This MCQ module is based on: Humidity, Condensation & Cloud Types
TOPIC 20 OF 29
Humidity, Condensation & Cloud Types
🎓 Class 11
Social Science
CBSE
Theory
Ch 10 — Water in the Atmosphere
⏱ ~28 min
🌐 Language: [gtranslate]
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10.1 Why Water Vapour Is the Most Restless Constituent of the Air
The air around you contains water — even on the driest summer afternoon. You cannot see it, but you can feel it as the stickiness of a Mumbai monsoon evening, you can taste it in the salt-flecked breeze of a coastal town, and you can watch it appear as the foggy breath leaving your mouth on a Shimla winter morning. This invisible water vapour varies from zero up to four per cent by volume of the atmosphere — a tiny share by mass, yet large enough to drive almost every weather phenomenon we experience.
Water is present in the atmosphere in three states — gaseous (vapour), liquid (droplets) and solid (ice crystals). The moisture comes mainly from two sources: from water bodies through evaporation, and from plants through transpiration. There is therefore a continuous exchange of water between the atmosphere, the oceans and the continents through the chain of evaporation, transpiration, condensation and precipitation. We call this loop the hydrological cycle, and Chapter 10 is essentially a close-up of its atmospheric leg.
📖 Definition — Humidity
The water vapour present in the air is known as humidity?. It is expressed quantitatively in different ways. The amount of water vapour the air can hold is not unlimited — it depends almost entirely on the temperature of the air. Warm air can hold a great deal of water vapour; cold air, very little. This single rule is the master key to dew, fog, cloud and rain.
Range in Atmosphere
Water vapour: 0–4% by volume. Highest over warm tropical seas; nearly zero over polar deserts.
Two Sources
Evaporation from oceans, lakes and rivers — and transpiration from leaves of plants.
Three States
Gaseous (vapour), liquid (droplets) and solid (ice). The atmosphere recycles water through all three.
Temperature Rules
Warm air holds more vapour; cold air less. Cooling air to "dew point" forces vapour to condense.
10.2 Evaporation — Liquid Becomes Vapour
The amount of water vapour in the atmosphere is added or withdrawn through two opposite processes — evaporation and condensation. Evaporation? is the process by which water is transformed from the liquid to the gaseous state. Heat is the main cause of evaporation; the energy needed to lift water molecules out of the liquid phase comes from the warmth of the Sun, the warmth of the surrounding air, or even the warmth of the body of the water itself.
🔥 Latent Heat of Vaporisation
The latent heat of vaporisation? is the amount of heat energy required to convert a unit mass of a liquid into vapour without any change in temperature. For water this comes to about 600 calories per gram at ordinary surface temperatures. The heat seems to "disappear" into the vapour molecules — but it is not lost; it is stored as latent (hidden) energy and will be released again when the vapour condenses.
Three Factors That Decide How Fast Water Evaporates
Why does a drying sari dry faster on a hot, breezy afternoon than on a cool, still one? NCERT identifies three controls on the rate of evaporation.
(i) Temperature
An increase in temperature increases the water absorption and retention capacity of the given parcel of air — so warm air can swallow more vapour.
(ii) Area of Free Water
A wide reservoir or shallow tray loses water faster than a narrow well — more surface molecules can escape per second.
(iii) Dry vs Moist Air
If moisture content is low, the air still has potential to absorb and retain moisture; movement of air replaces the saturated layer with the unsaturated layer — greater air movement, greater evaporation.
Why Wind Helps
Wind sweeps away the layer of vapour that builds up just above the water surface, allowing fresh dry air to take its place — drying clothes love a breeze.
The Hydrological Cycle — Evaporation, Condensation, Precipitation
LET'S EXPLORE — Why Does a Wet Floor Dry Faster Under a Fan?
L3 Apply
Take a wet kitchen floor on a humid afternoon. Half the floor is left as it is; the other half has a fan blowing across it. Within minutes the fan-blown half is dry while the still half is still glistening. Using the three NCERT factors of evaporation, explain in two sentences which of the three is doing most of the work here, and why the fan does not actually warm the floor.
