Köppen’s Climate Classification — A/B/C/D/E/H Groups

11.1 Why We Classify the World's Climates

Imagine standing on a busy port in Mumbai during July — the air is hot, sticky and the monsoon clouds are spilling rain across the harbour. Now picture yourself the same week in Riyadh — the same Sun is overhead but the air is dust-bright, the temperature crosses 45°C and there is no rain in sight. A week later, in Reykjavík, Iceland, the same Sun gives only a thin warmth, the wind off the Atlantic is cool and a fine drizzle falls. Three places, the same planet, the same Sun — and three completely different worlds. Climate is the long-term pattern that links each place to a specific kind of weather, vegetation and way of life. To make sense of this enormous variety, geographers must classify. They reduce the planet's atmospheric chaos to a small set of recognisable groups — and once a place is identified by its group, we can predict its rainfall season, its forests, its crops and even its building styles.

Three Approaches to Classifying Climate

Geographers have used three broad approaches to sort the world's climates. Each begins with a different question.

This chapter focuses on the most widely used scheme — the empirical classification proposed by the German-born climatologist Wladimir Köppen.

11.2 Köppen's Scheme of Classification of Climate

The most widely used classification of climate today is the empirical scheme developed by V. Köppen^?. He had noticed something simple but powerful — there is a close link between the natural vegetation of a region and its climate. Tropical rainforests, savannas, deserts, deciduous forests, taiga and tundra each occupy a particular envelope of temperature and rainfall. Köppen therefore selected specific values of mean annual and mean monthly temperature and precipitation, related these values to the distribution of vegetation, and used the boundaries to delimit climatic groups. He published his system in 1918 and refined it over the following decades. It remains the most popular scheme of climate classification in textbooks and atlases today.

To keep the system tidy, Köppen used a clever letter code. Capital letters name the major group; the first small letter describes the seasonality of precipitation; and a second small letter may be added to describe the severity of summer or winter temperature.

The Five Major Climatic Groups

Köppen recognised five major climatic groups. Four of them — A, C, D and E — are distinguished on the basis of temperature; only group B is distinguished on the basis of precipitation (where potential evaporation exceeds the rainfall received). Capital letters A, C, D and E together delineate the humid climates, while B covers the dry climates.

11.3 Group A — Tropical Humid Climates

The Tropical Humid climates lie between the Tropic of Cancer and the Tropic of Capricorn. Because the Sun is overhead throughout the year and the Inter Tropical Convergence Zone (ITCZ) regularly sweeps across these latitudes, the climate is hot and humid. The annual range of temperature is very low — sometimes only 1–2°C — while the annual rainfall is high. Köppen split this group into three types: Af (tropical wet), Am (tropical monsoon) and Aw (tropical wet and dry).

Tropical Wet Climate (Af)

The Af — tropical wet climate is found near the equator. Its three main heartlands are the Amazon Basin in South America, western equatorial Africa (the Congo Basin) and the islands of the East Indies (Indonesia, Malaysia, Philippines). A significant amount of rainfall occurs in every month of the year, usually as convectional thunder showers in the afternoon. Temperature is uniformly high and the annual range is negligible. The maximum temperature on any given day hovers around 30°C while the minimum is around 20°C. The climate supports tropical evergreen forests with a dense canopy and exceptional biodiversity.

Tropical Monsoon Climate (Am)

The Am — tropical monsoon climate is found over the Indian sub-continent, the north-eastern part of South America and northern Australia. Heavy rainfall occurs mostly in summer when moisture-laden winds blow inland; winter is dry. (A detailed account of this monsoonal regime is given in the companion volume India: Physical Environment.) Most of peninsular India, where rainfall is heavily concentrated in June–September, falls under this type.

Tropical Wet and Dry Climate (Aw)

The Aw — tropical wet and dry climate occurs north and south of the Af belt. On the western side of continents it borders the dry climate; on the eastern side it borders Cf or Cw. Extensive Aw zones are found north and south of the Amazon forest in Brazil and adjoining parts of Bolivia and Paraguay, in Sudan and in south of Central Africa. The annual rainfall is considerably less than in Af and Am climates and is also more variable. The wet season is shorter and the dry season is longer, with the drought being more severe. Temperature is high throughout the year, and the diurnal range of temperature is greatest in the dry season. Deciduous forest and tree-shredded grasslands (savanna) occur in this climate.

11.4 Group B — Dry Climates

The Dry climates (B) are characterised by very low rainfall — not adequate for the growth of plants. They cover a very large area of the planet, extending across large latitudes from 15° to 60° north and south of the equator. Köppen separated dry climates from humid climates by a single rule: potential evaporation must exceed precipitation.

Steppe (BS) and Desert (BW) — and the h/k Subdivision

Dry climates are divided into steppe or semi-arid (BS) and desert (BW), and each is further subdivided by latitude:

• BSh — subtropical steppe and BWh — subtropical desert at latitudes 15°–35° (h = hot).
• BSk — mid-latitude steppe and BWk — mid-latitude desert at latitudes 35°–60° (k = cold).

Subtropical Steppe (BSh) and Subtropical Desert (BWh)

Subtropical steppe (BSh) and subtropical desert (BWh) share common temperature and precipitation traits. Located in the transition zone between humid and dry climates, the steppe receives slightly more rainfall than the desert — adequate enough for the growth of sparse grasslands. Rainfall in both is highly variable, and the variability affects life in the steppe much more than in the desert, often causing famine. Rain in the desert occurs as short, intense thundershowers and is ineffective in building soil moisture. Fog is common in coastal deserts that border cold currents. Maximum summer temperatures are extremely high — the highest shade temperature of 58°C was recorded at Al Aziziyah, Libya, on 13 September 1922. Both annual and diurnal ranges of temperature are large.

