UPSC Prelims 2024 Q12: Geography – Atmosphere & Climatology
With reference to "water vapour", which of the following statements is/are correct?
1. It is a gas, the amount of which decreases with altitude.
2. Its percentage is maximum at the poles.
Select the answer using the code given below:
- (a) 1 only
- (b) 2 only
- (c) Both 1 and 2
- (d) Neither 1 nor 2
Answer: (a) 1 only
Explanation
Water Vapour — Properties and Distribution
| Statement | Claim | Verdict | Why |
|---|---|---|---|
| 1 | Water vapour is a gas; its amount decreases with altitude | TRUE | Water vapour enters the atmosphere via evaporation and transpiration at the surface. It is heaviest near the surface (where evaporation occurs) and decreases rapidly with altitude. About 90% of all water vapour is found in the lowest 5 km of the atmosphere. |
| 2 | Water vapour's percentage is maximum at the poles | FALSE | Water vapour content is maximum near the EQUATOR (3-4% of air volume). The equatorial zone has high temperatures driving intense evaporation. Poles are cold → low evaporation → very dry air (< 0.2% water vapour). |
Water Vapour Distribution by Latitude:
| Region | Approx. Water Vapour % | Reason |
|---|---|---|
| Equatorial | 3-4% | High temperature → high evaporation |
| Tropical | 2-3% | Warm, humid |
| Mid-latitudes | 1-2% | Moderate |
| Polar | < 0.2% | Cold → very little evaporation |
Statement 1 only is correct. Answer: 1 only.
Answer: (a) 1 only.
Why each option is right or wrong
- (a) CORRECT. Statement 1 is true (water vapour decreases with altitude — concentrated near surface). Statement 2 is false (maximum water vapour is at the equator, not the poles).
- (b) Incorrect. 'Only 2' would mean water vapour is maximum at poles — the exact opposite. Polar air is extremely dry due to cold temperatures and minimal evaporation.
- (c) Incorrect. 'Both 1 and 2' would require polar air to have maximum water vapour — polar regions are among the driest on Earth. The Sahara is dry, but the polar air in terms of water vapour content is even lower.
- (d) Incorrect. 'Neither' would mean water vapour does NOT decrease with altitude — it clearly does, with ~90% found in the lowest 5 km of atmosphere.
Key point
Water vapour decreases with altitude (concentrated near surface). Maximum water vapour = EQUATOR (warm, high evaporation). Minimum = poles (cold, dry). Statement 1 TRUE, Statement 2 FALSE.
Remember
Water vapour decreases with altitude (90% in lowest 5 km) and is maximum near the equator (3–4%), not the poles. Polar air is extremely dry.
Common mistakes
- Thinking water vapour is maximum at the poles — poles are cold deserts with very low evaporation and minimal atmospheric moisture.
- Not knowing the vertical distribution — water vapour is surface-concentrated; the upper atmosphere is extremely dry.
- Confusing total precipitation with water vapour content — some mid-latitude areas have high precipitation but the instantaneous water vapour % is lower than equatorial regions.
Exam tip
Water vapour: concentrated NEAR SURFACE (90% below 5 km), MAXIMUM at EQUATOR (3–4%), MINIMUM at poles. Warm air holds more water vapour — equatorial warmth drives high evaporation and humidity.
Sources
- NCERT Class 11 Geography — Chapter 8: Composition and Structure of Atmosphere
- IMD — Atmospheric Moisture Distribution