Why are dewdrops not formed on a cloudy night?

Answer: (b) Clouds reflect the Earth’s radiation.

Clouds send the Earth’s outgoing heat back down, so the ground doesn’t cool to the dew point.

The idea

Dew point is the temperature at which air becomes saturated when cooled without changing its water vapour content. Dew forms when a surface cools to that threshold and vapour condenses on it. Clouds absorb outgoing infrared radiation and emit some downward, reducing the surface cooling needed for dew. The exam uses “reflection” as shorthand for this return of heat.

Ground emits infrared at night → Clouds absorb some outgoing energy → Clouds emit infrared downward → Ground loses less heat overall → Surface may stay above the dew point

Why the others are wrong

(a) Clouds do absorb some heat, but the key answer UPSC gave is that they send the heat back down.

(c) The reverse — cloudy nights are WARMER.

(d) Wind has nothing to do with it here.

The trap

(a) is partly true physics, so it tempts. The accepted answer is that clouds turn the heat BACK, keeping the ground warm.

Elimination technique

Dew needs surface cooling. The option claiming lower surface temperatures on cloudy nights predicts more condensation, so it cannot explain the stated absence of dew. Then distinguish the return of heat from the mere absorption of radiation.

Also know

Clear skies, light winds and moist air favour dew, though clouds do not make dew impossible in every situation. Fog is made of droplets suspended near the ground; dew collects on a surface. Hoar frost forms when water vapour deposits as ice on a sufficiently cold surface. Frozen dew is liquid dew that freezes later.