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Thornthwaite, Charles Warren


(b. Pinconning, Michigan, 7 March 1899;

d. Arlington, Virginia, 11 June 1963), geography, climatology.

Thornthwaite changed the course of climatology in the twentieth century and influenced the development of the modern field of climatology. He devised the eponomous Thornthwaite climate classification system by modifying and enhancing the famous Köppen climate system. Thornthwaite was the founder of the field of applied climatology. He advocated the study of microclimates, or topoclimatology, and his water budget model has been used by hydrologists throughout the world.

Early Years. The Thornthwaite family name came originally from Cumberland County in the Lake District in northwestern England. Thornthwaite’s parents, Ernest and Mildred Thornthwaite, lived on a farm near Pinconning in central Michigan. All four of their children were born on the farm; Charles Warren Thornthwaite was the eldest. The Thornthwaites, like many of the other enterprising settlers in Michigan, placed a high value on education. There was no high school in Pinconning, so Thornthwaite went to live in Mount Pleasant in order to attend high school. He worked as a janitor of the Methodist church and lived in a room in the church.

After graduating from high school in 1918, Thornthwaite was inducted into the Army Cadet Corps. World War I ended shortly thereafter and the corps was disbanded, so he enrolled at Central Michigan Normal School (now Central Michigan University) in Mount Pleasant, intending to become a schoolteacher. Thornthwaite was able to get financial assistance from the Student’s Army Training Corps. While in college, he met his future wife, Denzil Slentz, as well as John Leighly, who became an important figure in Thornthwaite’s life and work. Leighly had come to Central Michigan to obtain his high school teaching certificate. The two men met during a school break, when both were members of a crew that unloaded lumber from boxcars; they formed a lifelong friendship.

Thornthwaite enjoyed music and mathematics and sang in the college chorus. One of his professors introduced him to grand opera, which remained as one of Thornthwaite’s passions. Thornthwaite graduated in 1922 and obtained a job teaching science at the high school in Owosso, Michigan. He took courses in the summers of 1923 and 1924 at the University of Michigan. In 1924 Thornthwaite and his college friend Leighly drove to California in a Model T Ford. This in itself was a geographic adventure! Upon arrival, both men entered the University of California at Berkeley to pursue graduate work with the distinguished geographer Carl Sauer. Sauer, who was fluent in German, encouraged both Thornthwaite and Leighly to read German and to become acquainted with German scientists such as climatologist Wladimir Köppen. At Berkeley, Sauer taught an introductory course in physical geography. He rejected the popular geographic concepts of Ellen Churchill Semple (the first female geographer who stated that man is a product of Earth’s surface, and that the choices people made in response to the environment were predetermined) and was known for his work in cultural geography. The department attracted many distinguished visiting geographers.

Thornthwaite and Slentz were married in California in 1925. Because money was a problem for the new couple, Thornthwaite started to do some part-time work for the Kentucky Geological Survey. Because Thornthwaite needed to be in Kentucky and Sauer was interested in the geography of the area, Sauer suggested that Thornthwaite conduct his doctoral research on the city of Louisville. Thornthwaite finished his course work at Berkeley in 1927 and was awarded his doctoral degree in 1930 with a dissertation titled “Louisville, Kentucky: A Study in Urban Geography.”

The Climate Classification System. Thornthwaite was hired as a professor of geography at the University of Oklahoma in Norman in 1927. At that time, geography was classified in the “mathematical and natural sciences” group of the College of Arts and Sciences. Thornthwaite taught four or five courses each semester at the University of Oklahoma until the spring of 1934. He also continued to work part time for the Kentucky Geological Survey through 1930. His first article on meteorology; “The Polar Front in the Interpretation and Prediction of Oklahoma Weather” was published in 1929 in the Proceedings of the Oklahoma Academy of Science. This work was done in an era before the U.S. Weather Bureau had adopted the Bergen school methods of analysis, which presented new methods of forecasting, including the inclusion of air mass and frontal analysis.

The first article by Thornthwaite that attracted worldwide attention was his work on climate classification, “The Climates of North America according to a New Classification,” published in 1931. Thornthwaite was very familiar with the widely used Köppen system and had studied it extensively at Berkeley. Köppen had developed the first numerical classification of climate based on actual observations of temperature and precipitation that also described the general relationships of vegetation and soils to climate. Thornthwaite pointed out that Köppen did not consider his classification system a finished product due to the omission of a moisture effectiveness factor. For example, in the Köppen system, equal amounts of annual rainfall produce forest vegetation in Siberia and desert plants in Africa. Thornthwaite also pointed out that Köppen had no category for a subhumid climate. It was this fact more than any other that prompted Thornthwaite to recognize the work that needed to be done. In his 1931 article, Thornthwaite observed that the “effective” temperature and precipitation were more accurate climate descriptors than were actual measurements of temperature and precipitation.

