CDS Geography · Climatology, Atmosphere and Weather
Humidity, Clouds and Precipitation
How the moisture in the air is measured, how it turns into dew, fog and rain, and how to name a cloud from its look and its height.
Why this matters
Twelve CDS questions from 2018 to 2025: 3 EASY, 9 MODERATE. Eight of them ask you to name, match or order clouds, five of those in 2023 and 2024 alone, so the cloud tables on this page are the best value in the chapter.
Concept 1 of 4: Humidity and relative humidity
Air holds water as an invisible gas, and warm air can hold more of it than cold air. Relative humidity compares what the air holds with what it could hold at its present temperature. Cool the air without adding water and the relative humidity climbs, until at the dew point the air is full and the vapour starts to condense.
Definition
The measures:
- Humidity: the moisture content of the air (water vapour) at a given time and place.
- Absolute humidity: the actual weight of water vapour in a volume of air, in grams per cubic metre.
- Relative humidity: the moisture present as a percentage of the most the air could hold at that temperature. So it needs both the absolute humidity and the temperature.
- Air holding all it can is saturated (relative humidity 100 per cent). The temperature at which this happens is the dew point.
- Fog, cloud and dew are products of condensation, not humidity itself.
Relative humidity
Worked example
A cubic metre of air holds 12 g of water vapour. At its temperature it could hold 20 g. What is its relative humidity?
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2024 · CDS (I) 2024 — General Knowledge · Q84Moderate
Example 1 · Climatology, Atmosphere and Weather · Humidity, Clouds and Precipitation
How can one determine the relative humidity on a particular day at a given place?
Humidity is the vapour, not the droplets
Humidity is the water vapour in the air. Suspended droplets are fog or cloud, and moisture deposited on surfaces is dew: these are what condensation makes, not humidity itself.
Relative humidity needs two numbers
Neither temperature alone nor absolute humidity alone gives the relative humidity. You need the vapour present and the temperature, which sets how much the air could hold.
Concept 2 of 4: Dew, frost, fog and how rain forms
When moist air is cooled to its dew point, the vapour turns back to water on whatever is nearby: on grass it is dew, on dust particles in the air near the ground it is fog, higher up it is cloud. To fall as rain, the tiny cloud droplets must grow much bigger, and there are two ways for that to happen.
Definition
Forms of condensation:
- Dew: water drops on cool surfaces (grass, leaves) when the dew point is above freezing.
- Frost: ice on cold surfaces when the dew point is below freezing.
- Fog: a cloud with its base at or very near the ground. Radiation fog forms when the land cools by radiation at night. Advection fog forms when moist air blows over a cold surface and is chilled by contact.
- Mist is like fog but holds more moisture and thins visibility less.
How cloud droplets grow into rain:
- Collision and coalescence: big drops sweep up small ones as they fall. This is how rain forms in warm clouds, those that do not reach the freezing level.
- Ice-crystal (Bergeron) process: in clouds that rise above the freezing level, ice crystals grow at the expense of water drops, then fall and melt.
| Form | How it forms |
|---|---|
| Dew | Vapour condenses on cool surfaces above freezing |
| Frost | Vapour freezes on surfaces below freezing |
| Radiation fog | Ground cools by radiation at night and chills the air |
| Advection fog | Moist air blows over a cold surface |
| Rain by collision and coalescence | In clouds that stay below the freezing levelQ CDS 2018 (I): the collision and coalescence process applies to clouds that do not extend beyond the freezing level. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2023 · CDS (II) 2023 — General Knowledge · Q24Moderate
Example 2 · Climatology, Atmosphere and Weather · Humidity, Clouds and Precipitation
Consider the following statements on 'Fog' :
1. Fog is simply a cloud that forms close to the ground.
2. Radiation fog is associated with radiation cooling of the land at night.
3. Advection fog forms when moisture is blown over a cold surface and is chilled by contact.
Which of the statements given above are correct ?
Warm-cloud rain is collision and coalescence
Collision and coalescence works in clouds that do not reach the freezing level. Clouds that rise above it make rain mainly through ice crystals. It does not apply to all clouds.
