Koppen Climate Classification of India and Thornthwaite

Koppen Climate Classification: Meaning, Basis and History

What Is the Koppen Climate Classification?

The Koppen climate classification is an empirical classification, one built on observed data, particularly temperature and precipitation. Wladimir Koppen noticed a close relationship between the distribution of vegetation and climate. He selected values of temperature and precipitation that matched the edges of the main vegetation zones and used them as the boundaries between climate types. That is why the scheme still helps a student predict what grows in a place. Its groups are, in effect, vegetation belts described in numbers.

Climate classifications follow one of three broad approaches, and it helps to place Koppen among them:

• Empirical: Based on observed data, particularly temperature and precipitation; Koppen's scheme uses mean annual and mean monthly values.
• Genetic: Organises climates by their causes, such as air masses and wind systems.
• Applied: Made for a specific purpose, such as agriculture or water planning.

A climatic region, in any of these schemes, is an area with a broadly uniform climate produced by the same combination of factors. Temperature and rainfall are the two elements that decide the class in every scheme.

History of the Koppen Scheme from 1884 to Koppen-Geiger

Koppen's scheme is the most widely used climate classification in the world. Wladimir Koppen (1846 to 1940) first published it in 1884 and modified it several times, notably in 1918 and 1936. The German climatologist Rudolf Geiger changed it again in 1954 and 1961, so the modern version is often called the Koppen-Geiger classification. The name is spelt Köppen in German and Koppen or Koeppen in English.

NCERT dates the scheme to 1918, the year of its fully worked version: 1884 is the first publication and 1918 the major revision.

Later schemes grew out of Koppen's. Glenn Trewartha published a modified version in 1966, revised in 1980, to redraw the middle latitudes, because Koppen's C group was too general. C. W. Thornthwaite, who learned Koppen's system at the University of California, Berkeley, published a new classification of North American climates in 1931 and a rational classification based on water loss in 1948.

Five Major Climate Groups and Koppen Letter Codes

Climate Groups A to E: Temperature and Rainfall Thresholds

Koppen recognised five major climatic groups, four based on temperature and one on precipitation. The capital letters A, C, D and E mark humid climates, and B marks dry climates. The thresholds for A, C, D and E are mean monthly temperatures, the average of a month across many years; group B is defined by rain against evaporation.

The dry group is defined differently from the other four. A place is dry when its rain is low in comparison with its temperature, because a hot place loses more water to evaporation than a cool one. Within group B, the capital letter S marks the steppe or semi-arid climate, where dryness is less, and W marks the desert or arid climate, where dryness is more.

Koppen Small Letters: Season of Rain and Degree of Heat

Each group is divided into types by small letters. The second letter usually tells when the dry season falls (in group B it tells steppe or desert), and a third letter tells how hot or severe the temperature is. Reading a code is simply reading these letters in order.

Tropical and Dry Climates of the World: Af, Am, Aw, BS and BW

Tropical humid climates lie between the Tropic of Cancer and the Tropic of Capricorn. The sun is overhead throughout the year and the Inter Tropical Convergence Zone (ITCZ) stays close, so these climates are hot and humid, with a very low annual range of temperature and high annual rainfall.

• Tropical wet (Af): Found near the equator in the Amazon Basin, western equatorial Africa and the islands of the East Indies. Significant rain falls in every month as afternoon thundershowers, the maximum temperature on any day is around 30 °C and the minimum around 20 °C. These climates usually occur within 10° latitude of the equator and have no natural seasons.
• Tropical monsoon (Am): Found over the Indian subcontinent, north-eastern South America and northern Australia. Heavy rain falls mostly in summer and the winter is dry.
• Tropical wet and dry, or savanna (Aw): Found north and south of the Af belt, as in Brazil beyond the Amazon forest, Sudan and south of Central Africa. The wet season is shorter, the dry season longer and the drought more severe; deciduous forest and grassland with scattered trees grow here.

The dry climates cover a very large area, from 15° to 60° north and south of the equator. At low latitudes, from 15° to 30°, they lie under the subtropical high, where sinking air and temperature inversion stop rain from forming. In the middle latitudes, from 35° to 60°, they are confined to continental interiors that maritime winds do not reach. That gives four types, namely subtropical steppe (BSh) and subtropical desert (BWh) at 15° to 30°, and mid-latitude steppe (BSk) and mid-latitude desert (BWk) at 35° to 60°.

The steppe receives slightly more rain than the desert, enough for sparse grassland, but its rain is highly variable. That variability affects life in the steppe much more than in the desert and has often caused famine.

People use the steppes and prairies in different ways. On the Eurasian steppes, temperate grasslands with hot summers and cold winters, the traditional dwelling of the nomad is the yurt, and pastoral nomadism has its core region from North Africa and the Arabian peninsula into Mongolia and Central China. The North American prairies are instead a centre of extensive commercial grain farming.

