Water in the Atmosphere, Class 11 Geography: Humidity, Clouds and Rainfall

What Water in the Atmosphere is about

Weightage: This chapter is part of Unit IV, Climate, of Fundamentals of Physical Geography, which carries 8 of the 70 theory marks in the school annual exam together with Composition and Structure of Atmosphere, Solar Radiation, Heat Balance and Temperature and Atmospheric Circulation and Weather Systems. Map work carries a further 5 marks for each book.Source: CBSE Geography (029) syllabus 2026-27, Class XI course structure

Key concepts

Humidity, saturation and dew point

NCERT section Water in the Atmosphere (introduction)

Water vapour in the air varies from zero to four per cent by volume. Water exists in air in three forms: gaseous, liquid, solid. It comes from water bodies (evaporation) and plants (transpiration) and returns through condensation and precipitation.

Humidity = water vapour present in the air.

Term	Meaning	Unit/notes
Absolute humidity	Actual amount (weight) of water vapour per unit volume of air	g/m³; differs from place to place
Relative humidity	Percentage of moisture present compared with the full capacity at a given temperature	%; changes as temperature changes capacity; greatest over oceans, least over continents
Saturated air	Air holding moisture to its full capacity at a given temperature; cannot hold any more	-
Dew point	The temperature at which saturation occurs in a sample of air	°C

Key idea: the air's capacity to hold water vapour depends entirely on its temperature. Warming air raises capacity (relative humidity falls); cooling lowers capacity (relative humidity rises until the dew point is reached).

Worked example

Suppose a parcel of air contains 12 g/m³ of water vapour and could hold 20 g/m³ at its present temperature. Find its relative humidity. What happens to the relative humidity if the air cools without losing moisture? (2 marks)

Try it first, then show the solutionSolution

1. Relative humidity = 12 ÷ 20 × 100 = 60 per cent.
2. Cooling lowers the capacity, so the same moisture forms a larger percentage: relative humidity rises; at the dew point it reaches saturation.

Answer: 60 per cent; it rises on cooling until the air is saturated at the dew point.

Evaporation and condensation

NCERT section Evaporation and Condensation

Evaporation: water changes from liquid to gas; heat is the main cause.

• Latent heat of vaporisation: heat energy needed to convert a unit mass of liquid into vapour without a change in temperature.
• Evaporation increases with: higher temperature (greater capacity to absorb and retain moisture), low moisture content of the air, and air movement, which replaces the saturated layer with unsaturated air (the more the air moves, the greater the evaporation).

Condensation: water vapour changes to water; caused by loss of heat.

• When moist air cools, its capacity to hold vapour may be exceeded and the excess condenses.
• In free air it happens around very small hygroscopic condensation nuclei: dust, smoke and sea salt are good nuclei because they absorb water.
• It also happens when moist air touches a colder object, or when temperature is close to the dew point.
• It depends on the amount of cooling and the relative humidity; influenced by volume, temperature, pressure and humidity.

Condensation takes place when:
1. Air temperature falls to the dew point with volume constant;
2. Both volume and temperature are reduced;
3. Moisture is added through evaporation.
The most favourable condition is a decrease in air temperature.

Condensation can occur with the dew point below or above freezing.

Worked example

Why do clothes dry faster on a windy day than on a still day at the same temperature? (2 marks)

Try it first, then show the solutionSolution

1. Evaporation from the clothes saturates the thin layer of air next to them.
2. Wind replaces this saturated layer with unsaturated air that can absorb more moisture, so evaporation continues faster.

Answer: Moving air removes the saturated layer, increasing evaporation.

Dew and frost

NCERT section Evaporation and Condensation

Forms of condensation are classified by temperature and location.

Dew

• Moisture deposited as water droplets on cooler surfaces of solid objects (stones, grass blades, leaves), not on nuclei in the air.
• Ideal conditions: clear sky, calm air, high relative humidity, cold and long nights.
• The dew point must be above freezing.

Frost

• Forms on cold surfaces when condensation takes place below freezing (0°C): the dew point is at or below freezing.
• Excess moisture is deposited as minute ice crystals instead of droplets.
• Ideal conditions for white frost: the same as for dew, except that the air temperature must be at or below freezing.

