Africa is home to the largest hot desert on Earth and some of the driest inhabited land anywhere, but the continent isn’t uniformly dry. The Sahara alone covers roughly 9.2 million square kilometers of North Africa, and arid or semi-arid conditions stretch across much of the Horn of Africa, southern Africa, and the Sahel. The dryness comes from a combination of global atmospheric patterns, ocean currents, mountain barriers, and ocean temperature cycles that each suppress rainfall in different regions.
How Global Air Circulation Creates Desert Belts
The single biggest reason so much of Africa is dry has to do with how the atmosphere moves heat around the planet. Near the equator, intense solar heating causes air to rise, cool, and drop its moisture as heavy tropical rain. That now-dry air flows outward at high altitude, then sinks back toward the surface around 30 degrees north and south latitude. As it descends, it compresses and warms, making it even less likely to produce rain. This creates a permanent belt of high pressure at those latitudes.
The results are visible on any world map. Nearly every major desert sits along the 30-degree lines: the American Southwest, Australia’s interior, the Arabian Peninsula, and, most dramatically, the Sahara. North Africa sits squarely under this descending air. Northwestern Sudan, deep in the Sahara, receives no more than about 3 to 32 millimeters of rain in an entire year. That’s roughly one inch at most. The same mechanism helps dry out southwestern Africa, where the Namib and Kalahari deserts lie near 30 degrees south.
The Rain Belt That Moves With the Seasons
Africa’s wet and dry seasons are governed by a narrow band of converging winds near the equator called the Intertropical Convergence Zone, or ITCZ. This band follows the sun’s position, shifting northward during the Northern Hemisphere summer and southward during the Southern Hemisphere summer. Where the ITCZ sits, warm air rises and rain falls. Where it has moved away from, dry conditions dominate.
This means most of tropical and subtropical Africa doesn’t get steady rain year-round. Instead, regions experience distinct wet and dry periods depending on how far the ITCZ travels. The Sahel, the transitional zone just south of the Sahara, gets its rain only during a few summer months when the ITCZ pushes far enough north. For the rest of the year, it bakes under dry, sinking air. If the ITCZ doesn’t migrate as far north in a given year, the Sahel gets even less rain than usual, sometimes triggering severe drought.
Relative humidity numbers across the continent reflect this pattern clearly. Arid areas of the Sahel and northeast Africa average less than 30% relative humidity with temperatures above 30°C. Semi-arid zones in the transitional belt between desert and coast sit at 30 to 60% humidity. Only the equatorial zones and the coast of the Gulf of Guinea consistently reach 60 to 80%.
Cold Ocean Currents Along the Coast
You might expect that being next to an ocean would bring moisture and rainfall, but along Africa’s southwestern coast the opposite happens. The Benguela Current carries cold water northward from the Antarctic along the coast from roughly 34°S to 19°S. This cold surface water chills the air directly above it, creating a stable, cool layer that suppresses the rising motion needed to form rain clouds. Fog rolls in regularly, but actual rainfall is almost nonexistent.
This is why the Namib Desert, one of the oldest deserts on the planet, sits right on the Atlantic coastline. Research published in Nature traced the intensification of this upwelling system to a cooling period in the late Miocene, millions of years ago, which triggered and deepened southern African aridity. The same trade winds that sustain the high-pressure belt also drive the coastal upwelling, linking the cold current directly to the broader atmospheric circulation.
Mountains That Block Moisture
In North Africa, the Atlas Mountains of Morocco and Algeria create a rain shadow that starves the interior of precipitation. Westerly winds carry moisture inland from the Atlantic, but the mountain range intercepts it. The windward side of the Atlas gets enough rain to support grasslands and wetlands. The leeward side, facing the Sahara, gets dramatically less.
NASA satellite data from a September 2024 rain event illustrated this starkly. The Atlas Mountains received 32 to 100 millimeters of rainfall during one storm, with ground stations recording up to 250 millimeters over two days at one location in the southern foothills. But just 300 miles south of the mountains, monthly rainfall totals dropped to about 100 millimeters, and further into the interior, they fell to near zero. The mountains act as a wall, with wet conditions on one side and desert on the other.
Ocean Temperatures and East African Drought
East Africa, particularly the Horn of Africa, faces its own distinct dryness problem tied to temperature patterns in the Indian Ocean. A climate cycle called the Indian Ocean Dipole (IOD) alternates between phases where the western Indian Ocean is warmer than the east (positive phase) and phases where the pattern reverses (negative phase).
During positive phases, the warm water near Africa’s east coast fuels atmospheric convection, pulling in moisture and often bringing heavy rain or even flooding during the October-to-December “short rains” season. During negative phases, the western Indian Ocean cools and the eastern side warms, shifting convective activity away from Africa. The result is drought. The 2021 negative IOD event caused drought conditions in East Africa comparable to severe events in 1996, 1998, 2005, and 2016. Climate projections suggest that more extreme negative IOD events are likely in the future, meaning droughts in the Horn of Africa could become both more frequent and more severe.
Human Activity Is Making It Worse
Africa’s dryness isn’t purely natural. Deforestation and overgrazing strip vegetation, which exposes bare, lighter-colored soil. That lighter surface reflects more sunlight back into the atmosphere (a property called albedo), which reduces the heating of the ground. Less surface heating means less rising air, and less rising air means less rainfall. Research in West Africa found a strong relationship between albedo and rainfall, with the statistical correlation between the two explaining about 80% of the variation. Tree cover and albedo were similarly linked, at about 66%.
This creates a feedback loop: less rain leads to less vegetation, which leads to higher albedo, which leads to even less rain. The severe West African drought of the 1970s and 1980s, combined with land-use changes, accelerated land degradation and desert expansion across the Sahel, devastating food and water security. Over the broader period from 1950 to 2015, the Sahara expanded by about 8%, with its southern boundary advancing roughly 100 kilometers deeper into the Sahel. An earlier study using precipitation records estimated the Sahara grew by 10% over the entire 20th century.
This expansion isn’t uniform or permanent in every location. Some areas have seen partial recovery of vegetation during wetter periods. But the overall trend has been toward more arid land, particularly where human pressure on the landscape is greatest.