The humid tropical group of climates found at low latitudes is largely controlled by equatorial and tropical air masses. Every month of the year has an average temperature above 64F (18C), and there is no winter season. In these tropical systems, the primary periodic energy flux is diurnal: temperature variation from day to night is greater than from season to season. Average annual rainfall is heavy and exceeds annual evaporation, but varies in amount and in seasonal and areal distribution.
Two types of climates are differentiated on the basis of seasonal distribution of precipitation. The tropical wet (or rainforest) climate has ample rainfall through 10 or more months of the year. The tropical wet-and-dry (or savanna) climate has a dry season more than 2 months long.