Cloudy nights are warmer compared to clear cloudless nights because clouds

Answer: (b) Reflect the heat given off by the earth

Clouds absorb and re-radiate (“reflect”) the Earth’s outgoing heat back down, keeping nights warmer.

The idea

Night-time surface temperature depends on the balance between outgoing and incoming infrared radiation. A clear sky permits a greater net loss of surface heat. Clouds absorb infrared and emit some of it back down, reducing that loss. This is re-emission rather than mirror-like reflection of heat.

Sunlight stops after sunset → Surface continues emitting infrared → Cloud absorbs outgoing radiation → Cloud emits infrared downward → Net surface heat loss is smaller → The night is often warmer

Why the others are wrong

(a) There are no “cold waves from the sky”.

(c) Clouds don’t produce heat.

(d) Close, but the key frames it as returning Earth’s heat.

Elimination technique

Clouds are not an independent heat source, and there are no falling “cold waves from the sky”. Reject those explanations; the decisive mechanism is returning some of the energy emitted by the surface.

Also know

Clear, calm nights favour radiation fog, dew and frost because near-ground cooling is stronger. Low cloud or fog can also reduce cooling once it forms. Clouds commonly cool daytime conditions by reflecting sunlight, so their day and night effects differ. A cloudy night is not guaranteed to be warm if a cold air mass is arriving.