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Water in the Atmosphere

NCERT Class 11 · Geography Based on NCERT Class 11 Geography textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explains the presence of water vapour in the atmosphere, its forms, and the processes of evaporation, condensation, and precipitation. It covers humidity, dew point, and the various forms of condensation like dew, frost, fog, mist, and clouds. The big idea is that water continuously cycles between the atmosphere, oceans, and continents, and the amount of moisture in the air determines weather phenomena. A student must understand how temperature, pressure, and nuclei influence condensation and precipitation.
Key Concepts (28)
Humidity
Water vapour present in the air.
e.g. On a humid day in Mumbai, you feel sticky because the air has a lot of water vapour.
Absolute humidity
The actual amount of water vapour present in the atmosphere, expressed as weight of water vapour per unit volume of air in grams per cubic metre.
e.g. If a cubic metre of air contains 10 grams of water vapour, the absolute humidity is 10 g/m³.
Relative humidity
The percentage of moisture present in the atmosphere as compared to its full capacity at a given temperature.
e.g. If air at 30°C can hold 30 g/m³ but actually holds 15 g/m³, relative humidity is 50%.
Saturated air
Air containing moisture to its full capacity at a given temperature, incapable of holding any additional moisture.
e.g. On a foggy morning, the air is saturated and cannot hold more water vapour.
Dew point
The temperature at which saturation occurs in a given sample of air.
e.g. If air cools to 15°C and becomes saturated, 15°C is the dew point.
Evaporation
The process by which water is transformed from liquid to gaseous state, caused by heat.
e.g. Water in a puddle disappears on a sunny day due to evaporation.
Latent heat of vaporisation
The amount of heat energy required to convert a unit mass of a liquid into vapour without a change in temperature.
e.g. When sweat evaporates from your skin, it absorbs latent heat, cooling you down.
Condensation
The transformation of water vapour into water, caused by loss of heat.
e.g. Water droplets form on a cold glass of water because water vapour in the air condenses.
Hygroscopic condensation nuclei
Very small particles like dust, smoke, and salt from the ocean that absorb water and help condensation occur in free air.
e.g. In polluted cities, smoke particles act as nuclei, leading to fog formation.
Dew
Moisture deposited in the form of water droplets on cooler surfaces of solid objects like stones, grass blades, and plant leaves.
e.g. In winter mornings, you see water droplets on grass — that is dew.
Frost
Excess moisture deposited in the form of minute ice crystals on cold surfaces when condensation takes place below freezing point (0°C).
e.g. In Shimla during winter, car windshields may have a layer of ice crystals — that is frost.
Fog
A cloud with its base at or very near the ground, reducing horizontal visibility to less than 1 km.
e.g. In Delhi during winter, thick fog often reduces visibility to a few metres.
Mist
Similar to fog but contains more moisture, limiting visibility to between 1 km and 2 km.
e.g. On hill stations like Ooty, mist often covers the mountains in the morning.
Smog
A condition when fog is mixed with smoke.
e.g. In industrial cities like Kanpur, smog is common in winter due to pollution.
Cloud
A mass of minute water droplets or tiny crystals of ice formed by condensation of water vapour in free air at considerable elevations.
e.g. White fluffy clouds in the sky are cumulus clouds.
Cirrus
Thin, detached clouds formed at high altitudes (8,000-12,000 m), having a feathery appearance, always white in colour.
e.g. On a clear day, you may see wispy, feathery clouds high up — those are cirrus.
Cumulus
Clouds that look like cotton wool, formed at a height of 4,000-7,000 m, existing in patches with a flat base.
e.g. On a summer afternoon, you see fluffy white clouds that look like cotton balls — cumulus clouds.
Stratus
Layered clouds covering large portions of the sky, formed due to loss of heat or mixing of air masses with different temperatures.
e.g. A grey, overcast sky with no distinct cloud shapes is often due to stratus clouds.
Nimbus
Black or dark gray clouds, extremely dense and opaque to the sun's rays, often low enough to seem to touch the ground.
e.g. Dark, rain-bearing clouds before a storm are nimbus clouds.
Precipitation
The release of moisture after condensation of water vapour, when condensed particles grow in size and fall to the earth's surface due to gravity.
e.g. Rain, snow, sleet, and hail are all forms of precipitation.
Rainfall
Precipitation in the form of water.
e.g. The monsoon rains in Kerala are a classic example of rainfall.
Snowfall
Precipitation in the form of fine flakes of snow, occurring when temperature is lower than 0°C.
e.g. In Gulmarg, snowfall occurs in winter.
Sleet
Frozen raindrops and refrozen melted snow-water, formed when a layer of air above freezing overlies a subfreezing layer near the ground.
e.g. In some hill stations, rain turns into small ice pellets before reaching the ground — that is sleet.
Hailstones
