GeographyICSE Class 9 · Humidity

Humidity, Condensation and Types of Rainfall (ICSE Class 9 Geography)

Updated 17 October 2026 · Draft, not yet fact-checked

Humidity is the water vapour in the air. Absolute humidity is the mass of vapour per unit volume of air; relative humidity is the percentage of vapour present compared with the most the air could hold at that temperature. Cooling to the dew point causes condensation (dew, frost, fog, mist, clouds), and precipitation falls as rain, snow, sleet or hail, with convectional, relief or cyclonic rainfall.

Understand Humidity, Condensation and Types of Rainfall

1. Humidity. *Absolute humidity* is the mass of water vapour per unit volume of air (grams per cubic metre). *Relative humidity* = (actual vapour content / maximum the air can hold at that temperature) x 100. Warm air can hold more vapour than cold air. When air is saturated its relative humidity is 100 per cent, and the temperature is the *dew point*.

2. Condensation. Cooling below the dew point turns vapour into liquid or ice on nuclei such as dust. Forms: *dew* (drops on cool surfaces on clear calm nights), *frost* (when the dew point is below freezing, vapour turns directly to ice crystals), *mist and fog* (condensation near the ground; fog is thicker, reducing visibility to less than 1 km), *clouds* (condensation at height; e.g. cirrus, cumulus, stratus, nimbus).

3. Precipitation. *Rain* (liquid drops), *snow* (ice crystals), *sleet* (rain and snow mixed or frozen rain drops) and *hail* (lumps of ice in thunderclouds).

4. Types of rainfall.

*Convectional*: heated moist air rises, cools and condenses, giving heavy afternoon showers with thunder in equatorial regions.
*Relief (orographic)*: moist winds forced up a mountain give rain on the windward side; the leeward side is a *rain shadow*.
*Cyclonic (frontal)*: air rises in a cyclone, or where warm air rides over cold at a front, giving widespread rain.

Humidity, Condensation and Types of Rainfall: key facts

Relative humidity: $R H = a c t u a l v a p o u r / ma x im u m c a p a c i t y x 100∣ S a t u r a t e d : 100 p er ce n t$
Condensation forms: $D e w ∣ F r os t ∣ M i s t ∣ F o g ∣ C l o u d s$
Precipitation: $R ain ∣ S n o w ∣ S l ee t ∣ H ai l$
Rainfall types: $C o n v ec t i o na l ∣ R e l i e f (or o g r a p hi c) ∣ C y c l o ni c (f r o n t a l)$

Humidity, Condensation and Types of Rainfall 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 of water

Suppose a glass of chilled water on a hot day gets droplets on its outside.

Explain why.

Try it first, then show the solutionSolution

The air near the glass is cooled below its dew point.
Water vapour condenses on the glass.

Answer: Vapour condenses on the cold surface.

Memory hook: Cool below dew point.

Tricks to remember Humidity, Condensation and Types of Rainfall

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

Dew point = condense point
At dew point, vapour starts to condense.
Use it when: Meaning.
Windward wet, leeward dry
Relief rain falls on the windward slope.
Use it when: Relief rainfall.

Humidity, Condensation and Types of Rainfall: solved examples

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

Example 1: Define · 2 marks · Short answerBoard level

Define relative humidity and give its formula. [2]

Try it first, then show the solutionSolution

Percentage of water vapour present to the maximum possible at that temperature.
RH = actual / capacity x 100.

Answer: RH = (actual vapour / maximum vapour) x 100.

Example 2: Calculate RH · 3 marks · Short answerHarder board level

At 25 degrees C, 1 m3 of air can hold 23 g of vapour but contains 11.5 g. Find the relative humidity and say what happens if it cools until it reaches the dew point. [3]

Try it first, then show the solutionSolution

RH = 11.5 / 23 x 100 = 50 per cent.
On cooling, capacity falls and RH rises.
At the dew point RH is 100 per cent and condensation begins.

Answer: RH = 50 per cent; at the dew point it reaches 100 per cent and vapour condenses.

Example 3: Fog and clouds · 3 marks · Short answerHarder board level

Compare fog and a cloud in position and cause. [3]

Try it first, then show the solutionSolution

Fog forms near the ground; cloud forms at height.
Both form by condensation on cooling.
Fog is a cloud at ground level.

Answer: Both are condensation; fog is near the ground, cloud is high.

Example 4: Relief rainfall · 4 marks · Long answerHarder board level

Explain with a labelled sketch description how relief rainfall and a rain shadow form. [4]

Try it first, then show the solutionSolution

Moist winds meet a mountain and rise.
They cool and condense, giving rain on the windward side.
Winds descend on the leeward side, warm and dry.
The leeward area is a rain shadow.

