In the Thornthwaite moisture index, category B denotes humid climates.
Thornthwaite’s 1948 system uses the moisture index, Im, to compare water surplus and deficit while accounting for potential evapotranspiration. Category B covers humid climates, with Im values from 20 up to but not including 100. It is subdivided into several ranges, including B4, B3, B2, B1, and B, depending on the exact moisture-index value.
Category A is even wetter and is called perhumid, with Im of 100 or more. Category C is subhumid, category D is semiarid, and category E is arid. This ordering is a common source of mistakes because the letter B does not simply mean “the wettest” category; it identifies the broad humid class below perhumid A.
The broader Thornthwaite classification also includes thermal-efficiency categories based on annual potential evapotranspiration. That makes it different from a rainfall-only scheme: two places with similar precipitation can receive different classifications if temperature and atmospheric water demand produce different evapotranspiration levels.