GeographyISC Class 11 · Atmospheric Moisture

Evaporation, Condensation and Humidity (ISC Class 11 Geography)

Updated 17 October 2026 · Fact-checked

Evaporation turns liquid water to vapour, absorbing latent heat; condensation reverses it when air cools to its dew point on nuclei, releasing latent heat. Absolute humidity is vapour mass per volume of air; relative humidity is actual vapour divided by capacity at that temperature, times 100, and rises as air cools.

Evaporation absorbs latent heat and condensation releases it; with the same vapour, cooling air raises relative humidity until it hits the dew point.

Illustration drawn for QuestionBuddy.

Understand Evaporation, Condensation and Humidity

Evaporation. Change from liquid to vapour; it takes latent heat from the surroundings and so cools the surface. It increases with high temperature, dry air, wind, a large open water surface and low pressure.

Condensation. Change from vapour to liquid (or directly to ice). It needs air cooled to its dew point and tiny nuclei (dust, smoke, salt). It releases the latent heat absorbed in evaporation, warming the air and powering thunderstorms and cyclones. Cooling occurs by adiabatic expansion of rising air (most important for clouds), contact with cold surfaces at night, mixing with colder air and advection over a cold surface.

Humidity.

Absolute humidity: mass of vapour per unit volume of air (g/m3); changes when air expands or contracts.
Specific humidity: mass of vapour per unit mass of air (g/kg); does not change with expansion.
Relative humidity (RH) = actual vapour content / capacity at that temperature x 100. With the same vapour, RH falls when air warms and rises when it cools, so it is lowest in early afternoon and highest around dawn. RH of 100 per cent is saturation; the temperature at which unsaturated air becomes saturated on cooling is the dew point.
RH is high over oceans and humid tropics, low over hot deserts; high RH makes heat uncomfortable because sweat evaporates slowly.
Measured by a hygrometer, such as the wet-and-dry bulb thermometer (psychrometer): a bigger difference between readings means drier air.

Precipitation begins when droplets or ice crystals become too heavy to stay suspended.

Evaporation, Condensation and Humidity: key facts

Relative humidity: $R H ($
Absolute and specific humidity: Absolute: g per m3 | Specific: g per kg
Evaporation controls: Temperature
Dryness
Wind
Open water
Low pressure
Condensation needs: Cooling to dew point
Nuclei
Releases latent heat

How to answer Evaporation, Condensation and Humidity questions

Humidity questions mix definitions (2 to 3 marks) with RH sums (3 to 4 marks).

1Write the RH formula and state both quantities in g per m3.
2Substitute and compute the percentage.
3For a temperature change, update the capacity but keep vapour constant.
4Interpret: saturated, rising or falling RH, or dew point.

Evaporation, Condensation and Humidity 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.

Dew point reasoning

Suppose air at 30 degrees C holds 18 g/m3 of vapour and its capacity at 30 degrees C is 30 g/m3.

Find its RH, and say what happens when it cools to a temperature where capacity is 18 g/m3.

Try it first, then show the solutionSolution

RH = 18/30 x 100 = 60 per cent.
At the temperature where capacity is 18 g/m3 the RH is 100 per cent, which is the dew point.

Answer: 60 per cent; the air reaches its dew point at that cooler temperature.

Memory hook: Vapour same, capacity falls.

Wet laundry

Suppose clothes dry quickly on a warm, windy, dry day but slowly on a damp, still, cool day.

Name the factors of evaporation that explain this.

Try it first, then show the solutionSolution

Warmth, wind and dry air speed evaporation.
Damp, cool, still air slows it.

Answer: Temperature, wind and dryness control evaporation.

Memory hook: Hot, windy, dry: fast drying.

Tricks to remember Evaporation, Condensation and Humidity

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

Warm air thirsty
Warm air can hold more vapour, so RH drops when it warms.
Use it when: RH with temperature.
Absolute changes, specific stays
Absolute humidity changes with volume; specific does not.
Use it when: Contrast.

Evaporation, Condensation and Humidity: solved examples

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

Example 1: Definition · 1 mark · MCQBoard level

Relative humidity is the ratio of:
(a) vapour to dry air in grams
(b) actual vapour to capacity at that temperature
(c) rainfall to evaporation
(d) temperature to pressure

Try it first, then show the solutionSolution

Percentage of capacity.

Answer: (b) Actual vapour to capacity at that temperature.

Example 2: Latent heat · 3 marks · Short answerBoard level

Explain why evaporation cools a surface and where condensation releases heat.

Try it first, then show the solutionSolution

Evaporation takes latent heat from the surroundings.
Cooling occurs at the surface.
Condensation releases the stored latent heat to the air.

