Deserts are arid regions defined by an extreme lack of precipitation rather than solely by temperature. These environments cover a substantial portion of the planet’s landmass. Deserts vary widely, encompassing hot, sand-swept regions and cold, ice-covered landscapes. Classification into types, such as subtropical, coastal, and polar, helps distinguish the climatic mechanisms that create these vast, dry zones.
Identifying the Largest Subtropical Desert
The largest subtropical desert in the world is the Sahara, dominating the northern expanse of the African continent. This immense arid zone covers approximately 3.6 million square miles (9.2 million square kilometers), making it the largest hot desert globally. It is the third-largest desert overall, smaller only than the polar deserts of Antarctica and the Arctic. The Sahara sprawls across parts of nearly a dozen countries, including Algeria, Chad, Egypt, Libya, Mali, Mauritania, Morocco, Niger, Sudan, and Tunisia.
The term “subtropical” is an important qualifier because the Antarctic Polar Desert, while significantly larger, is classified differently due to its climate mechanisms. The Sahara is defined by low annual rainfall; much of the central area is hyper-arid and virtually rainless, often receiving less than two inches (50 millimeters) per year.
The Climate Mechanisms That Create Subtropical Deserts
The formation of the Sahara and other subtropical deserts results directly from the Hadley Cell circulation, a large-scale atmospheric phenomenon. This process begins near the equator, where intense solar heating causes warm, moist air to rise, creating a low-pressure zone. As the air ascends, it cools, and water vapor condenses, resulting in the heavy rainfall characteristic of tropical rainforests.
Once the air loses its moisture, the dry air moves poleward in the upper atmosphere, sinking back toward the surface at approximately 30 degrees latitude. This descent creates persistent high-pressure systems, often called the subtropical ridge. As the air sinks, it is compressed and warms adiabatically, increasing its capacity to hold moisture.
This warm, dry, descending air suppresses cloud formation and precipitation, leading to arid conditions. The Sahara’s position beneath the sinking arm of the Northern Hemisphere Hadley Cell is the primary factor explaining its hot desert climate. Factors like continentality and the rain shadow effect of coastal mountain ranges also reinforce aridity.
Unique Geographical Features of the Sahara
The physical landscape of the Sahara is composed of three primary surface types, making it far more complex than the common image of endless sand dunes suggests. A significant portion consists of hamadas, which are vast, elevated plateaus of bedrock and rock rubble scoured clean by wind erosion. Another common feature is the reg, expansive plains covered in gravel and pebbles, representing the remnants of eroded rock.
The iconic sand seas, known as ergs, are actually a minor component, covering only about 25% of the Sahara’s total area. These ergs feature massive, shifting sand dunes that can reach heights of over 590 feet (180 meters). The desert is also characterized by extreme temperature fluctuations, known as a high diurnal range, due to the lack of moisture and cloud cover.
During the day, summer temperatures can soar above 104°F (40°C), with ground temperatures sometimes exceeding 160°F (71°C). The heat radiates quickly into the clear night sky, causing temperatures to plummet. Temperatures sometimes drop below freezing in the winter months, especially at higher elevations. Geological evidence indicates the Sahara has cycled through periods of greening, with its current arid state solidifying around 2500 BCE.