CDS Geography · Formula sheet

Climatology, Atmosphere and Weather formulas

2 formulas, 16 reference tables and 34 common traps for CDS Geography Climatology, Atmosphere and Weather, grouped by subtopic.

Full notes with worked examples

Composition and Layers of the Atmosphere

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The gases of the air and where they came from

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 vapour

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 atmosphere

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 layer

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.

Common traps

Argon, not carbon dioxide, is third

Carbon dioxide gets the most attention, but argon (0.93 per cent) is about 25 times as plentiful. In any ranking of gases by volume, argon comes right after nitrogen and oxygen.

Ascending means smallest first

A ranking item that asks for ascending order wants the scarcest gas first. Write the shares beside each gas before you read the options, then check whether the option runs up or down.

Variable, not permanent

Nitrogen, oxygen and argon are permanent gases with a fixed share. Water vapour is variable: its share changes with place, season and height.

The troposphere is thickest at the equator

Strong heating at the equator lifts air higher, so the troposphere is about 18 km thick there and only about 8 km at the poles.

Coldest is the top of the mesosphere

The lowest temperature in the atmosphere is near the top of the mesosphere (about minus 100 °C at 80 km), not at the tropopause and not in the stratosphere.

The hole is over Antarctica, the cause is CFCs

Options mix the pairs. The ozone hole is over the Antarctic, not the Arctic, and it is caused mainly by CFCs, not by carbon dioxide (which warms the planet but does not destroy ozone).

Ozone Day is in September

The International Day for the Preservation of the Ozone Layer is 16 September, the date the Montreal Protocol was signed. A statement putting it in November is false.

Insolation and Temperature

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Lapse rate and adiabatic change

Temperature at a height, using the normal lapse rate

T_h = T_0 - 6.5 \times h

$T_0$ temperature at the lower place, in °C
$h$ height climbed, in kilometres
$T_h$ temperature at the higher place, in °C

Insolation, albedo and global dimming

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 inversion

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

Common traps

The curve makes insolation uneven

A statement that insolation is evenly spread because the Earth is curved is false. The curve is exactly why it is uneven: straight rays at the equator, slanting rays near the poles.

Dimming cools, it does not change pressure

Global dimming is about less sunlight and a fall in temperature caused by particulates. Options that tie it to a rise or fall in air pressure, or to a rise in temperature, are wrong.

Adiabatic means no heat is exchanged

In an adiabatic change the air's temperature changes only because it expands (cools) or is compressed (warms). Latent heat, absorption of sunlight and advection are other ways of changing temperature, not adiabatic ones.

Different heat wave limits for hills

The heat wave limit is 40 °C for the plains but 30 °C for the hills, because hills are cooler to begin with. A statement giving both limits correctly is true.

Inversion means stability

Cold, heavy air under warm air cannot rise, so an inversion makes the lower air stable. A statement that it causes instability is false.

A fast fall with height spreads smog

Smog builds up under an inversion lid. A rapid fall of temperature with height (a steep normal lapse rate) does the opposite: it makes air rise and carries pollutants away.

Pressure, Winds and Cyclones

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Pressure gradient and the Coriolis force

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 cells

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 cyclones

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 storms

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.

Common traps

Left in the south, right in the north

The Coriolis force turns winds to the right in the northern hemisphere and to the left in the southern. A statement that it turns winds to the right in the southern hemisphere is false.

No Coriolis force at the equator

The Coriolis force is zero at the equator and greatest at the poles. This is why tropical cyclones never form right on the equator.

Hadley air sinks at 30°, not at the equator

The ITCZ, doldrums and equatorial trough are where Hadley cell air rises. It descends at the subtropical high-pressure belts near 30°.

All the listed factors count

NCERT names heating by latitude, the distribution of continents and oceans, and the rotation of the Earth among the factors behind the planetary winds. A statement item listing all three expects all three.

The eye is the centre, not the front

The eye of a cyclone is its calm centre, where the air sinks and the sky clears. It is not the front edge of the storm.

Hurricanes are tropical

Hurricanes are tropical cyclones of the Atlantic. They form over warm tropical seas, not in the middle latitudes.

Bora and Mistral are different valleys

Both are cold winds draining off mountains. The Bora reaches the Adriatic coast; the Mistral blows down the Rhone valley of France.

Kalbaisakhi is Bengal, mango showers are the south-west

Kalbaisakhi storms belong to West Bengal and Assam. The pre-monsoon showers of Kerala and coastal Karnataka are the mango showers.

Humidity, Clouds and Precipitation

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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

Dew, frost, fog and how rain forms

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 height

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 thundercloud

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.

Common traps

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.

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.

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.

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.

Climate Types and Koppen's Scheme

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Koppen's letters

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 India

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 features

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.

Common traps

W is desert, S is steppe

Inside the dry group B, capital W means desert (arid) and capital S means steppe (semi-arid). Do not mix them with the small w (winter dry season) and s (summer dry season).

Marine west coast summers are mild

Cfb, the marine west coast type, has no dry season and mild (warm and cool) summers. A 'severely cold summer' describes no Koppen C type.

South India is not extreme

Near the sea and the equator, South India has small daily and annual ranges and stays hot all year. 'Extreme climatic conditions' is the false statement about it.

BWhw is only the far west

The hot desert type BWhw covers only extreme western Rajasthan. The rest of the dry north-west (north-western Gujarat, parts of Rajasthan and Punjab) is the semi-arid BShw.

The Mediterranean is dry in summer

Most climates get their rain in summer. The Mediterranean is the exception: its summer is dry and its winter is wet. An option saying 'dry winter' is wrong.

Monsoon-like winds outside the tropics

A monsoon-like reversal of winds caused by winter anticyclones points to the cold climate with dry winters of north-east Asia, not to the tropical monsoon.

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