Small rounded solid pieces of ice formed when raindrops pass through colder layers, having several concentric layers of ice.
e.g. During a severe thunderstorm in Punjab, hailstones the size of peas may fall.
Convectional rain
Rainfall caused by heated air rising in convection currents, expanding, cooling, and condensing to form cumulus clouds, often with thunder and lightning.
e.g. Afternoon thunderstorms in Bengaluru during summer are convectional.
Orographic rain
Rainfall caused when saturated air mass is forced to ascend over a mountain, leading to condensation; windward slopes get heavy rain, leeward slopes are rain-shadow areas.
e.g. The western slopes of the Western Ghats receive heavy orographic rain, while the eastern side (rain-shadow) is dry.
Cyclonic rain
Rainfall associated with cyclones, as explained in Chapter 9.
e.g. The rainfall during a depression in the Bay of Bengal is cyclonic.
Rain-shadow area
The area situated on the leeward side of a mountain that gets less rainfall.
e.g. The Deccan plateau east of the Western Ghats is a rain-shadow area.
How It Works
Evaporation
  1. Heat energy is absorbed by water, causing molecules to gain kinetic energy.
  2. Water molecules escape from the liquid surface into the air as water vapour.
  3. The rate of evaporation increases with higher temperature, lower moisture content, and greater movement of air.
  4. Latent heat of vaporisation is absorbed without a change in temperature.
💡 Evaporation is a cooling process because it absorbs latent heat; students often confuse it with condensation which releases heat.
Condensation
  1. Moist air is cooled, reducing its capacity to hold water vapour.
  2. When temperature reaches dew point, air becomes saturated.
  3. Excess water vapour condenses into liquid form around hygroscopic condensation nuclei (dust, smoke, salt particles).
  4. Condensation can also occur when moist air contacts a colder object or when moisture is added to air through evaporation.
💡 Condensation requires cooling and nuclei; it does not happen in perfectly clean air. Students often forget the role of nuclei.
Formation of Dew
  1. During clear, calm, cold nights, surfaces like grass and stones cool rapidly.
  2. Air in contact with these surfaces cools to dew point (above freezing point).
  3. Moisture condenses directly on these cooler surfaces as water droplets.
💡 Dew forms on surfaces, not in the air; ideal conditions are clear sky, calm air, high relative humidity, and long nights.
Formation of Frost
  1. Same conditions as dew, but air temperature is at or below freezing point (0°C).
  2. Dew point is at or below freezing point.
  3. Excess moisture deposits as minute ice crystals instead of water droplets.
💡 Frost is frozen dew — the key difference is temperature below 0°C.
Formation of Fog and Mist
  1. Temperature of an air mass containing large quantity of water vapour falls suddenly.
  2. Condensation takes place within the air mass on fine dust particles (nuclei).
  3. Fog forms with base at or near ground, reducing visibility to less than 1 km.
  4. Mist contains more moisture and limits visibility to between 1 km and 2 km.
💡 Fog is drier than mist; mist has thicker moisture layers around nuclei. Smog is fog mixed with smoke.
Formation of Clouds
  1. Water vapour in free air at considerable elevations condenses around nuclei.
  2. Minute water droplets or tiny ice crystals form a visible mass.
  3. Clouds are classified by height, expanse, density, and transparency into cirrus, cumulus, stratus, and nimbus.
💡 Clouds form in free air, not on the ground; nimbus clouds are rain-bearing and dark.
Precipitation
  1. Continuous condensation in free air causes condensed particles to grow in size.
  2. When the resistance of air fails to hold them against gravity, they fall to earth's surface.
  3. Forms include rainfall (liquid), snowfall (solid flakes), sleet (frozen raindrops), and hail (layered ice pellets).
💡 Precipitation requires particles to grow large enough to overcome air resistance; not all condensation leads to precipitation.
Convectional Rainfall
  1. Air near the surface is heated and becomes light.
  2. It rises in convection currents, expands, and loses heat.
  3. Condensation occurs, forming cumulus clouds.
  4. Heavy rainfall occurs with thunder and lightning, but does not last long.
💡 Common in equatorial regions and summer afternoons; students often forget it is associated with cumulus clouds.
Orographic Rainfall
  1. Saturated air mass encounters a mountain and is forced to ascend.
  2. As it rises, it expands, temperature falls, and moisture condenses.
  3. Windward slopes receive heavy rainfall.
  4. On the leeward side, air descends, temperature rises, and capacity to hold moisture increases, creating a rain-shadow area.
💡 The key is windward vs. leeward; students often mix up which side gets more rain.
Exam Traps — Don't Make These Mistakes
⚠ Confusing absolute and relative humidity
✗ Relative humidity is the actual amount of water vapour in the air.
→ Relative humidity is a percentage comparing actual moisture to the maximum possible at that temperature, not the actual amount.
✓ Absolute humidity is the actual weight of water vapour per unit volume (g/m³), while relative humidity is the percentage of moisture present compared to full capacity at a given temperature.