Answer: Rain on the windward slope; dry rain shadow on the leeward side.

Example 5: Case: afternoon thunder · 5 marks · Case-basedHOTS / competency

Case. In an equatorial town, hot sunny mornings are followed by heavy thundery showers every afternoon. (i) Name the rainfall. [1] (ii) Describe the process. [3] (iii) Why does it not occur at night? [1]

Try it first, then show the solutionSolution

(i) Convectional.
(ii) Strong heating makes moist air rise, cool and condense into cumulonimbus.
(iii) Cooling at night stops convection.

Answer: (i) Convectional (ii) Rising, cooling, condensing air (iii) No strong heating at night.

Where students lose marks in Humidity, Condensation and Types of Rainfall

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

Mistake: Using RH formula in the wrong order
Capacity put on top.
Do this: Actual over capacity.
Usually costs: 1 mark.
Mistake: Mixing frost and snow
Both ice.
Do this: Frost forms on surfaces; snow falls.
Usually costs: 1 mark.
Mistake: Forgetting rain shadow
Only windward rain.
Do this: Mention the leeward dry area.
Usually costs: 1 mark.

Exam tips

Show RH as a fraction then percentage.
Name the process for each form: condensation or sublimation.
Draw relief rainfall with arrows.
Give a place example for each rainfall type.

Practice questions on Humidity, Condensation and Types of Rainfall

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
Frost is formed when:
(a) dew point is above 0 degrees C
(b) dew point is below 0 degrees C
(c) air is saturated and warm
(d) fog is thick
Hint
Freezing.
Answer
(b).
Q2 · 1 mark · Very short answerBoard level
Name the instrument for measuring humidity. [1]
Hint
Hygro.
Answer
A hygrometer.
Q3 · 2 marks · Very short answerBoard level
Differentiate between dew and mist. [2]
Hint
Surface/air.
Answer
Dew is liquid drops on surfaces; mist is tiny droplets suspended in air.
Q4 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Warm air has a higher capacity for water vapour.
Reason (R): Warm air expands and its molecules move faster.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Capacity.
Answer
(a).
Q5 · 3 marks · Short answerHarder board level
State three differences between convectional and relief rainfall. [3]
Hint
Cause.
Answer
Convectional: heating, equatorial, thundery; relief: mountains, windward side, with a rain shadow.
Q6 · 2 marks · Very short answerBoard level
A room at 20 degrees C holds 8.5 g of vapour per m3 when its capacity is 17 g per m3. Find the relative humidity. [2]
Hint
Calculate.
Answer
8.5 / 17 x 100 = 50 per cent.
Q7 · 2 marks · Very short answerHarder board level
Why does hail form only in thunderclouds? [2]
Hint
Updraughts.
Answer
Strong updraughts carry drops up to freezing levels repeatedly, building layers of ice.
Q8 · 3 marks · Short answerHarder board level
Explain why the leeward side of the Western Ghats is dry. [3]
Hint
Rain shadow.
Answer
Moist winds shed rain on the windward side, then descend as dry winds on the leeward side.
Q9 · 4 marks · Case-basedHOTS / competency
Case. A morning in winter has thick fog at an airport. (i) Define fog. [1] (ii) Name the problem for flights. [1] (iii) Describe the cause. [2]
Hint
Visibility.
Answer
(i) A cloud at ground level of tiny droplets. (ii) Poor visibility. (iii) Cooling of air near the ground below dew point on clear calm nights.
Q10 · 4 marks · Case-basedHOTS / competency
Case. A student leaves a bowl of cold water in a warm humid kitchen. (i) What appears on the outside? [1] (ii) Explain. [2] (iii) Would it happen in dry desert air? Why? [1]
Hint
Dew point.
Answer
(i) Water droplets. (ii) Humid air touching the bowl cools below dew point. (iii) Less likely, because dry air has little vapour.

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.RH at saturation?Show answer
100 per cent.
2.Rain on windward side?Show answer
Relief rainfall.
3.Instrument for humidity?Show answer
Hygrometer.
4.Thundery afternoon rain?Show answer
Convectional.
5.Temperature at which condensation begins?Show answer
Dew point.

Humidity, Condensation and Types of Rainfall: frequently asked questions

What is a nucleus?

A tiny particle on which vapour condenses.

Is fog the same as mist?

Fog is denser.

What is sleet?

Frozen rain or rain mixed with snow.