UPSC Prelims 2012 Q49: Geography – Atmosphere & Climatology
Normally, the temperature decreases with the increase in height from the Earth's surface,
because
1. The atmosphere can be heated upwards only from the Earth's surface.
2. There is more moisture in the upper atmosphere.
3. The air is less dense in the upper atmosphere.
Select the correct answer using the codes given below:
- (a) 1 only
- (b) 2 and 3 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Answer: (c) 1 and 3 only
Explanation
Why temperature falls with height
In the troposphere, temperature falls by about 6.5°C for every kilometre of height.
This is the normal lapse rate.
| Statement | Verdict | Reason |
|---|---|---|
| 1. The atmosphere can be heated upwards only from the Earth's surface | Correct | Air is heated mainly by terrestrial radiation from the ground, not directly by the Sun |
| 2. There is more moisture in the upper atmosphere | Incorrect | Moisture decreases with height; most water vapour lies in the lowest layers |
| 3. The air is less dense in the upper atmosphere | Correct | Thin air holds less heat |
How the air is heated:
| Process | Role |
|---|---|
| Terrestrial radiation | Long-wave heat from the ground, absorbed by carbon dioxide and water vapour |
| Conduction | Heats the layer touching the ground |
| Convection | Carries heat upward |
Answer: (c) 1 and 3 only.
Why each option is right or wrong
- (a) Incorrect. Statement 3 is also a reason; the air is less dense higher up.
- (b) Incorrect. Statement 2 is false; there is less moisture in the upper atmosphere.
- (c) CORRECT. The air is heated from the Earth's surface, and thin air holds less heat.
- (d) Incorrect. Statement 2 is false; moisture decreases with height.
Key point
Normal lapse rate: 6.5°C per kilometre. The air is heated from below by terrestrial radiation; thin upper air with little moisture and dust holds less heat.
Remember
Lapse rate 6.5°C per km; air is heated from the ground.
Common mistakes
- Thinking the Sun heats the air directly; the ground does most of it.
- Assuming there is more moisture higher up because clouds are there.
- Forgetting that the fall in temperature stops at the tropopause.
Exam tip
The atmosphere is heated from below, like a pan on a stove. The farther from the ground, the cooler it is.
Sources
- NCERT Class XI — Fundamentals of Physical Geography, Ch 9 (Solar Radiation, Heat Balance and Temperature)
- G.C. Leong — Certificate Physical and Human Geography, Ch on Climate