CDS Geography · Free notes

Climatology: CDS Geography

Climatology has 55 past-year questions in CDS General Knowledge, from 2016 (II) to 2026 (II): 9 EASY, 41 MODERATE and 5 HARD. It is a growing chapter: 23 of the 55 came in the 2024, 2025 and 2026 papers. Most items are statement sets or 'identify it from these features' items, and their sentences are close to NCERT Class 11, so the notes follow NCERT's order and keep its wording. Clouds, Koppen's letters, temperature inversion and ozone come back again and again: learn those tables exactly, because a wrong option is often an NCERT sentence with one word changed.

The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.

All 55 past questions CDS home All formulas on one page

Subtopic notes

Formula & revision sheet

2 formulas · 16 reference tables · 34 gotchas across all subtopics — the exam-eve cheat-sheet

Composition and Layers of the Atmosphere

Reference tables (4)

The gases of the air and where they came from9 rows

Gas	Share by volume (per cent)
Nitrogen	78.08
Oxygen	20.95
Argon	0.93
Carbon dioxide	0.036
Neon	0.002
Krypton	0.001
Helium	0.0005
Xenon	0.00009
Hydrogen	0.00005

Water vapour3 rows

Place	Water vapour in the air
Warm, wet tropics	Up to about 4 per cent
Dry, cold deserts and polar regions	Less than 1 per cent
Higher up in the atmosphere	Less than near the groundQ CDS 2026 (II): water vapour is variable and decreases with altitude, and is under 1 per cent in deserts and polar regions. Both true.

Layers of the atmosphere5 rows

Layer	Height	Temperature with height	Remember it for
Troposphere	Ground to 8 km (poles) or 18 km (equator)	Falls	All weather
Stratosphere	Up to about 50 km	Rises	Ozone layer, calm air for jets
Mesosphere	About 50 to 80 km	Falls	Coldest air, near its top
Thermosphere and ionosphere	About 80 to 400 km	Rises	Reflects radio waves
Exosphere	Above the thermosphere	Very thin air	Outermost layer

The ozone layer4 rows

Question	Answer
Where is most ozone?	In the stratosphere, about 90 per cent of itQ Asked twice: CDS 2017 (II) and 2021 (I). Not the troposphere, not the ionosphere.
Does the stratosphere let ultraviolet in?	No, its ozone absorbs most of itQ CDS 2017 (II): ultraviolet causes skin cancer, but the stratosphere blocks it rather than letting it in.
Where is the ozone hole, and what causes it?	Over Antarctica, mainly CFCsQ CDS 2024 (II): Antarctic and CFCs, not the Arctic and not carbon dioxide.
Ozone Day	16 September (Montreal Protocol signed, 1987)Q CDS 2020 (II): the false statement put Ozone Day in November.

Watch out for (7)

Argon, not carbon dioxide, is third
→ The gases of the air and where they came from
Ascending means smallest first
→ The gases of the air and where they came from
Variable, not permanent
→ Water vapour
The troposphere is thickest at the equator
→ Layers of the atmosphere
Coldest is the top of the mesosphere
→ Layers of the atmosphere
The hole is over Antarctica, the cause is CFCs
→ The ozone layer
Ozone Day is in September
→ The ozone layer

Insolation and Temperature

Formulas (1)

Lapse rate and adiabatic change · Temperature at a height, using the normal lapse rate $T_h = T_0 - 6.5 \times h$

Reference tables (2)

Insolation, albedo and global dimming5 rows

Surface	Share of sunlight it reflects
Fresh snow	About 80 to 90 per cent, the highestQ CDS 2022 (I): a snow covered mountain has the highest albedo, above a sandy desert, bare rock or a mangrove forest.
Desert sand	About 30 to 40 per cent
Grassland	About 20 to 25 per cent
Dense forest	About 5 to 15 per cent
Open water with the Sun high	Under 10 per cent

