A desert does not have to be hot
A desert is defined by how little rain falls, not by how hot it is. Most geographers count a place as a desert if it gets less than 250 millimetres of rain in a year, and some deserts are bitterly cold. By that measure the largest desert in the world is Antarctica, and the second is the Arctic. The largest hot desert is the SAHARA in North Africa, which is about the size of the United States.
The GOBI in Mongolia and China is a cold desert, where winter temperatures fall far below freezing and snow can dust the DUNE fields. The ATACAMA in Chile is the driest non-polar place on Earth; some weather stations there have never recorded rain, and its soil is so like the soil of Mars that the American space agency tests its Mars rovers there. Parts of it are so dry that dead animals and plants barely rot.
Most deserts are not sand
In films, deserts are endless rolling dunes of SAND, but only about a quarter of the Sahara is sand. Most of it is bare rock, gravel and stony plains. Dunes move, though: the wind blows sand up one gentle side and it slides down the steep far side, so a dune creeps forward, sometimes many metres a year. Some dunes even make a deep humming or booming sound when sand avalanches down them, which travellers once blamed on spirits.
A MIRAGE, the shimmering lake that seems to lie ahead on a hot day, is light bent by layers of hot air near the ground, so you are really seeing a reflection of the sky. An OASIS is real water, where an underground spring reaches the surface, and it can support palm trees, farms and even towns.
The Sahara used to be green
Between about 11,000 and 5,000 years ago, a slight wobble in the Earth's orbit brought heavier summer rains to North Africa, and the Sahara was grassland dotted with lakes, home to hippos, giraffes and crocodiles. Ancient rock paintings in the middle of today's desert show people swimming and herding cattle. The Sahara still affects places far away: every year, winds carry millions of tonnes of its dust across the Atlantic, and the minerals in it help fertilise the Amazon rainforest.
Life in the driest places
The NAMIB, along the coast of south-western Africa, may be the oldest desert in the world, dry for tens of millions of years. Almost no rain falls, but fog rolls in from the sea, and one small beetle survives by standing on a dune crest head-down in the morning fog, letting droplets gather on its back and run down into its mouth. People have lived in deserts for thousands of years too, many as a NOMAD moving with herds of CAMEL, goats or sheep between grazing and water. Our desert animals page covers the creatures of the KALAHARI, the MOJAVE, the SONORAN, the THAR, the Australian OUTBACK and beyond, and how they cope with DROUGHT, SCORCHING heat, DUST and the DRY, BARREN, ARID land, where a CACTUS stores water in its stem.