Temperate, Cold and Polar Climates: Cwa, Cs, Cfa, Cfb, D and E

Warm temperate climates extend from 30° to 50° of latitude, mainly on the eastern and western margins of the continents, with warm summers and mild winters. Where a climate sits on the continent decides whether its rain comes in summer, in winter or all year.

The Mediterranean climate is the classic dry-summer type. The subtropical high sits over it in summer, and the westerly winds bring rain in winter. The marine west coast climate is its opposite in rhythm. Because of the sea, temperatures stay moderate and rain falls in every season. The humid subtropical Cfa climate of southern and eastern China, called the China type in older textbooks, also has rain through the year, from summer thunderstorms and winter frontal rain.

• Tundra (ET): Named after its low-growing mosses, lichens and flowering plants; the subsoil is permanently frozen as permafrost.
• Ice cap (EF): Interior Greenland and Antarctica, below freezing even in summer.

Mountain climates sit outside this sequence. Temperature falls with height, so a single tropical mountain can carry several climates one above another, from nearly tropical at its foot to tundra above the snow line. This vertical zonation is why tropical highlands do not fit a single Koppen letter.

Koppen Climate Classification of India: Eight Climatic Regions

How Many Climatic Regions Does India Have Under Koppen?

India has eight climatic regions under the Koppen climate classification. The whole of India has a monsoon type of climate, but temperature and rainfall combine differently from region to region, and these combinations are the sub-types of the monsoon climate. Koppen's capital letters, with the small letters f, m, w, h, c and g, turn them into eight codes.

Full Forms of Koppen Climate Codes in India

Each code is read letter by letter:

• Amw: A tropical, m monsoon (rain forest despite a dry season), w dry season in winter. In words, a tropical monsoon climate with a short dry season.
• As: A tropical, s dry season in summer. A tropical monsoon climate with a dry summer, found only on the Coromandel coast.
• Aw: A tropical, w winter dry season. The tropical savanna, or tropical wet and dry, climate.
• BShw: B dry, S semi-arid or steppe, h hot (low-latitude), w dry winter. A hot semi-arid steppe climate.
• BWhw: B dry, W arid or desert, h hot, w dry winter. A hot desert climate.
• Cwg: C warm temperate, w dry winter, g Gangetic plain type. A warm temperate monsoon climate with a dry winter.
• Dfc: D cold snow forest, f no dry season, c fewer than four months above 10 °C. A cold humid winter with a short summer.
• E: The polar type, where every month averages below 10 °C.

Tropical Climatic Regions of India: Amw, As and Aw

The three tropical codes cover the Peninsula and the coasts, where the coldest month stays at 18 °C or above. What separates them is how long the dry season lasts and when it comes.

• Amw on the west coast: The coast south of Goa, where the south-west monsoon strikes the Western Ghats head on. The tropical monsoon strip along the Malabar coast and the Western Ghats typically receives more than 2,000 mm of rain a year, and December to March are the driest months.
• As on the Coromandel coast: The one part of India whose dry season falls in summer. The coast lies parallel to the Bay of Bengal branch and in the rain shadow of the Arabian Sea branch during the south-west monsoon, and gets its main rain in October and November, when the north-east monsoon crosses the Bay of Bengal and picks up moisture.
• Aw over the Peninsular plateaus: Most of the plateau south of the Tropic of Cancer. The rainy season lasts from June to September, with 750 to 1,500 mm a year, and the long dry winter and hot early summer let heat waves build in May.

Dry Climatic Regions of India: BShw Semi-Arid Steppe and BWhw Hot Desert

The north-west is the only part of India in Koppen's dry group, because evaporation there takes more water than the rain supplies. Both codes carry h, for a hot low-latitude dry climate, and w, for a dry winter.

• BShw, semi-arid steppe: North-western Gujarat, parts of western Rajasthan and Punjab, a transitional belt between the true desert and the humid plains. The south-west Punjab, west Haryana and Kathiawar belt receives 300 to 650 mm a year, and that rain is very unreliable.
• BWhw, hot desert: Extreme western Rajasthan, the heart of the Thar. Annual rain totals less than 300 mm, comes in bursts in July, August and September, and is so erratic that a place wet one year may see no rain for the next couple of years. May and June average around 35 °C, with daily maxima occasionally above 50 °C.

The cause is the same one that places the world's hot deserts at low latitudes. The north-west lies under the subtropical high, where sinking air keeps the sky clear, and the Arabian Sea branch of the monsoon that crosses Saurashtra and Kachchh passes over west Rajasthan along the Aravallis, causing only scanty rain.

Climate of the Ganga Plain and Himalaya: Cwg, Dfc and E

North of the Tropic of Cancer, winters are cool enough to take India out of the tropical group. The Ganga plain falls in group C, and the Himalaya, rising through every belt of temperature, reaches groups D and E.

