UPSC Prelims 2023 Q62: Geography – Atmosphere & Climatology
- (a) The total amount of insolation received at the equator is roughly about 10 times of that received at the poles
- (b) Infrared rays constitute roughly two - thirds of insolation
- (c) Infrared waves are largely absorbed by water vapour that is concentrated in the lower atmosphere
- (d) Infrared waves are a part of visible spectrum of electromagnetic waves of solar radiation
Answer: (c) Infrared waves are largely absorbed by water vapour that is concentrated in the lower atmosphere
Explanation
Earth's Atmosphere — Insolation and Infrared
| Option | Claim | Verdict | Correct Fact |
|---|---|---|---|
| A | Total insolation at equator ~10x that at poles | FALSE | The equator receives about 2–3 times more annual insolation than poles, not 10 times. The actual multiplier depends on sun angle and day length variations. |
| B | Infrared rays = ~2/3 of insolation | FALSE | Solar radiation reaching Earth's surface is approximately 50% visible light, 45% infrared, and 5% ultraviolet — not 2/3 infrared. |
| C | Infrared waves are largely absorbed by water vapour in lower atmosphere | TRUE | Water vapour, CO₂, and other greenhouse gases in the lower troposphere absorb longwave infrared radiation emitted by Earth's surface. This is the greenhouse effect mechanism. Water vapour is the most abundant GHG. |
| D | Infrared waves are part of visible spectrum | FALSE | Infrared is beyond the red end of the visible spectrum (wavelength >700 nm). The visible spectrum is ~380–700 nm. Infrared is invisible to the human eye. |
Answer: (c) Infrared waves are largely absorbed by water vapour that is concentrated in the lower atmosphere.
Why each option is right or wrong
- (a) Incorrect. Equatorial insolation is higher than polar, but not by a factor of 10 — the ratio is approximately 2–3 times, not 10 times.
- (b) Incorrect. Infrared constitutes about 45% of incoming solar radiation, not two-thirds.
- (c) CORRECT. Infrared waves (especially longwave/terrestrial infrared) are largely absorbed by water vapour and other greenhouse gases concentrated in the lower atmosphere.
- (d) Incorrect. Infrared rays are beyond the visible spectrum (wavelength > 700 nm) — they are invisible to the human eye.
Key point
Infrared = absorbed by water vapour in lower atmosphere (greenhouse effect). Infrared is NOT visible (beyond red end of spectrum). Equatorial insolation ≈ 2–3x polar (not 10x). Infrared ≈ 45% of solar radiation (not 2/3).
Remember
Infrared waves from Earth's surface are largely absorbed by water vapour concentrated in the lower atmosphere — this is the primary greenhouse effect mechanism. Infrared radiation is beyond the visible spectrum, not part of it.
Common mistakes
- Thinking infrared is part of the visible spectrum — it is just beyond the red (hence 'infra-red'), invisible to humans but felt as heat.
- Accepting the '10 times' ratio for equator-pole insolation without questioning — the actual ratio is much lower.
- Not connecting water vapour to infrared absorption — water vapour is the most important greenhouse gas, absorbing longwave infrared radiation from Earth's surface.
Exam tip
Visible spectrum: 380–700 nm (ROYGBIV). Infrared: >700 nm (invisible, felt as heat). Absorbed by: water vapour (main GHG) in lower atmosphere. This is the fundamental basis of the greenhouse effect. Statement C = greenhouse effect mechanism.
Sources
- NCERT Geography Class 11 — Chapter 9: Solar Radiation
- NASA — Earth's Energy Budget