UPSC Prelims 2024 Q12: Water Vapour by Altitude & Latitude Explained
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:
- 1 only
- 2 only
- Both 1 and 2
- Neither 1 nor 2
Answer: (a) 1 only
Statement-wise explanation
- 1: correct — NCERT (Class XI, Fundamentals of Physical Geography, 'Composition and Structure of Atmosphere') calls water vapour a variable gas in the atmosphere that decreases with altitude. Evaporation from oceans and land adds it at the surface. Air also gets colder with height, so it can hold less vapour. Nearly all of the atmosphere's water vapour sits in the troposphere.
- 2: incorrect — NCERT says water vapour can make up about 4% of air by volume in the warm, wet tropics but less than 1% in dry, cold deserts and polar regions. It decreases from the equator towards the poles. Cold polar air can hold very little vapour, so the poles have the lowest share, not the highest.
How to eliminate
Statement 2 can be judged from basic physics: warm air holds more water vapour than cold air. The tropics are warm and have large evaporating oceans, while the poles are cold and are often called 'cold deserts' because they get so little precipitation. So 'maximum at the poles' is wrong. Statement 1 follows from the same logic: temperature falls with height in the troposphere and the moisture comes from the surface, so the amount drops with altitude. With 2 wrong and 1 right, only one option remains. Watch for one trap: polar air can have high relative humidity because it is close to saturation. But relative humidity is not the actual amount of vapour, and the statement is about the amount (percentage of air).
Know more
Water vapour: the variable gas
Dry air has a nearly fixed mix: nitrogen about 78%, oxygen about 21%, argon about 0.93% and carbon dioxide about 0.04% by volume. Water vapour is left out of that 'dry air' list because its share changes so much from place to place, from almost zero to about 4%.
| Factor | Effect on water vapour | Reason |
|---|---|---|
| Altitude | Decreases upward | Moisture comes from surface evaporation, and colder air at height holds less |
| Latitude | Decreases from equator to poles | Higher temperature and more evaporation in the tropics |
| Land vs sea | Higher over oceans and wet regions | More water available to evaporate |
| Season | Higher in warm seasons | Warm air holds more vapour |
Why it matters
- Greenhouse role: Water vapour absorbs part of the incoming sunlight and much of the heat the Earth gives off. It acts like a blanket, keeping the Earth from getting too hot or too cold.
- Weather: Almost all of it is in the troposphere, which is why clouds, rain and storms happen there. Its condensation releases latent heat, which drives storms and affects whether the air is stable or unstable.
- Humidity terms: Absolute humidity is the actual amount of vapour in a given volume of air. Relative humidity is how close the air is to saturation at its temperature. Polar air can have high relative humidity and still contain very little vapour.
Exam tip: For any atmospheric gas, ask how it varies with height and with latitude. Water vapour and ozone are the classic exceptions to the 'well-mixed' rule.
NCERT Concepts to Revise
- Water vapour: The question is about this gas and how its amount changes with height and latitude.
- Composition of the atmosphere: Water vapour is the variable part of atmospheric composition, unlike the fixed shares of nitrogen and oxygen.
- Humidity: The statements are about humidity, meaning how much water vapour the air actually holds.
- Relative humidity: Helps you tell high relative humidity apart from a large amount of water vapour, a common trap with cold regions.