Quick summary
- The Earth's atmosphere is an envelope of gases, water vapor and dust particles held in place by gravity.
- Gases: nitrogen (78.08%), oxygen (20.95%), argon (0.93%), carbon dioxide (0.036%), ozone and trace gases.
- Vertical structure: five layers based on temperature variation - troposphere, stratosphere, mesosphere, thermosphere (which contains the ionosphere) and exosphere.
Why this topic matters
This is the foundation of climatology in UPSC CSE and BPSC. Questions regularly appear on the tropospheric lapse rate, the height of the tropopause at different latitudes, ozone absorption of ultraviolet radiation, and the reflection of radio waves by the ionosphere.
1. Composition of the Atmosphere
- Permanent gases: Nitrogen (78.08%) and oxygen (20.95%) make up 99% of clean dry air. Argon (0.93%) is the third most abundant gas.
- Variable gases:
- Carbon dioxide (0.036%): Transparent to incoming solar radiation (shortwave) but opaque to outgoing terrestrial radiation (longwave), which makes it a key greenhouse gas.
- Water vapor: Decreases from the equator toward the poles and with altitude. It absorbs radiation and acts as a blanket that insulates the Earth.
- Dust particles: Act as hygroscopic nuclei, the surfaces around which water vapor condenses to form clouds and fog. They also scatter sunlight, adding to the colors of sunrise and sunset.
2. Thermal Structure of the Atmosphere
2.1 Troposphere
- Average height: 13 km. It is about 8 km at the poles and 18 km at the equator, because strong convection currents carry air higher at the equator.
- Contains about 75% of the atmosphere's mass and almost all of its water vapor and dust.
- All weather phenomena (clouds, rain, storms) occur here.
- Normal lapse rate: temperature decreases with height at 6.5 °C per 1,000 m (1 °C per 165 m).
- Boundary: tropopause. Its temperature is about -80 °C over the equator and -45 °C over the poles.
2.2 Stratosphere
- Extends from the tropopause up to 50 km.
- Free from clouds and convection, which makes it ideal for jet aircraft.
- Contains the ozone layer (15-30 km), which absorbs harmful solar ultraviolet (UV) radiation. This is why temperature increases with height in this layer.
- Boundary: stratopause.
2.3 Mesosphere, Thermosphere and Exosphere
- Mesosphere (50-80 km): Temperature decreases with height, reaching about -100 °C at 80 km (the mesopause, the coldest part of the atmosphere). Meteors burn up in this layer as they enter the atmosphere.
- Thermosphere and ionosphere (80-400 km): Temperature rises rapidly with height. The ionosphere lies within it and contains electrically charged particles (ions) that reflect radio waves back to Earth, which makes long-distance radio communication possible.
- Exosphere: Beyond 400 km. It is extremely thin and merges into outer space, with hydrogen and helium as the dominant gases.
Key terms and definitions
| Term | NCERT-standard definition |
|---|---|
| Normal lapse rate | The rate at which air temperature decreases with increasing altitude in the troposphere (6.5 °C per 1,000 m). |
| Hygroscopic nuclei | Microscopic dust and salt particles around which atmospheric water vapor condenses to form water droplets. |
| Ionosphere | The part of the thermosphere (80-400 km) containing ionized particles that reflect radio waves. |
How this is asked in the exam
- Height of the tropopause: It is higher at the equator (18 km) because intense heat creates strong convection currents that push air upward. At the poles (8 km), cold, dense air stays low.
- Temperature trend by layer: a very common statement-based question. The temperature pattern changes from layer to layer (see the memory aid below).
- Functions of layers: ozone and UV absorption (stratosphere), reflection of radio waves (ionosphere), burning of meteors (mesosphere).
Memory aids
- Layer order: "The Strong Man Throws Everything" - Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere.
- Temperature pattern: falls, rises, falls, rises. Troposphere ↓, Stratosphere ↑, Mesosphere ↓, Thermosphere ↑.
- Gas order: N₂ > O₂ > Ar > CO₂.
Common mistakes and confusions
- Temperature decreases in the troposphere, increases in the stratosphere (ozone absorbs UV), decreases in the mesosphere and increases in the thermosphere.
- The ionosphere is not a separate sixth layer. It lies inside the thermosphere.
- The 0.036% figure for carbon dioxide is the NCERT value. Use it for exam answers, but remember that the real concentration is higher today and still rising.
Test yourself
1. In which layer do clouds, rain and storms occur?
2. Why is the tropopause higher at the equator than at the poles?
3. Which layer reflects radio waves back to Earth?
4. Why does temperature increase with height in the stratosphere?
Quick revision checklist
- I can recall the order of gases: N₂ > O₂ > Ar > CO₂.
- I understand why the tropopause is higher at the equator than at the poles.
- I can state the function of the ozone layer (stratosphere) and the ionosphere (radio reflection).
- I can give the temperature trend for all four layers.