Koeppen climate classification

Also called: Koppen climate classification · Topic: World Climate Classification: Koeppen's Scheme · NCERT: Class 11, Ch 11 "World Climate and Climate Change"

Meaning

The Koeppen climate classification is an empirical scheme (built only from measured data, not from the causes of climate). Wladimir Koeppen set out its letter code in 1918, and in 1936 he revised it with Rudolf Geiger. It sorts the world's climates into five groups, A, B, C, D and E, using mean annual and mean monthly temperature and precipitation. Each group splits into types marked by extra letters.

It matters because it is still the most widely used climate scheme. Its climate boundaries are drawn to match vegetation boundaries, so the climate map closely follows the map of natural plant zones.

Explanation

How Koeppen built it: from vegetation to numbers

• Core idea: "climate is best defined by native vegetation." Koeppen designed his formulas to match the vegetation zones that people first began mapping in the late 19th century [2].
• Natural plants "record" the climate of a place over many years.
• Where forest gives way to grassland, or where trees stop growing, some temperature or rainfall limit has been crossed.

• Koeppen turned those limits into fixed numbers. These numbers became the borders of his climate classes.

• Result: the climate map closely matches the map of biomes (large natural zones of plants and animals, such as tropical rainforest or tundra).

• Inputs: only mean annual and mean monthly temperature and precipitation. Weather stations record these values everywhere, so any place in the world can be classified.
• Timeline:
• 1884: Koeppen's first published work on world climate zones [2].
• 1900: his first scheme, based on vegetation zones.
• 1918: the familiar letter-code scheme.
• 1936: revised with Rudolf Geiger, which is why it is called the "Koeppen-Geiger" scheme.
• Up to 1940: Koeppen kept revising it with Geiger until his death [2].
• 2007: a revised world map by Peel, Finlayson and McMahon [2].
• 2018: new world maps at about 1 km resolution. They cover 1980-2016, and also 2071-2100 under RCP8.5 (a high-emission scenario), using 32 climate models [3]. These maps matched station records with 80.0% accuracy, against 66.1-73.4% for earlier maps [3].

The five major groups (capital letters)

Group	Name	Rule
A	Tropical	Coldest month 18°C or more
B	Dry	Potential evaporation is more than 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. They are set by temperature.
• Only B is set by moisture.
• If more water could evaporate than falls as rain, the land stays short of water.
• Trees cannot survive there, so grassland, scrub or bare desert takes over.

• Here dryness, not cold, decides what plants can grow [2].

• There is no single rainfall figure for B. The dry limit is worked out from the mean annual temperature and from which season gets most of the rain. A hotter place needs more rain to stay out of group B [3].

• C/D boundary: NCERT gives −3°C. Many modern maps use a coldest month of 0°C instead [3].
• E boundary: the 2018 maps define E as a warmest month below 10°C [3]. This means the same as "every month below 10°C".
• Why 10°C separates D from E:
• Trees need a warm season long enough to grow new wood each year.
• If even the warmest month stays below about 10°C, trees cannot finish their growth.
• So the 10°C isotherm of the warmest month (a line joining places where the warmest month averages 10°C) roughly follows the tree line. It matches the poleward tree line, where taiga meets tundra at about 60-70°N, and the tree line on mountains such as the Himalaya.

The letter code: reading the second and third letters

• Second letter: when the dry season comes
• f: no dry season, so rain falls in every month (A, C, D).
• m: monsoon, meaning a short dry season and heavy rain the rest of the year (A).
• w: dry winter (A, C, D).
• s: dry summer (mainly C; also A and D).
• S: steppe, meaning semi-arid (B only).

• W: desert, meaning arid (B only).

• Third letter: how hot it is

• a: hot summer, with the warmest month 22°C or more (C, D).
• b: warm summer, with the warmest month below 22°C (C, D).
• c: cool summer (C, D).
• d: very cold winter (D).
• h: hot, with an annual mean of 18°C or more (B only).

• k: cold, with an annual mean below 18°C (B only).

• Group E uses a second capital letter instead:

• ET (tundra): warmest month between 0°C and 10°C.

