GeographyCBSE Class 11 · Water in the Atmosphere

Humidity, Condensation and Clouds (Class 11 Geography, Fundamentals of Physical Geography, Chapter 10)

Updated 17 October 2026 · Fact-checked

Water vapour forms 0 to 4 per cent of the air by volume. Absolute humidity is the weight of vapour per unit volume (g per cubic metre); relative humidity is the percentage of the air's full capacity actually filled at that temperature. Cooling air to its dew point makes it saturated, and further cooling gives condensation as dew, frost, fog, mist or clouds (cirrus, cumulus, stratus, nimbus).

Cooling shrinks the air's capacity until relative humidity hits 100% at the dew point; extra cooling condenses vapour into dew, frost, fog or cloud.

Illustration drawn for QuestionBuddy.

Understand Humidity, Evaporation, Condensation, Dew, Frost, Fog and Clouds

Why this topic matters. Humidity terms, the dew/frost condition and the fog/mist/cloud comparisons are asked as 1-3 mark definitions and as reasoning items.

1. Humidity. Water is present in the air as gas, liquid and solid; the vapour share is 0 to 4 per cent by volume. Absolute humidity is the actual weight of vapour in a unit volume of air (g per cubic metre) and differs from place to place. Relative humidity is the percentage of moisture present compared with the full amount the air can hold at that temperature. It is highest over oceans and lowest over continents. Air holding its full capacity is saturated; the temperature at which a sample becomes saturated is the dew point. Capacity rises with temperature, so warming the air lowers relative humidity and cooling raises it.

2. Evaporation. Liquid turns to gas. Heat is the main cause; the latent heat of vaporisation is the heat needed to change a unit mass of liquid to vapour without a change in temperature. Evaporation increases with higher temperature, low moisture content of the air and moving air (which carries away the saturated layer).

3. Condensation. Vapour turns to liquid because of loss of heat. In free air it occurs on hygroscopic nuclei (dust, smoke, sea salt). It happens when (i) the temperature falls to the dew point at constant volume, (ii) both volume and temperature fall, or (iii) moisture is added by evaporation. A fall in temperature is the most favourable cause.

4. Forms of condensation. Dew: droplets on cool solid surfaces such as grass blades; needs a clear sky, calm air, high relative humidity and a long cold night, with the dew point above freezing. Frost: minute ice crystals deposited when the dew point is at or below freezing. Fog: a cloud with its base at or near the ground, visibility under 1 km; mist has more moisture, thicker moisture layers on each nucleus and visibility of 1 to 2 km. Smog is fog mixed with smoke over urban and industrial areas.

5. Clouds. A cloud is a mass of minute water droplets or ice crystals formed by condensation in free air at height. Cirrus: 8,000 to 12,000 m, thin, feathery, always white. Cumulus: 4,000 to 7,000 m, cotton-wool patches with a flat base. Stratus: layered, covering large parts of the sky, formed by loss of heat or mixing of air masses. Nimbus: black or dark grey, dense and opaque, at middle levels or very near the ground.

Humidity, Evaporation, Condensation, Dew, Frost, Fog and Clouds: key facts

Humidity terms: Absolute humidity g per cubic metre
Relative humidity percentage of full capacity at that temperature
Dew point temperature of saturation; Vapour is 0 to 4 per cent of air by volume.
Dew versus frost: Dew: dew point above 0 degrees C, water droplets | Frost: dew point at or below 0 degrees C, ice crystals; Both need clear sky and calm air.
Fog versus mist: Fog: visibility under 1 km | Mist: visibility 1 to 2 km, more moisture; Smog: fog mixed with smoke.
Cloud heights: $C i r r u s : 8, 000 t o 12, 000 m ∣ C u m u l u s : 4, 000 t o 7, 000 m ∣ S t r a t u s : l a y er e d ∣ N imb u s : d a r k, d e n se, r ain - b e a r in g$; Types by height, expanse and density.

Absolute humidity	g per cubic metre
Relative humidity	percentage of full capacity at that temperature
Dew point	temperature of saturation

How to answer Humidity, Evaporation, Condensation, Dew, Frost, Fog and Clouds questions

A relative humidity item is a ratio converted to a percentage.

1Write relative humidity as actual vapour divided by capacity at that temperature, multiplied by 100.
2Recompute capacity when the temperature changes; actual vapour stays the same unless moisture is added or removed.
3State whether the result is above or below saturation.
4If it would exceed 100 per cent, say condensation begins at the dew point.