Temperature inversion5 rows

Feature	Inversion
Temperature with height	Rises
Effect on the lower air	Stable, no vertical mixingQ CDS 2025 (I): surface inversion causes stability, not instability, and lasts a few hours until sunrise.
Main reason smog lingers	The inversion lid holds pollutants near the groundQ CDS 2022 (I): the inversion lid is the most important factor, ahead of long nights or many pollution sources.
Other name for surface inversion	Radiation inversion
Ideal conditions	Long winter night, clear sky, still air

Watch out for (6)

The curve makes insolation uneven
→ Insolation, albedo and global dimming
Dimming cools, it does not change pressure
→ Insolation, albedo and global dimming
Adiabatic means no heat is exchanged
→ Lapse rate and adiabatic change
Different heat wave limits for hills
→ Lapse rate and adiabatic change
Inversion means stability
→ Temperature inversion
A fast fall with height spreads smog
→ Temperature inversion

Pressure, Winds and Cyclones

Reference tables (4)

Pressure gradient and the Coriolis force6 rows

Feature	Fact
Strong pressure gradient on a weather map	Closely spaced isobarsQ CDS 2024 (II): closely spaced isobars, not widely spaced ones.
Coriolis deflection, northern hemisphere	To the right
Coriolis deflection, southern hemisphere	To the left
Coriolis force at the equator	Zero
Coriolis force at the poles	Greatest
Highest winter pressure in India	The north-west, for example JaisalmerQ CDS 2018 (I): Jaisalmer, ahead of Leh, Chennai and Guwahati.

Pressure belts and the three cells7 rows

Belt or cell	Latitude	What happens
Equatorial low (doldrums, ITCZ)	0 to 5°	Hot air rises; trade winds meet
Subtropical high	About 30°	Air of the Hadley cell sinksQ CDS 2020 (I): Hadley cell air rises at the equator and descends at the subtropical high-pressure cells.
Subpolar low	About 60°	Westerlies and polar easterlies meet
Polar high	About 90°	Cold air sinks
Hadley cell	0 to 30°	Driven by strong heating at the equator; gives the trade winds
Ferrel cell	30 to 60°	Gives the westerlies
Polar cell	60 to 90°	Gives the polar easterlies

Tropical and temperate cyclones4 rows

Name	Where
Cyclones	Indian Ocean
Hurricanes	Atlantic Ocean and the Caribbean
Typhoons	Western Pacific and South China Sea
Willy-willies	Off north-western Australia

Local winds and storms7 rows

Wind or storm	Kind	Where
Bora	Cold, dry, blows down the mountains	Eastern shore of the Adriatic SeaQ CDS 2020 (I): Bora, not the Mistral or the Bise.
Mistral	Cold	Rhone valley, France
Bise	Cold, dry	Switzerland
Loo	Hot, dry, summer afternoons	Plains of north India
Kalbaisakhi	Evening thunderstorms before the monsoon	West Bengal and Assam
Mango showers	Pre-monsoon showers	Kerala and coastal Karnataka
Blizzard	Violent snowstorm	High snowy mountains and polar landsQ CDS 2021 (II): of four states, only Arunachal Pradesh, with its high Himalaya, needs a blizzard warning.

Watch out for (8)

Left in the south, right in the north
→ Pressure gradient and the Coriolis force
No Coriolis force at the equator
→ Pressure gradient and the Coriolis force
Hadley air sinks at 30°, not at the equator
→ Pressure belts and the three cells
All the listed factors count
→ Pressure belts and the three cells
The eye is the centre, not the front
→ Tropical and temperate cyclones
Hurricanes are tropical
→ Tropical and temperate cyclones
Bora and Mistral are different valleys
→ Local winds and storms
Kalbaisakhi is Bengal, mango showers are the south-west
→ Local winds and storms

Humidity, Clouds and Precipitation

Formulas (1)

Humidity and relative humidity · Relative humidity $\text{RH} = \frac{\text{water vapour present}}{\text{most water vapour the air can hold at that temperature}} \times 100$

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

Climate Types and Koppen's Scheme

Reference tables (3)

