24 Climatic Classification

M K Nanda

1. Learning outcomes
2. Introduction
3. Types of climatic classification
3.1. Köppen’s classification
3.2. Thornthwaite’s Rational Classification
3.3. Comparison of Koeppen’s and Thornthwaite’s Schemes
4. Climatic subdivisions of India
4.1. Climatic regions of India as per Koeppen’s Scheme
4.2. Climatic regions of India as per Thornthwaite’s Scheme
4.3. Agroclimatic zones of India
5. Summary

1. Learning outcomes

• After studying this module, you shall be able to:
• know about the systematic way of defining climate of a region know about the types of climatic classification
• know about climatic classification of Köppen and Thornthwaite know about the agro-climatic zones of India

2. Introduction

Climate classifications, the formalization of systems that recognize, clarify, and simplify climatic similarities and differences between geographic areas in order to enhance the scientific understanding of climates. Such classification schemes rely on efforts that sort and group vast amounts of environmental data to uncover patterns between interacting climatic processes.

The earliest known climatic classifications were those of Classical Greek times. Such schemes generally divided Earth into latitudinal zones based on the latitudes, i.e. the Equator, the Tropics of Cancer and Capricorn, and the Arctic and Antarctic circles, respectively and on the length of day. Modern climate classification has its origins in the mid-19th century, with the first published maps of temperature and precipitation over Earth’s surface, which permitted the development of methods of climate grouping that used both variables simultaneously.

3. Types of climatic classification

Different schemes of climatic classification are broadly differentiated as either empiric or genetic methods. Empirical methods make use of observed environmental data, such as temperature, humidity, and precipitation, or simple quantities derived from them (such as evaporation). In contrast, genetic methods classify climate on the basis of its causal elements, the activity and characteristics of all factors (circulation systems, fronts, jet streams, solar radiation, topography etc.) that give rise to the spatial and temporal patterns of climatic data. Hence, while empirical classifications are largely descriptive of climate, genetic methods are explanatory. However, for all practical applications empirical classifications are widely adopted. In this section two classic and most widely accepted climatic classification schemes of Koppen and Thornthwaite have been discussed.

3.1 Köppen’s classification

The most popular empirical classification is given by Wladimir Köppen, in 1900 and several revised versions thereafter. Koeppen’s scheme used certain critical values of temperatures of the warmest and the coldest months and of rainfall of the wettest and the driest months. His climatic divisions generally coincide with vegetational divisions. Koppen (1936) divided the world climate into the following 5 principal groups.

A: Tropical rainy climate

Temperature of the coolest month does not exceed 18 0C

Af – Tropical rainforest (equatorial climate): Warm Temperature throughout with. mean value exceeding 27°C, abundant rainfall (annual average 250 cm), suitable for luxuriant vegetation. Prevails over Amazon basin, Zaire basin and south-east Asia.

Aw – Tropical savanna: Mean annual temperature 23°C, wet summers (due to convectional rainfall) and dry winter with annual rainfall 160 cm. Floods and droughts are common. Vegetation is tropical grassland or savanna with scattered deciduous trees. Prevails over Sudan, Veld plateau and the tropical grasslands of Australia.

Am – Monsoon type: Seasonal reversal of winds, associated with alternate periods of rainfall and drought with a short dry season.. This climate is experienced over the Pacific coast of Colombia, Guinea coast of west Africa, south-east Africa, south and south-east Asia and northern Australia.

As – Dry summer: A rare climatic type prevailing over some rainshadow areas along eastern coast of southern India in Tamil Nadu and Orissa that remain dry during summer monsoon and receive winter rainfall from retreating monsoons.

B: Dry climates:

Potential evaporation exceeds precipitation and constant water deficiency is experienced.

Bwh – Desert (Low Latitude) Climate: Sub-tropical high pressure region with mean annual temperature is 38°C and scanty and erratic rainfall. Vegetation varies with the soil type. This climate is experienced over southwest USA, north Africa (Sahara), west Asia, Thar desert, and central Australia.