Thornthwaite used the term effective temperature to relate the rate of plant growth to temperature and the term effective precipitation to indicate that plant growth was not only dependent on the amount of precipitation but also on the amount of water lost by evaporation and transpiration. He calculated effective precipitation from measurements of precipitation and evaporation published in the U.S. Weather Bureau Bulletin to obtain a “P-E index,” which provided a relationship between precipitation and evaporation for any location where the data was available. Thornthwaite also derived a “T-E index,” expressing the empirical relationship between temperature and evaporation. Köppen approved of Thornthwaite’s attempts to design a more complete foundation for the classification of climate. Subsequently, Thornthwaite began to apply the new classification system globally. He acquired climate data from four thousand stations from around the globe, applied his classification system, and produced a world map indicating the various climates. This very complicated map was not included in the commonly used climate classification system.

Soil Conservation. In 1934 Thornthwaite resigned from the University of Oklahoma and started to work with the Study of Population Distribution of the University of Pennsylvania. The opportunity to work on this study in Washington, D.C., with the University of Pennsylvania allowed him to leave Oklahoma, where he may have had some working tension with one of his colleagues. His work Internal Migration in the United States, published in 1934, was an intensive study of population and migration of American-born white and black populations, complete with maps. His work at the University of Pennsylvania also resulted in a collaborative publication on migration and economic opportunity in the Great Plains, his only published work on this topic.

In July 1935 Thornthwaite, on the recommendation of Sauer, became the director of the new Climatic and Physiographic Research Section of the Soil Conservation Service, Department of Agriculture. Thornthwaite moved to Arlington, Virginia, with his wife and three daughters. During his time he was involved in several projects, including the establishment of a climatic research center in western Oklahoma and involvement with the Polacca Wash project in Arizona. Through the establishment of the climatic research center, two hundred weather stations were established in an area of about 1,800 square miles (about 4,600 sq. km). The fact that Thornthwaite recommended such a dense network of climatic stations is an indication that as early as 1935 he realized the importance of mesoscale meteorology. The Polacca Wash project yielded important scientific results. His monograph “Climate and Accelerated Erosion in the Arid and Semiarid Southwest, with Special Reference to the Polacca Wash Drainage Basin, Arizona,” with C. F. Stewart Sharpe and Earl F. Dosch, appeared in 1942, summing up the principal findings concerning the rate of gully erosion and rainfall frequency and intensity in a dry climate.

Most importantly, Thornthwaite’s work with the Soil Conservation Service heightened his interest in the relationship between precipitation and evaporation and the loss of water from land surfaces to air by evaporation and transpiration. The central idea behind this revision was evapotranspiration, which was a new term in climatology at that time. His interpretation and concept of evapotranspiration and the hydrologic cycle was presented in “An Approach toward a Rational Classification of Climate,” published in 1948 in the Geographical Review. This paper introduced a new approach to climate classification, but also presented fundamental ideas on water storage in the soil and evapotranspiration. The paper was also exceptional in its portrayal of the hydrologic cycle applied to the United States.

Seabrook and the Institute for Climatic Research. Thornthwaite decided to move to Seabrook Farms in New Jersey as an irrigation consultant in 1946. Seabrook Farms was at the time a leading producer of frozen vegetables. His decision was based on the decreasing interest in the Soil Conservation Service and his conversations with John Seabrook, the general manager of the farms. Upon his arrival, he stood in the factory yard, surrounded by trucks filled with shelled peas. He noted that the factory was operating at full capacity, yet the trucks in the yard seemed to be fixed in place. Trucks stood there in the sun for hours, and he recognized that this delay would not improve the freshness of the peas. He launched a major research project to study the “pea problem.” He arrived at a system that involved variable harvest times, which was successfully implemented in 1947. After the success of this operation, the system was expanded to include all crops grown at Seabrook Farms. Thornthwaite transformed his information on planting and harvest dates of the various crops to a growth-unit slide rule that he called a “crop meter.” He became particularly interested in agricultural climatology and conducted a variety of studies in this area that included daily plant readings.

Thornthwaite also worked on obtaining a mathematical expression for the variation of temperature, moisture, and wind with height to learn about turbulence near the ground. He utilized crop duster planes and the smoke puffer, a device that would shoot a puff of smoke into the air so that its duration of visibility could be a direct measurement of the turbulence structure of the lower layer of the atmosphere. This instrument helped him study and clarify some of the complexities in the understanding of atmospheric turbulence. The disposal of industrially polluted effluent by means of spray irrigation, which was pioneered by Thornthwaite at Seabrook Farms and is possibly the largest system ever installed for an industry, provided a radically different solution to a complex problem of growing importance to industry. Thornthwaite was very happy and productive at Seabrook Farms from 1946 to 1952. The results of his work were described in both farm publications and scientific journals.

