climate and geography: Africa 213 geographical features One of Africa’s unique characteristics is that it straddles the equator, with large expanses of land on either side In the north the African coast borders the Mediterranean Sea, where the winter rains are dependable To the south this coastal region borders the great Sahara, which over the course of the planet’s long history has fluctuated between savanna and desert; by medieval times the Sahara had returned to being desert, and in the early 21st century it was the largest hot desert in the world, spanning approximately 1,700 miles north to south and 4,000 miles east to west, from the Atlantic Ocean to the Red Sea The Sahara is far from static in size, having been observed to expand and retreat in its boundaries from tens to hundreds of miles within a decade The Sahara separates North Africa from sub-Saharan Africa to the south By no means uniform in terms of physical features, the desert features oases and river valleys (such as the great Nile River valley) as well as higher-altitude areas that provided water and more lush vegetation to inhabitants and travelers in medieval times The Sahel region, running east to west just south of the Sahara, is characterized by semidry savanna grasslands and forests Farther south, the land gradually comes to feature more lightly wooded savanna areas, which feed into the central African rain forests Rain forests are also found on Africa’s western coast The central part of the continent thins from east to west down to the southern tip of the cape Africa’s ecological zones are more closely correlated to altitude to the south of the equator than to the north In the east mountains and plateaus begin in Ethiopia and run southward into present-day South Africa During at least the first part of medieval times this area was savanna grassland and woodland, and much of it was home to the tsetse fly, which limited the herding of large animals until consistent and substantial forest clearing transformed the landscape The principal deserts in the south are the Kalahari and Namib deserts While mountainous areas are present, Africa is not known for grand mountain ranges However, one of the continent’s major features is the Great Rift Valley, which has long been a bed of volcanic activity, running north to south in eastern Africa from the Red Sea to Mozambique Elongated lakes, such as Lake Malawi, Lake Turkana, and Lake Tanganyika, characterize this region Several mountainous areas disrupt the raised plateau spanning much of the continent to the sides of the rift, including the Atlas Mountains in the northeast, the Ethiopian Highlands in the east, and the Drakensberg in the south Principal peaks include Mount Kilimanjaro, at 19,340 feet, in present-day Tanzania; Ras Dashen, at 14,872 feet, in Ethiopia; and Mount Cameroon, at 13,435 feet, in Cameroon Most of Africa’s highland areas were produced by volcanic activity; exceptions include the Atlas and Drakensberg mountains, in the north and south, respectively Little folding or merging of tectonic plates has occurred in Africa because the continent is positioned atop one relatively stationary tectonic plate The continent’s major rivers include the Nile River in the northeast, the Niger River in the west, the Congo River in west-central Africa, and the Zambezi River in south-central Africa Since much of the continent (especially in the east and south) is characterized by plateau landscape, many of the major rivers reach the ocean with dramatic waterfalls that drop down the steep escarpments characterizing much of the coastline The coastline itself is relatively smooth, with few coves that serve as good natural harbors, and provided relatively little opportunity to access the interior during medieval times Climate The major factors, primarily terrestrial, that influenced climate in Africa during medieval times have indeed been influential throughout the continent’s long history These factors included topography, location, and the size of the continent Such characteristics remained constant throughout the medieval period and contributed to the ecological patterning of tropical rain forest, open savanna grassland, desert, and the coastal Mediterranean terrain Owing to Africa’s location straddling the equator, such that it lies mostly between latitudes 35 degrees north and south and receives a great deal of solar energy, one of the dominant climatic concerns across much of the continent is the amount of rainfall The long boundary between the Sahel and the Sahara is based on rainfall and is therefore not fixed The Sahara sometimes is defined as the area that receives less than inches of rainfall annually, while the Sahel receives up to 20 inches of rainfall annually South of the Sahel is moister savanna, which receives up to 40 inches of rainfall each year This ecological zone transitions into more forested areas and tropical rain forests in the Congo River basin, where the annual rainfall can reach approximately 200 inches Such rain forests also are found along Africa’s west coast Rainfall and associated ecological factors vary based on elevation, and in the early 21st century some of the mountain peaks in Africa were snow covered year round Ocean currents and associated air currents play a substantial role in regional climatic variation in Africa, since they alternately bring moisture and warmth from the equatorial Atlantic or dryness and cold from the north The Guinea Current brings rainfall and a warm climate to western Africa and the warm Mozambique Current does the same in southeastern Africa The Canary Current carries dry, cool air to the western coastal area of the Sahara Desert, while the Benguela