Here, we discus three schemes of climatic regionalisation of India.
Koeppen’s scheme is based on the monthly values of temperature and precipitation. Koeppen identified five major climatic types—tropical climates, dry climates, warm climates, snow climates and ice- climates. He used letter symbols A, B, C, D and E to denote these climatic types.
These five types can be further subdivided into sub-types on the basis of seasonal variations in the distribution pattern of rainfall and temperature. Koeppen used small letters such as m. w or h to define these sub-types.
Based on Koeppen’s method (Fig 13.31), India can be divided into the following climatic regions:
2. Monsoon type with dry season in summers (As)
3. Tropical savannah type (Aw)
4. Semi-arid 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)
Koeppen’s Amw type of climate prevails over the western coast of India, south of Goa. The as 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 north-eastern 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).
Thornthwaite’s scheme is based on the concept of water balance. If the rainfall of a place is less than the water which is lost through evaporation and transpiration, the place has a water deficit. If it is more than the need, then there is a surplus.
Thornthwaite evolved a formula to work out the monthly values of water surplus and water deficit. Areas having water surplus in all the months of the year have a humid climate and areas having water deficit in all the months of the year have an arid climate. In between these two extremes, there are other types with varying degrees of water surplus or water deficit.
On the basis of Thornthwaite’s method, the following climatic regions can be identified:
1. Perhumid (A)
2. Humid (B)
3. Moist sub-humid (C2)
4. Dry sub-humid (Cj)
5. Semi-arid (D)
6. Arid (E) (Fig 13.32)
The perhumid region (A) lies along the west coast of India, south of Goa and some parts of north-eastern India. The humid climate (B) prevails all along the coast adjoining A, and over northern West Bengal, and the neighbouring parts of north-eastern India. The moist sub-humid (C2) climate prevails along the Western Ghats. It is also found over Orissa and West Bengal.
The Ganga valley and north-eastern parts of Central India have a dry sub-humid type of climate (C1). The peninsular interior, western Madhya Pradesh, Haryana and Punjab have a semi-arid type of climate (D). Saurashtra, Kachchh and Rajasthan, on the other hand, experience an arid climate (E).
Trewartha’s classification of climate, which is a modified form of Koeppen’s scheme, corresponds with the vegetative, agricultural and even geographical regions of India, in a fairly satisfactory manner. Four major climatic groups (A, B, C, and H) which are further subdivided into seven climatic types have been recognised.
They are as follows:
A Tropical Rainy Climatic Group
B Dry Climatic Group
BS—Tropical Steppe (semi-arid)
C Humid Mesothermal Climatic Group
Caw—Sub-Tropical Humid (Dry Winters)
H Mountain Climate:
The climatic letters A, B, C and H stand for the major groups of climate and the other letters designate the sub-divisions of major groups.
A stands for tropical rainy climate with persistently high temperature which is not less than 18° C in the coolest month.
B is a dry climate of those regions where the rate of evaporation is more than the moisture received from precipitation.
C represents humid sub-tropical or humid mesothermal climate.’ The temperature of the coldest month is between 18° C and 0° C.
a indicates hot summers with the warmest month having over 22° C temperature.
h is used when mean annual temperature is 18° C.
m stands for heavy but seasonal monsoon rainfall; dry period is very short.
s means steppe or semi-arid climate.
w stands for a desert.
The tropical monsoon type of climate (Am) is found over the Western Ghats, western Nagaland and Tripura. The mean maximum and minimum temperatures here are 27° C and 18° C respectively. This region receives an annual rainfall of 250 cm.
The tropical savannah climate (Aw) covers almost the whole of Deccan Plateau except a narrow strip of rain-shadow area in the east of the Western Ghats, north-eastern Guajrat, southern Madhya Pradesh, southern Bihar, Orissa, Andhra Pradesh and Tamil Nadu. This region receives and annual rainfall of 100 cm and the mean maximum temperature here is 45° C and the mean minimum temperature is 18° C.
The tropical steppe (semi-arid) type of climate (BS) covers interior Karnataka, central Maharashtra, western Andhra Pradesh – and interior Tamil Nadu. This region receives an annual rainfall of less than 75 cm and the mean maximum and means minimum temperatures here are 32° C and 23° C respectively.
The sub-tropical steppe (Bsh) type covers an area from Punjab to Kachchh, which is characterised by an annual rainfall which fluctuates between 50 cm and 75 cm. The mean maximum and mean minimum temperatures here are 46° C and 6° to 10° C respectively.
The sub-tropical desert type of climate (Bwh) prevails over western Rajasthan and Kachchh. This area receives a low annual rainfall of 12.5 cm and the mean maximum and mean minimum temperatures here are 48° C and 12° C respectively.
The sub-tropical humid (dry winters) type of climate (Caw) covers the Punjab foothills, Uttar Pradesh, Bihar, West Bengal, Assam and Arunachal Pradesh. The annual rainfall varies here from 62.5 cm in the plains to upto 250 cm in the east. The mean maximum and minimum temperatures here are 46° C and 10° C respectively.
The mountain type of climate (H) prevails in mountain areas of Kashmir and Arunachal Pradesh. The northern slopes receive a low rainfall of 8-10 cm annually due to the rain-shadow effect, while the southern slopes receive 250 cm rainfall. The mean maximum temperatures vary between 10° C and 15° C and the mean minimum temperatures go below zero.