World Climate Classification: Koeppen's Scheme

1. Classifying climate: approaches, Koeppen's logic and his letter code

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• Three approaches to classifying climate (Class 11, World Climate and Climate Change):
• Empirical classification: uses observed values, mainly temperature and precipitation. It does not ask what causes them. Koeppen's scheme is the standard example.
• Genetic classification: groups climates by their causes, such as air masses, pressure belts and circulation. Strahler's air-mass scheme is the standard example.

• Applied classification: built for one purpose, such as farming, building design or aviation.

• Thornthwaite classification (1931, revised 1948; beyond NCERT): uses precipitation effectiveness and potential evapotranspiration. Potential evapotranspiration is the water a place could lose to evaporation and plant transpiration, so the scheme compares the water a place gets with the water it could lose. It has also been applied to India. Trewartha (1968) changed Koeppen's scheme and added H for highlands.

• Koeppen climate classification: Wladimir Koeppen (1846-1940) was a Russian-German climatologist and botanist. He saw that vegetation boundaries follow certain temperature and rainfall values, so he used those values as the limits of his classes. His first version (1900) was based on vegetation zones. The familiar scheme came in 1918 and was revised with Rudolf Geiger in 1936, which is why it is also called "Koeppen-Geiger". It uses mean annual and mean monthly temperature and precipitation.

Group	Rule
A Tropical	Coldest month 18°C or more
B Dry	Potential evaporation exceeds precipitation
C Warm temperate	Coldest month between −3°C and 18°C
D Cold snow forest	Coldest month −3°C or below
E Polar	Every month below 10°C

• A, C, D and E are humid groups and are defined by temperature. Only B is defined by moisture.
• Koeppen seasonality letters (second letter, showing the dry season): f = no dry season; m = monsoon with a short dry season; w = dry winter; s = dry summer. In group B, S = steppe (semi-arid) and W = desert (arid).
• Koeppen temperature letters (third letter): a = hot summer (warmest month 22°C or more); b = warm summer; c = cool summer; d = very cold winter. In group B, h = hot (annual mean 18°C or more) and k = cold.
• The 10°C isotherm of the warmest month roughly matches the tree line, both towards the poles and up mountains. That is why it divides D from E (beyond NCERT).
• Trap: m is a moisture letter, not a temperature letter.

2. Group A: tropical humid climates (Af, Am, Aw)

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• Tropical humid climates (A): the coldest month averages 18°C or more. They lie between the Tropic of Cancer and the Tropic of Capricorn. The Sun is high overhead all year and the ITCZ (Inter Tropical Convergence Zone, the belt where the trade winds of the two hemispheres meet and rise) brings rain. Rainfall is high and the annual range of temperature is very low, often under 3°C near the equator.
• Tropical wet climate (Af):
• Where: the Amazon basin, western equatorial Africa (the Congo basin) and the East Indies islands (Indonesia, Malaysia).
• Rain: falls every month as afternoon convectional thundershowers. The driest month gets 60 mm or more (beyond NCERT).
• Temperature: the daily maximum is about 30°C and the minimum about 20°C. So the range within a day is larger than the range across the year ("night is the winter of the tropics").

• Vegetation: tropical evergreen forest with a dense, layered canopy and the highest biodiversity (see biodiversity-conservation).

• Tropical monsoon climate (Am):

• Where: the Indian subcontinent, north-eastern South America and northern Australia.
• Climate: heavy summer rain and a short dry winter. The driest month is under 60 mm, but total rain is heavy enough for forest to survive.

• In India it covers the west coast south of Goa (shown as Amw on the India map). Details are in indian-climate-monsoon.

• Tropical wet and dry climate (Aw) (savanna):

• Where: north and south of the Af belt. It borders dry climates on the western side of continents and Cf or Cw climates on the eastern side.
• NCERT examples: Brazil north and south of the Amazon forest, Bolivia, Paraguay, Sudan, and the region south of Central Africa. Beyond NCERT: the Llanos of Venezuela, northern Australia and most of peninsular India.
• Climate: rain is lower than in Af and Am, and it varies from year to year. The wet season is short. The dry season is long, with severe drought. The daily temperature range is largest in the dry season.
• Vegetation: deciduous forest and tree-studded grassland. NCERT's "tree-shredded" is a misprint. Savanna life is covered in world-physical-features.

