Thornthwaite devised a new method for climatic classification. It was based on water balance. He found out the water balance for each month for all the places. If the water balance had a water surplus situation it is called Humid and if it is water deficient it is called Arid.
He also found out many other types between these two and had their relationship with natural vegetation because, according to him, every climatic type was represented by vegetation. Indices were calculated.
According to this type of classification, the following climatic regions have been indentified along with indices mentioned there in:
(1) A. It is prehumid type with an index of +100. It is represented by rain forest type of vegetation and is found in the Malabar Coast where the S.W Monsoons strike against the Western Ghats and provide heavy rainfall. It is also found in the north-east India.
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(2) B. It is humid type of climate and the area is forested but not that of equatorial type. This is found in areas which surround a type.
(3) C2. It is called moist sub-humid. Grasslands are a characteristic of this type of climate. It is found around the eastern edge of the Western Ghats, West Bengal, Orissa, etc.
(4) C1– It is called Dry sub-humid. Dry grassland is the characteristic of this climatic type. This climatic type is found in the Ganga Valley, the north-eastern part of Central India, etc.
(5) D. It is called semi-arid. It is steppe-like in nature. This type of climate is usually found in Punjab, Haryana, north-western region, western M.P and the interior part of the Peninsula.
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(6) E. It is called arid type of climate. Dry grass and stunted bushes are found in this type of climate. This climate is found in Rajasthan, Kutchch and Saurashtra (Gujarat).