2019 GS Paper I, all questions

UPSC Prelims 2019 · GS Paper I

Question 44: Geography

Why are dewdrops not formed on a cloudy night?

aClouds absorb the radiation released from the Earth’s surface.
bClouds reflect back the Earth’s radiation.
cThe Earth’s surface would have low temperature on cloudy nights.
dClouds deflect the blowing wind to ground level.

AnswerUPSC’s answer is (b).

Dew forms when the ground loses heat quickly by radiation on a clear, calm night and cools below the dew point. On a cloudy night the clouds act as a blanket: they send the Earth's outgoing radiation back towards the surface, so the ground stays warmer and does not cool enough for dew to form. Remember the conditions for dew: clear sky, calm air, long cold nights and high humidity.

Why each option is right or wrong
  • (a) Clouds do absorb some radiation, but UPSC takes the key reason to be that they send it back.
  • (b) Correct: clouds reflect the Earth's radiation back, keeping the ground warm.
  • (c) The surface is warmer, not cooler, under clouds at night.
  • (d) Wind deflection is not the cause.

Worth knowing: Clouds also absorb and re-radiate the Earth's heat, so option (a) is arguable; UPSC's key chose (b).

Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.

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Source: UPSC, Civil Services (Preliminary) Examination 2019, General Studies Paper I, Series A. The paper on upsc.gov.in.