Why Deserts Form in Different Parts of the World

Strong sunlight near the equator warms moist air and makes it rise. As the air rises, it cools, water vapour condenses and heavy rain often falls. The now drier air moves away from the equator high in the atmosphere.

Around 30 degrees north and south, much of this air sinks. Sinking air becomes warmer and can hold more water vapour, which makes cloud formation less likely. These broad zones of descending dry air help explain why many large deserts sit in the subtropics.

This pattern is part of global atmospheric circulation. It does not draw perfect desert bands around the planet because oceans, mountains, winds and seasons complicate the picture. Still, it gives you a useful first clue when you compare world maps. The Sahara, Arabian Desert and deserts of central Australia all sit partly within these dry latitude zones, though local conditions shape each one.

Moist air may be forced upward when it reaches a mountain range. As it climbs, it cools and releases rain or snow on the side facing the incoming wind. By the time the air crosses the peak, it has lost much of its moisture.

The air then descends on the other side, warms and becomes drier. This sheltered area is called a rain shadow. A wet forest and a dry landscape can therefore exist on opposite sides of the same mountain range.

Rain shadows show why distance alone is not enough to explain climate. Two places at similar latitudes can receive very different rainfall because the path of moving air is different. When you study a dry region, look upwind and check whether mountains have already squeezed moisture from the air.

Cold ocean currents can cool the air above them and reduce evaporation. The cool, stable air may produce fog near a coast but little rain, helping coastal deserts form. Far inland, places may also be dry because moist ocean air loses water before travelling such a great distance. The Atacama Desert is a famous example where cold coastal water, stable air and nearby mountains combine to limit rainfall.

Use four questions to investigate a desert:

• •Is it near a zone of sinking air?
• •Do mountains create a rain shadow?
• •Does a cold ocean current pass nearby?
• •Is the region far from a major moisture source?

Because geographers define a desert by how little precipitation it receives, some of the driest places on Earth are cold. A common working definition is an area receiving less than about 250 millimetres of precipitation a year, although definitions vary. By that measure, Antarctica is the largest desert in the world, since the very cold air holds little moisture and snowfall across most of the interior is extremely low. The Gobi in Asia is another example, with bitterly cold winters caused by its altitude, its great distance from the sea and the cold Siberian air that settles over it.

Most people also picture deserts as seas of sand dunes, but sand seas cover only part of the world's dry lands. Much desert surface is bare rock plateau, gravel plains, dry salt flats or rocky mountains. Deserts often have large daily temperature swings for a related reason. With few clouds and little water vapour in the air, sunlight heats the ground strongly by day, and at night the heat escapes quickly back into space. A hot afternoon can therefore be followed by a surprisingly cold night.

Deserts are not permanent features. Between roughly eleven thousand and five thousand years ago, much of what is now the Sahara was grassland and scrub, with lakes, rivers and wetlands. Scientists call this period the African Humid Period. Evidence comes from several directions: old lake sediments buried under the sand, fossil remains of hippos, crocodiles and fish far from any water today, and pollen from plants that need more rain than the region now receives.

Rock art adds a human record. Paintings and engravings in places such as the Tassili n'Ajjer in Algeria show cattle herds, swimming figures and wildlife that could not survive there now. The main cause was slow change in Earth's orbit, which altered how much summer sunshine reached the Northern Hemisphere. Stronger summer heating pulled the West African monsoon further north and brought rain deep into the region. As the orbital pattern shifted, the rains retreated and the land dried, showing that the same circulation that makes deserts can also move them.

Wind cannot lift most sand grains high into the air. Instead, a grain is picked up, carried a short distance and dropped, striking the surface and knocking other grains into motion. This bouncing movement is called saltation, and most of it happens close to the ground, within about a metre. That is partly why rocks in windy deserts are often worn most strongly near their base. The British army engineer Ralph Bagnold studied blown sand in the Libyan Desert and in wind tunnels in the 1930s, and a dune field on Mars visited by NASA's Curiosity rover was named the Bagnold Dunes in his honour.

Dune shapes depend on how much sand is available and how steady the wind is. Where sand is limited and the wind blows mainly from one direction, crescent-shaped barchan dunes form, with their horns pointing downwind. Sand is pushed up the gentle windward slope and slides down the steeper slip face on the far side, so the whole dune creeps forward, with small dunes moving faster than large ones. Where winds come from several directions, star-shaped dunes can build to great heights. Roads, railways and oases in sandy deserts sometimes need fences or planted vegetation to hold back migrating dunes.

It is easy to assume wind is the main sculptor of deserts, since it builds dunes and blasts rock. Yet many geologists argue that running water does most of the shaping, even where rain is rare. When rain does come, it often falls as short, intense storms. Sparse plants do little to slow the water, and baked or crusted ground lets little soak in, so it rushes over the surface, picking up sand, gravel and boulders. It cuts steep-sided channels known as wadis in North Africa and Arabia, and arroyos in the Americas, which stay dry most of the time but carry much of the sediment away from desert mountains.

These flash floods are one of the real dangers of dry regions. A storm over distant hills can send a surge of muddy water down a channel where the sky overhead is clear, so travellers are advised never to camp in dry riverbeds. Where a channel leaves the mountains and spreads onto flatter ground, the water slows and drops its load, building a fan-shaped apron of gravel and sand called an alluvial fan. Water draining into basins with no outlet evaporates, leaving salt flats such as those in Death Valley. Many dramatic desert landforms are therefore records of rare but powerful water.

Deserts form wherever the atmosphere delivers very little precipitation over time. Subtropical sinking air, mountain rain shadows, cold currents and inland distance can all contribute. Focus on the movement of air and moisture, and dry landscapes begin to make geographic sense.