Question 53 from UPSC CSE 2019 - Geography

Question

Why are dewdrops not formed on a cloudy night?

IASCODEX · Accepted Answer

Correct Option: B. Clouds reflect back the Earth's radiation.

Strategy: Focus on the function of clouds relative to Earth's outgoing radiation. On clear nights, heat escapes freely, enabling surface cooling and dew formation. On cloudy nights, clouds act as a barrier that reflects or radiates much of this energy back to the surface, which prevents strong cooling and thus dew formation. Option B captures this insulating effect.

Key Info: Dew formation and the role of clouds are governed by radiative heat transfer processes.

- **Dew formation:** Occurs when surfaces cool at night by emitting infrared radiation, dropping their temperature below the dew point and causing atmospheric moisture to condense.
- **Clear nights:** The lack of clouds allows for unobstructed radiation loss from the surface, leading to strong cooling and conditions favorable for dew.
- **Cloudy nights:** Clouds act as radiative blankets. They absorb and re-radiate longwave emissions from Earth, reflecting energy back to the surface. This insulation maintains higher near-surface temperatures, preventing the cooling necessary for dew.
- **Influencing conditions:** Dew is favored on cool, calm, clear nights and inhibited by warm, windy, or cloudy conditions.
- **Net cloud effect:** The primary impact of clouds, especially thick, low-lying ones, is to retain heat and warm the surface layer at night. They do not significantly deflect wind.

IASCODEX · Answer

Option A: Clouds absorb the radiation released from the Earth's surface.

IASCODEX · Answer

Option C: The Earth's surface would have low temperature on cloudy nights.

IASCODEX · Answer

Option D: Clouds deflect the blowing wind to ground level.