Bwk – Mid-Latitude Deserts: These climatic conditions prevail over Takla Makan (China) and Gobi desert (Mongolia) and are similar to the low- latitude desert conditions.

Bsh and Bsk – Semiarid and Steppe: Mean annual temperature is around 21°C and rainfall a meager 30 cm. These regions are dry due to an interior location and absence of mountain barriers across the path of prevailing winds. These climatic conditions prevail over in the deep interiors of landmasses, such as Eurasia and North America.

C: Humid mesothermal/Warm temperate rainy

Mild winters; mean temperature of coldest month is below 18°C but above -3°C and that of the warmest month is above 10°C.

Cfa – Humid subtropical or China type climate: Hot and humid summer and mild winter with average annual temperature is 20°C and well distributed rainfall (100 cm). Hurricanes and typhoons are common. Prevails within 25° to 45° latitude on east coast in both hemispheres e.g., south-east USA, southern Brazil, Uruguay, Argentina, and south-eastern Africa, eastern coastal belt of Australia, eastern China and Japan.

Cfb – Marine west European climate: Characterised by on shore oceanic influences, short cool summers, mild winters with average annual temperature around 10°C and. and rainfall is 140 cm. Weather is variable and unpredictable. Prevails between 45° latitude and 65° latitude on west coast in both hemispheres e.g., Western Europe, narrow coastal belt in North and South America, -south-eastern Australia and New Zealand.

Cs – Mediterranean climate: Warm and dry summer (mean temperature 20°C- 27°C) due to sub-tropical high pressure conditions, mild winter (temperature 4°C to 10°C). with rainfall from low pressure cyclones (annual rainfall 40 cm- 60 cm). Prevails within 25° and 45° latitudes on west coasts in both hemispheres—over central California, central Chile, Mediterranean region, southern South Africa, southeastern and southwestern Australia.

D: Humid microthermal or Cold forest climates:

Severe winters, temperature of the coldest month is below -3°C and warmest month, above 10°C.

Df – Cool east coast climate: Hot and humid summer (mean temperature 25°C), influenced by tropical maritime air masses, cold winter (mean temperature -4°C to 0°C), . variable precipitation – convectional rainfall during summer and snowfall in winter. Prevails between 45° and 65° latitude on east coasts, over north-eastern USA, lower Danube plains, Korea, Japan, northern China.

Ds – Taiga climate: Short summer (temperature between- 10°C and 15°C), long and cold winters and low precipitation as influenced by continental polar air masses. Prevails over the belts from Alaska to Newfoundland and from Norway to Kamchetka peninsula sub-Arctic region. ‘Taiga’ actually refers to the softwood coniferous forest cover.

Dw – Continental type climate: Short and cool summer (temperatures 10° to 21°C), long and cold winters (temperatures below 0°C) and variable rainfall mostly during summers and snowfall during winter. Prevails in deep interiors of the continents between Taiga and the mid-latitude deserts over Poland and the Baltic states, Russian plains, northern states of USA and the southern states of Canada.

E: Polar climates:

Temperature of the warmest month is below 10°C. There is no warm season.

ET – Tundra Climate: Experienced over coastal fringes of the Arctic Ocean. Short, cool summer, long, cold winter and meager precipitation that limitTaiga vegetation.

Ef – Ice Cap: Areas permanently covered with snow. Average temperature of the warmest month is below 0°C. These conditions occur over the poles and the interiors of Greenland.

H: Highlands climate:

Prevails over the mountainous regions of Rockies, Andes, Alps and the Himalayas. Vegetational zoning from foothills upwards is similar to latitudinal change. High insolation, low temperature, low pressure, high precipitation and larger diurnal ranges at higher altitudes.

