This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
The savanna is often mistaken for a simpler cousin of the desert, a place that is merely hot and dry. It is neither. What actually draws the boundary of a savanna is not temperature at all, since mean monthly temperatures sit above 18C year round with only a modest swing between wet and dry seasons, but rainfall, and specifically the shape of rainfall over a year. A savanna gets real rain, 800 to 1600mm a year by some measures, enough to grow grass tall enough to hide a lion. What it does not get is rain spread evenly through the year. Almost all of it falls in a single wet season, and for months at a stretch, sometimes with under 60mm falling in the driest month, almost none falls at all.
That seasonal whiplash is what keeps a savanna from becoming a forest. Trees can survive the dry season, but not enough of them can establish fast enough during the brief wet season to close a canopy before the rain shuts off again, so the biome settles into an equilibrium of scattered trees standing over a nearly continuous grass layer, rather than tipping all the way to forest or all the way to bare ground. Fire is part of what holds that balance in place. Grass that cures out in the long dry season burns readily, and fire on savanna, including the African savanna, is common enough to count as an ordinary feature of the biome, one that keeps woody growth from closing back over the grass rather than a rare event.
The biome forms a band on both sides of the equator, sitting between the closed canopy tropical rainforest nearer the equator and the true desert farther away, on every continent that reaches those latitudes: East Africa's open plains, Brazil's Cerrado, northern Australia's tropical savanna. Two ecosystems already on this atlas, the Serengeti and the Cerrado, were classified under the temperate Grassland biome before this entry existed; both are correctly savanna, and the record now reads that way.