Question map
Why are dewdrops not formed on a cloudy night?
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] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 24: Hydrological Cycle (Water Cycle) > Explanation: > p. 331
- [2] FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 10: Water in the Atmosphere > Dew > p. 87
- [3] Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 14: Climate > FACTORS INFLUENCTNG TEMPERATURE > p. 135
PROVENANCE & STUDY PATTERN
Full viewThis 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.
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?
- Statement 2: In the context of dew formation on cloudy nights, do clouds reflect or backscatter Earth's outgoing longwave radiation toward the surface, reducing nighttime cooling?
- Statement 3: In the context of dew formation on cloudy nights, is the Earth's surface temperature lower on cloudy nights compared to clear nights?
- Statement 4: In the context of dew formation on cloudy nights, do clouds deflect or redirect blowing wind down to ground level, increasing near-surface wind speeds?
- 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.
- Describes clouds as 'blanketing' outgoing radiated heat, reducing night cooling.
- Connects cloud cover with moderating diurnal temperature range through absorption/retention of terrestrial heat.
- 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.
- Explicitly says cloudy nights are warm because clouds reflect outgoing long-wave radiation back toward the earth.
- Directly links that reduced nighttime cooling under clouds to suppressed dew formation.
- Describes high clouds blocking/reflecting most outgoing long-wave radiation (greenhouse effect), showing clouds can trap Earth's infrared.
- Contrasts high and low clouds to show clouds alter longwave balance, supporting the mechanism that clouds affect nocturnal cooling.
- Defines clear sky as an ideal condition for dew, implying clouds inhibit the radiational cooling needed for dew formation.
- Provides the observational consequence (dew forms on clear, cold nights) that aligns with reduced cooling under cloudy skies.
- Explains that clear air transmits shortwave solar radiation but absorbs/redistributes longwave from the surface.
- States the atmosphere radiates energy back toward the surface, implying clouds/atmosphere reduce nighttime surface cooling.
- Specifically states clouds absorb Earth's longwave radiation and emit infrared both to space and back to the surface.
- Says the radiation trapped and sent back to the surface adds to other surface-reaching energy, implying warmer surface on cloudy nights.
- Describes that during clear nights vegetation cools by radiation to or below the dew point, causing dew to form.
- Notes larger objects lose heat more slowly and may not reach dew point on clear nights, illustrating stronger surface cooling on clear nights than on cloudy nights.
Lists ideal conditions for dew: clear sky, calm air, cold and long nights — implying dew is associated with stronger nocturnal cooling under clear skies.
A student could compare typical night cooling under clear vs cloudy conditions (using local temperature records) to see which nights get 'cold and long' enough for dew.
Explains that surface (radiation) cooling and inversions occur most often on clear nights when ground cools rapidly by radiation.
Combine with the rule that rapid ground cooling implies lower surface temperatures on clear nights than on nights with reduced radiative loss (e.g., cloudy nights).
States water vapour (like CO2) absorbs long-wave terrestrial radiation and plays a significant role in the insulating action of the atmosphere.
Extend by noting clouds contain/are associated with moisture and can trap outgoing long-wave radiation, so cloudy nights should lose less heat to space than clear nights.
Presents the multiple-choice idea that clouds absorb/reflect Earth's radiation — two mechanisms often cited to explain why dew is not formed on cloudy nights.
A student could treat these as candidate mechanisms and check observational data (nighttime temperature differences, dew occurrence) to see which mechanism predicts warmer surfaces on cloudy nights.
Notes clear skies promote rapid night radiation and large diurnal temperature range (big night cooling) in certain climates.
Use this as an example that clear nights can be much colder than cloudy nights in regions with low atmospheric insulation, so compare climates/locations to test the statement broadly.
- Explains that the boundary-layer wind profile governs hourly mean wind speeds near the ground.
- States near-ground velocity depends on surface roughness and height, implying surface-layer processes control wind speed rather than clouds redirecting flow.
