A Revised Thornthwaite-Type Global Climate Classification

Although the Köppen climate classification is the most common climate classification in use today, the 1948 Thornthwaite classification is frequently cited as an improved climate classification system for its rational approach. However, the Thornthwaite classification is infrequently used because it tends to be too complex for use in everyday settings and world maps of the classification were never produced. This paper will present global maps of all four components of the 1948 Thornthwaite climate classification—a long-time wish of John "Russ" Mather, to whom this paper is dedicated. In addition, a revised Thornthwaite-type climate classification is presented with the intent of providing a more rational climate classification for everyday use in a classroom setting. This classification uses an amended version of the Thornthwaite moisture index, not only to delineate climatic moisture gradients but also to define a single seasonality index responsive to mean seasonal variation in both thermal and moisture conditions. Replacing the two cumbersome seasonality indicators in the original Thornthwaite classification with one variable greatly improves the utility of the classification. Results from this classification are compared to the Köppen and original Thornthwaite climate classification schemes.