From AMS Glossary
A humidity province of Thornthwaite's 1931 climate classification, defined by precipitation-effectiveness index values of 32–63, and designated letter code C.
Based on its typical vegetation, the subhumid climate is sometimes called grassland, or prairie, climate. In 1948, Thornthwaite divided this category into moist subhumid and dry subhumid climate, with moisture index values of 0 to +20 and -20 to 0, respectively. The zero value of moisture index also divides, overall, the moist climates from the dry climates. This type of climate and the semiarid climate are very susceptible to drought conditions.
Thornthwaite, C. W. 1931. The climates of North America according to a new classification. Geogr. Rev.. 21. 633–655.
Thornthwaite, C. W. 1948. An approach toward a rational classification of climate. Geogr. Rev.. 38. 55–94.