• Cwg, the Ganga plain type: The Ganga plain, eastern Rajasthan, northern Madhya Pradesh and most of the North-east. Most rain comes from the south-west monsoon. The dry winter is broken only by weak temperate cyclones from the Mediterranean, whose meagre rain is highly beneficial for rabi crops in Punjab, Haryana, Delhi and western Uttar Pradesh.
• Dfc, Arunachal Pradesh: A cold humid winter with a short summer, in which the coldest month falls to minus 3 °C or below, there is no dry season (f), and fewer than four months average above 10 °C (c).
• E, the polar type: Jammu and Kashmir, Himachal Pradesh and Uttarakhand, at heights where every month averages below 10 °C.

Height, not latitude, makes these mountain climates. Temperature falls by about 6.5 °C for every 1,000 metres of height, so the Himalaya runs from nearly tropical in the foothills to tundra above the snow line within a short distance. North of the main range, the trans-Himalayan belt of Ladakh has a cold desert climate, barren and wind-blown, because the mountains keep the monsoon out. Agra and Darjeeling lie on the same latitude, yet January averages 16 °C in Agra and only 4 °C in Darjeeling.

Factors, Monsoon and Seasons Behind India's Climatic Regions

Factors That Shape the Climatic Regions of India

The eight regions exist because six factors that control the climate of India act on it in different combinations, and each one draws a regional boundary.

Monsoon Climate and the Four Seasons of India

The thread joining all eight regions is the monsoon, a climate marked by the seasonal reversal in the direction of winds. The south-west monsoon of June to September brings most of India's rain, and the north-east monsoon of October and November waters the Tamil Nadu coast; the ITCZ, the jet streams and the oceans together drive the Indian monsoon.

The India Meteorological Department describes the year as four seasons, and each Koppen code describes how these four seasons feel in one place.

1. Cold weather season: Sets in by mid-November in the north; December and January are the coldest months, and the mean daily temperature stays below 21 °C over most of northern India.
2. Hot weather season: March to May; the heat belt moves north, and temperatures around 48 °C are not uncommon in the north-west in May.
3. South-west monsoon season: June to September; the rains begin abruptly with the burst of the monsoon.
4. Retreating monsoon season: October and November; clear skies and the oppressive "October heat" in the north, and the rainiest months of the year on the eastern side of the Peninsula.

The traditional Indian calendar divides the same year into six two-month seasons, Vasanta, Grishma, Varsha, Sharada, Hemanta and Shishira, a cycle that fits north and central India but not the south, where the seasons vary little.

Thornthwaite Climate Classification: Moisture Index and Water Budget

Thornthwaite's 1931 and 1948 Schemes

The Thornthwaite climate classification grew out of Koppen's. C. W. Thornthwaite, an American geographer and climatologist, learned Koppen's system at Berkeley and set out to correct its flaws. His first scheme, published in 1931 for the climates of North America, was inspired by Koppen. His 1948 paper, "An Approach Toward a Rational Classification of Climate", became his most cited work and is still in use worldwide.

The idea behind both is that rain matters to plants only after evaporation and transpiration have taken their share. Two places with the same rainfall can have very different water for crops if one is much hotter. Thornthwaite therefore built his classes on potential evapotranspiration, the water that evaporation from the soil and transpiration from plants would remove if water were always available, and compared it with the rain that actually falls.

Thornthwaite Moisture Index, Thermal Regimes and the Water Balance

The moisture index compares this demand with the supply, and the lower the value of the index in an area, the drier it is. Humid regions receive more precipitation than evaporation takes over a year, and arid regions lose more to evaporation than they receive.

• Moisture classes: Hyperhumid, humid, subhumid, subarid, semi-arid (index values of minus 20 to minus 40) and arid (values below minus 40).
• Microthermal regime: Low annual mean temperatures, generally between 0 °C and 14 °C, with short summers and low potential evaporation.
• Mesothermal regime: Neither persistent heat nor persistent cold, with potential evaporation between 57 and 114 centimetres.
• Megathermal regime: Persistent high temperatures and abundant rainfall, with potential annual evaporation above 114 centimetres.

In 1955 Thornthwaite and J. R. Mather extended the idea into the water balance, a simple method for estimating water surplus, runoff and the water that recharges an aquifer. About 33 per cent of the Earth's landmass is arid or semi-arid in this scheme. Because the index measures water actually available to plants, it is used to study the number of herbivore and mammal species an area supports and in studies of climate change. The United States Geological Survey distributes a Thornthwaite monthly water-balance program as a research tool, an assessment tool and a tool for classroom instruction.

