Köppen Climate Classification

The Köppen climate classification is the world's most widely used system for sorting places into climate types. It uses just two things that anyone can measure: temperature and rainfall, month by month. Based on these, every place on Earth gets a short letter code, such as Aw (tropical savanna) or BWh (hot desert). Geographers, ecologists and agricultural planners use it to compare climates and to see how they are changing.

Why do we need a climate classification?

The world has thousands of weather stations, each with years of numbers. Without a common system, it is hard to say whether Chennai and Mombasa have "the same kind" of climate, or whether a place has changed over time. A classification works like a library catalogue: it groups similar climates under one label. Once places are labelled, we can map them, compare them, and track if a label shifts, which is exactly what climate scientists now do to measure climate change.

Where did it come from?

The system was created by Wladimir Köppen (1846-1940), a Russian-born German climatologist who was also trained in botany (the study of plants). He first published it in 1884 and revised it several times, including in 1918 and 1936. Köppen noticed that natural vegetation (forests, grasslands, deserts) follows climate closely, so he chose temperature and rainfall limits that match where these vegetation types change. Later, the German climatologist Rudolf Geiger revised it in 1954 and 1961. That is why it is often called the Köppen-Geiger classification.

How does it work?

Each climate code has up to three letters.

• The first letter (capital) gives the main climate group, based mostly on temperature (or, for group B, on dryness).
• The second letter tells you about the rainfall pattern, for example whether there is a dry season and when it comes.
• The third letter adds detail about heat, for example how hot the summer is or whether a dry climate is hot or cold.

Think of it like a postal address: country, then city, then street. "A" is the country (tropical), "w" is the city (dry winter), and so on.

The five main groups

• A: Tropical. Every month has an average temperature of 18°C or more. There is no real winter. Example: most of peninsular India.
• B: Dry (arid). Here the key test is not temperature but water. More water could evaporate than falls as rain. The exact dryness limit depends on temperature, because hot places lose water faster. B splits into BW (desert, very dry) and BS (steppe or semi-arid, a little wetter, with grasses).
• C: Temperate (warm, mild winters). The coldest month is cool but not freezing for long (in the common modern definition, the coldest month averages above 0°C, or above -3°C in Köppen's original scheme, but below 18°C).
• D: Continental (cold winters). The coldest month averages below 0°C (or -3°C), and the warmest month is above 10°C.
• E: Polar. Even the warmest month averages below 10°C. ET is tundra (warmest month between 0°C and 10°C), EF is ice cap (below 0°C).

Common second and third letters

• f: rain in all seasons, no dry season.
• m: monsoon type, a short dry season but very heavy rain in the wet season.
• w: dry season in winter.
• s: dry season in summer.
• h: hot. In Köppen's original scheme this meant a mean annual temperature above 18°C. For example, BSh is a hot semi-arid steppe.
• k: cold (for dry climates, the opposite of h).
• g: a Ganges-type pattern, where the hottest month comes before the summer solstice and the main rainy season. Köppen used it for the Ganga plain.

Tropical savanna vs hot semi-arid: the two zones in today's news

• Aw (tropical savanna): warm all year (A), with a clear dry season in winter (w). In this type, the driest month gets less than 60 mm of rain. Tall grasses with scattered trees grow naturally. Annual rain is moderate, often around 75 cm or more in India's savanna belt.
• BSh (hot semi-arid steppe): hot (h), but dry enough to fall in group B. Rain is too low or too uncertain to support forests; short grasses and thorny bushes grow. Farming depends heavily on irrigation or the occasional good monsoon.

So when a place moves from BSh to Aw, it means rainfall has risen enough that the place is no longer "dry" by Köppen's water test.