Koppen's letters14 rows

Code	Name	Main feature
Af	Tropical wet	No dry season
Am	Tropical monsoon	Short dry season
Aw	Tropical wet and dry (savanna)	Winter dry seasonQ CDS 2020 (II): Aw is the tropical savannah climate, not Am, Cwg or Dfc.
BSh	Subtropical steppe	Low-latitude semi-arid
BWh	Subtropical desert	Low-latitude arid
BSk	Mid-latitude steppe	Mid-latitude semi-aridQ CDS 2025 (II): three of four pairs from this table were right; marine west coast was wrongly given a severely cold summer.
BWk	Mid-latitude desert	Mid-latitude arid
Cfa	Humid subtropical	No dry season, warm summer
Cs	Mediterranean	Dry, hot summer
Cfb	Marine west coast	No dry season, warm and cool summer
Df	Humid continental	No dry season, severe winter
Dw	Subarctic (cold climate with dry winters)	Winter dry and very severe
ET	Tundra	No true summer
EF	Ice cap	Perennial ice, no month above 0 °C

Koppen types in India8 rows

Koppen type	Region of India
Amw	West coast south of Goa
As	Coromandel coast of Tamil Nadu
Aw	Most of the peninsular plateaus
BShw	North-western Gujarat, parts of western Rajasthan and Punjab
BWhw	Extreme western Rajasthan
Cwg	Ganga plain, eastern Rajasthan, northern Madhya Pradesh, most of the north-east
Dfc	Arunachal Pradesh
E	Jammu and Kashmir, Himachal Pradesh, Uttarakhand

World climate types from their features4 rows

Climate	Tell-tale feature	Where
Tropical wet and dry (savanna)	Short wet season, long dry season	Sudan; Paraguay, Bolivia, Brazil
Mediterranean	Dry summer, rainy winter	West sides of continents, 30° to 40°Q CDS 2019 (II): the most noticeable feature is the dry summer, not a dry winter.
Cold climate with dry winters	Winter anticyclone, monsoon-like wind reversal, 12 to 15 cm a year	North-east AsiaQ CDS 2025 (II): named from three statements, ahead of tundra and steppe.
Tundra	Very short warm season, few species	Arctic fringes of North America and EurasiaQ CDS 2018 (I): low biodiversity, and the short warm season is the reason.

Watch out for (6)

W is desert, S is steppe
→ Koppen's letters
Marine west coast summers are mild
→ Koppen's letters
South India is not extreme
→ Koppen types in India
BWhw is only the far west
→ Koppen types in India
The Mediterranean is dry in summer
→ World climate types from their features
Monsoon-like winds outside the tropics
→ World climate types from their features

PYQ weightage by concept

18 concepts · 55 PYQs — where the marks actually sit, so you know what to drill first

Composition and Layers of the Atmosphere12 PYQs · 22%

Concept	PYQs	Share
The ozone layer	5	9%
The gases of the air and where they came from	4	7%
Water vapour	2	4%
Layers of the atmosphere	1	2%

Insolation and Temperature9 PYQs · 16%

Concept	PYQs	Share
Temperature inversion	4	7%
Insolation, albedo and global dimming	3	5%
Lapse rate and adiabatic change	2	4%

Pressure, Winds and Cyclones11 PYQs · 20%

Concept	PYQs	Share
Pressure gradient and the Coriolis force	3	5%
Pressure belts and the three cells	3	5%
Local winds and storms	3	5%
Tropical and temperate cyclones	2	4%

Humidity, Clouds and Precipitation12 PYQs · 22%

Concept	PYQs	Share
The four basic cloud forms and the thundercloud	5	9%
Cloud families by height	3	5%
Humidity and relative humidity	2	4%
Dew, frost, fog and how rain forms	2	4%

Climate Types and Koppen's Scheme11 PYQs · 20%

Concept	PYQs	Share
World climate types from their features	5	9%
Koppen's letters	4	7%
Koppen types in India	2	4%

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