A climatic region is a homogeneous climatic condition which is the result of combined effects of climatic factors. The two important factors of climatic classification are temperature and rainfall.
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Koppen’s Climatic Classification:
The following five climatic regions have been identified in India according to Koppen’s climatic classification: He used letters A, B, C, D and E to denote these climatic types:
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A – Tropical climate (with mean monthly temperature over 18°C).
B – Dry climate (If dryness is less, semi desert (S); if it is more, desert (W).
C – Warm climate (with mean temperature between 18°C and 3°C).
D – Snow climate (with mean temperature under -3°C).
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E – Ice Climate (mean temperature under -10°C).
Koppen further sub-divided these types on the basis of seasonal variations e.g. f (sufficient precipitation) m (rain forest despite a dry monsoon season) w (dry season in winter) h (dry and hot) c (less than 4 months with mean temperature over 10°C) and g (Gangetic plain).
Moisture Index of Thornthwaite:
Thornthwaite index is based on the concept of water balance. He found out the water balance for each month for all the places. If the water balance had a water surplus solution, it is called humid and if it is water deficient it is called arid.
He also used English letters to classify different types of climate.
As per Dr. Trewartha’s modified form of Koppen’s classification, India can be divided into following climatic regions.
(i) Tropical Rain Forest (AM):
This type of climate is found on the west coastal plain and Sahyadris and in parts of Assam. The temperatures are high, not falling below 18.2°C even during winter and rising to 29°C in April and May, the hottest months. Dense forests and plantation agriculture with crops like tea, coffee and spices are the characteristic vegetation in the area.
(ii) Tropical Savanna (AW):
Most of the peninsula, except the semiarid zone in the leeside of the Sahyadris experiences this type of climate. A long dry weather lasting through winter and early summer and high temperatures remaining above 18.2°C even during the winter season and rising as high as 32°C in summer are the chief characteristics of this climate. Nagpur has a mean temperature of 35.4°C for May which is the hottest month and 20.7°C for December the coldest month in the year. The natural vegetation all over the area is savanna.
(iii) Tropical Semi-arid Steppe Climate (BS):
The rain-shadow belt, running southward from central Maharashtra to Tamil Nadu, in the leeside of the Sahyadris and Cardamom Hills come under this type of climate of low and uncertain rainfall, and high temperatures varying from 20° to 23.8°C for December and 32.8°C for May. Agriculturally, the climate is suitable only for dry farming and livestock rearing.
(iv) Tropical and Sub-Tropical Steppe (BSh):
This type of climate occurs over a broad crescent from Punjab to Kachchh between the Thar Desert to its west and the more humid climates of the Ganga Plain and the Peninsula to its east and south respectively. The climate, therefore, is transitional between these two areas. The annual rainfall is not only low but it is also highly erratic.
(v) Tropical Desert (BWH):
The western parts of Barmer, Jaisalmer and Bikaner districts of Rajasthan and a good part of Kachchh form the sandy wastes of the Thar which experiences a typical desert climate. Ganganagar has recorded a maximum temperature of 50°C, the highest record.
(vi) Humid Sub-Tropical with winter (CAW):
A large area to the south of the Himalayas, east of the tropical and sub-tropical steppe and north of the tropical savanna running in a long belt from Punjab to Assam with a south-westward extension into Rajasthan east of the Aravalli Range, has this type of climate. Winters are dry except for a little rain received from the westerly depressions.
(vii) Mountain Climate (H):
The Himalayan and Karakoram ranges experience this type of climate with sharp contrasts between the temperatures of the sunny and shady slopes, high diurnal range of temperatures and high variability of rainfall. The trans-Himalayan region, Ladakh, where the south-west monsoon fails to reach, has a dry and cold climate and a sparse and stunted vegetation.