The fan does not raise the temperature of the floor; what it does is replace the layer of saturated air sitting just above the wet film with a fresh layer of relatively unsaturated air. As NCERT puts it, "movement of air replaces the saturated layer with the unsaturated layer; hence, the greater the movement of air, the greater is the evaporation." So among the three controls — temperature, area of free water, and air movement — it is the third that dominates here. The temperature is the same on both halves and so is the area of the wet patch; only the wind speed differs. This is also exactly why a desert cooler works (it pulls dry air over wet pads) and why clothes dry on a windy roof in winter even when the air is cold.
10.3 Condensation — Vapour Becomes Water
The transformation of water vapour into water is called condensation?. It is the exact reverse of evaporation, and the energy bookkeeping is also reversed: condensation is caused by the loss of heat. When moist air is cooled, it may reach a level at which its capacity to hold water vapour ceases. The excess water vapour then condenses into the liquid form. The latent heat that the vapour absorbed during evaporation is now released back into the surrounding air — which is why condensation slightly warms the air around a forming cloud.
In free air, condensation does not happen on nothing. It always needs a tiny solid or liquid surface for the molecules to gather upon. Cooling around very small particles called hygroscopic condensation nuclei? is what produces clouds and fog in the open atmosphere. Particles of dust, smoke and salt from the ocean are particularly good nuclei because they absorb water readily.
Condensation also takes place when moist air comes in contact with some colder object — like the cold wall of a glass bottle straight from the fridge — and it may also take place when the air temperature is close to the dew point. Condensation, therefore, depends upon the amount of cooling and the relative humidity of the air. It is influenced by the volume of air, temperature, pressure and humidity. NCERT lists four situations in which it takes place:
🌫️ Four Situations Producing Condensation
- (i) When the temperature of the air is reduced to dew point with its volume remaining constant.
- (ii) When both the volume and the temperature are reduced.
- (iii) When moisture is added to the air through evaporation.
- (iv) The most favourable condition is the decrease in air temperature below the dew point.
🔑 Dew Point
The air containing moisture to its full capacity at a given temperature is said to be saturated — incapable of holding any additional moisture. The temperature at which saturation occurs in a given sample of air is known as the dew point?. Cool air below its dew point and condensation must begin.
10.4 Measuring Humidity — Three Useful Numbers
Humidity can be expressed in three different ways depending on what we want to compare. NCERT names three measures.
(a) Absolute Humidity — How Much Water Is There?
The actual amount of water vapour present in the atmosphere is known as the absolute humidity. It is the weight of water vapour per unit volume of air, expressed in grams per cubic metre (g/m³). The ability of the air to hold water vapour depends entirely on its temperature, so absolute humidity differs from place to place on the surface of the earth — generally greater over warm oceans and least over cold continental interiors.
(b) Specific Humidity — Weight per Weight
Specific humidity expresses the weight of water vapour per unit weight of air, in grams per kilogram (g/kg). Because it compares mass to mass, specific humidity does not change when the air is compressed or expanded — only when vapour is added or removed. Meteorologists use it whenever air parcels are rising or sinking.
(c) Relative Humidity — How Full Is the Sponge?
The relative humidity? is the percentage of moisture present in the atmosphere as compared to its full capacity at a given temperature. With a change of air temperature, the capacity to retain moisture increases or decreases — and therefore relative humidity is also affected. Relative humidity is greater over the oceans and least over the continents (because oceans are huge sources of moisture and continents are not).
Absolute Humidity
Weight of vapour per unit volume of air. Unit: g/m³. Tells you the actual moisture present.
Specific Humidity
Weight of vapour per unit weight of air. Unit: g/kg. Unaffected by pressure changes.
Relative Humidity
Moisture in air ÷ maximum it could hold at that temperature × 100. Unit: %. The "feel" number.
Hygrometer
The instrument that measures relative humidity — used at every weather station and in many homes.
🌍 Why Coastal Air Feels Sticky
Mumbai in July often sits at 85–95 per cent relative humidity. The air is almost saturated, so sweat from your skin cannot evaporate — and the body's main cooling system stalls. In contrast, Jodhpur in May may be 40 °C but only 20 per cent relative humidity; sweat evaporates instantly, drinking water out of you, but the heat feels "dry". This is why temperature alone never tells the whole comfort story.