Worked example

On two clear, calm winter nights, the dew points are 4°C and -2°C. What forms on the grass each morning? (2 marks)

Try it first, then show the solutionSolution

1. With a dew point of 4°C (above freezing), moisture condenses as droplets: dew.
2. With a dew point of -2°C (below freezing), it is deposited as tiny ice crystals: frost.

Answer: Dew on the first night, frost on the second.

Fog, mist and smog

NCERT section Evaporation and Condensation

Fog: when an air mass with much water vapour cools suddenly, condensation takes place within itself on fine dust particles. Fog is a cloud with its base at or very near the ground: a mini cloud with nuclei of dust, smoke and salt.

	Fog	Mist
Visibility	Less than 1 km	1-2 km
Moisture	Drier	More moisture; each nucleus has a thicker layer of moisture
Where	Where warm currents of air meet cold currents	Frequent over mountains, where rising warm air meets cold slopes

Smog: in urban and industrial centres, smoke provides plenty of nuclei for fog and mist; fog mixed with smoke is smog.

Worked example

Visibility on a highway falls to 600 m one morning and to 1.5 km the next. Classify each. (1 mark)

Try it first, then show the solutionSolution

1. Fog limits visibility to less than 1 km: 600 m is fog.
2. Mist limits it to between 1 and 2 km: 1.5 km is mist.

Answer: Fog the first morning, mist the second.

Clouds

NCERT section Clouds

Cloud: a mass of minute water droplets or tiny ice crystals formed by condensation of water vapour in free air at considerable heights. Grouped by height, expanse, density and transparency into four types:

Type	Height	Features
Cirrus	8,000-12,000 m (highest)	Thin, detached, feathery, always white
Cumulus	4,000-7,000 m	Like cotton wool; in patches, scattered; flat base
Stratus	-	Layered, cover large parts of the sky; form by loss of heat or mixing of air masses of different temperatures
Nimbus	Middle levels or very near the surface	Black or dark grey; extremely dense, opaque to sunlight; sometimes seem to touch the ground; shapeless masses of thick vapour

Combinations:

• High clouds: cirrus, cirrostratus, cirrocumulus
• Middle clouds: altostratus, altocumulus
• Low clouds: stratocumulus, nimbostratus
• Extensive vertical development: cumulus, cumulonimbus

Worked example

A pilot flying at 10 km sees thin, white, feathery clouds above the aircraft. Name them and explain why they are white. (2 marks)

Try it first, then show the solutionSolution

1. Thin, detached, feathery clouds at 8,000-12,000 m are cirrus.
2. They are thin, so they are not dense or opaque enough to look grey; NCERT describes them as always white.

Answer: Cirrus; thin high clouds that are always white.

Precipitation and its forms

NCERT section Precipitation

Precipitation: continuous condensation in free air makes particles grow; when air resistance can no longer hold them against gravity, they fall. Release of moisture after condensation = precipitation, in liquid or solid form.

Form	How it forms
Rainfall	Precipitation as water
Snowfall	When temperature is below 0°C: fine flakes; moisture released as hexagonal crystals that form snowflakes
Sleet	Frozen raindrops and refrozen melted snow-water: a layer of above-freezing air over a sub-freezing layer near the ground; raindrops leaving the warm air freeze in the cold air below and reach the ground as small ice pellets no bigger than raindrops
Hail	Raindrops released by clouds solidify into small rounded pieces of ice while passing through colder layers; hailstones have several concentric layers of ice

Sleet and hail are limited and sporadic in time and space.

Worked example

How does sleet differ from hail in its formation and size? (2 marks)

Try it first, then show the solutionSolution

1. Sleet: raindrops from a warm layer freeze while falling through a sub-freezing layer near the ground; pellets are no bigger than the raindrops.
2. Hail: raindrops solidify into rounded ice pieces while passing through colder layers, building several concentric layers of ice.

Answer: Sleet: small pellets frozen near the ground; hail: layered ice balls formed in colder layers.

Types of rainfall

NCERT section Types of Rainfall

By origin, three types:

1. Convectional rain

• Heated air becomes light and rises in convection currents, expands, loses heat, condenses; cumulus clouds form.
• Heavy rain with thunder and lightning, but short-lived.
• Common in summer or the hotter part of the day; very common in equatorial regions and interiors of continents, especially in the northern hemisphere.