11.5 Group C — Warm Temperate (Mid-Latitude) Climates

The Warm Temperate (mid-latitude) climates (C) extend from 30° to 50° latitude, mainly on the eastern and western margins of continents. These regions generally have warm summers and mild winters. NCERT groups them into four types:

• Cwa — Humid subtropical (dry in winter and hot in summer);
• Cs — Mediterranean;
• Cfa — Humid subtropical (no dry season, mild winter);
• Cfb — Marine west coast climate.

Humid Subtropical Climate (Cwa) — the China Type

The Cwa — humid subtropical climate occurs poleward of the Tropics of Cancer and Capricorn — chiefly in the North Indian plains and the South China interior plains. The climate resembles the Aw type, except that winters here are not warm but cool. Most of north India — Punjab, Haryana, Uttar Pradesh, Bihar — falls under this type.

Mediterranean Climate (Cs)

As the name suggests, the Cs — Mediterranean climate occurs around the Mediterranean Sea, along the west coast of continents in subtropical latitudes between 30°–40°. Examples include Central California, Central Chile, and the south-eastern and south-western coasts of Australia. These areas come under the influence of the subtropical high in summer and the westerly winds in winter. The climate is therefore characterised by hot, dry summers and mild, rainy winters. Monthly average temperature in summer is around 25°C and in winter below 10°C. The annual precipitation ranges between 35–90 cm. Olives, citrus, vines and cork oak typify the natural and cultivated vegetation.

Humid Subtropical (Cfa) Climate

The Cfa — humid subtropical climate lies on the eastern parts of continents in subtropical latitudes. In this region the air masses are generally unstable and produce rainfall throughout the year. It is found in the eastern United States of America, southern and eastern China, southern Japan, north-eastern Argentina, coastal South Africa and the eastern coast of Australia. Annual averages of precipitation vary from 75–150 cm. Thunderstorms in summer and frontal precipitation in winter are common. Mean monthly temperature in summer is around 27°C; in winter it varies from 5°–12°C. The daily range of temperature is small.

Marine West Coast Climate (Cfb)

The Cfb — marine west coast climate lies poleward of the Mediterranean climate on the west coast of continents. The main areas are north-western Europe, the west coast of North America (north of California), southern Chile, south-eastern Australia and New Zealand. Due to the marine influence, temperatures are moderate, and winter is warmer than its latitude would suggest. The mean temperature in summer months ranges from 15°–20°C, and in winter 4°–10°C. The annual and daily ranges of temperature are small. Precipitation occurs throughout the year, ranging widely from 50–250 cm. Damp, mild, foggy and cloudy weather is the popular image of this climate — and it is the home of the temperate deciduous and mixed forest.

11.6 Group D — Cold Snow-Forest Climates

The Cold Snow-forest climates (D) occur in the large continental area of the northern hemisphere between 40°–70° north latitudes in Europe, Asia and North America. Because the Southern Hemisphere has no land mass at these latitudes (only ocean), there is no D climate south of the equator. The severity of winter is more pronounced in higher latitudes. NCERT divides them into two types:

• Df — cold climate with humid winter (humid continental);
• Dw — cold climate with dry winter (Eurasian taiga / subarctic).

Cold Climate with Humid Winters (Df)

The Df — cold climate with humid winter occurs poleward of the marine west coast climate and the mid-latitude steppe. Winters are cold and snowy. The frost-free season is short. Annual ranges of temperature are large. Weather changes are abrupt and short. Towards the pole, winters become more severe. This is the climate of the great boreal coniferous forest belt across Canada and the European plain.

Cold Climate with Dry Winters (Dw) — Eurasian Subarctic

The Dw — cold climate with dry winter occurs mainly over north-eastern Asia. Here the development of a pronounced winter anticyclone (the Siberian High) and its weakening in summer produce a monsoon-like reversal of wind. Toward the pole, summer temperatures become lower and winter temperatures fall extremely low — many locations experience below-freezing temperatures for up to seven months in a year. Precipitation occurs in summer; the annual precipitation is low — only 12–15 cm. The vegetation is the taiga, the world's largest forest belt of larch, pine and spruce.

11.7 Group E — Cold Polar Climates

The Polar climates (E) exist poleward beyond 70° latitude. Köppen identified two types: Tundra (ET) and Ice Cap (EF).

Tundra Climate (ET)

The tundra climate (ET) takes its name from its vegetation — low-growing mosses, lichens and flowering plants that hug the ground out of the wind. This is the region of permafrost, where the sub-soil is permanently frozen. The short growing season and water-logging during the brief summer support only these low-growing plants. During summer, the tundra regions enjoy a very long duration of daylight — at the highest latitudes, the Sun does not set for weeks (the "midnight Sun"). The treeline ends here.

Ice Cap Climate (EF)

The ice cap climate (EF) occurs over the interior of Greenland and Antarctica. Even in summer the temperature stays below freezing point. The area receives very little precipitation. Snow and ice gradually accumulate; the mounting pressure causes the deformation of the ice sheets and they break. Pieces move out as icebergs that float in the Arctic and Antarctic waters. Plateau Station, Antarctica (79°S), portrays this climate — its annual mean temperature is one of the coldest in the world.