Dew never forms on a cloudy night, no matter how cold and damp the air feels. Clouds act like a blanket over the Earth, and that single fact explains a common UPSC exam question.
🏛️ Must Know
Why dew forms on clear nights but not on cloudy ones
- Cause Dew forms when the ground and objects near it lose heat by radiating it into space. This is called terrestrial or outgoing longwave radiation.
- Clear night On a clear night, this heat escapes freely upward. The ground surface cools quickly, often below the surrounding air’s dew point.
- Cloudy night Clouds absorb this outgoing radiation and re-radiate a large part of it back down. This keeps the surface warmer than it would be under a clear sky.
- Result Because the surface does not cool enough, air near it never reaches its dew point. So water vapour does not condense, and dew does not form.
Good to Know
📘 Good to Know
The dew point and the conditions dew actually needs
- Dew point The dew point is the temperature at which air becomes saturated with water vapour. Below it, excess vapour condenses into liquid droplets.
- Best conditions Dew forms best on clear, calm nights with high humidity. Calm air matters because wind mixes warmer air down and stops the surface cooling as much.
- Surface matters Dew forms more easily on good radiators like grass and leaves than on materials like rock or concrete. Grass loses heat fast because it has a large surface area and low heat capacity.
📝 Previous Year Questions
UPSC CSP 2019 — Why dewdrops do not form on a cloudy night
- UPSC 2019 UPSC asked why dewdrops are not formed on a cloudy night. The correct answer is that clouds reflect back the Earth’s radiation, keeping the surface too warm for dew to form. See UPSC CSP 2019 GS Paper I, Q44. View this question.
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Great to Know
🌟 Great to Know
Dew, frost, and fog: related but different phenomena
- Frost If the surface cools below freezing point, water vapour deposits directly as ice instead of liquid dew. This is called frost, or hoar frost.
- Fog link Radiation fog forms by the same basic process as dew, radiative cooling of the surface, but occurs when the whole layer of air near the ground cools and saturates, not just solid surfaces.
- Farming use Farmers watch for clear, calm nights because they bring frost risk to crops. Cloudy nights, in contrast, protect crops by keeping the surface warmer.
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