THINK ABOUT IT — Same Vapour, Different Relative Humidity
L4 Analyse
A parcel of air at 30 °C contains 15 g/m³ of water vapour. At this temperature the maximum the air could hold is 30 g/m³. The same parcel cools overnight to 18 °C, where its maximum capacity drops to 15 g/m³. (a) What is the relative humidity at 30 °C? (b) What happens to the relative humidity by morning? (c) What forms on the lawn?
(a) RH at 30 °C = (15 ÷ 30) × 100 = 50 per cent. The air is "half-full". (b) As the parcel cools, the numerator (actual vapour) stays the same but the denominator (capacity) shrinks. By 18 °C the capacity equals the actual content (15 g/m³) — RH = 100 per cent — the air is saturated and 18 °C is the dew point. (c) Any further cooling forces excess vapour out. On the cool grass blades it appears as dew; on cool car roofs, the same. If the surface were below 0 °C, it would appear as frost instead. This is the textbook "saturation by cooling" mechanism.
10.5 Forms of Condensation — Dew, Frost, Fog, Mist, Cloud
After condensation the moisture in the atmosphere takes one of several forms — dew, frost, fog and clouds. NCERT classifies these forms on the basis of temperature and location (whether they form on a solid surface or up in free air). Crucially, condensation can take place when the dew point is lower than the freezing point or higher than the freezing point — and the resulting form is different in each case.
(a) Dew — Droplets on Grass
When the moisture is deposited in the form of water droplets on the cooler surfaces of solid objects — rather than on nuclei in the air above — such as stones, grass blades and plant leaves, it is known as dew?. The ideal conditions for its formation are a clear sky, calm air, high relative humidity, and cold and long nights. For dew to form, the dew point must be above the freezing point.
(b) Frost — Ice on Grass
Frost? forms on cold surfaces when condensation takes place below the freezing point (0 °C). In this case the dew point is at or below the freezing point. The excess moisture is deposited not as water droplets but as minute ice crystals. The conditions for the formation of white frost are the same as those for dew, except that the air temperature must be at or below the freezing point.
(c) Fog — Cloud at Ground Level
When the temperature of an air mass containing a large quantity of water vapour falls all of a sudden, condensation takes place within itself on fine dust particles. So fog is a cloud with its base at or very near the ground. Fog reduces horizontal visibility on the earth's surface to less than 1 km. In urban and industrial centres smoke provides plenty of nuclei which help the formation of fog and mist. When fog is mixed with smoke it is described as smog — the dense, choking blanket that hangs over Delhi and other north Indian cities through the winter.
(d) Mist — Lighter Cousin of Fog
Mist is similar to fog but limits visibility to between 1 km and 2 km. The only difference between the mist and the fog is that mist contains more moisture than the fog — in mist each nucleus contains a thicker layer of moisture. Mists are frequent over mountains as the rising warm air up the slopes meets a cold surface. Fogs are drier than mist and they are prevalent where warm currents of air come in contact with cold currents — for example, off the coast of Newfoundland where the warm Gulf Stream meets the cold Labrador Current.
| Form | Where it forms | Dew point vs freezing point | Visibility |
|---|---|---|---|
| Dew | On cool ground surfaces (grass, leaves, stones) | Above 0 °C | Not applicable |
| Frost | On cool ground surfaces | At or below 0 °C | Not applicable |
| Fog | In free air, base at/near ground | Variable; usually above 0 °C | Less than 1 km |
| Mist | In free air, base at/near ground; common on mountains | Variable; usually above 0 °C | 1 to 2 km |
| Cloud | In free air, at considerable elevations | Variable | Cloud base above ground |
Dew vs Frost — How a Few Degrees Make All the Difference
SOURCE — From the NCERT Textbook
L2 Understand
NCERT writes: "Such a condition when fog is mixed with smoke, is described as smog. The only difference between the mist and fog is that mist contains more moisture than the fog. In mist each nuceli contains a thicker layer of moisture. Mists are frequent over mountains as the rising warm air up the slopes meets a cold surface." Explain in your own words why mountain stations like Mussoorie or Mahabaleshwar see daily mist after a sunny morning, and why Delhi sees smog instead of clean fog in winter.