⚠ Thinking condensation only happens when air is cooled
✗ Condensation only occurs when air temperature decreases.
→ Condensation can also occur when moisture is added to the air through evaporation, or when volume and temperature both change.
✓ Condensation takes place when temperature is reduced to dew point, when both volume and temperature are reduced, or when moisture is added to the air through evaporation.
⚠ Mixing up fog and mist
✗ Fog and mist are the same thing.
→ They differ in visibility and moisture content: fog reduces visibility to less than 1 km and is drier; mist limits visibility to 1-2 km and contains more moisture.
✓ Fog reduces horizontal visibility to less than 1 km, while mist limits it to between 1 km and 2 km. Mist contains more moisture than fog.
⚠ Confusing sleet and hail
✗ Sleet and hail are the same thing.
→ Sleet is frozen raindrops or refrozen melted snow-water, formed when warm air overlies subfreezing air. Hailstones are formed by raindrops passing through colder layers and have concentric layers of ice.
✓ Sleet is small ice pellets from frozen raindrops, while hailstones are larger, rounded pieces of ice with concentric layers.
⚠ Thinking all clouds bring rain
✗ All clouds produce precipitation.
→ Only nimbus clouds (and cumulonimbus) are dense and dark enough to produce significant precipitation; cirrus, cumulus, and stratus clouds may not produce rain.
✓ Nimbus clouds are extremely dense and opaque, and are associated with precipitation. Other cloud types like cirrus and cumulus may not produce rain.
⚠ Misidentifying rain-shadow area
✗ The windward side of a mountain is the rain-shadow area.
→ The windward side receives heavy rainfall; the leeward side, where air descends and warms, is the rain-shadow area.
✓ The rain-shadow area is on the leeward side of the mountain, which gets less rainfall because descending air warms and increases its capacity to hold moisture.
Compare & Contrast
Definition and measurement
Absolute humidity
Relative humidity
Absolute humidity is the actual weight of water vapour per unit volume (g/m³), while relative humidity is the percentage of moisture present compared to the full capacity at a given temperature.
Visibility and moisture content
Fog
Mist
Fog reduces horizontal visibility to less than 1 km and is drier, while mist limits visibility to between 1 km and 2 km and contains more moisture around each nucleus.
Temperature and form of deposition
Dew
Frost
Dew forms as water droplets when dew point is above freezing point (0°C), while frost forms as ice crystals when dew point is at or below freezing point.
Cause and mechanism
Convectional rainfall
Orographic rainfall
Convectional rainfall is caused by heated air rising in convection currents, while orographic rainfall is caused by saturated air forced to ascend over a mountain barrier.
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 SA of 30 words (3 marks), 1 LA of 150 words (5 marks), total approximately 10 marks
Most asked: Relative humidity and absolute humidity definitions · Dew point and saturated air · Types of rainfall (convectional, orographic, cyclonic) · Forms of condensation (dew, frost, fog, mist, clouds) · World distribution of rainfall patterns
Don't skip: Explain the three types of rainfall with diagrams; Differentiate between absolute and relative humidity; Describe the forms of condensation and conditions for their formation; Discuss the world distribution of precipitation
One-Line Revision (read aloud before the exam)
  1. Water vapour in the atmosphere is called humidity, measured as absolute humidity (g/m³) or relative humidity (percentage).
  2. Saturated air holds maximum moisture at a given temperature; the temperature at which saturation occurs is the dew point.
  3. Evaporation transforms liquid water into vapour and absorbs latent heat of vaporisation.
  4. Condensation transforms water vapour into liquid and requires cooling and hygroscopic condensation nuclei like dust or salt.
  5. Dew forms on cool surfaces when dew point is above 0°C; frost forms as ice crystals when dew point is at or below 0°C.
  6. Fog is a ground-level cloud reducing visibility to less than 1 km; mist reduces visibility to 1-2 km and contains more moisture.
  7. Smog is fog mixed with smoke, common in urban and industrial areas.
  8. Clouds are classified into cirrus (high, feathery), cumulus (cotton-like, flat base), stratus (layered), and nimbus (dark, rain-bearing).
  9. Precipitation occurs when condensed particles grow too heavy for air resistance and fall as rain, snow, sleet, or hail.
  10. Convectional rain is caused by heated air rising; orographic rain occurs on windward slopes of mountains; cyclonic rain is associated with cyclones.
  11. The leeward side of a mountain is a rain-shadow area with less rainfall.
  12. World rainfall decreases from equator to poles; coastal areas receive more rain than interiors.
  13. Between 35°-40° N/S, eastern coasts get more rain; between 45°-65° N/S, western margins get more rain due to westerlies.
  14. Equatorial belt and windward slopes of mountains receive over 200 cm rainfall annually; rain-shadow areas receive less than 50 cm.

