Arid Very dry. Arid regions, such as deserts, have too little water to support plant life.

Arroyo A dry creek, stream bed or gulch in an area with an arid or semi-arid climate that fills and flows after it receives rainwater during the short rainy season. It is also called a wash. Flash floods are common in arroyos following thunderstorms.

Barchan A crescent-shaped sand dune that slowly shifts along with the wind.

Butte A desert hill with very steep sides and a flat top. A butte is made up of hard rock that does not easily wear away. It is like a mesa but smaller.

Cold desert An area with less than 25 centimetres (10 inches) of snow or rainfall a year, and with an average temperature of less than 10°C (50°F). For example, Antarctica is a cold desert, as it receives very little snow.

Desert A desert is a dry area that receives less than 25 centimetres (10 inches) of rain per year. Most people think of deserts as vast sandy regions, but only 20% of the world’s deserts are sandy. The rest are bare rock, or covered with gravel. When rain does fall in occasional storms, there is no soil to soak it up so it quickly dries up. Some deserts are hot all year round and some are always cold. In many deserts, strong winds blast sand at the rocks, carving out amazing shapes.

Desert pavement A hard, thin desert surface of densely packed pebbles and rocks. Desert pavements form when sand and small rocks are blown away, leaving behind heavier lumps of rock.

Erg A sandy area of desert, often containing fields of sand dunes.

Hamada A flat, rocky area of desert, with little or no sand covering.

Hoodoo A strangely shaped column of rock. Hoodoos are made up of layers of soft and hard rock, which the wind and sand erodes at different rates, producing bizarre shapes and colours, and sometimes a stripy appearance.

Hot desert An area with less than 25 centimetres (10 inches) of rainfall a year, and with an average temperature of over 20°C (68°F). Hot deserts are cold at night because they do not have any cloud cover to trap heat.

Mesa A desert mountain with a flat top and very steep sides. Mesas are formed when layers of soft rock are worn away by wind and water, revealing a block of harder rock. Mesas take their name, the Spanish word for “tabletop”, from their distinctive, flat-topped appearance.

Natural arch A rock that is shaped like an arch. Natural arches form when the wind forces open a crack in a rock face.

Oasis A lush area of water and trees in the middle of a desert. An oasis forms where groundwater comes to the surface at a spring.

Plateau A flat, raised area of land. Plateaus form when the movement of tectonic plates pushes up whole sections of flat rock. Buttes and mesas are both types of plateau.

Playa A flat, dried-up lake, sometimes called a salina. During a rare rainstorm, the playa may briefly fill up with rainwater.

Rain shadow A rain shadow is an area of land that receives little rain because it lies on the far side of a mountain range to the prevailing winds. Winds blow moist air over the mountains, where moisture condenses in the cold and falls as rain or snow. By the time wind reaches the far side of the mountains, it has lost most of its moisture.

Salt flat A layer of salt crystals that forms in a dried-out lake. During a brief rainstorm, salts in the rocks of the desert dissolve in the warm rainwater. A pool of water may collect in an area of low ground. When it dries out, a layer of salt is left behind. After each rainfall the layer of salt becomes thicker.

Sand Tiny pieces of rock. Sand is formed when larger rocks are worn down by wind and rain.

Sand dune A heap, or bank, of sand that has been shaped by the wind. Sand dunes form where sand, blown by the wind, is stopped by an obstacle, such as a boulder, a plant or a change in the shape of the shape of the land.

Semi-desert A region that has a semi-arid climate, one that lies between an arid and a humid climate. Semi-deserts occur in areas with hot temperatures all year round, and very little rain, such as in parts of Australia, India and Africa. They usually have short wet seasons, and are often found on the edges of deserts. The vegetation is dominated by scrubland or dry grasses.

Wadi A steep-sided gorge. A wadi is formed when fast-flowing water from rare rainstorms wears away desert rocks.

Wind erosion The gradual wearing away of the land caused by the wind. Wind erosion gradually blows away soil and sand. Strong winds may hurl sand against rocky surfaces, where it acts like sandpaper, wearing away the rock. This process is called abrasion.

Zeugen A mushroom-shaped rock in the desert. It is formed by wind erosion. Most of the sand carried by the wind is found about a metre above the ground. Zeugens form when wind-blown sand blasts away the base of a boulder, leaving a narrow, top-heavy rock formation. Zeugens are sometimes called rock pedestals.

Consultant: Ian Fairchild