Sudan is the third largest country in Africa. It is bordered by Egypt to the north, the Red Sea to the northeast, Eritrea and Ethiopia to the east, South Sudan to the south, the Central African Republic to the southwest, Chad to the west and Libya to the northwest. The country is located in an immense basin sloping gently down towards the North plateau and mountains, while the North-eastern part is characterised by mountains and hills sloping towards the Red Sea coast. The principal physical feature of Sudan is the river Nile and its tributaries. The Nile system runs through from South to North. The huge water flow of the river Nile comes from the Blue Nile, Sobat and Atbora rivers, while the White Nile contributes only about thirty percent of its annual flow because it loses a large amount of its water due to evaporation. Meadows and pastures take up a quarter of the land area and forests and woodlands about one fifth. Sudan is a hot country. The central region has the highest mean annual temperatures of the world. The North of the country is arid and semiarid.