What makes a place a desert?

Low precipitation. A common rule of thumb is less than about 250 mm of rain or snow a year, though scientists often also consider how fast water evaporates. Temperature does not decide it, which is why Antarctica counts as a desert.

How do sand dunes move?

Wind blows sand grains up the gentle side of a dune and over the crest, where they slide down the steep slip face. As sand is moved from one side to the other, the whole dune creeps downwind, small ones by up to tens of metres a year.

What is desert pavement?

A surface of closely packed pebbles and stones. It forms as wind removes fine particles, or as windblown dust collects under the stones and lifts them, leaving a stony layer that protects the soil beneath.

A dry riverbed that only carries water after rain. Wadis can fill suddenly and dangerously during flash floods, even when the storm is far away.

Can I explore a desert in the app?

Yes. You can land in a desert and explore its dunes and rocky plains in interactive 3D.

Sand seas, bare rock and the art of surviving without water

Desert

3D landscape · Erg Chebbi, Sahara

Land here and explore →

A desert is defined by dryness, not heat. A common rule of thumb is that a desert receives less than about 250 mm of rain or snow a year, though scientists often use a measure that also counts how quickly water evaporates. By that kind of definition, deserts and other drylands cover roughly a third of Earth's land. In this app you can land among the dunes and explore a desert in interactive 3D.

The famous hot deserts include the Sahara, the Arabian Desert, the Namib and the Atacama. Yet most desert surfaces are rock, gravel and dry plains rather than sand, and some of the largest deserts are cold. Deserts are shaped by wind, rare but violent floods and huge swings in temperature, and the plants and animals that live there have evolved remarkable ways to find, store and save water.

Key facts

Definition: Under ≈ 250 mm of precipitation a yearA common rule of thumb; aridity indices are also used
Share of Earth's land: Roughly one thirdIncluding semi-arid and cold deserts
Largest hot desert: The SaharaAbout 9 million km², similar in size to the USA
Largest desert overall: AntarcticaAbout 14 million km² of cold desert
Driest non-polar desert: The AtacamaSome weather stations have recorded almost no rain for years
Oldest desert: The NamibOften said to have been arid for tens of millions of years
Tallest dunes: Over 300 mFound in places such as the Namib and parts of the Sahara and China
Highest air temperature: 56.7°CDeath Valley, 1913 — the official record, though some experts question it
Sand cover: Only about 20–25% of the SaharaMost of it is rock, gravel and plains

The great hot deserts

The Sahara stretches across North Africa from the Atlantic to the Red Sea. It is the largest hot desert in the world, about 9 million km², and contains sand seas called ergs, rocky plateaus called hamadas, gravel plains and volcanic mountains. To its east, the Arabian Desert covers much of the Arabian Peninsula and includes the Rub' al Khali, or Empty Quarter, the largest continuous expanse of sand on Earth.

In south-western Africa, the Namib runs along the Atlantic coast and is often described as the oldest desert, dry for tens of millions of years. Its red dunes, such as those around Sossusvlei, are among the tallest anywhere. Across the Atlantic, the Atacama in Chile is the driest non-polar place on Earth; some of its stations have gone years without measurable rain, and its soils are so barren that NASA has used it to test equipment for Mars.

How dunes form and move

Dunes form where there is loose sand and wind strong enough to move it. Most sand travels by saltation: grains are lifted, carried a short distance and dropped, knocking other grains into the air as they land. Sand is blown up the gentle windward slope of a dune and tumbles down the steep downwind face, called the slip face, which settles at an angle of around 30–35 degrees.

Because sand is constantly removed from one side and added to the other, whole dunes creep downwind. Small crescent-shaped barchan dunes can travel tens of metres a year, while giant dunes move far more slowly. The shape of a dune depends on the supply of sand and the pattern of the wind. Barchans form where winds are steady and sand is scarce, long linear or seif dunes form where winds come from two main directions, and star dunes, with several radiating arms, grow where the wind shifts around the compass.

Singing sand

Some dunes boom or hum when sand avalanches down their slip faces, producing a low note that can be heard far away. The effect depends on the size, dryness and sorting of the grains, and its exact physics is still discussed by researchers.