MCQs — 10 Questions with Answers

Q1. Water vapour constitutes what percentage by volume of the atmosphere?

  • A. Zero to four per cent ✓
  • B. Five to ten per cent
  • C. Fifteen to twenty per cent
  • D. More than twenty-five per cent

Answer: A — The study material explicitly states that water vapour varies from zero to four per cent by volume of the atmosphere.

Q2. At which temperature does water vapour begin to condense into liquid form?

  • A. Room temperature (25°C)
  • B. The dew point temperature of that air mass ✓
  • C. Always at 0°C
  • D. When temperature reaches -40°C

Answer: B — Condensation occurs specifically when air is cooled to its dew point, which is the temperature at which saturation occurs for that particular air mass.

Q3. Relative humidity is highest over which surface type?

  • A. Sandy deserts
  • B. Mountain peaks
  • C. Oceans and water bodies ✓
  • D. Urban centres with concrete

Answer: C — The material states that relative humidity is greater over the oceans and least over the continents due to continuous evaporation from water bodies.

Q4. Which of the following is NOT a correct pair of condensation form and its formation condition?

  • A. Dew forms when dew point is above 0°C on solid surfaces
  • B. Frost forms when dew point is at or below 0°C on solid surfaces
  • C. Fog forms when visibility reduces to less than 1 km and is a ground-level cloud
  • D. Mist forms only over mountains and has visibility less than 1 km ✓

Answer: D — Mist has visibility between 1-2 km (not less than 1 km), and while common over mountains, it also forms over oceans and other regions where warm air meets cold surfaces.

Q5. Study the cloud formation sequence: air cools → water vapour condenses → cloud forms. Which factor is most critical to ensure this sequence occurs?

  • A. Increase in air pressure only
  • B. Presence of condensation nuclei (dust, salt, smoke particles) ✓
  • C. Increase in wind speed
  • D. Decrease in relative humidity

Answer: B — Condensation nuclei are essential; water vapour condenses around these particles in free air, and without them, condensation cannot occur even if air cools below dew point.

Q6. Cumulus clouds form at altitudes between 4,000-7,000 metres and appear as scattered cotton-wool patches. What does this indicate about their formation condition?

  • A. They form in areas of stable, continuous cooling throughout the layer
  • B. They form in localised convective heating creating updrafts in specific patches ✓
  • C. They form due to mixing of air masses at the tropopause
  • D. They form when wind shear prevents cloud spreading

Answer: B — Cumulus clouds' scattered, patchy appearance indicates localised convective heating creating separate updrafts, unlike the continuous layers of stratus clouds formed by widespread cooling.

