UPSC Prelims 2001 Q111: Geography – Atmosphere & Climatology
Cloudy nights are warmer compared to clear cloudless nights, because clouds
- (a) prevent cold waves from the sky from descending on Earth
- (b) reflect back heat given off by Earth
- (c) produce heat and radiate it towards Earth
- (d) absorb heat from the atmosphere and send it towards Earth
Answer: (b) reflect back heat given off by Earth
Explanation
Why cloudy nights are warmer than clear nights
| Point | Explanation |
|---|---|
| At night | The Earth loses heat by terrestrial (long-wave) radiation |
| Clear sky | Heat escapes to space; the ground cools fast |
| Cloudy sky | Clouds absorb and send back the outgoing heat to the Earth |
| Result | Cloudy nights stay warmer; clear nights bring frost and dew |
Clouds trap the heat given off by the Earth and return it downward, much like a blanket. On clear nights this heat escapes into space, so the surface cools quickly. The key words are 'reflect back heat given off by Earth'.
Answer: (b) reflect back heat given off by Earth.
Why each option is right or wrong
- (a) Incorrect. There are no 'cold waves' descending from the sky.
- (b) CORRECT. Clouds return the Earth's outgoing heat.
- (c) Incorrect. Clouds do not produce heat of their own.
- (d) Incorrect. Clouds do not take heat from the air to send to Earth; they return Earth's own radiation.
Key point
Clear, calm winter nights give the strongest ground frost and radiation fog.
Remember
Cloudy nights warmer: clouds send back the Earth's heat.
Common mistakes
- Thinking clouds produce heat of their own.
- Imagining 'cold waves' falling from the sky.
- Forgetting that the Earth radiates heat at night.
Exam tip
Clouds act as a blanket: they keep the Earth's own heat in.
Sources
- NCERT Class XI — Fundamentals of Physical Geography, chapter 'Solar Radiation, Heat Balance and Temperature'
- India Meteorological Department — Glossary: terrestrial radiation