UPSC Prelims 2001 · General Studies
Question 111: Science and Technology
Cloudy nights are warmer compared to clear cloudless nights, because clouds
AnswerThe answer is (b).
Cloudy nights are warmer because clouds act as a blanket: they send back the heat that the Earth radiates after sunset instead of letting it escape into space, so the ground cools less. On a clear night the heat escapes freely and the temperature falls more. Remember the same idea in the greenhouse effect and in frost forming on clear, still nights.
Why each option is right or wrong
- (a) There are no cold waves descending from the sky; the difference is in outgoing heat.
- (b) Right as per the key: clouds send back the heat given off by the Earth.
- (c) Clouds do not produce heat.
- (d) Close in physics, since clouds absorb and re-emit the Earth's heat, but the key prefers (b).
Worth knowing: Strictly, clouds absorb the heat radiated by the Earth and re-emit part of it downward rather than mirror it, so (d) is arguably as good as (b).
Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2001, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.