The significant aspect of Koeppen’s classification scheme is that it uses measurable and visible physical, elements like temperature and precipitation and their combined interaction with vegetation as the basis of classification. Koeppen’s scheme uses letter symbols to denote various characteristics, which is practical and convenient. However it ignores other factors, such as cloudiness, wind, rainfall intensity, currents and, above all, the air masses which form the basis of modern climatology. It is also difficult to explain the existence of different vegetation types within the same climatic division and similar vegetation types in different climatic divisions.

3.2  Thornthwaite’s rational classification

The main limitation of Koppen’s classification is the lack of rational basis for selecting temperature and precipitation values for different climatic zones. Thornthwaite’s (1948) improved the same by introducing water balance concept in his classification scheme. He compared the potential evapotranspiration, PET (defined as the amount of water that could evaporate and transpire from a vegetated landscape without restrictions other than the atmospheric demand) with precipitation and computed ‘moisture index’ which considers the water surplus (s) and water deficit (d) which occur in different seasons in most places. Water surplus means seasonal addition to sub soil moisture that is being used by the crop at a reduced rate of transpiration during deficit period.

Thus, the climate of a place is defined on the basis of (i) Potential evapotranspiration i.e. the combines loss of moisture from vegetation surface as evaporation and transpiration and , (ii) Seasonal variation of effective moisture and (iii) Average annual thermal efficiency. According to Thornthwaite’s classification scheme, Humidity Index =  ,   and Aridity Index =  , where

s = Monthly water surplus calculated as the sum of the monthly differences between precipitation and potential evapotranspiration when precipitation is greater than evapotranspiration

d = Monthly water deficit calculated as sum of monthly values of potential evapotranspiration for those months when precipitation is less than evapotranspiration

n = Water need

Thornthwaite assumed that a surplus of 6 inch of water will counteract a deficit of 10 inch and thus gave higher weightage to humidity index. Thus Moisture Index is given by

Table 1. Climate types and moisture index in Thornthwaite’s classification

Thornthwaite has considered moisture index above zero as moist climate (A, B4, B3, B2, B1 and C2) and moisture index below zero as dry climate (C1, D and E). He has given subdivisions to express the extent of dry period under moist climate and extent of moist period under dry climate.

Table 2. Subdivisions of moist and dry climates in Thornthwaite’s classification

Thornthwaite considered potential evaporation as Thermal Efficiency (T-E index) and used it as a thermal limit in his climatic classification scheme.

3.3.       Comparison of Koeppen’s and Thornthwaite’s Schemes

There are certain similarities as well as some distinct differences between the schemes of Koeppen and Thornthwaite. Both the schemes are based on empirical investigation. While Koeppen had considered vegetation to be a direct indicator of the totality of climate, Thornthwaite has given indirect recognition to the vegetational aspects through the concept of evapotranspiration which includes transfer of water from plants to atmosphere.

Both have used temperature and rainfall as basic atmospheric elements controlling climate. Koeppen considered the absolute values of critical climatic determinants, temperature and rainfall as recorded at different places. Thornthwaite. on the other hand, considered them through Thermal  Efficiency and ‘Precipitation Effectiveness’, calculated by using evapotranspiration and moisture indices.

Thornthwaite gave his scheme in 1931 and modified it thrice—in 1933, 1948 and 1955—each one being an improvement over the previous one. Thornthwaite’s scheme is more widely used in applied climatology.

4. Climatic subdivisions of India

The vast expanse of India across the latitudes and longitudes, the presence of large mountain ranges, long rivers and wide range of soil types have led to diverse climatic conditions. Monsoon also plays a very significant role in modulating the seasonality. Considering the deep association of natural vegetation as well as agriculture and other economic activities and livelihood in general with the climate, it is important to characterize the climate at regional scale by adopting some rational basis for the identification and judicious exploration of potential natural resources.