2. Orographic (relief) rain

• A saturated air mass meets a mountain, is forced to rise, expands, cools and condenses.
• Windward slopes get more rain.
• On the other side the air descends and warms, its capacity to hold moisture rises, so the leeward slopes stay dry.
• The leeward area with less rain is the rain-shadow area.

3. Cyclonic (frontal) rain

• Associated with extra-tropical cyclones and fronts (Chapter 9): warm air is lifted over cold air along fronts, cools and gives rain.

Worked example

Why is the leeward side of a mountain range dry? (2 marks)

Try it first, then show the solutionSolution

1. The moist air shed most of its moisture as orographic rain while rising up the windward slope.
2. Descending the leeward side, the air warms, its capacity to hold moisture increases and it does not condense, so the leeward side is a rain-shadow area.

Answer: Descending, warming air absorbs rather than releases moisture: rain shadow.

World distribution of rainfall

NCERT section World Distribution of Rainfall

General patterns:

• Rainfall decreases steadily from the equator towards the poles.
• Coastal areas get more than continental interiors.
• Oceans get more than land, being great sources of water.
• 35°-40° N and S: rain heavier on eastern coasts, decreasing westward.
• 45°-65° N and S: because of the westerlies, rain falls first on western margins, decreasing eastward.
• Where mountains run parallel to the coast, rain is greater on the coastal plain (windward) and decreases leeward.

Precipitation regimes (annual):

Amount	Regions
Over 200 cm (heavy)	Equatorial belt; windward slopes of mountains on western coasts in the cool temperate zone; coastal areas of monsoon lands
100-200 cm (moderate)	Interior continental areas; coastal areas of continents (moderate)
50-100 cm	Central parts of tropical lands; eastern and interior parts of temperate lands
Under 50 cm (very low)	Rain-shadow zones of continental interiors and high latitudes

Seasonal distribution is important for judging effectiveness. Rain is evenly distributed through the year in the equatorial belt and western parts of cool temperate regions.

Worked example

Why do western coasts between 45° and 65° latitude receive more rain than eastern coasts? (2 marks)

Try it first, then show the solutionSolution

1. In these latitudes the westerlies blow from the ocean onto the western margins of continents.
2. They give rain first on the western side, and rainfall decreases eastward as the air loses moisture.

Answer: The westerlies bring ocean moisture to western coasts first.

Water in the Atmosphere in real life

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

Topics in this chapter

• Humidity, Evaporation, Condensation, Dew, Frost, Fog and Clouds
• Precipitation, Types of Rainfall and World Distribution of Rainfall

Water in the Atmosphere: key facts

Water vapour in air

0-4% by volume

Three forms: gas, liquid, solid.

Relative humidity

$RH($

Greatest over oceans, least over continents.

Absolute humidity

Weight of water vapour per unit volume of air (g/m³)

Actual amount present.

Condensation conditions

• •Cool to dew point (volume constant)
• •reduce volume and temperature
• •add moisture by evaporation

Most favourable: fall in temperature.

Fog vs mist

$Fog:visibility<1km|Mist:1-2km$

Mist holds more moisture.

Cloud heights

Cirrus 8,000-12,000 m; cumulus 4,000-7,000 m; nimbus middle levels to near the ground

Stratus: layered sheets.

Rainfall types

$Convectional(heating)|orographic(mountain)|cyclonic/frontal(fronts)$

Rain shadow on the leeward side.

Rain by coast

35°-40°: east coasts wetter | 45°-65°: west coasts wetter (westerlies)

Rain decreases from equator to poles.

Rainfall regimes

$>200cm|100-200cm|50-100cm|<50cm$

Equatorial and monsoon coasts heaviest; rain-shadow interiors and high latitudes lowest.

Water in the Atmosphere: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Water in the Atmosphere

The question patterns to expect, and the method that works for each.

Tricks to remember Water in the Atmosphere

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

• Forms of condensation

  'Dewy Frosty Foggy Clouds': dew, frost, fog/mist, clouds.

  Use it when: Listing forms.

• Cloud types

  'Can Cows Sing Nicely': Cirrus (highest), Cumulus, Stratus, Nimbus (lowest, darkest).

  Use it when: Cloud types and order.

• Precipitation forms

  'Rain Snows Sleet Hail' from warm to icy.

