Deserts of the world

A desert is defined by dryness, not heat. Geographers usually describe a desert as a place that receives very little precipitation, often less than 250 millimetres a year, and where more water could evaporate than actually falls.

By that definition the largest desert in the world is Antarctica, followed by the Arctic. The largest hot desert is the Sahara, which covers much of northern Africa and is roughly the size of the United States.

• Subtropical high pressure: air rising at the equator descends around 30 degrees north and south, warming and drying as it sinks, which produces the Sahara, Arabian, Kalahari and Australian deserts.
• Rain shadows: mountains block moist winds, as the Himalayas help dry the Gobi and Tibetan regions.
• Distance from the sea: air loses its moisture long before reaching the interior of large continents.
• Cold ocean currents: chilly coastal water cools the air above it and prevents rain, creating coastal deserts such as the Atacama and Namib.

Sand seas, called ergs, are the classic image of a desert, but they make up only a fraction of the world's deserts. Much desert land is rocky plateau, gravel plain or dry mountain. Only about a fifth of the Sahara is covered by sand dunes.

Wind shapes deserts by moving sand and blasting rock, but water is surprisingly important. Rare, intense storms cause flash floods that carve dry riverbeds called wadis or arroyos. Where water collects and evaporates, it can leave salt flats, such as Salar de Uyuni in Bolivia, the largest in the world.

Hot deserts can be scorching by day and cold at night. Dry air and clear skies let sunlight heat the ground intensely, but at night there is little moisture or cloud to hold the heat in, so it escapes quickly. Daily temperature swings of 20 degrees Celsius or more are common.

The Atacama in Chile is one of the driest non-polar places on Earth; some weather stations there have recorded almost no rain for years. Its soils are so dry that scientists use them to test instruments designed to search for life on Mars.

Desert plants save water in many ways. Cacti in the Americas and similar-looking euphorbias in Africa store water in thick stems, and their spines reduce water loss and deter animals. Many plants have deep or widespread roots, tiny leaves or waxy coatings. Some seeds lie dormant for years and sprout only after heavy rain, producing sudden carpets of flowers.

Animals avoid the heat by being active at night or around dawn and dusk, sheltering in burrows by day. Camels tolerate large changes in body temperature and can lose a large share of their body water without harm. The kangaroo rat of North America may never need to drink, getting water from seeds and from its own metabolism.

People have lived in deserts for thousands of years, as nomadic herders, oasis farmers and traders. Caravans of camels crossed the Sahara carrying gold, salt and enslaved people, connecting West Africa with the Mediterranean.

Today deserts hold large cities such as Cairo, Riyadh and Phoenix, sustained by rivers, groundwater and desalination, and they host some of the world's largest solar power plants. Desertification, the degradation of dry but productive land through overgrazing, poor farming and drought, threatens the livelihoods of many people living in the world's drylands.