A desert is any region that receives less than 250 millimeters (about 10 inches) of rainfall per year. That single threshold is what separates a desert from other biomes, and it means deserts aren’t limited to the hot, sandy landscapes most people picture. Antarctica, covered in ice and averaging temperatures well below freezing, is the largest desert on Earth at 14.2 million square kilometers. What defines a desert isn’t heat or sand. It’s dryness.
Why Deserts Form
Most of the world’s major deserts exist because of large-scale patterns in how air circulates across the planet. Near the equator, warm air rises, carrying moisture with it. As that air climbs higher into the atmosphere, it cools and drops its water as tropical rain. The now-dry air moves away from the equator and sinks back toward the surface at roughly 30 degrees latitude, both north and south. This sinking air creates zones of high pressure where clouds rarely form, which is why so many of the world’s iconic deserts, including the Sahara, the Arabian Desert, and parts of Australia, sit in a belt around those latitudes.
Mountains create deserts through a different mechanism. When moisture-laden winds hit a mountain range, the air is forced upward. As it rises, it cools, and the water it carries condenses into clouds and falls as rain or snow on the windward side. By the time the air crosses the peaks and descends on the other side, it has lost most of its moisture. The landscape in that “rain shadow” dries out. The Great Basin Desert in the western United States formed this way, sitting in the shadow of the Sierra Nevada.
Cold ocean currents produce a third type. Along certain coastlines, upwelling cold water chills the air above it, creating fog but very little actual rain. Chile’s Atacama Desert, one of the driest places on the planet, receives as little as 5 millimeters of rainfall per year in its most extreme stretches. Winter fogs roll in from the Pacific but rarely deliver meaningful precipitation.
What Desert Landscapes Actually Look Like
Sand dunes cover only about 20 percent of the world’s desert surfaces. The popular image of endless rolling dunes is the exception, not the rule. Nearly 50 percent of desert terrain consists of flat, gravelly plains where wind has stripped away fine particles and left behind loose pebbles and cobbles. The rest is a mix of rocky plateaus, dry lake beds, salt flats, and sparse scrubland.
Desert soils, classified as aridisols, are too dry to support most conventional plant life. Because so little water moves through them, soluble minerals like calcium carbonate, gypsum, and various salts accumulate in layers rather than washing away. Salt-rich desert soils are generally unsuitable for agriculture unless the salts are flushed out first. These mineral accumulations give desert landscapes their characteristic pale, chalky, or crystalline surfaces.
Temperature Swings
Hot deserts experience some of the most dramatic daily temperature shifts of any environment. During the day, temperatures average around 38°C (just over 100°F), but at night they can plummet to roughly -4°C (about 25°F). That’s a swing of more than 40°C in a single day. The lack of moisture in the air is what makes this possible: water vapor acts as an insulating blanket, trapping heat near the surface. Without it, heat escapes rapidly after sunset.
Not all deserts are hot, of course. Polar deserts like Antarctica and parts of the Arctic are bitterly cold year-round. Midlatitude deserts, found between 30 and 50 degrees from the equator in continental interiors far from any ocean, experience both scorching summers and freezing winters. The Gobi Desert in Mongolia regularly drops below -30°C in January.
How Plants Survive With Almost No Water
Desert plants have evolved a set of overlapping strategies to cope with extreme dryness. Many develop deep root systems that reach water sources far underground, while others spread shallow roots over wide areas to capture brief rainfall before it evaporates. Cacti and other succulents store water in their tissues, using thick, waxy outer layers to minimize water loss.
One of the most effective adaptations involves a shift in when plants “breathe.” Most plants open tiny pores in their leaves called stomata during the day to take in carbon dioxide for photosynthesis. But opening stomata in desert heat means losing enormous amounts of water to evaporation. Many desert species flip this schedule entirely, opening their stomata only at night when temperatures are cooler and humidity is higher. They store the carbon dioxide chemically and use it for photosynthesis the next day. Some species take an even simpler approach: they replace leaves with thorns or spines, cutting water loss dramatically while relying on green stems for photosynthesis.
How Animals Handle the Heat
For desert animals, the most straightforward survival strategy is avoidance. The vast majority of desert rodents, roughly 84 percent of species studied, are nocturnal, spending the hottest hours underground in burrows where temperatures are far more moderate. Burrow-dwelling, or fossorial, behavior is considered one of the most important adaptations for surviving in deserts. Underground shelters can be 20 to 30 degrees cooler than the surface during peak heat.
Animals that must be active during the day rely on a combination of behavioral and physiological tricks. Lizards shuttle between sun and shade to regulate body temperature. Some desert mammals have evolved lighter-colored fur that reflects solar radiation while still providing insulation against heat radiating from the ground. Many desert animals also produce highly concentrated urine and dry feces, conserving every possible drop of water. Some small rodents can survive their entire lives without ever drinking, getting all the moisture they need from the seeds they eat.
The World’s Major Deserts
Deserts cover roughly one-third of the Earth’s land surface, spread across every continent. The polar deserts are the largest by area: Antarctica alone accounts for 14.2 million square kilometers, and the Arctic desert covers much of Greenland and northern Canada. Among hot deserts, the Sahara dominates at about 9 million square kilometers, stretching across nearly the entire width of northern Africa.
- Subtropical deserts sit near 30 degrees latitude, created by sinking dry air. The Sahara, Arabian, and Kalahari deserts fall into this category.
- Rain shadow deserts form on the sheltered side of mountain ranges. The Great Basin and Patagonian deserts are prominent examples.
- Coastal deserts develop where cold ocean currents suppress rainfall. The Atacama and Namib deserts are the best known.
- Midlatitude deserts occupy continental interiors far from moisture sources, with extreme seasonal temperature ranges. The Gobi and Karakum deserts are typical.
Each type has a distinct feel. Coastal deserts are often blanketed in fog, subtropical deserts are brutally sunny, and midlatitude deserts swing between summer heat and winter cold. What unites them all is the same basic fact: less than 250 millimeters of rain per year, and an ecosystem shaped entirely around scarcity.