History of the Thornthwaite Climate Classification System (Credit: Encyclopedia, Iowa State Agriculture)

​On March 7, 1899 in Pinconning, Michigan, Charles Warren Thornthwaite was born to Ernest and Mildred Thornthwaite. Like all other settlers in the area, education had a high value, and with Pinconning not having a school, Charles Thornthwaite moved to Mt. Pleasant, MI in order to attend school. Growing up in a farming family, Thornthwaite had great interest in agriculture and what goes into farming such as the weather and climate of an area. After high school, Thornthwaite attended Central Michigan Normal School, which is now Central Michigan University, and graduated in 1922. Upon graduating, Thornthwaite and a friend from CMU, John Leighly, both traveled to California to attend graduate school at UC-Berkeley. Their mentor, Carl Sauer, was a renowned geographer and encouraged both students to study the climate classification of Vladimir Koppen. With Thornthwaite’s interest in climatology and climate’s relation to agriculture already high, he had no hesitation to study the classification made by the German climatologist.

Upon graduating from UC-Berkeley, Thornthwaite moved to Louisville, Kentucky to work for the Kentucky Geologic Survey. Because of Thornthwaite’s interest in geography and climate, his mentor, Sauer, suggested that Thornthwaite conduct doctoral research on the city of Louisville. In 1930, he was awarded a doctoral degree with a dissertation titled “Louisville, Kentucky: A Study in Urban Geology.” While conducting his research in Louisville, he was also hired as a professor of geography at the University of Oklahoma in 1927. While at OU, and before his final doctoral dissertation, Thornthwaite published an article on meteorology titled: “The Polar Front in the Interpretation and Prediction of Oklahoma Weather” in 1929. This was published in the Proceedings of the Oklahoma Academy of Science. This article was the first article that captured nationwide attention for Thornthwaite and his classifications.
In 1931, based on his research in Oklahoma, Thornthwaite published “The Climates of North America According to a New Classification.” From the knowledge he learned at UC-Berkeley about the Koppen system, Thornthwaite noted that Koppen did not consider how moisture can play a factor in climate classification. Also, Thornthwaite noted that Koppen did not have a category for a sub-humid climate. Based off of these two findings, Thornthwaite decided to write his new classification article. In the 1931 article, he noted that evaporation and transpiration along with temperature and precipitation were better indicators than just temperature and precipitation. Thus, the Thornthwaite Climate Classification system was born.
​
This system divides climate into groups according to the vegetation characteristics found within them. Thornthwaite found that climate and vegetation are directly correlated in his studies, thereafter basing his system on this fact. The system which is the sum of monthly P/E (precipitation and evaporation) is defined into five different humidity provinces associated with vegetation. A P/E index of more than 127 (wet) indicates rain forest; 64–127 (humid) indicates forest; 32–63 (subhumid) indicates grassland; 16–31 (semi-arid) indicates steppe; less than 16 (arid) indicates desert.

From creating a new climate classification system to becoming the first president of the Commission for Climatology of the World Meteorological Organization, C. Warren Thornthwaite gave his life to climate science. He provided climate science with a whole new array of science and data to be observed. The Thornthwaite Climate Classification system is still heavily used today to calculate climate systems. 

​To learn more about other interesting stories related to global climate issues, be sure to click on the following link: www.globalweatherclimatecenter.com/climate

©2018 Weather Forecaster Alec Kownacki