Thornthwaite Climate Classification of India and IMD's Aridity Anomaly Index

Thornthwaite's 1948 scheme was applied to India and its neighbourhood in a 1956 paper in the Indian Journal of Meteorology and Geophysics, the journal of the India Meteorological Department, renamed Mausam in 1979, using climatic data from about 250 stations. Two findings followed that earlier schemes had missed. Most parts of India are more arid than earlier classifications showed, and the country's thermal efficiency is more than adequate to support forest everywhere, so what limits vegetation is the seasonal distribution of rainfall.

The India Meteorological Department still uses Thornthwaite's idea of aridity, the water deficiency a plant suffers, to monitor agricultural drought during the monsoon. The method has three steps:

1. Water balance: Rainfall is first used by plants for evapotranspiration; any excess recharges the soil until it reaches field capacity, and only then does the surplus run off.
2. Aridity index: The water deficit, potential minus actual evapotranspiration, as a percentage of potential evapotranspiration; potential evapotranspiration is computed by Penman's equation. It is worked out on a weekly or biweekly basis.
3. Aridity anomaly: The actual aridity of the week minus the normal for stations representing different agroclimatic zones. A positive anomaly of 1 to 25 marks mild, 26 to 50 moderate and above 50 severe agricultural drought.

Difference Between Koppen and Thornthwaite Climate Classification

Both schemes use temperature and precipitation, but they ask different questions. Koppen asks whether a place crosses fixed values that match vegetation boundaries; Thornthwaite asks how much of the rain is left for plants once evaporation and transpiration are counted.

Both schemes survive because Koppen's letters are quick to read on a map and in an answer, while Thornthwaite's index tells a planner how much water crops will actually find, which is why drought monitoring in India follows Thornthwaite and textbooks keep Koppen's codes.

Trewartha's Scheme, Climate Change and Economic Life

Trewartha Climate Classification and the Middle-Latitude Problem

The Trewartha climate classification, first published in 1966, is a modified Koppen-Geiger scheme. It keeps Koppen's tropical and polar climates and redraws the middle latitudes by counting the months with a mean temperature of 10 °C or higher. In Trewartha's scheme E stands for boreal, and the polar climates move to F.

• C, subtropical: Eight or more months at 10 °C or higher.
• D, temperate: Four to seven months at 10 °C or higher.
• E, boreal: One to three months at 10 °C or higher.

The change answers the main criticism of Koppen, that the single C group was too general. Standard Koppen puts London or New York in the same C group as Brisbane or New Orleans, despite great differences in seasonal temperatures and native plants; counting warm months separates them by the length of the growing season.

Climatic Regions, Monsoon Climate and Economic Life in India

India's climatic regions are sub-types of one monsoon climate, and that shared monsoon is the axis around which the whole agricultural cycle revolves. Indian agriculture is based on the south-west monsoon, so the monsoon sets the rhythm of rural livelihoods across the country.

• Growing season: Except in the Himalaya, every part of the country has temperatures above the threshold for crops throughout the year.
• Crop variety: Regional variations in the monsoon climate help India grow many types of crops.
• Winter rain: Rain from temperate cyclones in north India is highly beneficial for rabi crops.
• Hazards: Variability of rainfall brings droughts or floods every year in some part of the country, and a sudden monsoon burst causes soil erosion over large areas.
• Ways of life: Regional climatic variation shows in India's vast variety of food, clothes and house types.

Koppen Classes as a Record of Climate Change

Because each Koppen class matches a vegetation type, a shift of classes on the map signals a shift in what can grow. Between 1950 and 2010 about 5.7 per cent of all land area worldwide moved from wetter and colder classes to drier and hotter ones, a change that natural variation alone cannot explain. Koppen-Geiger maps that run from 1901 to 2100 under a worst-case emission scenario show tropical climates expanding to higher latitudes, dry climates spreading into temperate zones and polar climates shrinking dramatically.

Sources

• NCERT: India: Physical Environment (Class XI), Climate (current edition)
• NCERT: India: Physical Environment (Class XI), Climate (earlier edition), Climatic Regions of India and Table 4.1
• NCERT: Fundamentals of Physical Geography (Class XI), World Climate and Climate Change
• NCERT: Fundamentals of Human Geography (Class XII), Primary Activities
• NCERT: Exploring Society: India and Beyond (Class VIII)
• India Meteorological Department (Pune): Drought and the Aridity Anomaly Index
• India Meteorological Department: Climatic Types of India According to the Rational Classification of Thornthwaite (Indian Journal of Meteorology and Geophysics, 1956)
• USGS Open-File Report 2007‑1088: A monthly water-balance model
• NOAA Science On a Sphere: Koppen-Geiger Climate Changes, 1901 to 2100
• Wikipedia: Köppen climate classification
• Wikipedia: Climate classification
• Wikipedia: Climate of India
• Wikipedia: Charles Warren Thornthwaite
• Wikipedia: Trewartha climate classification
• Wikipedia: Thar Desert
• UPSC: Civil Services Examination question papers