UPSC Prelims 2024 Q1: Geography – Atmosphere & Climatology
- (a) Both Statement - I and Statement - II are correct and Statement - II explains Statement - I
- (b) Both Statement - I and Statement - II are correct, but Statement - II does not explain Statement - I
- (c) Statement - I is correct, but Statement - II is incorrect
- (d) Statement - I is incorrect, but Statement - II is correct
Answer: (d) Statement - I is incorrect, but Statement - II is correct
Explanation
How the atmosphere is heated
| Radiation | Wavelength | Absorbed by the atmosphere? |
|---|---|---|
| Incoming solar (insolation) | Short-wave | Mostly passes through; air is largely transparent to it |
| Terrestrial (from Earth's surface) | Long-wave (infrared) | Largely absorbed by CO₂, water vapour and other greenhouse gases |
The Sun heats the ground first.
The ground then radiates long-wave energy, and this is what heats the air from below.
That is why temperature falls with height in the troposphere (about 6.5°C per km).
Statement I — incorrect. The atmosphere is heated more by terrestrial radiation than by incoming solar radiation.
Statement II — correct. CO₂ and other greenhouse gases are good absorbers of long-wave radiation.
Answer: (d) Statement - I is incorrect, but Statement - II is correct.
Why each option is right or wrong
- (a) Incorrect. Statement I is false: the atmosphere is heated mainly by terrestrial, not solar, radiation.
- (b) Incorrect. Both are not correct; Statement I reverses the main source of atmospheric heat.
- (c) Incorrect. It is the other way round: Statement I is false and Statement II is true.
- (d) CORRECT. The air is heated from below by terrestrial long-wave radiation, which greenhouse gases absorb.
Key point
Atmosphere is heated indirectly — by long-wave terrestrial radiation absorbed by greenhouse gases, not by direct sunlight.
Remember
The atmosphere is heated more by terrestrial radiation than by incoming sunlight.
Common mistakes
- Assuming the Sun heats the air directly; the air is nearly transparent to short-wave sunlight.
- Forgetting why the lapse rate exists: air is heated from the ground up, so it is colder higher up.
- Thinking the 'II explains I' link holds; Statement II actually supports the opposite of Statement I.
Exam tip
'Atmosphere is heated from below.' Remember this NCERT line and Statement I falls at once.
Sources
- NCERT Class XI — Fundamentals of Physical Geography, Ch 9 (Heating and Cooling of Atmosphere)
- Savindra Singh — Climatology (Prayag Pustak Bhawan)