Deserts are regions that receive less than 250 millimeters (about 10 inches) of rainfall per year. They cover roughly one-third of Earth’s land surface and exist on every continent, including Antarctica. While most people picture endless sand dunes and scorching heat, deserts are far more varied than that. Some are frozen, some are rocky, and some sit along cool coastlines.
How Deserts Are Defined
The defining feature of a desert is aridity, not temperature. Any landscape averaging under 250 millimeters of annual precipitation qualifies as arid, and landscapes receiving between 250 and 500 millimeters are classified as semiarid. That low moisture can come in the form of rain, snow, or fog. What ties all deserts together is a water deficit: more moisture evaporates or sublimates than falls from the sky in a given year.
Why Deserts Form
Most of the world’s hot deserts sit near 30 degrees north or south of the equator, and that’s no coincidence. Warm, moist air rises near the equator, drops its rain over tropical forests, then flows outward at high altitude. By the time it descends back to the surface around 30 degrees latitude, it has lost nearly all its moisture. This sinking, dry air creates a persistent high-pressure zone where rain is rare. The Sahara, the Arabian Desert, and much of the Australian outback all owe their existence to this atmospheric pattern.
Mountains create deserts through a different mechanism called the rain shadow effect. When moisture-laden air hits a mountain range, it’s forced upward, cools, and releases its water as precipitation on the windward side. By the time the air crosses to the other side, it has little moisture left. Joshua Tree National Park in California sits in one of these rain shadows, blocked from Pacific storms by peaks over 10,000 feet tall.
Other deserts form simply because they’re too far from any ocean for moisture to reach them. The Gobi Desert in Mongolia and northern China is a classic example, isolated deep in the interior of a continent.
The Four Main Types
Deserts fall into four broad categories based on their climate and location.
- Hot and dry deserts are what most people think of. The Sahara, Sonoran, Mojave, and Chihuahuan deserts all fit here. Summers are extremely hot, winters are mild, and rainfall is sparse year-round. Surface temperatures in the Lut Desert of Iran and the Sonoran Desert of Mexico have been measured by satellite at 80.8°C (177.4°F), the hottest land surfaces on Earth.
- Semiarid deserts are slightly cooler and receive a bit more precipitation. The sagebrush country of Utah and Montana’s Great Basin falls into this category. Summer temperatures average between 21 and 27°C (70 to 81°F), and winters bring low but not negligible rainfall.
- Coastal deserts form where cold ocean currents cool the air along a shoreline, preventing rain from forming even though the ocean is right there. Chile’s Atacama Desert is the best-known example. Winters are cool, and summers are moderately warm, with averages ranging from 13 to 24°C.
- Cold deserts may seem like a contradiction, but Antarctica and the Arctic easily qualify. Antarctica receives so little precipitation that it is technically the largest desert on Earth. Cold deserts have frigid winters with snowfall and short, cool summers.
The World’s Largest Deserts
When ranked by area, polar deserts dominate the list. The Antarctic Ice Sheet covers about 13.96 million square kilometers (5.39 million square miles), making it the single largest desert. The Arctic follows closely at 13.7 million square kilometers. The Sahara is the largest hot desert at 8.6 million square kilometers, roughly the size of the entire United States. The Arabian Desert covers 2.3 million square kilometers, and the Gobi rounds out the top five at 1.3 million square kilometers.
What Desert Landscapes Actually Look Like
Only about 25 percent of desert terrain is sand. The rest is a mix of gravel plains, rocky plateaus, dry riverbeds, and salt flats. Geographers use specific terms for these surfaces: an erg is a sand sea filled with dunes, a reg is a flat plain covered in stones and gravel, and a hamada is a barren rocky plateau. Near mountain foothills, desert surfaces can be over 90 percent gravel. Farther out toward the basin floor, the ground shifts to finer sediment, and sand dunes may form.
Dry riverbeds called wadis (or arroyos in the Americas) cut through many deserts. They’re bone dry most of the year but can channel flash floods after sudden storms, reshaping the landscape in hours.
How Plants Survive Extreme Dryness
Desert plants have evolved remarkable strategies for dealing with scarce water. Succulents like cacti store water in specialized tissues called hydrenchyma, and their water content can reach 90 to 95 percent of their body weight. That stored water keeps them alive through months without rain.
Many desert plants also photosynthesize differently from typical vegetation. Most plants open tiny pores in their leaves during the day to absorb carbon dioxide, but this also lets water escape. Desert-adapted species flip that schedule: they open their pores at night, when temperatures are cooler and less water is lost, and store the captured carbon dioxide as an acid. During the day, they keep their pores sealed and use the stored carbon dioxide internally. Around 30,000 plant species worldwide use this strategy.
Root systems offer another survival advantage. Some desert trees send roots incredibly deep to reach permanent groundwater. A tree called Boscia albitrunca in the Kalahari Desert has been documented with roots extending 68 meters underground. Other plants spread shallow, horizontal root systems that fan out several meters to capture brief surface moisture from light rains.
How Animals Beat the Heat
Most desert animals avoid the worst of the heat rather than enduring it. The single most common behavioral adaptation is nocturnality. Studies tracking large desert mammals like the Arabian oryx show they become almost completely inactive during the day in hot, dry summers, shifting all their activity to nighttime while still maintaining the same total daily activity levels. Smaller creatures, including kangaroo rats, many reptiles, and burrowing owls, spend daytime hours underground in burrows where temperatures are far cooler and humidity is higher.
Some desert insects take a different approach entirely, laying eggs that stay dormant for months or even years until conditions improve. Fairy shrimp in temporary desert pools do the same, hatching only when rare rains fill depressions in the rock. Even something as simple as staying on the shady side of a twig, as some desert insects do throughout the day, can mean the difference between survival and overheating.
Water in the Desert
Deserts aren’t completely devoid of water. Oases form where geological faults or permeable rock layers allow groundwater to reach the surface. Egypt’s Siwa Oasis, for example, has around 200 natural springs fed by deep aquifers, discharging roughly 442,000 cubic meters of water per day. These springs have supported human settlement for thousands of years.
Underground aquifers beneath many deserts hold enormous volumes of water, some of it deposited thousands of years ago when the climate was wetter. These fossil water reserves supply agriculture and cities across the Middle East, North Africa, and the American Southwest, though they are being drawn down far faster than they recharge.