The atmosphere is a thin veil that makes life on earth possible. — Anonymous
7.1 Composition of the Atmosphere
The atmosphere is a mixture of gases, water vapour, and aerosols:
| Gas | Approximate proportion |
|---|---|
| Nitrogen (N₂) | 78% |
| Oxygen (O₂) | 21% |
| Argon | 0.93% |
| Carbon dioxide (CO₂) | ~0.04% (rising) |
Water vapour (0–4%) and ozone (O₃) are variable but vital. Aerosols (dust, pollen, pollutants) affect visibility and climate.
7.2 Structure of the Atmosphere
Layers by temperature change:
| Layer | Altitude | Characteristics |
|---|---|---|
| Troposphere | 0–12 km | Weather; temperature decreases with height |
| Stratosphere | 12–50 km | Ozone layer; temperature increases |
| Mesosphere | 50–80 km | Coldest layer; meteors burn here |
| Thermosphere | 80–400 km | Very hot; auroras; ionosphere |
| Exosphere | 400+ km | Merges with outer space |
The tropopause separates troposphere from stratosphere.
7.3 Importance of the Atmosphere
- Life support — oxygen for respiration; CO₂ for photosynthesis
- Climate regulation — greenhouse effect traps heat
- Protection — ozone absorbs harmful UV radiation
- Weather — clouds, rain, wind generated in troposphere
Rising CO₂ from fossil fuels and deforestation intensifies the greenhouse effect, contributing to global warming—a major concern for India's agriculture and coastal cities.
7.4 Atmospheric Pollution
Primary pollutants: CO, SO₂, NOₓ, particulate matter, volatile organic compounds.
Secondary pollutants: smog, acid rain, ground-level ozone.
Indian cities (Delhi, Mumbai, Kolkata) face severe air pollution from vehicles, industry, crop burning, and dust. Air Quality Index (AQI) monitors health risk.
Chapter Summary
- Atmosphere: mainly nitrogen and oxygen; water vapour and CO₂ are crucial
- Layers: troposphere (weather), stratosphere (ozone), mesosphere, thermosphere
- Atmosphere supports life, regulates climate, and shields from UV
- Pollution threatens health and climate; AQI used for monitoring
- Rising greenhouse gases linked to anthropogenic climate change
Exercises (NCERT)
- Exercise 1 — composition of the atmosphere
- Exercise 2 — layers of the atmosphere and their features
- Exercise 3 — causes and effects of atmospheric pollution in India