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Q44 (IAS/2019) Geography › World Physical Geography › Atmospheric moisture Official Key

Why are dewdrops not formed on a cloudy night?

Result
Your answer: —  ·  Correct: B
Explanation

The correct answer is option B because cloudy nights are warm due to the reflection of outgoing long-wave radiation (towards the earth) by the clouds[1], and consequently dew formation is suppressed on a cloudy night[1]. The ideal conditions for dew formation are clear sky, calm air, high relative humidity, and cold and long nights[2]. On cloudy nights, clouds act as a blanket by reflecting the Earth's outgoing radiation back to the surface, preventing the surface from cooling sufficiently to reach the dew point temperature. Clouds, especially thick cumulus and stratus clouds affect the temperature of a place by absorbing the incoming solar insolation in the day, and blanketing the out-going radiated heat of the earth[3]. This blanketing effect keeps the surface warmer than it would be on a clear night, preventing the temperature drop necessary for moisture to condense as dew on surfaces.

Sources
  1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 24: Hydrological Cycle (Water Cycle) > Explanation: > p. 331
  2. [2] FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 10: Water in the Atmosphere > Dew > p. 87
  3. [3] Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 14: Climate > FACTORS INFLUENCTNG TEMPERATURE > p. 135
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Q. Why are dewdrops not formed on a cloudy night? [A] Clouds absorb the radiation released from the Earth's surface. [B] Clouds reflect ba…
At a glance
Origin: Books + Current Affairs Fairness: Moderate fairness Books / CA: 5/10 · 5/10
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This is a foundational Physical Geography question directly from NCERT Class XI. It tests the 'Conditions for Condensation' rather than just the definition. The strategy is simple: Memorize the prerequisites for weather phenomena (Dew, Frost, Fog) and understand the 'Heat Budget' mechanism that disrupts them.

How this question is built

This question can be broken into the following sub-statements. Tap a statement sentence to jump into its detailed analysis.

Statement 1
In the context of dew formation on cloudy nights, do clouds absorb longwave (infrared) radiation emitted by the Earth's surface and thereby reduce nighttime surface cooling?
Origin: Direct from books Fairness: Straightforward Book-answerable
From standard books
Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 24: Hydrological Cycle (Water Cycle) > Explanation: > p. 337
Presence: 5/5
“• High clouds are thin clouds (albedo: 25-30%) whereas low clouds are thick clouds (albedo: 70- 80%).• High clouds let through most of the incoming short-wave radiation (visible light) and block and reflect most of the outgoing long-wave radiation (heat or infrared radiation by earth) — the greenhouse effect.• Low, thick clouds are excellent reflectors of solar radiation, and they do block or absorb some of the outgoing long-wave radiation. However, they emit nearly as much infrared radiation to space as would to the surface. Thus, they reflect more heat than they trap, having a net cooling effect. So (d) Neither is the answer.”
Why this source?
  • Explicitly states high clouds block/reflect most outgoing long-wave terrestrial radiation (the greenhouse effect).
  • Notes low, thick clouds block or absorb some outgoing long-wave radiation, affecting surface heat balance.
  • Directly links cloud type and radiative interaction with Earth’s longwave emission, supporting reduced nocturnal cooling.
Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 14: Climate > FACTORS INFLUENCTNG TEMPERATURE > p. 135
Presence: 5/5
“As a whole, dry soils like sand are very sensitive to temperature changes, whereas wet soils, like clay, retain much moisture and warm up or cool down more slowly. 7. Cloud cover. Clouds, especially thick cumulus and stratus clouds affect the temperature of a place by absorbing the incoming solar insolation in the day, and blanketing the out-going radiated heat of the earth as illustrated in Fig. 14.11(a) and (b). This partly explains why day temperatures in equatorial regions with their thick layer clouds are never unbearable, while that of the cloudless deserts experience scorching heat of over 49 °C (l20 °F).”
Why this source?
  • Describes clouds as 'blanketing' outgoing radiated heat, reducing night cooling.
  • Connects cloud cover with moderating diurnal temperature range through absorption/retention of terrestrial heat.
FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 10: Water in the Atmosphere > Dew > p. 87
Presence: 4/5
“When the moisture is deposited in the form of water droplets on cooler surfaces of solid objects (rather than nuclei in air above the surface) such as stones, grass blades and plant leaves, it is known as dew. The ideal conditions for its formation are clear sky, calm air, high relative humidity, and cold and long nights. For the formation of dew, it is necessary that the dew point is above the freezing point.”
Why this source?
  • States dew formation requires clear sky, calm air and long cold nights.
  • Implicates that cloudiness prevents sufficient surface cooling needed for dew, consistent with clouds reducing nighttime radiative loss.
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Statement analysis

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Statement analysis

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