3. Group B: dry climates and coastal deserts

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• Dry climates (B): potential evaporation is greater than precipitation. Group B covers the largest area of any group, roughly a quarter to a third of the land (the exact share depends on the dataset), between 15° and 60° in both hemispheres. It occurs in three settings:
• Under the subtropical highs (15°-30°): sinking air and temperature inversion (air near the ground is colder than the air above it, the reverse of the usual pattern) stop rain from forming.
• On west coasts beside cold currents: the dry belt reaches closer to the equator and right down to the coast (Peru-Chile).

• In mid-latitude interiors (35°-60°): these places are cut off from moist sea winds, often by surrounding mountains.

• Subdivisions: BS = steppe (semi-arid) and BW = desert (arid). By latitude, h covers 15°-35° and k covers 35°-60°.

• Subtropical desert climate (BWh) and subtropical steppe climate (BSh):
• Rain is scanty and highly variable. In deserts it falls as short, intense showers that do not build up soil moisture.
• BSh lies on the desert margins and gets enough rain for sparse grassland. Its variable rain often brings famine, as in the Sahel (the 1967-77 drought; see climate-change).
• Examples: the Sahara, Arabia, the Thar, the Kalahari and the Great Australian deserts.
• Annual and daily temperature ranges are both large.

• Temperature record: (NCERT: 58°C at Al Aziziyah, Libya, 13 September 1922; now: the WMO invalidated that reading in 2012, and the accepted world record is 56.7°C at Furnace Creek, Death Valley, USA, 10 July 1913).

• Coastal desert: cold currents chill the air from below. Cold air near the ground cannot rise, so the air stays stable, rainless and foggy.

• Examples: the Atacama (Humboldt or Peru current), the Namib (Benguela current), the coastal Sahara (Canary current), Baja California (California current) and Western Australia (West Australian current). See ocean-movements.

• Fog-catching nets (camanchaca, Chile) and fog-drinking Namib beetles are popular examples.

• Mid-latitude steppe climate (BSk): semi-arid interiors with hot summers, cold winters and short grass. Examples: the Eurasian steppes and the Great Plains.

• Mid-latitude desert climate (BWk): arid interiors and rain shadows (the dry side of a mountain range, away from the moist wind). Examples: the Gobi, the Taklamakan, Patagonia (in the rain shadow of the Andes), the Great Basin and Ladakh. Summers are hot and winters severely cold.

4. Group C: warm temperate climates (Cwa, Cs, Cfa, Cfb)

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• Warm temperate climates (C): the coldest month averages between −3°C and 18°C. They lie between 30° and 50° on the eastern and western edges of continents. Summers are warm and winters mild.
• Why east and west coasts differ:
• West coasts: in summer the subtropical high brings dry weather (Cs). Further from the equator, the westerlies bring rain all year (Cfb).

• East coasts: warm currents and onshore summer winds bring rain all year (Cfa). Where a monsoon dominates, rain falls in summer and winter is dry (Cwa).

• Humid subtropical climate with dry winter (Cwa):

• Where: just poleward of the tropics, mainly the north Indian plains and the interior plains of south China.
• Climate: hot, rainy summers and dry winters. It is like Aw, but winters are cooler, because the coldest month is below 18°C (NCERT wrongly says winters are "warm").

• The Ganga plain is shown as Cwg on the India map.

• Mediterranean climate (Cs):

• Where: west coasts between 30° and 40°: around the Mediterranean Sea, central California, central Chile, south-western and southern Australia, and the Cape region of South Africa (beyond NCERT).
• Climate: the subtropical high gives hot, dry summers (about 25°C). The westerlies give mild, rainy winters (below 10°C). Rain is 35-90 cm a year.
• Vegetation: evergreen scrub, called maquis (Mediterranean), chaparral (California) and fynbos (South Africa).

• Crops: citrus, olives and grapes. See world-physical-features and primary-activities-world.

• Humid subtropical climate (Cfa):

• Where: east coasts in subtropical latitudes: the eastern USA, southern and eastern China, southern Japan, north-eastern Argentina, coastal South Africa and eastern Australia.
• Climate: unstable air (warm, moist air that rises easily and forms rain clouds) brings rain all year, 75-150 cm. Summers bring thunderstorms and winters bring frontal rain.

• Temperature: about 27°C in summer and 5-12°C in winter. The daily range is small.

• Marine west coast climate (Cfb):

• Where: poleward of Cs on west coasts: north-western Europe, the west coast of North America north of California (including British Columbia), southern Chile, south-eastern Australia and New Zealand.
• Climate: the westerlies and warm currents, such as the North Atlantic Drift, make winters warmer than the latitude would suggest. Summers are 15-20°C and winters 4-10°C, so ranges are small.
• Rain falls all year and varies widely, from 50 to 250 cm.

