The latitude belt between 10 and 30 N. is intermediate between the equator and middle latitudes. This produces the tropical wet-dry savanna climate, which has a wet season controlled by moist, warm maritime tropical air masses at times of high sun, and a dry season controlled by continental tropical air masses at times of low sun (see Appendix 2, climate diagram for Key West, Florida). Trewartha (1968) classifies the tropical wet-dry climate as Aw, with the letter w signifying a dry winter.
Alternation of wet and dry seasons results in the growth of a distinctive vegetation known generally as tropical savanna. It is characterized by open expanses of tall grasses interspersed with hardy, drought-resistant shrubs and trees. Some areas have savanna woodland, monsoon forest, thornbush, and tropical scrub. In the dry season, grasses wither into straw and many tree species shed their leaves. Other trees and shrubs have thorns and small or hard, leathery leaves that resist loss of water.
Soils are mostly Histosols and Inceptisols. Heavy rainfall and high temperatures result in heavy leaching. Streamflow in these regions is subject to strong seasonal fluctuations, in striking contrast to the constant streamflow typical of rainforest climates. In the rainy season, extensive low-lying areas are submerged; in the dry season, streamflow dissipates, exposing channel bottoms of sand and gravel as mud flats dry out.
In North America, the Savanna Province Division is found in southern Florida, where habitats and fauna are strongly influenced by fluctuating water levels. Large numbers of birds are especially characteristic.