On a hill station, the morning Sun warms the valley floor. The warmed valley air, now lighter, rises up the slopes. As it climbs, it cools rapidly — and because the slopes are damp, the rising parcels are humid. By the time they meet the cooler ridge surface, the air is at or below dew point and condenses on the available nuclei. Each tiny nucleus collects a thick coat of moisture, giving us a true mist with visibility of one to two kilometres. In Delhi the situation is different. The early-morning radiation cooling of the surface produces a normal radiation fog, but the air is also full of soot, vehicle exhaust and crop-stubble smoke. The fog droplets condense onto these dirty nuclei and remain suspended much longer — the result is smog: dirtier than fog, more dangerous to breathe, and slower to lift even after sunrise.
10.6 Clouds — Floating Reservoirs of Condensed Water
A cloud? is a mass of minute water droplets or tiny crystals of ice formed by the condensation of water vapour in free air at considerable elevations. As the clouds are formed at some height over the surface of the earth, they take various shapes. According to their height, expanse, density and transparency or opaqueness, NCERT groups clouds under four basic types: (i) cirrus, (ii) cumulus, (iii) stratus and (iv) nimbus.
(i) Cirrus — The High Feather Clouds
Cirrus? clouds are formed at high altitudes (8,000–12,000 m). They are thin and detached clouds having a feathery appearance. They are always white in colour. Cirrus clouds are made entirely of ice crystals because the air is so cold at that altitude.
(ii) Cumulus — Cotton-Wool Clouds
Cumulus? clouds look like cotton wool. They are generally formed at a height of 4,000–7,000 m. They exist in patches and can be seen scattered here and there. They have a flat base and rise as puffy domes — the classic "fair-weather" cloud of summer afternoons.
(iii) Stratus — Sheet-Like Layers
As their name implies, stratus clouds are layered clouds covering large portions of the sky. They are generally formed either due to loss of heat (radiation cooling of a layer of air at night) or the mixing of air masses with different temperatures.
(iv) Nimbus — Dark Rain Clouds
Nimbus clouds are black or dark grey. They form at middle levels or very near the surface of the earth. These are extremely dense and opaque to the rays of the Sun. Sometimes the clouds are so low that they seem to touch the ground. Nimbus clouds are shapeless masses of thick vapour — they are the rain-bringers.
Compound Cloud Types — Combining the Four Basics
A combination of these four basic types gives rise to several compound types arranged by altitude.
| Family | Altitude | Cloud types | Typical look |
|---|---|---|---|
| High clouds | 8,000–12,000 m | Cirrus, Cirrostratus, Cirrocumulus | Wispy, white, ice crystals |
| Middle clouds | 4,000–7,000 m | Altostratus, Altocumulus | Sheet-like or patchy, water/ice mix |
| Low clouds | 0–4,000 m | Stratus, Nimbostratus, Stratocumulus | Grey, often producing drizzle/rain |
| Vertical development | 0 m up to 12,000 m | Cumulus, Cumulonimbus? | Towering, anvil-topped — thunderstorms |
Cloud Altitude Classification
Cloud altitude bands — typical heights in metres (NCERT classification)
DISCUSS — Identify the Cloud (NCERT Figures 10.1 & 10.2)
L2 Understand
The textbook prints two photographs and asks: "Identify these cloud types which are shown in Figure 10.1 and Figure 10.2." Without the photographs, walk through the four NCERT clues that you should always use to name a cloud — and then describe what each of the four basic types would look like through that checklist.
NCERT itself names the four discriminators: height, expanse, density, and transparency/opaqueness. Walk through them in turn. (1) Cirrus — high (8,000–12,000 m), narrow expanse, low density, transparent: a thin feather streak across blue sky. (2) Cumulus — middle (4,000–7,000 m), patchy expanse, moderate density, opaque centre but bright edges: cotton-wool tufts with flat base. (3) Stratus — low, very wide expanse covering most of the sky, layered, semi-transparent: an even grey sheet that turns the day flat. (4) Nimbus — middle/low, large expanse, very dense, fully opaque: dark grey or black, often touching the ground, releasing rain. Apply the four-step checklist to any photograph and you will name the cloud correctly.
