UPSC Prelims 2012 · GS Paper I
Question 49: Geography
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 :
AnswerUPSC’s answer is (c).
In the troposphere temperature falls with height (about 6.5 degrees Celsius per kilometre). The Earth's surface absorbs solar radiation and heats the air above it by radiation, conduction and convection, so the lowest air is warmest. Higher up, the air is thinner and less dense, so it holds less heat. The upper atmosphere has less moisture, not more.
- 1. The atmosphere is mainly heated from the Earth's surface, so lower air is warmer.
- 2. Moisture decreases with height.
- 3. Thinner air aloft holds less heat.
How you could have got this
- If you were sure statement 2 is not correct, (b) and (d) go: each of them includes 2.
What you needed to know: Moisture decreases with height.
- Then, if you were sure statement 3 is correct, (a) goes: it leaves 3 out.
What you needed to know: Thinner air aloft holds less heat.
Only (c) is left. You didn’t need statement 1.
Which options go is worked out from UPSC’s key and the options, not by a model.
Why each option is right or wrong
- (a) Incomplete: lower density of air aloft is also a reason.
- (b) Wrong: there is less moisture in the upper atmosphere, not more.
- (c) Correct: heating from below, and thinner air aloft.
- (d) Wrong: statement 2 is false, since the upper atmosphere has less moisture.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2012, General Studies Paper I, Series A. The paper on upsc.gov.in.