Humidity, Evaporation, Condensation, Dew, Frost, Fog and Clouds in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Cold glass in a warm room

Suppose a glass of chilled water is kept in a warm humid room and droplets form on its outer surface within minutes.

Which process is this, and what temperature is reached at the glass surface?

Try it first, then show the solutionSolution

Air touching the cold glass is cooled.
Its temperature falls to the dew point, so vapour condenses.

Answer: This is condensation; the glass surface is at or below the dew point of the room air.

Memory hook: Cold surface, wet surface.

Misty hill road

Suppose a driver climbs a hill road and finds that visibility drops to 1.5 km as warm air meets cold slopes.

Is this fog or mist, and why?

Try it first, then show the solutionSolution

Visibility is between 1 and 2 km.
Mist has more moisture on each nucleus and visibility of 1 to 2 km.

Answer: It is mist, not fog.

Memory hook: Under 1 km is fog, 1 to 2 km is mist.

Tricks to remember Humidity, Evaporation, Condensation, Dew, Frost, Fog and Clouds

Short memory aids. Say each one aloud once, then use it on a question the same day.

A-R-D
Absolute is weight, Relative is percentage, Dew point is the saturation temperature
Use it when: Humidity terms.
CuCiSN
Cirrus highest and white, Cumulus cotton, Stratus layers, Nimbus dark
Use it when: Cloud types.
Dew above, frost below
Dew point above 0 gives dew, at or below 0 gives frost
Use it when: Dew or frost.

Humidity, Evaporation, Condensation, Dew, Frost, Fog and Clouds: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Relative humidity · 1 mark · MCQBoard level

A reading notes that the air holds 9 g of vapour per cubic metre while it could hold 18 g at that temperature. The relative humidity is:
(A) 25 per cent
(B) 50 per cent
(C) 75 per cent
(D) 200 per cent

Try it first, then show the solutionSolution

Relative humidity = 9 / 18 x 100 = 50 per cent (1 mark).

Answer: (B)

Example 2: Dew point · 2 marks · Very short answerBoard level

Define dew point and state what happens when air is cooled below it. (2 marks)

Try it first, then show the solutionSolution

Temperature at which air is saturated (1 mark).
Excess vapour condenses on nuclei or surfaces (1 mark).

Answer: The dew point is the temperature at which a sample of air becomes saturated. Cooling below it makes the excess vapour condense into dew, frost, fog, mist or cloud.

Example 3: Warming and relative humidity · 3 marks · Short answerHarder board level

The air in a room holds 10 g of vapour per cubic metre when its capacity is 18 g. It is heated until its capacity becomes 25 g, no vapour being added. Find the change in relative humidity and explain the cause. (3 marks)

Try it first, then show the solutionSolution

Initial RH = 10/18 x 100 = 55.6 per cent (1 mark).
Final RH = 10/25 x 100 = 40 per cent (1 mark).
Capacity rises with temperature while actual vapour is unchanged, so RH falls by about 15.6 points (1 mark).

Answer: Relative humidity falls from about 55.6 to 40 per cent because warm air can hold more vapour while the actual amount stays 10 g.

Example 4: Assertion and reason: dew · 1 mark · Assertion-reasonHarder board level

Assertion (A): Dew forms most heavily on a clear, calm night.
Reason (R): A cloudy sky traps outgoing heat so that surfaces stay warmer.
(A) Both true, R explains A
(B) Both true, R does not explain A
(C) A true, R false
(D) A false, R true

Try it first, then show the solutionSolution

A clear sky lets surfaces cool fast, which favours dew; cloud cover keeps them warmer, so R explains why clear nights are better (1 mark).

Answer: (A)

Example 5: HOTS: frost on one field only · 5 marks · Long answerHOTS / competency

On a still, clear winter night two farms differ in the moisture of their air: Farm P has more humid air with a dew point of +4 °C, and Farm Q has drier air with a dew point of −2 °C. Farm P has droplets on its crop leaves and Farm Q has white crystals. Explain with reference to the dew point, and say what a farmer should expect at P on a later, colder night. (5 marks)

Try it first, then show the solutionSolution

Both surfaces cooled to the dew point (1 mark).
At P the dew point (+4 °C) is above 0 degrees C so water droplets formed: dew (1 mark).
At Q the dew point (−2 °C) is below 0 degrees C so vapour was deposited as ice crystals: frost (1 mark).
Other conditions are the same: clear sky, calm air, long cold night (1 mark).
On a colder night, if the surface at P falls below 0 degrees C, the vapour or dew can freeze and P can also show frost (1 mark).

