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UPSC Prelims 2012 · World Geography · Air, Winds and Climates
Normally, the temperature decreases with the increase in height from the Earth's surface, because Select the correct answer using the codes given below
- the atmosphere can be heated upwards only from the Earth's surface
- there is more moisture in the upper atmosphere
- the air is less dense in the upper atmosphere
- (a) 1 only
- (b) 2 and 3 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Show the answer and explanation
Answer: (c) 1 and 3 only
Heating from below and thinner air explain the lapse rate.
Statement by statement
1. ✓ CORRECT The air is heated from the ground up.
2. ✗ WRONG Upper air has LESS moisture.
3. ✓ CORRECT Thinner air holds and absorbs less heat.
The idea
In the troposphere, the surface is a major heating route for the air above it. Rising air also encounters lower pressure, expands and cools without needing to lose heat to its surroundings. This expansion is the clearest physical mechanism behind cooling with ascent. Lower density alone is not a complete explanation of temperature.
Air receives heat near the surface → Air rises → Surrounding pressure falls → The air parcel expands → Expansion uses internal energy → The parcel cools
Why the others are wrong
(a) Drops the thin-air reason.
(b) Accepts more moisture above.
(d) Accepts more moisture above.
Also know
The average environmental lapse rate is about 6.5 °C per kilometre, but actual profiles vary. Unsaturated rising air cools at about 10 °C per kilometre; saturated air cools more slowly because condensation releases latent heat. In an inversion, temperature rises with height. The stratosphere has a different temperature pattern because ozone absorbs sunlight.
🔒 Normal lapse rate ≈ 6.5 °C per km.
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