Why are dewdrops not formed on a cloudy night?
- (a) Clouds absorb the radiation released from the Earth's surface.
- (b) Clouds reflect back the Earth's radiation. ✓ UPSC's answer
- (c) The Earth's surface would have low temperature on cloudy nights.
- (d) Clouds deflect the blowing wind to ground level.
Why the answer is (b)
• Dew forms when the ground cools by radiating heat to space on a clear night until surface air reaches its dew point.
• Clouds act as a blanket: they absorb the Earth's outgoing long-wave radiation and re-radiate it back downward, so the surface stays warm and does not cool to the dew point — the official key words this as clouds 'reflecting back' the Earth's radiation, option (b).
• Hence option (b).
Why the other options are wrong
- (a) Clouds absorb the radiation released from the Earth's surface.
- Absorption alone is incomplete; re-radiation back to Earth prevents cooling (key prefers b).
- (c) The Earth's surface would have low temperature on cloudy nights.
- Cloudy nights are warmer, not colder, at the surface.
- (d) Clouds deflect the blowing wind to ground level.
- Clouds do not deflect wind to the ground.
Asked in the GS Paper I of the UPSC Civil Services Preliminary Examination 2019, held on 2 June 2019. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.