Desert pavement, wadis and rock

Many desert surfaces are covered by desert pavement: a tight mosaic of pebbles and stones fitted together almost like cobbles. One explanation is that wind blows away the fine particles and leaves the stones behind. Another, supported by research in the American south-west, is that windblown dust settles between and beneath the stones over thousands of years, lifting them up on a layer of fine soil. The stones are often coated in desert varnish, a dark, thin film of clay and manganese and iron oxides that builds up extremely slowly.

Wadis, called arroyos or washes in the Americas, are dry riverbeds that carry water only after rain. Because desert soils absorb water poorly and there is little vegetation to slow it, a storm can send a sudden flash flood racing down a wadi, sometimes far from where the rain fell. Wind-sculpted rocks, mesas, canyons and dry salt lakes such as the Bolivian Salar de Uyuni complete the desert landscape.

Climate: why deserts are dry

Most hot deserts lie roughly 15–30 degrees north and south of the equator, where air that rose over the tropics sinks back down. Sinking air warms and dries, suppressing cloud and rain. Other deserts form in the rain shadow of mountains, far from the sea, or along coasts where cold ocean currents cool the air so that it cannot rise to make rain, as in the Namib and Atacama.

Clear skies and dry air let heat pour in by day and escape quickly at night, so the daily temperature range can be enormous. Hot desert ground can be far hotter than the air above it, and satellites have measured land surface temperatures of around 70°C in places such as Iran's Lut Desert. Deserts also change over long timescales: between roughly 11,000 and 5,000 years ago, much of the Sahara was green, with lakes, grasslands and people herding cattle.

Life in the desert

Desert plants save water in many ways. Cacti in the Americas and similar succulents elsewhere store water in thick stems, have spines instead of leaves and open their pores at night. Some plants have very deep roots, while many annual flowers survive as seeds for years and then bloom briefly after rare rains. Namibia's welwitschia, with just two strap-like leaves, can live for more than a thousand years.

Animals often avoid the heat by being active at night or living underground. Camels can go for long periods without drinking and tolerate a rise in body temperature during the day. The fennec fox loses heat through its huge ears. In the Namib, some beetles collect drinking water by standing on dune crests in the morning fog and letting droplets run down their backs into their mouths.

Cold deserts in brief

A desert does not need to be hot. Antarctica receives so little snowfall over most of its interior that it is the largest desert on Earth, and its McMurdo Dry Valleys are among the driest places on the planet. The Arctic is also largely a polar desert.

Other cold deserts include the Gobi in Mongolia and China, where winters are bitterly cold, the Great Basin in the western United States and the Patagonian Desert in Argentina. These deserts often have dry, rocky surfaces, scattered shrubs and wide seasonal temperature ranges.

Surprising facts about Desert

The largest desert on Earth is not the Sahara but Antarctica, because very little snow falls across most of the continent.
Only around a fifth to a quarter of the Sahara is covered in sand; the rest is mostly rock, gravel and dry plains.
Dust from the Sahara crosses the Atlantic and helps fertilise the Amazon rainforest.
More people are thought to drown in deserts than die of thirst there, because flash floods arrive so suddenly.
Parts of the Atacama are so dry and lifeless that NASA has tested Mars rover instruments there.
Between roughly 11,000 and 5,000 years ago, the Sahara was a green land of lakes and grasslands, shown in rock art of swimmers and cattle.

Frequently asked questions

What makes a place a desert?: Low precipitation. A common rule of thumb is less than about 250 mm of rain or snow a year, though scientists often also consider how fast water evaporates. Temperature does not decide it, which is why Antarctica counts as a desert.
How do sand dunes move?: Wind blows sand grains up the gentle side of a dune and over the crest, where they slide down the steep slip face. As sand is moved from one side to the other, the whole dune creeps downwind, small ones by up to tens of metres a year.
What is desert pavement?: A surface of closely packed pebbles and stones. It forms as wind removes fine particles, or as windblown dust collects under the stones and lifts them, leaving a stony layer that protects the soil beneath.
What is a wadi?: A dry riverbed that only carries water after rain. Wadis can fill suddenly and dangerously during flash floods, even when the storm is far away.
Can I explore a desert in the app?: Yes. You can land in a desert and explore its dunes and rocky plains in interactive 3D.

Keep exploring

The landscapes in Ember’s Universe are procedural 3D scenes modelled on real places. The landforms, plants and light follow the real thing, but the details are generated, not mapped.

Sources