Answer: Both leaf surfaces cooled to the dew point on a clear, calm night. The air at P is more humid, with a dew point of +4 °C, above freezing, so droplets formed as dew. The drier air at Q has a dew point of −2 °C, below freezing, so minute ice crystals formed as frost. If the night turns colder and the surface at P falls below 0 °C, the vapour or dew can freeze and P will also show frost.

Where students lose marks in Humidity, Evaporation, Condensation, Dew, Frost, Fog and Clouds

The slips examiners see most often, and what to write instead.

Mistake: Mixing up absolute and relative humidity
Both have "humidity" in the name.
Wrong working
Absolute humidity is a percentage of capacity.
Right working
Absolute is weight per volume (g per cubic metre); relative is a percentage.
Do this: Attach a unit to each.
Usually costs: 1 mark.
Mistake: Saying dew forms on dust in the air
Confusing dew with fog and cloud.
Wrong working
Dew is condensation on nuclei in the sky.
Right working
Dew is deposited as droplets on cool solid surfaces; fog and cloud form on nuclei in the air.
Do this: Link dew with grass blades and leaves.
Usually costs: 1 mark.
Mistake: Wrong fog and mist visibility
Numbers not learnt.
Wrong working
Fog has visibility of 1 to 2 km.
Right working
Fog: under 1 km; mist: 1 to 2 km.
Do this: Memorise the pair.
Usually costs: 1 mark.

Exam tips

Give the unit of absolute humidity and say relative humidity is a percentage.
For dew and frost always state the dew point relative to freezing.
In cloud answers give the height range and one feature for each type.

Practice questions on Humidity, Evaporation, Condensation, Dew, Frost, Fog and Clouds

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which cloud is thin, feathery, always white and found at 8,000 to 12,000 m?
(A) cumulus
(B) stratus
(C) cirrus
(D) nimbus
Hint
Highest cloud.
Answer
(C)
Q2 · 2 marks · Very short answerBoard level
Distinguish absolute humidity from relative humidity. (2 marks)
Hint
Weight versus percentage.
Answer
Absolute humidity is the actual weight of water vapour per unit volume of air in g per cubic metre. Relative humidity is the percentage of the air's full capacity, at that temperature, that is actually filled.
Q3 · 3 marks · Short answerHarder board level
Under what three situations does vapour in the air condense? Which one is most favourable? (3 marks)
Hint
Dew point, volume, moisture.
Answer
Condensation occurs when (i) the air is cooled to its dew point at constant volume, (ii) both volume and temperature of the air are reduced, and (iii) moisture is added to the air by evaporation. A fall in temperature is the most favourable condition.
Q4 · 3 marks · Short answerHarder board level
Why are dust, smoke and sea salt important for cloud and fog formation, and how does this lead to smog over cities? (3 marks)
Hint
Hygroscopic nuclei.
Answer
These particles are hygroscopic: they absorb water, so vapour condenses on them in free air. Over cities smoke supplies abundant nuclei, so fog mixed with smoke forms smog.
Q5 · 4 marks · Short answerHOTS / competency
A valley resident sees a dense dark grey cloud so low that it seems to touch the hilltops, while earlier in the day thin white wisps floated far higher. Name both clouds and explain how the two differ in height, density and likely outcome. (4 marks)
Hint
Nimbus and cirrus.
Answer
The low, dark, dense and opaque cloud is nimbus, found at middle levels or very near the ground and associated with rain. The thin, feathery white wisps are cirrus, the highest cloud at 8,000 to 12,000 m, which is detached and does not give rain.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What is the dew point?Show answer
The temperature at which air becomes saturated.
2.Which cloud is always white and feathery?Show answer
Cirrus.
3.Where is relative humidity greatest?Show answer
Over oceans.
4.What is smog?Show answer
Fog mixed with smoke.

Humidity, Evaporation, Condensation, Dew, Frost, Fog and Clouds: frequently asked questions

What is latent heat of vaporisation?

The heat needed to change a unit mass of liquid into vapour without a change in temperature.

Can condensation occur below freezing?

Yes; the dew point can be below or above freezing.

Why does moving air increase evaporation?

It replaces saturated air near the water with unsaturated air.