The Sahara desert, as defined by the World Wide Fund for Nature (WWF), includes the hyper-arid center of the Sahara, between latitudes 18° N and 30° N. It is one of several desert and xeric shrubland ecoregions that cover the northern portion of the African continent. Setting The Sahara Desert is the world's largest hot, non-polar desert and is located in North Africa. It stretches from the Red Sea to the Atlantic Ocean. The vast desert encompasses several ecologically distinct regions. The Sahara Desert ecoregion covers an area of in the hot, hyper-arid center of the Sahara, surrounded on the north, south, east, and west by desert ecoregions with higher rainfall and more vegetation. The North Saharan steppe and woodlands ecoregion lies to the north and west, bordering the Mediterranean climate regions of Africa's Mediterranean and North Atlantic coasts. The North Saharan steppe and woodlands receives more regular winter rainfall than the Sahara Desert ecoregion. The South Saharan steppe and woodlands ecoregion lies to the south, between the Sahara Desert ecoregion and the Sahel grasslands. The South Saharan steppe and woodlands receive most of its annual rainfall during the summer. The Red Sea coastal desert lies in the coastal strip between the Sahara Desert ecoregion and the Red Sea. The western portion of the region is known as El Djouf, on the border of Mauritania and Mali. Some mountain ranges rise up from the desert and receive more rainfall and cooler temperatures. These Saharan mountains are home to two distinct ecoregions; the West Saharan montane xeric woodlands in the Ahaggar, Tassili n'Ajjer, Aïr, and other ranges in the western and central Sahara Desert and the Tibesti-Jebel Uweinat montane xeric woodlands in the Tibesti and Jebel Uweinat of the eastern Sahara. The surface of the desert ranges from large areas of sand dunes (erg), to stone plateaus (hamadas), gravel plains (reg), dry valleys (wadis), and salt flats. The only permanent river that crosses the ecoregion is the Nile River, which originates in east Africa and empties northwards into the Mediterranean Sea. Some areas encompass vast underground aquifers resulting in oases, while other regions severely lack water reserves. Climate The Sahara Desert features a hot desert climate (Köppen climate classification BWh). The Sahara Desert is one of the driest and hottest regions of the world, with a mean temperature sometimes over and the average high temperatures in summer are over for months