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Climatic types of India according to the rationalclassification of Thomthwaite

Y. P. SUBRAHMANYA~I

Department oj Grophysics, A7Idhra U7Iiversity, lI'aUair

(Receh't'd 30 June 1955)

only on the meteorological characteristicsof the overlying atmosphere but also on theamoun t of available moisture, apart fromthe fact that it is also influenced by the natureof the evaporat ing surface. In view of thevariou s difficulties involved in the experi­mental measurement of actual evapotranspi­ration, Thornthwaite (1948) introduced thepotential evapotranspiration which ho definesas the total amount of water that wouldevaporate and transpire if it were alwaysavailable for full usc. From a series of fieldinvestigations, Thornthwaite showed thatthis potential evapot ranspiration (abbreviated as I'.E.) depends primarily on th oclimat ic properties of the atmosphere andcau, therefore, be determined much moreeasily than the actual evapotranspirat ion.There are not, however, Illany sta tions in theworld today where measurements of I'.E.are made and in view of the urgent need forsuch data in various other fields, attemptsare , at present , being made to estimate th isquantity hy empirical methods. From acareful study of the wat er-balances of anumber of irrigated valleys in the NorthAmerican continent , Thornthwaito derived amathematical expression for the computationof P.E. from a knowledge of the meanmonthly temperature, provided the latitudeof the place is known. The equation in itsoriginal form is not easy to use but with thehelp of the nomogram s and tables devised

For a dctail ed analysis of all the climat ictypes , it is highly necessary to investigatethe hydrologic balance of the regiou underconsideration. Since the moisture availableto plants depends not only on the amount ofrainfall bnt also on the evapotranspirutionlosses, the main prohlem here is one of obtain­ing an index of precipitation effectivencs.s,Precipitation can be measured fairly accu­rately by means of raingauges but , till today,there is no satisfactory means of measuringactual evapotranspiration for it depends not

ABSTRAt.'T.Ueing the climat ic data of about 2,jO statiol\.'l, the climatell of India and the ne~hrouthoodAre elaSRificd according to t he lU48 scheme of Thomthwaite. The a~lysi4 scem." to "weal Of'~in Ieatureewhich no pl"l"",iool c~ification has shewn. I~ iR found that most Me('ho~ of I ndia are more and than indioeeted hy earlier schemes. The thermal efficiency rcgune of the country 11 more than adequate 10 supportIoreet ty pe of vegetat lon. Hmtted in maintE'n&nce and .. growth OIUY, by the eeuon~l. dist rib ut ion of rainfall.lt is (t'lt that a natural vegetation map of tbe country 1I very CfJltenhal for the va lidity of tho scheme to beverified.

The real purpose of a climat ic classificationis to define the climatic types in statisticalte rms in which the climate, as a geographicfactor, is to be regarded as having definite anduniform characteristics (Hare 1951). Sincesoil and climax vegetation are known toreflect faithfully the climati c features ofmost regions, modern systems of cla..ssifica­tion base their divisions on the ehamcteris­tics of natnral vegetation. Koppen was thofirst to give mnch thought to the classifica­tion of climates and his first paper dealingwith his comprehensive scheme appearedin tho year 1900. In a very int eresting articl eou tho problems in the classification ofclimates, Thornthwaite (19!3) traced thchistorical development of the Koppen systemand showed how it is not rational and whvit cannot be applied to the study of regionalclimates.