Composition and Layers of Atmosphere
NCERT Class 11 - Fundamentals of Physical Geography, Chapter 8
Composition and Layers of Atmosphere
The atmosphere is a mixture of gases, water vapour and dust particles held to the earth by gravity. It is divided into five layers by the way temperature changes with height, and nearly all weather occurs in the lowest one.
Composition
| Gas | Approximate share by volume |
|---|---|
| Nitrogen | About 78 per cent |
| Oxygen | About 21 per cent |
| Argon | About 0.93 per cent |
| Carbon dioxide | About 0.04 per cent |
| Other gases | Neon, helium, krypton, hydrogen and ozone in traces |
- Nitrogen dilutes oxygen and is essential for plant growth after it is fixed into compounds.
- Oxygen is needed for respiration and for combustion.
- Carbon dioxide is small in quantity but crucial - it is needed for photosynthesis and it absorbs outgoing long wave radiation, which keeps the earth warm.
- Water vapour varies from almost nothing over deserts to about four per cent in the humid tropics; it is the source of all precipitation and also a greenhouse gas.
- Dust particles act as nuclei of condensation, without which water vapour would not readily form droplets, and they scatter sunlight to produce the colours of sunrise and sunset.
- Ozone in the stratosphere absorbs harmful ultraviolet radiation and acts as a shield for life.
The five layers
| Layer | Extent and temperature | Key feature |
|---|---|---|
| Troposphere | Up to about 8 km at the poles and 18 km at the equator; temperature falls with height | All weather occurs here; lapse rate about 6.5 degrees per kilometre |
| Stratosphere | Up to about 50 km; temperature rises with height | Contains the ozone layer; stable air, so aircraft fly here |
| Mesosphere | Up to about 80 km; temperature falls sharply | The coldest layer; meteors burn up here |
| Thermosphere or ionosphere | About 80 to 400 km; temperature rises steeply | Ionised layers reflect radio waves; auroras occur here |
| Exosphere | Beyond the thermosphere | Extremely thin, gradually merging into outer space |
- The boundaries between layers are called pauses - tropopause, stratopause and mesopause.
- The troposphere is thicker at the equator because strong convection pushes the boundary higher, and thinner at the poles where the air is cold and dense.
- Temperature falls in the troposphere because the atmosphere is heated from below, by the earth's surface, not directly by the sun.
- Temperature rises in the stratosphere because ozone absorbs ultraviolet radiation and converts it to heat.
हिंदी संकेत: क्षोभमंडल में तापमान ऊंचाई के साथ घटता है क्योंकि वायुमंडल नीचे से, यानी पृथ्वी की सतह से गर्म होता है। समतापमंडल में तापमान बढ़ता है क्योंकि ओजोन पराबैंगनी किरणें सोखती है।
Exam me kaise aata hai
Factual questions ask the proportion of nitrogen and oxygen, the height of the troposphere at the equator and the poles, and which layer contains ozone. Match-the-following links layer with its characteristic. Statement-based questions test why temperature rises in the stratosphere.
UPSC / State PSC ke liye
Note that carbon dioxide and water vapour are tiny in quantity but decisive in effect, because they are transparent to incoming short wave radiation and opaque to outgoing long wave radiation - this single sentence explains both the natural greenhouse effect and global warming.
Yahan confuse hote hain
✗ Oxygen is the most abundant atmospheric gas | ✓ Nitrogen is, at about 78 per cent
✗ The troposphere is thickest at the poles | ✓ It is thickest at the equator and thinnest at the poles
✗ The stratosphere is the coldest layer | ✓ The mesosphere is the coldest layer
Ek nazar me
- Nitrogen about 78 per cent, oxygen about 21 per cent, argon about 0.93 and carbon dioxide about 0.04.
- Water vapour is the source of precipitation; dust particles are nuclei of condensation.
- Troposphere - about 8 km at the poles, 18 km at the equator; all weather occurs here.
- Lapse rate in the troposphere is about 6.5 degrees per kilometre.
- Stratosphere holds the ozone layer and temperature rises with height.
- Mesosphere is the coldest layer; meteors burn up here.
- Thermosphere reflects radio waves; the exosphere merges into space.