Savanna Climate

Picture the wide grasslands from the movie The Lion King. On the savanna, the year splits into two big seasons. In the 'wet season,' it feels like the sky has sprung a leak and rain pours down almost every day. In the 'dry season,' barely a drop falls and the ground cracks open.

When the wet season comes, the parched plains turn green almost overnight. Grass shoots up taller than a person, and herds of zebras and wildebeests arrive to graze. But once the dry season starts, the puddles dry up and the grass turns yellow and withers. To survive, animals travel hundreds of kilometers to find places that still have water.

This back-and-forth between two extreme seasons has shaped plants as well as animals. Only plants like the baobab, which stores huge amounts of water inside its trunk, or the acacia, which keeps its leaves small to cut down on evaporation, can make it through the dry season.

The secret of the savanna climate lies in how air moves around the Earth. Near the equator, strong sunlight heats the air and it rises, building huge rain clouds. This band of rain clouds is called the 'intertropical convergence zone' (the area near the equator where air gathers and rises). A bit farther from the equator, there are high-pressure zones where dry air sinks.

Because the Earth orbits the sun while tilted, this band of rain clouds drifts north and south with the seasons. When the band moves over the savanna, the wet season arrives and rain pours down. When the band moves away and dry air sinks in, the rain shuts off and the dry season begins.

In other words, the savanna sits right between two extremes: rainforests, where it rains all year, and deserts, where it almost never rains. So it gets a taste of both kinds of weather, one after the other.

In the climate classification made by the German climatologist Köppen, savanna climate is written as 'Aw.' The 'A' stands for a warm tropical climate where even the coldest month averages 18 degrees Celsius or higher. The 'w' comes from a German word meaning dry in winter.

People often imagine the savanna as bare, treeless land. In reality, it's a woodland grassland, with short trees that can survive a long dry season standing here and there. The grass grows thick during the wet season, then dries out, and wildfires often break out (today, far more are started by people than by lightning). These fires play an important role: they keep thick forest from taking over and help the grassland stay a grassland.

Savannas aren't only in Africa. They spread across many continents, including the cerrado in Brazil, northern Australia, and parts of India. People in these regions have long lived in a distinctive way: farming with the rains and moving their livestock during the dry season.