🎯 Competency-Based Questions — Humidity, Condensation & Clouds
Case Stem. A weather observer in Shimla (1,950 m) records the following on a clear December evening. At 5 pm: temperature 14 °C, vapour content 8 g/m³, capacity at this temperature 12 g/m³. By 11 pm: temperature has fallen to 6 °C, capacity now 8 g/m³, vapour content unchanged. By 5 am: temperature falls to −1 °C; the observer notices a thin white deposit on the lawn. Use this scenario to answer Q1–Q4.
Q1. The relative humidity at 5 pm is —
L3 ApplyAnswer: (b) ≈ 67 per cent. RH = (actual ÷ capacity) × 100 = (8 ÷ 12) × 100 ≈ 66.7 per cent. The air is two-thirds saturated.
Q2. By 11 pm the vapour content (8 g/m³) equals capacity (8 g/m³). The temperature 6 °C is therefore the —
L3 ApplyAnswer: (c) Dew point. The dew point is the temperature at which a sample of air becomes saturated. Here that occurs at 6 °C — note that this is well above the freezing point.
Q3. The white deposit on the lawn at 5 am is most likely —
L4 AnalyseFrost. The dew point (6 °C) is above the freezing point, but by dawn the surface has fallen to −1 °C. Once the air at the surface is at or below 0 °C, condensation skips the liquid stage and goes straight to minute ice crystals — i.e. frost. NCERT lists clear sky, calm air, high RH and a long cold night as the ideal frost-forming conditions, all of which Shimla provides on a December dawn.
HOT Q. The same air parcel rises up the Himalayan slopes the next morning and reaches an altitude where its temperature falls to −10 °C. Predict what kind of cloud forms and at what altitude band it would lie. Justify using two NCERT criteria.
L6 CreateHint: A rising parcel that reaches −10 °C is well above the freezing point of water — at such heights NCERT identifies the middle cloud band (4,000–7,000 m) where altostratus and altocumulus form, or possibly the high band (8,000–12,000 m) of cirrus if the parcel rises further. The two NCERT criteria you can cite are height/altitude and density/opaqueness: a wispy, semi-transparent cloud at high altitude is cirrus; a thicker layer at middle altitude is altostratus. If forced upward strongly enough, the same parcel can build into a cumulonimbus and break into precipitation — taking us into the next part of the chapter.
⚖️ Assertion–Reason Questions — Class 11
Options:
(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.
(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.
Assertion (A): Wet clothes dry faster on a windy day than on a still day, even when the air temperature is the same.
Reason (R): Movement of air replaces the saturated layer above the wet surface with the unsaturated layer, so the greater the movement of air, the greater is the evaporation.
Answer: (A) — Both true and R precisely explains A. NCERT lists air movement as the third major control on evaporation; replacing saturated air with unsaturated air keeps the diffusion gradient steep.
Assertion (A): Frost forms on cold ground surfaces while dew forms on cool ground surfaces, even though the conditions for both are otherwise similar.
Reason (R): When the dew point is at or below the freezing point, excess moisture is deposited as minute ice crystals instead of liquid droplets.
Answer: (A) — Both true; R explains A. The thermal threshold of 0 °C is what decides whether condensation produces dew or frost, even though the meteorological recipe (clear sky, calm air, high RH, long night) is otherwise the same.
Assertion (A): Cirrus clouds are always white in colour and never produce rain at the surface.
Reason (R): Cirrus clouds form at altitudes of 8,000–12,000 m where temperatures are well below freezing, so they consist of tiny ice crystals rather than liquid droplets.
Answer: (A) — Both true and R is the correct explanation. The high-altitude ice crystals scatter light efficiently in all directions, giving the white feathery appearance; ice crystals at such heights rarely grow heavy enough to fall as rain at the surface.
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Class 11 Geography — Fundamentals of Physical Geography
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