Answer: Evaporation absorbs latent heat; condensation releases it to the air.

Example 3: RH sums · 4 marks · Long answerHarder board level

Air at 20 degrees C holds 12 g/m3 of vapour with capacity 24 g/m3. Find its RH. If it warms to a temperature where capacity is 30 g/m3, find the new RH. If it cools to where capacity is 12 g/m3, state the RH.

Try it first, then show the solutionSolution

RH = 12/24 x 100 = 50 per cent.
Warmer: 12/30 x 100 = 40 per cent.
Cooler to capacity 12: 12/12 x 100 = 100 per cent, the dew point.
RH falls on warming and rises on cooling.

Answer: 50 per cent; 40 per cent when warmed; 100 per cent at the dew point.

Example 4: Daily RH pattern · 4 marks · Long answerHarder board level

Explain why RH is usually lowest in the early afternoon and highest around dawn.

Try it first, then show the solutionSolution

Vapour content changes little in a day.
Afternoon air is warmest, so capacity is greatest.
Same vapour over greater capacity gives low RH.
At dawn the air is coolest, capacity least, so RH is highest.

Answer: Same vapour, capacity larger when warm: low RH in afternoon; high at dawn.

Example 5: Interpret a psychrometer · 5 marks · Case-basedHOTS / competency

Case (invented): A psychrometer shows a large difference between the wet and dry bulb readings at noon in a desert and a small difference in a humid coastal town. (a) Which place has drier air? (b) Explain with evaporation. (c) Say why humid heat feels worse.

Try it first, then show the solutionSolution

(a) The desert.
(b) Large difference means more evaporation cools the wet bulb, so the air is dry.
(c) In humid air sweat evaporates slowly, so the body cools poorly.

Answer: (a) Desert (b) more evaporation cools the wet bulb (c) sweat evaporates slowly.

Where students lose marks in Evaporation, Condensation and Humidity

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

Mistake: Dividing capacity by actual
RH is inverted.
Wrong working
24/12 x 100 = 200%
Right working
12/24 x 100 = 50%
Do this: RH = actual / capacity x 100.
Usually costs: 2 marks.
Mistake: Saying warm air has higher RH
Capacity effect forgotten.
Do this: With constant vapour, RH falls when air warms.
Usually costs: 1 mark.
Mistake: Forgetting units
g per m3 missing.
Do this: Write g per m3 for absolute humidity.
Usually costs: 1 mark.

Exam tips

Always show RH substitution.
Say capacity grows with temperature.
Define dew point with 100 per cent RH.
Include nuclei in condensation answers.

Practice questions on Evaporation, Condensation and Humidity

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 humidity does not change when air expands?
(a) Absolute
(b) Specific
(c) Relative
(d) Capacity
Hint
Per unit mass.
Answer
(b) Specific.
Q2 · 4 marks · Long answerHarder board level
Air holds 15 g/m3 and the RH is 75 per cent. Find the capacity. If it cools until its capacity is 16 g/m3, find the new RH.
Hint
Capacity = actual / RH.
Answer
Capacity = 15 / 0.75 = 20 g/m3. New RH = 15/16 x 100 = 93.75 per cent.
Q3 · 3 marks · Short answerHarder board level
A cup of water stands in a hot, dry, windy room. Explain why it evaporates quickly.
Hint
Evaporation factors.
Answer
High temperature, dry air far from saturation and wind removing moist air all increase evaporation.
Q4 · 3 marks · Short answerHarder board level
Distinguish between absolute and specific humidity.
Hint
Per volume versus per mass.
Answer
Absolute humidity is vapour mass per unit volume (g per m3) and changes when air expands; specific humidity is vapour mass per unit mass of air (g per kg) and stays constant.
Q5 · 4 marks · Case-basedHOTS / competency
Case: On a clear calm night, air near the ground cools until dew forms on grass. (a) Name the temperature reached. (b) State what RH is then. (c) Explain why nuclei are needed for clouds.
Hint
Dew point.
Answer
(a) The dew point. (b) 100 per cent. (c) Vapour condenses on dust, smoke or salt nuclei.

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.Unit of absolute humidity?Show answer
Grams per cubic metre.
2.100 per cent RH means?Show answer
Saturation.
3.Instrument for humidity?Show answer
Hygrometer or psychrometer.
4.Condensation releases?Show answer
Latent heat.
5.When is RH highest in the day?Show answer
Around dawn.

Evaporation, Condensation and Humidity: frequently asked questions

Why does evaporation cool?

It uses latent heat from the surface.

Does RH depend on temperature?

Yes, through capacity.

What are condensation nuclei?

Tiny particles such as dust, smoke and salt.