What is the largest desert in the world?

A desert is defined by precipitation, conventionally receiving less than about 10 inches or 250 millimeters annually, not by temperature or by the presence of sand. By that definition the Antarctic Polar Desert is the largest on Earth at roughly 5.5 million square miles, followed by the Arctic Desert, with the Sahara at about 3.6 million square miles ranking third overall and first among hot deserts. Antarctica's interior receives so little precipitation that parts of the McMurdo Dry Valleys are among the driest places on the planet, with some areas thought to have gone without meaningful rainfall for a very long time.

The mental image of a desert as an endless sea of dunes is also inaccurate. Sand seas cover only a modest fraction of the Sahara, with the rest consisting of rocky plateaus, gravel plains, mountains, and salt flats. Most of the world's deserts are rocky rather than sandy. Deserts also span an enormous temperature range, from the Sahara and Death Valley to the Gobi, which experiences severe winters, to the polar deserts.

Why deserts occur where they do follows a few mechanisms. Subtropical deserts including the Sahara, Arabian, and Australian deserts sit around 30 degrees latitude where descending dry air in the Hadley circulation suppresses rainfall. Rain shadow deserts form on the leeward side of mountain ranges that strip moisture from incoming air, which explains much of the American West. Coastal deserts such as the Atacama arise where cold ocean currents limit evaporation, and parts of the Atacama are the driest non-polar places on Earth. Continental interiors far from any ocean, like the Gobi, are dry simply because moisture never reaches them. Deserts are also expanding in some regions through desertification driven by land use and climate change, which affects the livelihoods of a substantial share of the world's population.