5. Group D: cold snow forest climates (Df, Dw)

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• Cold snow forest climates (D):
• Rule: the coldest month is −3°C or below and the warmest month is above 10°C.
• Where: large continental interiors between 40° and 70°N in Europe, Asia and North America.
• D climates are found only in the Northern Hemisphere. The southern continents narrow at these latitudes, so there is little land there, and the surrounding ocean keeps winters mild.

• Winters grow harsher towards the poles and further inland. Continentality (being far from the sea) makes both summers and winters more extreme.

• Vegetation: the colder D types carry the taiga, the boreal coniferous forest of pine, spruce, fir and larch. The warmer D types carry mixed forest. The taiga economy (lumbering, fur, pulp) is covered in world-physical-features.

• Cold climate with humid winters (Df):
• Where: poleward of the marine west coast and mid-latitude steppe climates.
• Climate: cold, snowy winters; a short frost-free season; large annual ranges; sudden, short-lived weather changes.

• Beyond NCERT: the Dfa and Dfb (humid continental) belts of the US Midwest, south-eastern Canada and eastern Europe grow maize and wheat and support dairy farming. Dfc covers most of Canada and Russia.

• Cold climate with dry winters (Dw):

• Where: mainly north-eastern Asia (eastern Siberia, Manchuria and northern Korea).
• Wind reversal:
  • Winter: a strong anticyclone (a high-pressure system) forms over Siberia, and dry, cold air flows outward from it.
  • Summer: the high weakens and the winds reverse, much like a monsoon.
• Rain: falls in summer and is low, 12-15 cm a year (NCERT).
• Winter: up to seven months below freezing.
• Beyond NCERT: Verkhoyansk (−67.8°C, 1892) and Oymyakon (−67.7°C, 1933) form the Northern Hemisphere's cold pole. They have the world's largest annual temperature range, over 60°C.

6. Group E: polar climates, and highland (H) climates

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• Polar climates (E): every month averages below 10°C. They lie poleward of about 70°.
• Tundra climate (ET):
• Rule: the warmest month is between 0°C and 10°C.
• Climate: there is no true summer, but summer daylight lasts very long.
• Vegetation: the climate is named after its vegetation of mosses, lichens and low flowering plants.

• Where: northern Canada, Alaska, Siberia and coastal Greenland.

• Permafrost: ground that stays frozen for at least two years in a row.

• Only a thin top "active layer" thaws in summer. Water cannot drain through the frozen ground below, so the surface becomes waterlogged and only low plants can grow.
• Permafrost lies under roughly a quarter of the Northern Hemisphere's land.

• As it thaws with warming, it releases methane and carbon dioxide, which cause more warming (see climate-change). The thaw also damages buildings, roads and pipelines (Norilsk fuel spill, 2020).

• Ice cap climate (EF):

• Rule: every month is below 0°C.
• Where: interior Greenland and Antarctica. NCERT's example station is Plateau Station, Antarctica (79°S).
• Precipitation is very low, so these are polar deserts.
• Snow piles up into ice sheets. The weight of the ice makes the sheets flow and break off at their edges.

• Lowest temperature: −89.2°C at Vostok, Antarctica (21 July 1983).

• Iceberg:

• An iceberg is a floating block of freshwater ice broken off a glacier or ice sheet. About nine-tenths of it lies below the water.

• The Labrador current carries Greenland icebergs south into North Atlantic shipping lanes. The sinking of the Titanic (1912) led to the International Ice Patrol (1914).

• H, highland climates (added by Trewartha; not in NCERT):

• Temperature falls with height, so climate belts stack one above another over short distances.
• Examples: the Andes, Tibet, the Himalaya and the East African highlands. Ladakh and the Himalayan peaks are India's examples (see india-structure-physiography).
• Koeppen maps often show these areas as ET or Dw.

7. Putting the map together: the idealised continent, Koeppen in India and the scheme's limits

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• The idealised (hypothetical) continent (beyond NCERT) shows where each type would sit on a simple model continent:
• Near the equator: Af, then Am and Aw moving poleward.
• West and centre under the subtropical highs: B. It reaches the coast and comes close to the equator where cold currents flow.
• West coast: Cs at 30°-40° and Cfb beyond 40°.
• East coast: Cfa or Cwa.

