Why are dewdrops not formed on a cloudy night?
Answer & explanation
Answer: (b) Clouds reflect back the Earth's radiation.
Dew forms only when the ground and grass cool below the dew point, and that needs a clear night in which the Earth's heat escapes freely to space. On a cloudy night the clouds send the Earth's heat back down, so the surface stays warmer and no dew forms.
- ✓ (b) Clouds hold back the Earth's outgoing radiation and return part of it towards the surface, keeping the ground from cooling enough for water vapour to condense on it. UPSC treats this as the cause.
- ✗ (a) This is the tempting option. Clouds do take in some of the Earth's radiation, but the answer UPSC accepts is that they send it back, which is what keeps the surface warm.
- ✗ (c) It is the reverse of the truth: with cloud cover the surface stays warmer, and it is a clear, calm, cold night that gives low surface temperature and dew.
- ✗ (d) Dew needs calm air, but clouds do not act by blowing wind down to the ground; the deciding factor is heat loss.
Remember · Dew needs a clear, calm, cold night; cloud cover returns the Earth's heat and stops the surface cooling to the dew point.
📘 Read it in NCERT: Class 11 Fundamentals of Physical Geography, Ch 10 (practise this chapter)
Sources
- NCERT Class 11 · Fundamentals of Physical Geography, Chapter 10 “The ideal conditions for its formation are clear sky, calm air, high relative humidity, and cold and long nights.”
- Clouds and Radiation, NASA Earth Observatory ↗ “When a cloud absorbs longwave radiation emitted by the Earth's surface, the cloud reemits a portion of the energy to outer space and a portion back toward the surface.”
- Glossary, National Weather Service (NOAA), entry 'Radiational Cooling' ↗ “At night, the Earth suffers a net heat loss to space due to terrestrial cooling. This is more pronounced when you have a clear sky.”
Question and answer: UPSC's official GS Paper I (2019, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). ·