Charles Warren Thornthwaite was one of the most influential climatologists of the 20th century. His work on the water balance concept remains a legacy today in environmental and scientific disciplines. Scientists used his water balance methods to depict regional climates and to solve environmental and geographical problems specifically related to agriculture and water balance. Thornthwaite realized that a more logical approach to the moisture factor in climate was needed. As a result, he developed the concept of potential evapotranspiration (PE)—evaporation and transpiration from a uniformly covered ground surface of vegetation that would be adequately supplied with water. Empirical relationships were developed to determine PE based on [Page 2822]simple measures of air temperature and a latitude factor to adjust for day length and variables either measured ...