The climatic classification of India in the light of popularly used Koppen’s approach and Thornthwait’s approach as well as the agroclimatic classification adopted by National Commission of Agriculture is being discussed in this section

4.1.       Climatic regions of India as per Koeppen’s scheme

Based on Koeppen’s method, India has eight climatic regions as described in figure 1. Those are

1. Monsoon type with short dry season (Amw),
2. Monsoon type with dry season in summers (As),
3. Tropical savannah type (Aw),
4. Semiarid steppe climate (Bshw),
5. Hot desert type (Bwhw)
6. Monsoon type with dry winters (Cwg),
7. Cold humid winter type with short summers (Dfc)
8. Polar type (E)

As per the Koeppen’s scheme the Amw type of climate prevails over the western coast of India, south of Goa. This type characterised by dry summers is experienced along the Coromandel Coast. The dry climate prevails over two parts in India. The interior peninsula, Rajasthan and parts of Haryana have Bshw type of climate, while the extreme western Rajasthan experiences Bwhw type of climate. Most of the peninsular plateau has tropical savannah type of climate (Aw). The plain of India falls under the warm temperate type of climate with dry winters (Cwg). The northeastern India falls under Dfc type of climate. Here, the winters are cold and humid, while the summers are short. Kashmir and the adjoining mountain ranges have a polar type of climate (E).

4.2. Climatic regions of India as per Thornthwaite’s scheme

On the basis of Thornthwaite’s method, the climate of India falls under six different groups. Those are, 1- Perhumid (A),

2- Humid (B4 to B1),

3- Moist sub-humid (C2 ),

4- Dry sub-humid (C1),

5- Semiarid (D) and

6- Arid (E)

According to the Thornthwaite’s climatic classification scheme India have both moist and dry climates. Under moist climate the perhumid region (A) lies along the west coast of India, south of Goa and some parts of northeastern India. The humid climate (B) prevails all along the coast adjoining regions of west coast, northern part of West Bengal, and the neighbouring parts of northeastern India. The moist subhumid (C2) climate prevails along the Western Ghats and also over Orissa and West Bengal. The Ganga valley and northeastern parts of Central India have a dry subhumid type of climate (C1). The peninsular interior, western Madhya Pradesh, Haryana and Punjab have a semiarid type of climate (D). The regions of Saurashtra, Kuchchh and Rajasthan, on the other hand, experience an arid climate (E).

4.3. Agro-climatic zones of India

Agriculture is the most important economic activity of India that supports livelihood to the majority of Indian population. For participatory planning and development in agriculture, it is important to characterize the climate of India as per the requirement of crop production. An “Agro-climatic zone” is a land unit in terms of major climates, suitable for a certain range of crops and cultivars. The planning aims at scientific management of regional resources to meet the food, fiber, fodder and fuel wood without adversely affecting the status of natural resources and environment (FAO, 1983). Agrocliamatic zonation of India has been accomplished by different institutions e.g., Indian Council of Agricultural Research (ICAR), Planning Commission, National Agricultural Research Project (NARP) under ICAR, National Bureau of Soil Survey and Land Use Planning (Agro -ecological regions by the NBSS & LUP).

5. Summary

• Climate classification is an effort to recognize, clarify, and simplify climatic similarities and differences between geographic areas in order to enhance the scientific understanding of climates. The earliest known climatic classifications were those of classical Greek times which divide the earth into latitudinal zones.
• The most popular empirical classifications of modern times are of Koppen and Thornthwaite.
• Köppen classified world climate into 6 major groups namely, Tropical rainy, Dry, Humid-mesothermal, Humid-microthermal, Polar and Highlands Climate. These climatic groups largely coincides with the vegetation distribution over the earth surface
• Thornthwaite has classified the world climate by using thermal efficiency and precipitation effectiveness calculated by using evapotranspiration and moisture indices.
• Agriculture being the most important economic activity of India different institutions e.g., Indian Council of Agricultural Research (ICAR), Planning Commission, National Agricultural Research Project (NARP) have delineated the Agrocliamatic zones of India based on soil type, climate as well as distribution of crops and natural vegetation.