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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. American Geographical Society is collaborating with JSTOR to digitize, preserve and extend access to Geographical Review.[With separate map, P1. I, facing p. 941 THE direction that the modern study of climate has taken has been dictated largely by the development of meteorological instruments, the establishment of meteorological observatories, and the collection of weather data. The catalogue of climatic elements consists of those that are customarily measured and usually includes temperature, precipitation, atmospheric humidity and pressure, and wind velocity. Increasingly, climatic studies have tended to become statistical analyses of the observations of individual elements. Because of this, climatology has been regarded in some quarters as nothing more than statistical meteorology. THE ROLE OF EVAPORATION AND TRANSPIRATIONBut the sum of the climatic elements that have been under observation does not equal climate. One element conspicuously missing from the list is evaporation. The combined evaporation from the soil surface and transpiration from plants, called "evapotranspiration," represents the transport of water from the earth back to the atmosphere, the reverse of precipitation. The rain gauge measures precipitation within acceptable limits of accuracy. We know reasonably well how rainfall varies from one place to another over the inhabited parts of the earth and also how it varies through the year and from one year to another. On the other hand, no instrument has yet been perfected to measure the water movement from the earth to the atmosphere, and consequently we know next to nothing about the distribution of evapotranspiration in space or time.We cannot tell whether a climate is moist or dry by knowing the precipitation alone. We must know whether precipitation is greater or less than the water needed for evaporation and transpiration. Precipitation and evapotranspiration are equally important climatic factors. Since precipitation and evapotranspiration are due to different meteorological causes, they are not often the same either in amount or in distribution through the year. In some places more rain-falls month after month than evaporates or than the vegetation uses. The surplus moves through the ground and over it to form > DR. THORNTHWAITE, consulting climatologist, has spent the growing seasons of I946 and I947 in experiments "fitting crops to weather" (see p. 4). RATIONAL CLASSIFICATION OF CLIMATE methods of plant physiology. Nevertheless, evapotranspiration represents the return flow of water to the atmosphere and is thus an important meteorological process.The only method so far developed that measures the actual evapotranspiration from a field or any other natu...

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. American Geographical Society is collaborating with JSTOR to digitize, preserve and extend access to Geographical Review.[With separate map, PI. III, facing p. 654] D UURING the last decade a number of the sciences closely related to geography, in particular plant ecology, soil science, and geomorphology, have experienced remarkable development. The climatic basis of the distribution of plants, of soils, and of land forms has received considerable attention and has been reduced to a certain degree of order. It has been recognized by the scientists working in these fields that their development on a truly scientific basis is dependent on the consideration of climate in quantitative terms. As yet, however, their studies have dealt only with certain elements of climate of certain regions. The one classification of world climates developed on a quantitative basis which has been widely accepted has been devised by K6ppen.1 Koppen attempted to determine the limits of the arid climates on the basis of drainage patterns peculiar to arid regions, and he ascertained the combinations of mean annual temperature and precipitation that occur along their borders. His findings were reduced to a formula that expresses the mean annual depth of precipitation (in centimeters) at various mean annual temperatures (expressed in degrees Centigrade) along the moister limits of the dry climates.2The dry climates are designated steppe or desert depending on whether they receive more or less than half the critical amount of precipitation. K6ppen recognized that where there are distinct wet and dry seasons in summer or winter the period of occurrence is important, for summer rainfall is less effective than winter rainfall since evaporation is higher in summer than in winter. He allowed for variations in evaporation by increasing the precipitation factor of a given climatic division in the summer-rainy area and reducing it in the winter-rainy area.1Wladimir Koppen: Versuch einer Klassifikation der Klimate, vorzugsweise nach ihren Beziehungen zur Pflanzenwelt, Geogr.

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