The Horn of Africa (alternatively Northeast Africa, and sometimes Somali Peninsula; shortened to HOA) is a peninsula in East Africa that juts for hundreds of kilometers into the Arabian Sea, and lies along the southern side of the Gulf of Aden. It is the easternmost projection of the African continent. The term also refers to the greater region containing the countries of Eritrea, Djibouti, Ethiopia and Somalia. As such, it covers approximately 2,000,000 km² (772,200 sq mi) and is inhabited by about 90.2 million people (Ethiopia: 75 million, Somalia: 10 million, Eritrea: 4.5 million, and Djibouti: 0.7 million). Regional studies on the Horn of Africa are carried out, among others, in the fields of Ethiopian and Eritrean Studies as well as Somali Studies.
The Horn of Africa is almost equidistant from the equator and the Tropic of Cancer. It consists chiefly of mountains uplifted through the formation of the Great Rift Valley, a fissure in the Earth's crust extending from Turkey to Mozambique and marking the separation of the African and Arabian tectonic plates. Most of the region is mountainous due to faults resulting from the Rift Valley, with the highest peaks in the Simien Mountains of northwestern Ethiopia. Extensive glaciers once covered the Simien and Bale Mountains, but melted at the beginning of the Holocene. The mountains descend in a huge escarpment to the Red Sea and more steadily to the Indian Ocean. Socotra is a small island in the Indian Ocean off the coast of Somalia. Its size is 3,600 km² (1,390 sq mi) and it is a territory of Yemen, the southernmost country on the Arabian peninsula.
The lowlands of the Horn are generally arid in spite of their proximity to the equator. This is because the winds of the tropical monsoons that give seasonal rains to the Sahel and the Sudan blow from the west. Consequently, they lose their moisture upon reaching Djibouti and Somalia, with the result that most of the Horn receives little rainfall during the monsoon season. On the windward side in the west and center of Ethiopia, and the extreme south of Eritrea, monsoonal rainfall is heavy. In the mountains of Ethiopia, many areas receive over 2,000 mm (78 in) per year, and even Asmara receives an average of 570 mm (23 in). This rainfall is the sole source of water for many areas far from Ethiopia, most famously for Egypt, which — in terms of rainfall — is the driest nation on Earth.
In the winter, the northeasterly trade winds do not provide any moisture except in mountainous areas of northern Somalia, where rainfall in late autumn can produce annual totals as high as 500 mm (20 in). On the eastern coast, a strong upwelling and the fact that the winds blow parallel to the coast means annual rainfall can be as low as 51 mm (2 in).
Temperatures on the Red Sea coast are some of the hottest in the world, typically around 41°C (106°F) in July and 32°C (90°F) in January. On the east coast, owing to the upwelling, they are somewhat, cooler but still hot. As elevation increases, temperatures decrease, so that at Asmara, maxima are around 20°C (68°F), though frosts are frequent on cloudless nights. On the highest peaks of the Simien Mountains, however, temperatures rarely reach 14°C (57°F) and can be as low as –10°C (14°F) on cloudless nights.
The Kingdom of Aksum (also known as "Axum") was an ancient state located in the north of modern-day Ethiopia and parts of Eritrea that thrived between the 1st and 7th centuries. Due to the Horn's strategic location, it has been used to restrict access to the Red Sea in the past.
The region was also a source of biological resources during the Antiquity: The Ancient Egyptians, Greeks and Romans sent expeditions to the region for frankincense, myrrh, dragon's blood or cinnabar and took these commodities back along the Incense Route. Therefore the Romans called this region Regio Aromatica. It is believed to also contain the fabled Egyptian Land of Punt.
The Horn was also part of a network of ports that extended down the coast of Africa, from the Persian Gulf as part of a larger and ancient commerce route along the greater Indian Ocean rim.
Moreover, the region is regularly stricken by natural catastrophes, such as droughts or floods that hit rural areas particularly hard. As a result, the region has some of the world's highest levels of malnutrition and is continuously threatened with a major humanitarian crisis. Between 1982 and 1992, about two million people died in the Horn of Africa due to this combination of war and famine.
Some important ethno-linguistic groups in the Horn of Africa are:
The Horn of Africa is a Conservation International Biodiversity hotspot and one of the two entirely arid ones. However the Horn of Africa suffers largely from overgrazing and only 5% of its original habitat still remains. On Socotra, another great threat is the development of infrastructure.
The Horn of Africa holds more endemic reptiles than any other region in Africa, with over 285 species total (and about 90 species found exclusively in the region). Among endemic reptile genera, there are Haackgreerius, Haemodracon, Ditypophis, Pachycalamus and Aeluroglena. Half of these genera are uniquely found on Socotra. Unlike reptiles, amphibians are poorly represented in the region.