Q7. Latent heat of vaporisation is the energy required to convert liquid water into vapour. How would a decrease in atmospheric pressure affect evaporation rate?

  • A. Evaporation rate decreases because higher air density increases resistance
  • B. Evaporation rate increases because lower pressure reduces molecular resistance to escape ✓
  • C. Evaporation rate remains unchanged as pressure does not affect latent heat
  • D. Evaporation rate decreases because water molecules condense faster at low pressure

Answer: B — Lower atmospheric pressure reduces the force holding water molecules in the liquid state, making it easier for evaporation to occur even at lower temperatures.

Q8. Both statement 1 AND statement 2 must be true for the answer: Statement 1: Absolute humidity is the actual mass of water vapour per unit volume. Statement 2: Relative humidity depends on air temperature and is expressed as a percentage of saturation capacity.

  • A. Both statements are correct ✓
  • B. Statement 1 is correct but Statement 2 is incorrect
  • C. Statement 1 is incorrect but Statement 2 is correct
  • D. Both statements are incorrect

Answer: A — Both statements are accurate: absolute humidity is measured in g/m³ as actual water vapour mass, and relative humidity is a percentage that varies with temperature changes.

Q9. A meteorologist observes that the dew point is 15°C and current air temperature is 20°C. If the air temperature drops to 15°C, which condensation forms will appear and in what order?

  • A. Dew will form immediately on surfaces at the instant temperature reaches dew point ✓
  • B. Frost will form on all surfaces because 15°C is below the typical saturation point
  • C. Fog will form in free air first, then dew on surfaces as ground cools further
  • D. No condensation will occur because temperature is still above freezing

Answer: A — When air temperature equals the dew point (15°C), saturation occurs and condensation begins; since 15°C > 0°C, dew (water droplets) forms on cool surfaces, not frost.

Q10. Using NCERT climate data, if a coastal city has absolute humidity of 15 g/m³ at 25°C saturation capacity of 20 g/m³, and an inland city at same temperature has absolute humidity of 8 g/m³ with same saturation capacity, calculate which city has higher relative humidity and by what percentage difference.

  • A. Coastal city: 75%, inland city: 40%, difference = 35% ✓
  • B. Coastal city: 75%, inland city: 60%, difference = 15%
  • C. Both cities: 75%, no difference
  • D. Inland city: 85%, coastal city: 60%, difference = 25%

Answer: A — Relative humidity = (absolute humidity / saturation capacity) × 100; coastal = (15/20)×100 = 75%; inland = (8/20)×100 = 40%; difference = 35%, confirming higher humidity over oceans.

Flashcards

What is absolute humidity and how is it measured?

Absolute humidity is the actual mass of water vapour per unit volume of air, expressed in grams per cubic metre (g/m³).

Define relative humidity and state one condition affecting it.

Relative humidity is the percentage of moisture in air compared to its full capacity at a given temperature; it changes with air temperature and is higher over oceans than continents.

What is dew point and why is it important?

Dew point is the temperature at which water vapour in air becomes saturated and condensation begins; it determines when and where condensation will occur.

Name three factors that increase evaporation rate.

Increased temperature, low moisture content in air, and greater air movement (wind) all increase evaporation.

What are hygroscopic condensation nuclei and give one example.

Hygroscopic condensation nuclei are tiny particles that absorb water and trigger condensation; examples include dust, salt crystals from oceans, and smoke particles.

How do dew and frost differ in formation?

Dew forms when dew point is above freezing (0°C) as water droplets on surfaces, while frost forms below freezing as ice crystals.

What is the key difference between fog and mist?

Fog reduces visibility to less than 1 km and is drier, while mist reduces visibility to 1-2 km and contains more moisture per nucleus.

State the height range and appearance of cumulus clouds.

Cumulus clouds form at 4,000-7,000 metres, look like cotton wool, have flat bases, and appear scattered in patches.

Which cloud type is associated with heavy rainfall and why?

Nimbus clouds bring precipitation because they are extremely dense, opaque, dark grey, and form near ground level with thick concentrated water droplets.

Define precipitation and name its four forms.

Precipitation is the release of moisture from clouds to earth's surface; forms include rainfall (water), snowfall (ice crystals), sleet, and hail.

