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

[UPSC Civil Services Exam – 2012 Prelims]

(a) 1 only

(b) 2 and 3 only

(c) 1 and 3 only

(d) 1, 2 and 3

Answer: (c)

Explanation:

• Statement 1 is correct because the temperature decreases with an increase in elevation due to the decrease in pressure and the expansion of air molecules. This can only be heated from the Earth’s surface. The normal lapse rate is 1-degree Celsius temperature fall at a height of every 165 meters in the troposphere.
• Statement 3 is also correct because as the air density decreases with height, the temperature decreases at a predictable rate, with the air near the surface being heated and becoming lighter, and the air higher up cooling and becoming heavier.
• However, statement 2 is incorrect because it suggests that water vapor, which accounts for around 1% of air, decreases rapidly with altitude in the troposphere. In fact, water vapor varies greatly in the troposphere and does decrease with height, but not necessarily rapidly.

Therefore, option (c) is the correct answer.