Köppen's climate regions of India

Using this system, Indian geography textbooks (including NCERT) divide India into about nine climate types:

• Amw (monsoon type with a short dry season): the west coast south of Goa, which gets very heavy rain from the southwest monsoon.
• As (monsoon type with a dry season in the high-sun period): the Coromandel coast of Tamil Nadu and adjoining Andhra Pradesh, which gets much of its rain in winter from the northeast monsoon.
• Aw (tropical savanna): most of the peninsular plateau, away from the two coastal strips.
• BShw (semi-arid steppe): parts of north-western Gujarat, western Rajasthan's margins, Haryana and Punjab, and the rain-shadow areas east of the Western Ghats.
• BWhw (hot desert): most of western Rajasthan (the Thar).
• Cwg (monsoon with dry winter, Ganga type): most of the Ganga plain, eastern Rajasthan, parts of Madhya Pradesh and most of north-east India.
• Dfc (cold, humid winter with short summer): parts of Arunachal Pradesh and Sikkim.
• Et (tundra): the mountain areas of Uttarakhand and similar heights.
• E (polar type): the high mountains of Jammu and Kashmir, Ladakh and Himachal Pradesh.

Textbooks differ slightly on boundaries, so learn the logic (rainfall pattern and temperature) rather than memorising exact lines.

Why climate zones can shift

The Köppen label of a place is based on a long-term average, usually 30 years. This 30-year average is called a climate normal, and the World Meteorological Organization uses it as the standard period. If the 30-year average of rainfall or temperature changes, the label can change too. Scientists therefore compare old normals (such as 1951-1980) with new ones (such as 1991-2020).

A shift in Köppen zone is a simple, map-friendly way of showing climate change at the local level. Global studies have also used Köppen maps to project how zones may move by 2100.

India's position and use

The IMD keeps rainfall and temperature records for India's 36 meteorological subdivisions. These records allow climate zones to be mapped decade by decade. Zone maps feed into agro-climatic planning: deciding which crops suit which region, when to sow, and how much irrigation is needed. The Planning Commission divided India into 15 agro-climatic zones in 1989 for this kind of planning, and the Indian Council of Agricultural Research uses smaller agro-ecological units.

Commonly confused concepts

• Weather vs climate: Weather is the condition of the air on one day (today it is raining). Climate is the average pattern over about 30 years (Chennai usually gets heavy rain in November).
• Köppen vs Thornthwaite: Köppen uses only temperature and rainfall. C.W. Thornthwaite's system (1931, revised 1948) uses evapotranspiration, which is the total water lost from soil and plants to the air. Thornthwaite is more precise about water balance but harder to calculate.
• Köppen vs Trewartha: Glenn Trewartha modified Köppen's scheme (1960s) to better match vegetation in mid-latitudes and added a separate highland group (H).
• Aw vs As: Both are tropical savanna types. In Aw the dry season comes in winter; in As the dry season comes in the high-sun (summer) period, as on the Coromandel coast.
• BW vs BS: BW is true desert (very dry). BS is semi-arid steppe (dry, but less so).
• Agro-climatic zones vs Köppen zones: Köppen zones describe climate only. Agro-climatic zones also consider soil, water and cropping patterns.

Issues, criticism and the way forward

• Too simple for some uses: Köppen uses only monthly averages. It ignores how rain falls (a few heavy bursts or many gentle showers), humidity, wind and sunshine. Two places with the same code can feel very different for farming.
• Sharp boundaries: Nature changes gradually, but the system draws hard lines. A place just above a limit and one just below it get different labels.
• Vegetation-based limits: The limits were chosen to match European ideas of plant zones in the 19th and early 20th centuries, so they fit some regions better than others.
• Using shifts wisely: Scientists suggest using zone shifts as an early warning, along with detailed rainfall and temperature studies. The way forward is to update crop calendars, irrigation plans and drought-proofing schemes using fresh climate normals rather than old ones.

Concepts to Know

• Climate normal: The average of a weather value (like rainfall or temperature) over a 30-year period. It is the official "normal" against which any year is compared.
• Semi-arid: A climate that is dry but not a desert. There is some rain, enough for grasses and bushes, but not enough for forests.
• Savanna: A tropical grassland with scattered trees, with a wet season and a long dry season.
• Rain-shadow area: The dry side of a mountain range. Moist winds drop their rain on the side they hit first, so the far side stays dry. Example: the Deccan east of the Western Ghats.
• Evapotranspiration: The total water that goes into the air from the soil (evaporation) and from plant leaves (transpiration).
• Meteorological subdivision: One of the 36 regions into which the IMD divides India for weather records and forecasts.