Important Board Questions

Define absolute humidity and relative humidity. How do they differ from each other? [2 marks]

State exact definitions: absolute humidity = mass per volume (g/m³); relative humidity = percentage of saturation capacity. Key difference: absolute is actual amount, relative depends on temperature and capacity.

Explain the process of evaporation and condensation with reference to the water cycle. Why is latent heat important in this process? [5 marks]

Define evaporation (liquid → gas, driven by heat) and condensation (gas → liquid, triggered by cooling). Explain latent heat of vaporisation = energy needed to convert unit mass of liquid to vapour without temperature change. Link both to continuous water cycle between atmosphere, oceans, and continents through three steps: heat input → evaporation increase, air cooling → condensation, water droplet growth → precipitation.

Describe the different forms of condensation (dew, frost, fog, clouds) based on temperature and location. Explain why nimbus clouds bring heavy precipitation while cirrus clouds do not. [6 marks]

Create a table comparing: dew (>0°C, surfaces), frost (≤0°C, surfaces), fog (near ground, visibility <1 km), mist (1-2 km), and four cloud types by altitude. Explain nimbus density, altitude, and opaqueness trap thick water droplets that grow and fall; cirrus at high altitude (8,000-12,000 m) is thin, white, and composed of ice crystals too small/light to precipitate. Include how condensation nuclei role differs in each form.

Assertion-Reason Questions for Water in the Atmosphere

Q1. Assertion (A):

The ability of air to hold water vapour depends entirely on its temperature.

Reason (R):

Higher temperatures increase air's capacity to absorb and retain water vapour, while lower temperatures decrease this capacity.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true, and the reason correctly explains why temperature is the determining factor for moisture-holding capacity. The text explicitly states this relationship.

Q2. Assertion (A):

Condensation occurs most readily when air temperature decreases.

Reason (R):

Cooling reduces the air's moisture-holding capacity, allowing water vapour to transform into liquid or solid form when the dew point is reached.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both statements are true and correctly related. The reason accurately explains the mechanism behind condensation triggered by temperature decrease. Cooling is indeed the most favourable condition for condensation as stated in the chapter.

Q3. Assertion (A):

Dew and frost differ only in their temperature of formation.

Reason (R):

Dew forms when dew point is above freezing (0°C), resulting in water droplets, while frost forms when dew point is at or below freezing, resulting in ice crystals.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are correct. The primary difference between dew and frost is indeed the temperature threshold (freezing point), which determines whether condensation produces liquid droplets or ice crystals. The reason fully explains the assertion.

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Case-Based Questions (CBQ) for Water in the Atmosphere

Case Study:

During monsoon season in Kerala, meteorologists observed that a coastal city recorded an absolute humidity of 18 g/m³ while the saturation capacity at that temperature was 20 g/m³. Over the same 24-hour period, the average relative humidity was measured at 90%. By evening, with a sudden temperature drop of 3°C due to weather systems, the saturation capacity reduced to 16 g/m³. Visibility on highways dropped from 2 km to below 1 km, and the local meteorological department issued a fog warning.

Q1. What was the initial relative humidity reading if absolute humidity was 18 g/m³ and saturation capacity was 20 g/m³?

  • A. 85%
  • B. 90% ✓
  • C. 95%
  • D. 100%

Answer: B

Q2. After the temperature drop, what change would occur to the relative humidity even if absolute humidity remained at 18 g/m³?

  • A. Relative humidity would decrease because saturation capacity increased
  • B. Relative humidity would increase to over 112% because absolute humidity exceeds new capacity ✓
  • C. Relative humidity would remain constant because absolute humidity did not change
  • D. Relative humidity would decrease to exactly 90% regardless of capacity changes

Answer: B

Q3. The drop in visibility from 2 km to below 1 km indicates:

  • A. Mist changed to fog due to additional moisture addition
  • B. Dew was forming on the ground surface
  • C. Condensation in the air created fog from the previous mist conditions ✓
  • D. Wind speed increased, reducing visibility independently

Answer: C

Q4. Which of the following factors contributed most to fog formation during the evening?

  • A. Increase in absolute humidity through evaporation
  • B. Increase in wind speed bringing moisture from the ocean
  • C. Decrease in air pressure over the coastal region
  • D. Temperature decrease causing saturation and condensation on nuclei ✓

Answer: D

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