  Use it when: Forms of precipitation.

• Rainfall types

  'COC': Convectional, Orographic, Cyclonic.

  Use it when: Types of rainfall.

• Rainfall regimes

  '200-100-50': over 200 heavy, 100-200 moderate, 50-100 low, under 50 very low.

  Use it when: Regime table.

Water in the Atmosphere: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Water in the Atmosphere

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

• ✗Mistake: Confusing absolute and relative humidity

  Both describe moisture.

  Wrong working

  Relative humidity is the weight of water vapour per cubic metre.

  Right working

  Absolute humidity is g/m³; relative humidity is the percentage of capacity.

  ✓Do this: Absolute = amount; relative = percentage.

  Usually costs: 1 mark.

• ✗Mistake: Saying frost forms when the dew point is above freezing

  Students link frost only to cold air.

  Wrong working

  Frost forms when the dew point is above 0°C.

  Right working

  Frost needs the dew point at or below freezing; dew needs it above freezing.

  ✓Do this: Dew above zero, frost at or below.

  Usually costs: 1 mark.

• ✗Mistake: Reversing fog and mist visibility

  Similar features.

  Wrong working

  Mist reduces visibility to below 1 km.

  Right working

  Fog: below 1 km; mist: 1-2 km.

  ✓Do this: Fog is Foggier.

  Usually costs: 1 mark.

• ✗Mistake: Calling the windward side the rain shadow

  Side names confused.

  Wrong working

  The windward side is the rain-shadow area.

  Right working

  The windward side gets heavy rain; the leeward side is the rain shadow.

  ✓Do this: Lee = less.

  Usually costs: 1 mark.

• ✗Mistake: Placing the wetter coast wrongly by latitude

  Two latitude bands with opposite patterns.

  Wrong working

  Between 45° and 65°, east coasts are wetter.

  Right working

  35°-40°: east coasts wetter; 45°-65°: west coasts wetter (westerlies).

  ✓Do this: Westerlies wet the West at 45°-65°.

  Usually costs: 1 mark.

NCERT exercise guide: Water in the Atmosphere

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Water in the Atmosphere

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

1. Q1 · 1 mark · MCQExam level

   Drivers on a hill road can see about 1.5 km ahead through a thin, moist haze, while on the plains below visibility is under 1 km in a drier haze. The hill condition is:
   (a) fog, which holds more moisture than mist
   (b) mist, which holds more moisture than fog
   (c) smog, formed only in hills
   (d) frost

   Hint

   Visibility 1-2 km; mists are frequent over mountains.

   Answer

   (b) Mist: it contains more moisture than fog, limits visibility to 1-2 km and is frequent over mountains.

2. Q2 · 1 mark · MCQExam level

   Air at a given temperature that cannot hold any more moisture has reached its dew point. Its relative humidity is then:
   (a) 0 per cent
   (b) 50 per cent
   (c) 100 per cent
   (d) more than 100 per cent

   Hint

   Saturated air holds moisture to its full capacity.

   Answer

   (c) 100 per cent: the air is saturated.

3. Q3 · 1 mark · Assertion-reasonHarder exam level

   Assertion (A): Mist is frequent over mountains.
   Reason (R): Rising warm air up the slopes meets a cold surface.
   Choose (a)-(d).

   Hint

   NCERT's own reason.

   Answer

   (a). Both true; R explains A.

4. Q4 · 2 marks · Short answerExam level

   Name the three conditions under which condensation takes place and the most favourable one. (2 marks)

   Hint

   Volume, temperature, moisture.

   Answer

   (i) Temperature falls to the dew point with volume constant; (ii) both volume and temperature fall; (iii) moisture is added by evaporation. Most favourable: a decrease in air temperature.

5. Q5 · 2 marks · Short answerHarder exam level

   Suppose air at 30°C can hold 30 g/m³ and actually holds 15 g/m³. Find the relative humidity. If it cooled until its capacity was 15 g/m³, what would happen? (2 marks)

   Hint

   Actual ÷ capacity.

   Answer

   RH = 15 ÷ 30 × 100 = 50 per cent. If capacity fell to 15 g/m³, the air would be saturated (100 per cent): it would have reached its dew point, and further cooling would cause condensation.

