The Sahara Desert is the largest hot desert in the world. It covers approximately 9 million square kilometres across much of North Africa, although its exact area varies somewhat depending on how the desert boundary is defined and how transitional semi arid regions are treated. The Sahara extends from the Atlantic Ocean in the west to the Red Sea and parts of the eastern desert regions, and from the Mediterranean margins in the north toward the Sahel in the south. It is considerably smaller than Antarctica, which is the largest desert overall, but it is the largest desert classified as hot. Its enormous area makes it one of the dominant geographical and climatic features of Africa.
The Sahara is primarily a subtropical desert produced by large scale atmospheric circulation. Much of the region lies beneath the descending branch of the Hadley circulation, where air that has risen in tropical convergence zones eventually sinks in the subtropics. Descending air warms and becomes relatively drier, suppressing cloud formation and rainfall. The resulting subtropical high pressure regime is one of the principal reasons for the Saharas extreme aridity. Local conditions are also influenced by the Atlantic Ocean, Mediterranean Sea, Red Sea, mountain ranges and seasonal movements of the Intertropical Convergence Zone. The Sahara is therefore not dry because of one single geographical feature but because several climatic processes reinforce one another.
The Saharas landscape is remarkably varied. Large areas consist of gravel plains and rocky plateaus rather than sand. Sand seas, called ergs, occupy a smaller but visually striking proportion of the desert. Important landscapes include dunes, rocky hamadas, gravel plains, salt flats, dry valleys and isolated mountain massifs. The Tibesti Mountains and Ahaggar Mountains rise above surrounding desert surfaces and can receive more precipitation than lower areas. Oases occur where groundwater reaches the surface or can be accessed through wells. These variations create local habitats and allow human settlement in particular locations despite the generally severe aridity.
Sahara temperatures can be extremely high, particularly during summer, but temperature varies greatly with location, elevation, season and time of day. Clear skies and dry air allow strong daytime heating, while rapid nocturnal radiative cooling can produce substantial temperature declines after sunset. Higher mountain areas can be much cooler than lowland plains. Winter nights in some locations can approach or fall below freezing, especially at higher elevations. The Sahara therefore has an extreme thermal environment rather than simply constant heat. Record temperature claims associated with the desert must also be distinguished from verified meteorological observations because historical records sometimes involve measurement uncertainties.
The Sahara has not always had its present hyperarid character. During the African Humid Period, particularly between roughly 11,000 and 5,000 years ago, parts of what is now the Sahara were substantially wetter and supported lakes, rivers, wetlands, grasslands and abundant wildlife. Rock art and archaeological evidence document human occupation and animals such as cattle, giraffes and other species in landscapes that are now extremely dry. Changes in Earths orbital parameters altered seasonal solar radiation and influenced the African monsoon, contributing to major shifts in vegetation and rainfall. The Sahara therefore demonstrates that desert boundaries can change dramatically over geological and climatic timescales.
Human societies have occupied the Sahara for thousands of years. Ancient and medieval communities developed agriculture around oases, pastoralism in suitable regions and long distance trade routes crossing the desert. Camel domestication and widespread camel transport greatly facilitated trans Saharan trade, allowing goods such as salt, gold and other commodities to move between North and West Africa. Major historical centres developed around oasis systems and caravan routes. Modern populations live in cities, towns, villages and nomadic or semi nomadic communities, while contemporary economic activities include mining, petroleum and natural gas production, tourism, agriculture in irrigated areas and renewable energy development.
Saharan ecosystems support specialised plants and animals despite extreme aridity. Vegetation is concentrated around oases, seasonal watercourses, mountain regions and areas receiving comparatively higher rainfall. Species include drought tolerant shrubs, grasses, acacias and ephemeral plants. Animals include dromedary camels, fennec foxes, desert rodents, reptiles, insects and numerous migratory birds. Many species are adapted to nocturnal activity, water conservation and extreme temperatures. The desert is not biologically empty, but biodiversity is generally lower than in wetter tropical ecosystems and is concentrated in habitats where water is intermittently or permanently available.
The Sahara is the worlds largest hot desert and one of the most significant examples of a subtropical desert system. Its enormous size, low precipitation, extreme temperatures, varied landforms, ancient environmental changes and long human history make it important to climatology, geology, archaeology, ecology and geography. Its southern boundary grades into the Sahel, a semi arid transition zone rather than a simple sharp border. The Sahara should also be distinguished from the larger Antarctic desert, because Antarctica is a polar desert. The precise answer to largest hot desert is therefore the Sahara, while the answer to largest desert of any type is Antarctica.
