The largest deserts overall are the polar regions, with Antarctica holding the top spot. The largest desert outside of the Antarctic is the Sahara Desert. This vast North African expanse is the world’s largest non-polar desert, a formidable landscape of extreme heat and dryness. While the Arctic is the second largest desert overall, the Sahara remains the champion of the hot, subtropical desert category.
Understanding Desert Classification
The definition of a desert is not determined by temperature, but by precipitation. Specifically, a desert is an area that receives less than 250 millimeters (about 10 inches) of annual rainfall, where evaporation exceeds precipitation. This classification explains why the world’s two largest deserts are the polar regions, as the cold air holds very little moisture, resulting in minimal precipitation.
The three primary categories of deserts are based on geographical location. Polar deserts are characterized by consistently low temperatures and low precipitation, such as Antarctica and the Arctic. Cold Winter deserts, like the Gobi in Asia, experience hot summers but freezing winters. The third type is the Hot or Subtropical desert, exemplified by the Sahara. These are located near the Tropics of Cancer and Capricorn, where constant high-pressure systems create year-round arid conditions.
The Sahara The Largest Non-Polar Desert
The Sahara Desert is the largest hot desert on Earth, encompassing approximately 9.2 million square kilometers (3.6 million square miles), roughly the size of the continental United States or China. This immense desert dominates nearly a third of the African continent, stretching from the Atlantic Ocean in the west to the Red Sea in the east, and from the Mediterranean Sea in the north to the semi-arid Sahel region in the south.
The Sahara spans across the territories of eleven different nations:
- Algeria
- Chad
- Egypt
- Libya
- Mali
- Mauritania
- Morocco
- Niger
- Sudan
- Tunisia
- Western Sahara
Its topography is varied, consisting of three main surface features. Vast sand seas, known as ergs, cover about 25% of the surface and feature towering sand dunes. The remaining area is composed of flat, rock-strewn gravel plains called regs, or elevated, barren, rocky plateaus known as hamadas.
Geographical Factors Behind the Sahara’s Size
The primary reason for the Sahara’s colossal size and extreme aridity lies in a large-scale atmospheric circulation pattern known as the Hadley Cell. Warm, moist air rises near the equator, precipitates its moisture over the tropical rainforests, and then moves poleward as dry air high in the atmosphere. This dry air descends around the 30-degree latitude line, creating persistent high-pressure zones that suppress cloud formation and rainfall.
The Sahara is situated directly under the northern extent of this subtropical high-pressure belt, causing a constant downward flow of dry, warm air that prevents precipitation. The desert’s aridity is amplified by the rain shadow effect created by two distinct mountain ranges.
The Atlas Mountains in the northwest block moisture-laden air from the Atlantic Ocean and the Mediterranean Sea, forcing it to release moisture on the mountains’ northern slopes. The air descending on the Sahara side is extremely dry. Similarly, the cold Canary Current along the northwestern African coast stabilizes the atmosphere, inhibiting the formation of rain-producing clouds and reinforcing the dry conditions of the western Sahara.
Contextualizing the World’s Major Deserts
The Sahara’s size is best understood by comparing it to other major arid regions. The second-largest hot, subtropical desert is the Arabian Desert in Western Asia, covering approximately 2.3 million square kilometers (0.9 million square miles). The Sahara is nearly four times larger than this nearest subtropical rival.
Other significant non-polar deserts fall into the cold-winter category, such as the Gobi Desert, which spans parts of China and Mongolia at about 1.3 million square kilometers (0.5 million square miles). The Great Australian Desert is a complex of smaller deserts totaling about 1.4 million square kilometers (0.53 million square miles). In Southern Africa, the Kalahari Desert covers roughly 0.9 million square kilometers (0.35 million square miles).
The substantial difference in size highlights the unique combination of atmospheric and geographic factors that created Africa’s massive arid zone.