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Class 11 Composition and Structure of Atmosphere: cheat sheet
CBSE Social Science. What the air is made of, why water vapour, dust and carbon dioxide matter, and the layers of the atmosphere from the troposphere to the exosphere.
Key ideas
- The atmosphere is a mixture of gases, water vapour and dust; about 99% of its mass lies within 32 km of the surface.
- Nitrogen and oxygen make up almost all dry air; carbon dioxide is tiny but traps outgoing heat (the greenhouse effect).
- Water vapour, up to 4% in the warm wet tropics, falls with height and towards the poles; dust acts as hygroscopic nuclei for clouds.
- The troposphere holds all weather, dust and water vapour; it is thickest at the equator because convection carries heat high.
- The stratosphere holds the ozone layer, which absorbs ultra-violet rays.
- The mesosphere is the coldest layer; the ionosphere reflects radio waves back to the earth; the exosphere merges into space.
Remember
| Dry air by volume | nitrogen 78.08%, oxygen 20.95%, argon 0.93% |
| Share of a gas | volume × per cent ÷ 100 |
| Lapse rate (troposphere) | about 1 °C fall for every 165 m: temperature = surface − height ÷ 165 |
| Layer heights | troposphere to about 13 km, stratosphere to 50 km, mesosphere to 80 km, ionosphere 80–400 km |
| Tropopause | about −80 °C over the equator, −45 °C over the poles |
| Mesopause | about −100 °C at about 80 km |
Watch out
- Putting the ozone layer in the troposphere (it is in the stratosphere).
- Adding instead of subtracting with height: the troposphere gets colder upward.
- Saying the troposphere is thickest at the poles.
Worked example
Name the layer of the atmosphere at each height: (a) 305 km (b) 102 km
Working: troposphere to 13 km, stratosphere to 50 km, mesosphere to 80 km, ionosphere 80–400 km
Answer: (a) ionosphere (b) ionosphere
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