• Northern interiors and east: D, followed by ET and then EF towards the poles.

• Why this pattern matters: it is the map of pressure belts, winds and ocean currents drawn onto land (see pressure-winds-circulation and ocean-movements). Latitude, coast and continentality together predict the climate of any place, and Prelims statement questions are built on this logic.

• Koeppen's regions of India (only a pointer here; details in indian-climate-monsoon):

Code	Region
Amw	West coast south of Goa
As	Coromandel coast of Tamil Nadu (dry summer, winter rain)
Aw	Most of the peninsular plateau
BShw	Semi-arid parts of Rajasthan, Gujarat and Haryana; rain shadow of the Western Ghats
BWhw	Western Rajasthan
Cwg	Ganga plain (g = hottest month comes before the summer solstice)
Dfc	Sikkim, Arunachal Pradesh
E	Ladakh, and the peaks of Himachal Pradesh and Uttarakhand

• Answer-key nuance: the Class 11 exercise picks Am for most of the peninsula. This is debatable, because Aw covers the larger share.
• Strengths: the scheme is simple, based on data, closely linked to vegetation and easy to map. Beck et al. (2018, 2023) redrew it at 1 km resolution, for today and for 2071-2100.
• Limits:
• It ignores causes such as air masses and fronts, and it leaves out wind and cloud.
• It draws sharp boundaries, but in nature climates change gradually.
• Vegetation also depends on soil and history, not only on climate.

• It uses averages, which hide year-to-year variation and extremes.

• Climate change is moving Koeppen boundaries poleward and uphill (IPCC AR6 WGI; see climate-change).

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Exam angles

Prelims — high-yield facts and traps

• Group rules: A has its coldest month at 18°C or more. C has its coldest month between −3°C and 18°C. D has its coldest month at −3°C or below. In E every month is below 10°C. In B evaporation exceeds precipitation. Four groups are defined by temperature and only B by moisture. The humid set is A, C, D and E.
• Letters: f, m, w and s show the dry season. S and W stand for steppe and desert. h and k stand for hot and cold dry climates. a, b, c and d show how severe the temperature is.
• Type and region matches:
• Af: Amazon, Congo, Indonesia.
• Am: India, north-eastern South America, northern Australia.
• Cs: Mediterranean, California, central Chile, south-west Australia.
• Cfb: north-west Europe, British Columbia, New Zealand.
• Cfa: eastern USA, south China, southern Japan.
• Dw: north-eastern Asia.

• EF: interior Greenland, Antarctica.

• Coastal deserts and their cold currents: Atacama (Humboldt), Namib (Benguela), coastal Sahara (Canary), Baja California (California), Western Australia (West Australian).

• Koeppen's scheme is empirical: it is based on temperature and precipitation, with vegetation in mind. Genetic schemes classify by causes.
• India: Amw on the west coast south of Goa; As on the Coromandel coast; BWhw in western Rajasthan; Cwg in the Ganga plain; Aw over most of the peninsula.
• Records: the highest temperature is 56.7°C at Death Valley (1913). The lowest is −89.2°C at Vostok (1983).
• Statements marked FALSE:
• "D climates occur in both hemispheres" — FALSE.
• "Mediterranean climates get summer rain" — FALSE.
• "Al Aziziyah holds the world heat record" — FALSE.
• "Cwa winters are warmer than Aw winters" — FALSE.
• "m in Am is a temperature letter" — FALSE.
• "Tundra means ice cap" — FALSE (tundra has permafrost, not an ice cap).

Mains — GS-I themes

1. Explain the world pattern of climates using pressure belts, ocean currents and continentality (the idealised continent).
2. Why are the west coasts of continents dry in subtropical latitudes, while the east coasts are humid?
3. Compare A and B climates, and the vegetation of A and C climates (Class 11 exercises).
4. Discuss the strengths and limits of Koeppen's scheme, and compare empirical with genetic classification.
5. How is climate change shifting climate boundaries, and what does this mean for farming and biomes?

Current-affairs hooks

• Permafrost thaw in Siberia and Alaska, and its methane feedback.
• Record heat in deserts and the Mediterranean, and debates over new temperature records (verify current).
• Updated global Koeppen-Geiger maps showing tropical and dry zones expanding (verify current).
• Wildfires and droughts in Mediterranean-type (Cs) regions: California, Chile, Australia, Greece.
• Giant icebergs breaking off Antarctic ice shelves, such as A23a (verify current).