6. Q6 · 3 marks · Short answerHarder exam level

   Distinguish between convectional and orographic rainfall. (3 marks)

   Hint

   Cause of lifting, timing, place.

   Answer

   Convectional: air heated at the surface rises in convection currents; cumulus clouds; heavy short rain with thunder; summer afternoons, equatorial regions and continental interiors. Orographic: saturated air forced up a mountain; rain on windward slopes; leeward slopes dry (rain shadow).

7. Q7 · 3 marks · Short answerHOTS / competency

   Suppose a hill farmer notices that his upper fields stay dry on calm, clear winter nights while those in the hollow below are white with ice crystals. Using this and the previous chapter, explain. (3 marks)

   Hint

   Air drainage and frost.

   Answer

   On calm, clear nights cold dense air drains into hollows (air drainage), so the hollow becomes coldest. There the dew point is at or below 0°C, so moisture is deposited as ice crystals: frost. The upper slopes stay warmer, so no frost forms there.

8. Q8 · 5 marks · Long answerExam level

   A sky-watching club photographs four clouds: (i) cotton-wool patches with flat bases at about 5,000 m; (ii) a grey layer covering the whole sky; (iii) dark, dense clouds close to the ground with steady rain; (iv) white feathery wisps very high up. Explain how clouds form, and identify each with its features. (5 marks)

   Hint

   Height, shape, density.

   Answer

   Clouds are masses of minute water droplets or tiny ice crystals formed by condensation of water vapour on hygroscopic nuclei in free air at considerable heights. (i) Cumulus: like cotton wool, 4,000-7,000 m, in patches with a flat base. (ii) Stratus: layered clouds covering large parts of the sky, formed by loss of heat or mixing of air masses of different temperatures. (iii) Nimbus: black or dark grey, at middle levels or very near the surface, extremely dense and opaque, often giving rain. (iv) Cirrus: high (8,000-12,000 m), thin, detached and feathery, always white. Combinations give high, middle and low clouds and clouds of vertical development such as cumulonimbus.

9. Q9 · 2 marks · Short answerHOTS / competency

   Why do hailstones show concentric layers while sleet pellets do not? (2 marks)

   Hint

   Where and how each freezes.

   Answer

   Hail forms as raindrops solidify while passing through colder layers, building several concentric layers of ice. Sleet forms when raindrops simply freeze once while falling through a shallow sub-freezing layer near the ground, giving small pellets no bigger than the raindrops.

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. 1.How much of the atmosphere by volume can water vapour be?Show answer

   0-4 per cent.

2. 2.What is the dew point?Show answer

   The temperature at which saturation occurs in a given sample of air.

3. 3.What is latent heat of vaporisation?Show answer

   Heat needed to convert a unit mass of liquid into vapour without a change in temperature.

4. 4.Name three good condensation nuclei.Show answer

   Dust, smoke and sea salt.

5. 5.At what height do cumulus clouds form?Show answer

   4,000-7,000 m.

6. 6.In what shape are snow crystals?Show answer

   Hexagonal.

7. 7.What is a rain-shadow area?Show answer

   The leeward side of a mountain that receives less rainfall.

8. 8.Where is rainfall evenly distributed through the year?Show answer

   Equatorial belt and western parts of cool temperate regions.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Water vapour 0-4%; three forms; exchange by evaporation, transpiration, condensation, precipitation.
• ☐Absolute humidity g/m³; relative humidity % of capacity; saturated air; dew point.
• ☐Evaporation: heat, low moisture, moving air; latent heat of vaporisation.
• ☐Condensation: loss of heat; nuclei of dust, smoke, salt; most favourable: cooling.
• ☐Dew: dew point above 0°C; frost: at or below 0°C; both need clear, calm, humid long nights.
• ☐Fog < 1 km visibility, mist 1-2 km; smog = fog + smoke.
• ☐Clouds: cirrus 8-12 km, cumulus 4-7 km, stratus layers, nimbus dark and low.
• ☐Precipitation: rain, snow (hexagonal crystals), sleet, hail (concentric layers).
• ☐Rainfall: convectional, orographic (rain shadow), cyclonic.
• ☐Distribution: decreases poleward; east coasts wetter at 35°-40°, west at 45°-65°; regimes > 200 to < 50 cm.

Water in the Atmosphere: frequently asked questions