattern).
- The mesosphere is the coldest layer, not the troposphere.
Knowledge Check
2 questions · check your understanding
1. The ozone layer, which absorbs ultraviolet radiation, is located in the:
2. In which layer do all weather phenomena occur?
Prelims Pointers
- Nitrogen ~78%, Oxygen ~21%; carbon dioxide is only ~0.04% but vital for the greenhouse effect.
- The troposphere's temperature falls with height (normal lapse rate ~6.5°C per km).
- The ozone layer, which absorbs ultraviolet radiation, is in the stratosphere.
- The ionosphere (part of the thermosphere) reflects radio waves and hosts auroras.
Mains Angle
- 'The troposphere is the most important layer for life.' Discuss.
- Explain the significance of the ozone layer and the threats to it.
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