C. W. Thornthwaite Associates. Thornthwaite’s growing reputation led to the establishment of his own consulting firm, C. W. Thornthwaite Associates, a partnership with his brother-in-law, Floyd Slentz, in the early 1950s. In addition to the various consulting activities, Thornthwaite was involved with the design and construction of instruments in the company’s Laboratory of Climatology. During this time, five patents were issued to Thornthwaite for several improvements and modifications to instruments. A patent issued to him in 1941 (before the establishment of his consulting firm) was for improvements to an instrument that showed changes in the moisture concentration of a gas over time. This led to the ability to measure small changes in the dew point of the air. Another patent was for modifications to the dew point recorder and indicator in 1942. Three of the other patents were not officially issued until 1965 and 1966, after his death.

Other Activities. Thornthwaite chaired the American Geophysical Union’s committee on climatology in 1947 and 1948. He was elected the first president of the Commission for Climatology at the first congress of the World Meteorological Organization in 1951 and served a second term until January 1957. He was concerned about the creation of a world climate atlas. Thornthwaite felt that there was a growing interest in a global climate atlas among other technical commissions and regional associations. He started to build support, especially with the Food and Agriculture Organization and UNESCO. Even though he was able to obtain support for only one small set of water budget maps, he nevertheless was satisfied that climatology had taken steps in developing independently from meteorology (ironically, the field of climate dynamics was just emerging in the work of Guy S. Callendar and Gilbert Plass, among others).

In 1952 Thornthwaite was honored by the Association of American Geographers with its Outstanding Achievement Award. He served on a number of committees, including the Geography Advisory Committee to the Office of Naval Research of the National Research Council. His most significant publication of this period, “Topoclimatology,” was published in 1954. This paper had a considerable impact on climatology because it concluded that the study of climatology had to include the understanding and mapping of elements of Earth’s surface that affect the heat and water cycle, namely, topography, albedo, vegetation, and surface roughness. Thornthwaite was awarded the prestigious Cullum Geographical Medal of the American Geographical Society in 1959 in New York City.

Last Years. In the 1960s Thornthwaite was involved in the development of microclimatic instruments. He was chosen as the honorary president of the Association of the American Geographers from 1960 to 1961. During 1961 he began experiencing severe back pains, which interfered with his work schedule and increased during the last few months of his wife’s battle with cancer in 1962. He established a memorial scholarship for her at Central Michigan University. In 1963 he was awarded an honorary doctorate from his alma mater in recognition of his contributions to the fields of climatology and geography. He was unable to attend the ceremony, which took place nine days before his death, also from cancer, on 11 June 1963.



“The Polar Front in the Interpretation and Prediction of Oklahoma Weather.” Proceedings of the Oklahoma Academy of Science 9 (1929): 93–99.

“The Climates of North America according to a New Classification.” Geographical Review 21 (1931): 635–655.

Internal Migration in the United States. Philadelphia: University of Pennsylvania Press, 1934.

With Carter Goodrich, Bushrod W. Allin, et al. Migration and Economic Opportunity. Philadelphia: University of Pennsylvania Press, 1936.

With C. F. Stewart Sharpe and Earl F. Dosch. “Climate and Accelerated Erosion in the Arid and Semiarid Southwest, with Special Reference to the Polacca Wash Drainage Basin, Arizona.” U.S. Dept. of Agriculture, Technical Bulletin, no. 808, 1942.

“An Approach toward a Rational Classification of Climate.” Geographical Review 38 (1948): 55–94.

“Topoclimatology.” Proceedings of the Toronto Meteorological Conference 1953. London: Royal Meteorological Society, 1954, pp. 227–232.


Hare, F. Kenneth. “Charles Warren Thornthwaite, 1899–1963.” Geographical Review 53, no. 4 (1963): 595–597.

Leighly, John. “Charles Warren Thornthwaite, March 7, 1899–June 11, 1963.” Annals of the Association of American Geographers 54 (1964): 615–621.

Mather, John R., and Marie Sanderson. The Genius of C. Warren Thornthwaite. Norman and London: University of Oklahoma Press, 1996.

Wheeler, James O., and Stanley D. Brunn. “An Urban Geographer before His Time: C. Warren Thornthwaite’s 1930 Doctoral Dissertation.” Progress in Human Geography 26, no. 4 (2002): 463–486.

Sepideh Yalda

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Thornthwaite, Charles Warren

Thornthwaite, Charles Warren (1889–1963) An American climatologist who devised the system of climate classification that bears his name. Thornthwaite taught on the faculties of the Universities of Oklahoma (1927–43) and Maryland (1940–6), and at Johns Hopkins University (1946–55) before becoming director of the Laboratory of Climatology at Centerton, New Jersey, and professor of climatology at Drexel Institute of Technology, Philadelphia. He was president of the Section of Meteorology of the American Geophysical Union from 1941 to 1944 and in 1951 he was elected president of the Commission for Climatology of the World Meteorological Organization.

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