Why are dewdrops not formed on a cloudy night?

[UPSC Civil Services Exam – 2019 Prelims]

(a) Clouds absorb the radiation released from the Earth’s surface.

(b) Clouds reflect back the Earth’s radiation.

(c) The Earth’s surface would have low temperature on cloudy nights.

(d) Clouds deflect the blowing wind to ground level.

Answer: (b)

Explanation:

• Dew is formed when moisture condenses on cooler surfaces of solid objects such as stones, grass blades, and plant leaves.
• This process occurs when the dew point, which must be above freezing point, is reached under ideal conditions of clear sky, calm air, high relative humidity, and a long, cold night.
• When the sky is clear, there is more evaporation of water from trees and plants, resulting in more dew formation.
• Conversely, when the sky is cloudy, trees and plants do not cool as much during the night, leading to less dew formation.
• This is because the clouds reflect heat (earth’s radiation) back to the ground, which prevents it from getting cold enough for dew to form.

Therefore, option (b) is the correct answer.