Tropical Wet-and-Dry (Savanna) Climates

The tropical wet-and-dry climate, classified as Aw in the Köppen system, differs from the rainforest's Af regime in a single decisive respect: at least one month of the year receives less than roughly 60 millimeters of rainfall, producing a pronounced dry season, typically during the period of lower sun angle, that alternates with a markedly wetter season when the ITCZ migrates overhead. This alternation, rather than any large temperature swing — mean temperatures in savanna regions typically vary only modestly across the year, generally staying within the 20°C to 30°C range — is what defines the biome ecologically, since it is the dry season's length and severity, not the total annual rainfall, that determines whether an area can sustain closed forest or is instead pushed toward the open, grass-dominated landscape known as savanna. Total annual precipitation in Aw regions commonly ranges between 750 and 1,500 millimeters, concentrated overwhelmingly within the wet months, so that the same landscape can appear lush and green for several months and then turn brown and dust-dry for an equally extended dry period each year.