Concept 3 of 4: Cloud families by height
Clouds come in two basic shapes: flat layers (stratiform) and heaps (cumuliform). They are also grouped by height: high, middle and low. A cloud's name tells you both. 'Cirro' means high, 'alto' means middle, 'nimbo' or 'nimbus' means it brings rain or snow.
Definition
How clouds are named:
- Two basic forms: stratiform (layers) and cumuliform (heaps).
- High clouds (above about 6 km, 20,000 feet): cirrus, cirrostratus, cirrocumulus. They are made entirely of ice crystals.
- Middle clouds (about 2 to 6 km): altostratus, altocumulus.
- Low clouds (below about 2 km): stratus, stratocumulus, nimbostratus.
- Vertical clouds that span the heights: cumulus and cumulonimbus.
- A cloud with nimbo or nimbus in its name gives precipitation.
- Cirrocumulus makes small rippled patches like fish scales: the mackerel sky.
| Family | Height | Clouds |
|---|---|---|
| High | Above about 6 km (20,000 to 40,000 feet) | Cirrus, cirrostratus, cirrocumulusQ CDS 2024 (I): high, thin ice-crystal ripples at 20,000 to 40,000 feet, the 'mackerel sky', are cirrocumulus. |
| Middle | About 2 to 6 km | Altostratus, altocumulus |
| Low | Below about 2 km | Stratus, stratocumulus, nimbostratusQ CDS 2023 (II): lowest to highest runs nimbostratus, altostratus, cirrocumulus, cirrus. |
| Vertical | From low bases to great heights | Cumulus, cumulonimbus |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2023 · CDS (II) 2023 — General Knowledge · Q63Moderate
Example 3 · Climatology, Atmosphere and Weather · Humidity, Clouds and Precipitation
Consider the following statements about clouds :
1. Two major forms of clouds are stratiform and cumuliform.
2. According to the altitude, clouds are classified as high clouds, middle clouds and low clouds.
3. Stratus, nimbostratus and stratocumulus are types of high clouds.
4. Clouds having nimbo attached to their name produce precipitation.
Which of the statements given above are correct ?
Stratus clouds are low, not high
Stratus, stratocumulus and nimbostratus are low clouds. The high family is cirrus, cirrostratus and cirrocumulus: look for 'cirro' or 'cirrus' in the name.
Nimbostratus is low, though it is thick
Nimbostratus is a thick, dark rain cloud, but its base is low. In a lowest-to-highest ordering it comes before any 'alto' or 'cirro' cloud.
Concept 4 of 4: The four basic cloud forms and the thundercloud
Most cloud names are built from four Latin words: cirrus (a curl of hair), cumulus (a heap), stratus (a layer) and nimbus (a rain cloud). Learn the look that goes with each word. A cumulus that keeps growing upward becomes the thundercloud, cumulonimbus.
Definition
The four forms (NCERT Class 11):
- Cirrus: high (8 to 12 km), thin, feathery clouds of ice, white in colour.
- Cumulus: look like cotton wool, with a flat base; they lie in patches, generally at 4 to 7 km, and grow upward. Small ones are fair-weather clouds.
- Stratus: layered clouds that spread horizontally over large areas. A low stratus looks like fog lifted off the ground ('low ceiling').
- Nimbus: dark, thick, rain-giving clouds, low or middle level, sometimes very near the surface.
The thundercloud:
- A cumulus grows by convection into a towering cumulus congestus, and then into a cumulonimbus.
