Nature · Earth Science
Landscapes shaped by the absence of water
A desert is any region that receives very little precipitation — usually under 250 mm of rain a year — so that few plants and animals can survive. Deserts are not always hot: they are defined by dryness, not temperature, and cover roughly a third of Earth's land surface, from scorching sand seas to freezing polar plains.
Most deserts form where descending dry air suppresses rainfall (the subtropical belts near 30° latitude), in rain shadows behind tall mountains, deep in continental interiors far from the sea, or along cold ocean currents. With little moisture to bind it, the land is sculpted by wind and rare flash floods into dunes, rocky plains and carved rock.
Wind pushes sand up the gentle windward slope; it avalanches down the steep slip face, marching the whole dune slowly downwind.
Scorching sand seas beneath sinking air
Formed under the descending dry air of the subtropical high-pressure belts, these are the classic hot deserts. Clear skies mean blistering days and surprisingly cold nights as heat escapes.
Bone-dry shores beside cold seas
Cold currents chill the air offshore so it holds little moisture, leaving the coast parched even beside the ocean. Coastal fog rolls in and sustains specialised plants and animals.
Dry lands in the lee of mountains
When moist air is forced up a mountain range it drops its rain on the windward side, then descends warm and dry on the far side, leaving a desert in the mountain's shadow.
Frozen deserts of ice and rock
Deserts are defined by dryness, not heat — and the poles are extremely dry. Antarctica's interior receives less moisture than the Sahara, making it the largest desert of all.