• EF (ice cap): every month below 0°C.

• How to decode a code:

• First letter: find the group. Check the coldest month, or check dryness for B.
• Second letter: find when the dry season comes.
• Third letter: find how hot the summer is.

Where it occurs

The world

• Af: equatorial lowlands such as the Amazon basin and the Congo basin.
• BWh (hot desert): the Sahara.
• Cs (Mediterranean climate, with dry summers): the Mediterranean coasts.
• Dfc: the taiga belt of Siberia and Canada.
• ET: Arctic coasts. EF: Greenland and Antarctica.

India: every one of the five groups is found here

• Amw: India's west coast south of Goa (the Konkan-Malabar coast).
• Aw (savanna): most of peninsular India.
• As: the Coromandel coast of Tamil Nadu.
• BWhw: western Rajasthan.
• BSh: the interior of the Deccan plateau that lies in the rain shadow (the dry side of the hills, away from the rain-bearing wind).
• Cwg: the Ganga plain.
• Dfc: Arunachal Pradesh.
• E: the high Himalaya.

Don't confuse with

• Strahler's genetic classification: Strahler groups climates by their causes, which are air masses (huge bodies of air with fairly even heat and moisture) and pressure belts. Koeppen is empirical. He uses only measured temperature and rain and does not ask why the climate is the way it is.
• Thornthwaite classification (1931, revised 1948): this scheme uses potential evapotranspiration (PET), meaning the water a place could lose from soil and plants if water were always available. It also uses precipitation effectiveness, meaning how much rain is actually useful to plants. Koeppen did not use PET.
• Trewartha (1968): Trewartha changed Koeppen's scheme and added H for highland climates. Modified Koeppen-Geiger maps sometimes apply H to land above 1,500 m, but H was not part of the original system [2].
• Small w and capital W: small w means a dry winter, but capital W means desert. In the same way, capital S means steppe, but small s means a dry summer.

Prelims Hooks

• Only group B is defined by moisture, where potential evaporation is more than precipitation. Groups A, C, D and E are defined by temperature.
• Thresholds: in A, the coldest month is 18°C or more. In D, the coldest month is −3°C or below (many modern maps use 0°C [3]). In E, every month is below 10°C.
• Koeppen-Geiger refers to the 1936 revision with Rudolf Geiger. The familiar letter code dates from 1918, and the first vegetation-based scheme from 1900.
• Trap: m (monsoon) is a moisture letter, not a temperature letter. Also, h/k are used only with group B, and h means an annual mean of 18°C or more.
• India matches: Amw → west coast south of Goa. As → Coromandel coast. BWhw → western Rajasthan. Cwg → Ganga plain. Dfc → Arunachal Pradesh.
• The 10°C warmest-month isotherm roughly follows the tree line and separates D from E.

Mains Points

• Strengths (GS-I):
• It uses simple data recorded everywhere, so it works across the whole world.
• Its boundaries match vegetation and biome boundaries, which makes it useful for planning farming and land use.

• It is still the most used scheme and is now mapped at about 1 km resolution (2018) [3].

• Limitations (GS-I):

• It ignores the causes of climate, such as air masses and fronts. Genetic schemes like Strahler's fill this gap.
• It handles mountains poorly, which is why Trewartha added H.
• It uses rainfall but not the true water balance, which Thornthwaite's PET method covers.

• Its fixed thresholds draw sharp lines where real change is gradual.

• Climate change and India (GS-I/III):

• Koeppen zones are projected to shift by 2071-2100 under RCP8.5 [3].
• This warns of changes in crop belts, forest limits and the Himalayan tree line.
• Vegetation changes more slowly than climate, so a shift in zones shows that ecosystems are under climate stress. It does not mean the biome changes at once [3].
• India's range of climates, from Amw on the Konkan-Malabar coast to E in the high Himalaya, explains the variety in its farming patterns.

Related concepts

• Empirical classification
• Genetic classification
• Koeppen seasonality letters
• Koeppen temperature letters
• Thornthwaite classification

Read more

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