- Describes micrometeorological gusts and turbulence near the ground as governed by boundary-layer turbulence.
- Notes turbulence intensity near the ground varies with surface roughness, not from downward deflection by overhead clouds.
- Shows surface winds are shaped by pressure gradients and friction in the friction layer, with flow angles varying with surface roughness.
- Notes local terrain and friction strongly affect surface winds, supporting that near-surface wind behavior is controlled from below rather than by clouds above redirecting flow downward.
States ideal conditions for dew: clear sky and calm air — implying that calm near-surface winds favour dew formation and that cloudy nights correspond to different conditions.
A student could use this rule plus observations that cloudy nights often lack strong radiative cooling to infer whether surface winds are typically calmer or stronger under clouds.
Explains surface temperature inversion forms on clear nights when the ground cools rapidly — a process linked to calm near-surface conditions and dew/fog formation.
Combine with basic radiative physics to argue that clouds reduce ground cooling (less inversion), which a student could link to expected changes in surface wind mixing and speed.
Notes that wind circulation at the surface is often the opposite of circulation aloft (rising air with convergence aloft vs. subsidence/divergence) — showing vertical structure matters for surface winds.
A student could use this pattern to consider whether clouds (which imply ascent aloft in many situations) are associated with surface convergence or divergence and thus how they might affect near-surface wind.
Describes convective processes in thunderstorms where downdrafts can pull rotating air toward the ground (tornadogenesis) — an example where cloud-related processes bring stronger winds downward.
Use as an example that in some cloudy, convective situations downdrafts can increase surface winds, so a student should distinguish cloudy-stable nights from convective-cloud nights when judging the statement.
Says frontal activity (which produces cloudiness) is associated with wind shifts and changing wind at the surface — linking clouds with dynamic changes in near-surface wind.
A student could combine this with synoptic charts (basic outside knowledge) to test whether cloudy nights with fronts typically have stronger surface winds than non-frontal cloudy nights.
- [THE VERDICT]: Sitter. Direct hit from NCERT Class XI (Fundamentals of Physical Geography), Chapter 10: 'Water in the Atmosphere'.
- [THE CONCEPTUAL TRIGGER]: The topic of 'Condensation Forms' (Dew, Frost, Fog, Mist) and 'Terrestrial Radiation'.
- [THE HORIZONTAL EXPANSION]: Memorize the 'Sibling' thresholds: 1. Dew (Surface > 0°C); 2. Frost (Surface < 0°C); 3. Mist (Visibility > 1km, nuclei abundant); 4. Fog (Visibility < 1km); 5. Radiation Fog (Land, clear nights) vs Advection Fog (Sea, horizontal wind).
- [THE STRATEGIC METACOGNITION]: UPSC flips the script. Instead of asking 'What conditions favor dew?', they asked 'What prevents dew?'. Always study the *Inverse Conditions*—if Clear Sky = Dew, then Cloudy Sky = No Dew. Why? Because clouds act as a blanket.
Clouds intercept and re-radiate outgoing terrestrial longwave radiation, reducing net nocturnal surface cooling.
High-yield for questions on diurnal temperature range, night-time cooling, and local climate effects; links radiative physics to surface temperature and phenomena like dew and frost. Mastery helps answer questions on greenhouse effect at local and regional scales and distinguish effects of cloud type.
- Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 24: Hydrological Cycle (Water Cycle) > Explanation: > p. 337
- Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 14: Climate > FACTORS INFLUENCTNG TEMPERATURE > p. 135
Dew forms under clear skies, calm air, high relative humidity and long cold nights, so cloud cover inhibits dew by limiting surface radiative cooling.
Essential for questions on microclimates, surface processes and weather phenomena (dew, frost, fog); connects to nocturnal radiation balance and surface inversion concepts. Enables elimination-style reasoning in MCQs about when dew or frost will form.
- FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 10: Water in the Atmosphere > Dew > p. 87
- Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 22: Vertical Distribution of Temperature > Ground Inversion (Surface Temperature Inversion) > p. 301
Water vapour and the atmospheric column absorb a substantial fraction of outgoing longwave terrestrial radiation, contributing to heat retention above the surface.
Crucial for understanding Earth's heat budget, greenhouse gas roles, and radiation budget questions; links to broader topics like climate forcing and energy balance. Useful for quantitative and conceptual UPSC questions on incoming vs outgoing radiation and atmospheric heating.
- Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 20: Earths Atmosphere > Water Vapour > p. 272
- FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 8: Solar Radiation, Heat Balance and Temperature > Heat Budget of the Planet Earth > p. 69
Dew requires nocturnal radiative cooling to bring surface temperature to the dew point; clouds reduce that cooling and thus suppress dew.
High-yield for questions on local moisture phenomena, agricultural impacts, and nocturnal temperature patterns; links to radiation fog, frost, and surface energy balance. Mastery helps answer questions on when and why dew/fog form and how night-time temperatures vary with cloudiness.
- FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 10: Water in the Atmosphere > Dew > p. 87
- Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 24: Hydrological Cycle (Water Cycle) > Non-Adiabatic Temperature Changes > p. 330
Clouds modify both incoming solar and outgoing terrestrial radiation, including reflecting or absorbing outgoing longwave radiation that influences night-time temperatures.
Crucial for understanding diurnal temperature ranges, the greenhouse effect, and the planetary heat budget; useful for questions on climate forcing, cloud feedbacks, and energy balance. Enables reasoned answers about why cloudy nights are warmer and how clouds affect climate.
- Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 24: Hydrological Cycle (Water Cycle) > Explanation: > p. 337
- FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 8: Solar Radiation, Heat Balance and Temperature > Heat Budget of the Planet Earth > p. 69
High thin clouds tend to trap outgoing infrared (warming), while low thick clouds reflect more solar and have different infrared emission characteristics, producing differing net effects on surface temperature.
Important for nuanced answers distinguishing cloud types in meteorology and climatology questions; helps interpret why some clouds warm nights while others may produce net cooling effects during the day. Useful for case-based and comparative questions on cloud impacts.
- Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 24: Hydrological Cycle (Water Cycle) > Explanation: > p. 337
- Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 24: Hydrological Cycle (Water Cycle) > Explanation: > p. 331
Clouds and atmospheric water vapour absorb and re‑radiate long‑wave terrestrial radiation, reducing overnight cooling and keeping the surface warmer on cloudy nights.
High-yield: Explains why cloudy nights are generally warmer and why dew or frost are less likely under cloud cover. Connects to radiation balance, greenhouse effect, and diurnal temperature range; useful for questions on night-time temperature variation and cloud impacts.
- Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 20: Earths Atmosphere > Water Vapour > p. 272
- Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 20: Earths Atmosphere > Temperature balance > p. 280
Temperature Inversion. The conditions for Dew (Long nights, Clear sky, Calm air) are the EXACT same conditions required for 'Surface Temperature Inversion'. Expect a question linking Fog/Dew to Inversion layers in valleys.
The 'Outcome Logic' Hack. Dew requires a COLD surface. You need an option that explains why the surface is WARM. Option C says the surface is cold (which would actually *cause* dew), so eliminate. Option D (Wind) is mechanical, not thermal. Between A (Absorb) and B (Reflect back): 'Reflect back' implies the heat returns to the surface, actively warming it. Absorption (A) is passive; Reflection/Re-radiation (B) is the active 'blanket' mechanism that kills the dew.
GS-3 Agriculture & Water Management. Link 'Dew Formation' to 'Dryland Farming'. In arid regions (like Rajasthan or Israel), dew harvesting is a critical source of moisture for crops. Understanding radiative cooling helps in designing dew condensers.