Africa is the world's second largest continent with 20 percent of the Earth's land mass. The African continent is home to a variety of ecosystems, from hot deserts, to tropical rain forests. Approximately half of Africa is covered by savannas of some sort (about five million square miles), beginning just below the tropic of Cancer and continuing down to the tropic of Capricorn. The circulation of the atmosphere over Africa is dominated by areas of high pressure centered over adjacent oceans around the Tropics of Cancer and Capricorn. These areas produce winds from East to Northeast, over the Sahara and the Kalahari. These regions are arid because they are occupied by dry, subsiding air for most of the year. Moist air moving into Africa, mainly from the South Atlantic and the Indian Ocean, is monsoonal in character. The humid, unstable air moves inland in summer. The seasonality of the rainfall is an extremely important determining feature in the climate almost everywhere in Africa. Grasslands can be broken up into two main categories, the first being tropical grasslands, called savannas, and the second being temperate grasslands. Savannas are characterized by grassland with scattered individual trees.