Authentic Classroom Explanation by IAS Monk

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📍 The Question

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 correct answer using the code given below:

(a) 1 only
(b) 2 only
(c) Both 1 and 2
(d) Neither 1 nor 2

Correct Answer: (a) 1 only

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🎯 Theme of the Question

Physical Geography | Atmosphere | Variable Gases | Climatology

This question tests understanding of distribution and behaviour of water vapour in the atmosphere.

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🧠 Classroom Explanation

Water vapour is a variable gas in the atmosphere, meaning its quantity varies with place, time, and altitude.

Let us examine the statements one by one.

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🔹 Statement 1: Water vapour is a gas whose amount decreases with altitude. ✅

• Water vapour is concentrated in the lower layers of the atmosphere
• As altitude increases:
  • Temperature decreases
  • Air becomes thinner
  • Capacity to hold moisture reduces
• Hence, water vapour content decreases rapidly with height

✔️ Statement 1 is correct

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🔹 Statement 2: Its percentage is maximum at the poles. ❌

• Water vapour content depends on:
  • Temperature
  • Evaporation
• Warm regions can hold more moisture
• In the tropics, water vapour may reach up to 4% by volume
• In polar and cold regions, it is often less than 1%

Thus, water vapour is minimum at the poles, not maximum.

✘ Statement 2 is incorrect

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📊 Final Assessment

Statement	Status
Decreases with altitude	✅ Correct
Maximum at the poles	❌ Incorrect

👉 Only Statement 1 is correct

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❌ UPSC Elimination Logic

• UPSC checks:
  • Whether aspirants confuse temperature zones
• Cold air holds less moisture, not more
• Poles are dry, not humid

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🧩 Memory Hook

“Warm air holds water; cold air lets it go.”

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🧠 Prelims Strategy Insight

For atmospheric gases:

• Nitrogen & Oxygen → constant gases
• Water vapour & CO₂ → variable gases
• Always link temperature with moisture capacity

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🧭 IAS Monk Whisper

Moisture follows warmth — not latitude alone.