In 1953 Peveril Meigs divided the world's dry regions by rainfall. Extremely arid land has at least 12 months without rain, arid land gets under 250 millimeters a year, and semiarid land gets 250 to 500. As of November 2026, about one-third of Earth's land surface is desert. Many deserts lie near 30 degrees latitude, where air that rose and lost its water at the equator comes down dry. The Atacama Desert in Chile is one of the driest places on Earth. The driest parts of the Atacama Desert have no vegetation and not even the cyanobacteria found under stones in most other deserts. As of November 2026, rain of 1 millimeter or more may fall there only once every 5 to 20 years.

Sand dunes cover only about 10 percent of the world's deserts; others are dry expanses of rock, salt flats or even ice sheets. There are four basic types of desert: high pressure, rain shadow, interior continental and coastal. Sand dunes come in several forms, and the most common dune form on Earth and on Mars is the crescentic dune, and some crescentic dunes move faster than any other dune type; one group moved more than 100 meters per year between 1954 and 1959 in China's Ningxia Province. Linear dunes are sand ridges much longer than they are wide and may be more than 160 kilometers long. Star dunes grow upward rather than sideways; in China's Badain Jaran Desert they reach up to 500 meters and may be the tallest dunes on Earth.

Desert days and nights differ sharply, because in deserts about 90 percent of the day's accumulated heat radiates back toward the sky after sundown, which makes daily temperature ranges of 50 degrees or more. Many desert animals sleep in the heat of the day and come out at night, when it is cooler. In the desert heat, drink plenty of water and stay in the shade during the hottest part of the day. Cacti open their stomata only at night, when the plant is cool, so they lose less moisture through transpiration. Seeds of desert annuals can lie dormant for years if conditions are not right for sprouting. Some tiny desert animals survive drying out by cryptobiosis: they can lose up to 92 percent of their body water and recover in as little as half an hour once rehydrated.

Deserts also shape the air far beyond their edges. As of November 2026, the Sahara lifts upwards of 60 million tons of sand and soil into the air each year, and the dust travels across the Atlantic Ocean. The Sahara affects hurricanes too: easterly winds from the contrast between the hot, dry desert and the cooler, wetter coast to its south help start tropical waves. As of November 2026, the largest desert in the world is Antarctica, a cold desert of ice, and polar deserts hold great quantities of water, but most of it is locked in glaciers and ice sheets year-round, so little is available to plants and animals. A land that gets very little rain reaches across an ocean by wind.

Words to know

• cryptobiosis: hidden life
• arid: very dry
• crescentic: moon-shaped
• cyanobacteria: tiny bacteria
• semiarid: partly dry