- Cumulonimbus is a huge cauliflower-shaped tower whose top spreads into an anvil. It is the only cloud that brings hail, thunder and lightning.
| Cloud | Look | Weather |
|---|---|---|
| Cirrus | Thin, feathery, all ice | FairQ Matched twice: CDS 2018 (I) as 'feathery' and CDS 2022 (II) as 'entirely of ice particles'. |
| Cumulus | Cotton wool heaps with a flat base, in patches | Fair, if smallQ CDS 2025 (I): cotton wool look, flat base, patches, 4 to 7 km. |
| Stratus | Flat layer spreading sideways, like lifted fog | Dull, grey |
| Nimbus (and nimbostratus) | Dark, thick | Rain or snowQ CDS 2022 (II): nimbostratus matched with storm clouds. |
| Cumulonimbus | Towering cauliflower with an anvil top | Hail, thunder, lightningQ CDS 2024 (II): cumulonimbus grows from cumulus congestus. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2023 · CDS (I) 2023 — General Knowledge · Q103Easy
Example 4 · Climatology, Atmosphere and Weather · Humidity, Clouds and Precipitation
Consider the following characteristics of a cloud type :
1. They are born through convection.
2. Only cloud type that can produce hail, thunder and lightning.
3. They are large cauliflower-shaped towers, often 'anvil tops'.
Identify the type of cloud on the basis of the given characteristics.
Cumulus grows up, stratus spreads out
In a matching list, cumulus is the vertically growing cloud and stratus the horizontally spreading one. Swapping them is the usual wrong option.
Cumulonimbus comes from cumulus, not nimbostratus
A cumulonimbus is a cumulus that has grown very tall, through the cumulus congestus stage. It does not grow out of nimbostratus, altostratus or cirrocumulus.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Formulas (1)
- Humidity and relative humidity
Relative humidity
Reference tables (3)
Dew, frost, fog and how rain forms5 rows
| Form | How it forms |
|---|---|
| Dew | Vapour condenses on cool surfaces above freezing |
| Frost | Vapour freezes on surfaces below freezing |
| Radiation fog | Ground cools by radiation at night and chills the air |
| Advection fog | Moist air blows over a cold surface |
| Rain by collision and coalescence | In clouds that stay below the freezing levelQ CDS 2018 (I): the collision and coalescence process applies to clouds that do not extend beyond the freezing level. |
Cloud families by height4 rows
| Family | Height | Clouds |
|---|---|---|
| High | Above about 6 km (20,000 to 40,000 feet) | Cirrus, cirrostratus, cirrocumulusQ CDS 2024 (I): high, thin ice-crystal ripples at 20,000 to 40,000 feet, the 'mackerel sky', are cirrocumulus. |
| Middle | About 2 to 6 km | Altostratus, altocumulus |
| Low | Below about 2 km | Stratus, stratocumulus, nimbostratusQ CDS 2023 (II): lowest to highest runs nimbostratus, altostratus, cirrocumulus, cirrus. |
| Vertical | From low bases to great heights | Cumulus, cumulonimbus |
The four basic cloud forms and the thundercloud5 rows
| Cloud | Look | Weather |
|---|---|---|
| Cirrus | Thin, feathery, all ice | FairQ Matched twice: CDS 2018 (I) as 'feathery' and CDS 2022 (II) as 'entirely of ice particles'. |
| Cumulus | Cotton wool heaps with a flat base, in patches | Fair, if smallQ CDS 2025 (I): cotton wool look, flat base, patches, 4 to 7 km. |
| Stratus | Flat layer spreading sideways, like lifted fog | Dull, grey |
| Nimbus (and nimbostratus) | Dark, thick | Rain or snowQ CDS 2022 (II): nimbostratus matched with storm clouds. |
| Cumulonimbus | Towering cauliflower with an anvil top | Hail, thunder, lightningQ CDS 2024 (II): cumulonimbus grows from cumulus congestus. |
Watch out for (7)
- Humidity is the vapour, not the droplets→ Humidity and relative humidity
- Relative humidity needs two numbers→ Humidity and relative humidity
- Warm-cloud rain is collision and coalescence→ Dew, frost, fog and how rain forms
- Stratus clouds are low, not high→ Cloud families by height
- Nimbostratus is low, though it is thick→ Cloud families by height
- Cumulus grows up, stratus spreads out→ The four basic cloud forms and the thundercloud
- Cumulonimbus comes from cumulus, not nimbostratus→ The four basic cloud forms and the thundercloud
Test yourself on Climatology, Atmosphere and Weather
15 past CDS questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.