Explanation
- A
Clouds absorb the radiation released from the Earth’s surface.
(a) Clouds absorb the radiation released from the Earth’s surface. Clouds do interact with terrestrial longwave, but the standard dew explanation keyed by UPSC is reflection/return of Earth’s radiation that keeps the ground warmer, not this wording as the key.
- B
Clouds reflect back the Earth’s radiation.
(b) Clouds reflect back the Earth’s radiation. On a clear night the ground loses longwave heat to space and can cool below dew point; cloud cover returns (reflects/re-radiates) that outgoing radiation, so the surface stays warmer and dew does not form. That is the official key.
- C
The Earth’s surface would have low temperature on cloudy nights.
(c) Earth’s surface would have low temperature on cloudy nights. Cloudy nights are warmer at the surface, not colder; this reverses the energy balance, so it is not the key.
- D
Clouds deflect the blowing wind to ground level.
(d) Clouds deflect the blowing wind to ground level. Dew is a radiative-cooling condensation story, not a wind-deflection story, so this is not the key.
Summary. Official key is (b). Dew needs nocturnal radiative cooling of the ground below dew point. Clouds act as a blanket by sending Earth’s longwave radiation back, so droplets do not condense on grass. Cloudy nights are milder, not colder. Honour the stored letter (b).
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