Africa is the world's second-largest and second-most-populous continent. At about 30.2 million square kilometres, including adjacent islands, it covers six percent of the Earth's total surface area and 20.4 percent of the total land area. Africa's population is the youngest among all the continents; 50% of Africans are 19 years old or younger. The African climate ranges from tropical to subarctic on its highest peaks. Its northern half is primarily desert or arid, while its central and southern areas contain both savannah plains and very dense jungle (rainforest) regions. In between, there is a convergence, where vegetation patterns such as the Sahel zone and steppe dominate. Africa is the hottest continent on earth and 60% of the entire land surface consists of drylands and deserts.