What makes a desert?

What do you think of when you hear "desert"?

Maybe hot sand and camels. But a desert is any place that is very, very dry. It can be hot. It can be cold. It is always dry.

Many people picture a desert as hot sand under a blazing sun. But the U.S. Geological Survey (USGS) explains that deserts are called deserts because they are dry. They may be hot or they may be cold. They may be seas of sand, or wide plains of rock and gravel with a few plants. But deserts are always dry.

About one third of all the land on Earth is desert. That means that if you could walk across every piece of land in the world, you would spend about a third of your journey in deserts.

The USGS defines deserts as arid lands with meager rainfall that support only sparse vegetation and limited populations of people and animals, and stresses that heat is not part of the definition: deserts may be hot or cold, sandy or rocky, but they are always dry. Approximately one third of Earth's land surface is desert.

Deserts are not restricted by latitude, longitude or elevation; they occur from near the poles to near the Equator. China has both the highest desert, the Qaidam Depression at 2,600 m above sea level, and one of the lowest, the Turpan Depression at 150 m below sea level (USGS). Most of Earth's deserts lie in dry areas created by global atmospheric circulation.

Scientists measure how much rain falls in a year.

A rainforest gets lots and lots. A desert gets only a little. Some deserts get almost no rain for years.

To decide whether a place is a desert, scientists measure how much rain and snow falls there in a year. The USGS describes a system made in 1953 by Peveril Meigs that many scientists use. In it, deserts get less than 250 millimeters a year. That much water would be only 25 centimeters deep, less than the length of a school ruler. The very driest deserts go at least 12 months in a row with no rain at all.

NASA compares deserts with rainforests: rainforests get more than 2,000 millimeters of rain a year, while most deserts get less than 300. So a desert gets only about a tenth of the rain a rainforest gets.

There are almost as many definitions of deserts as there are deserts, the USGS notes, combining days of rain, annual totals, temperature, humidity and other factors. The widely accepted 1953 scheme of Peveril Meigs is based on precipitation: extremely arid lands have at least 12 consecutive months without rain; arid lands receive less than 250 mm a year; semiarid lands receive 250 to 500 mm. Arid and extremely arid lands are deserts, while semiarid grasslands are generally called steppes.

NASA's biome guide puts the contrast in round numbers: most deserts receive less than 300 mm a year, against more than 2,000 mm in rainforests, roughly 10 percent as much.

Antarctica is covered in ice and snow.

But very little new snow falls there each year. So Antarctica is a desert too! It is the driest place of all the continents.

Here is a surprise: Antarctica, the icy continent at the South Pole, is a desert. The Australian Antarctic Division calls it the coldest, windiest and driest continent on Earth. Most of what falls there is snow, but not very much of it. Melted down, a whole year's snow over the continent would make only about 150 millimeters of water, and in the high middle of the continent less than 50.

That is less than the 250 millimeters that counts as a desert. So even though Antarctica is covered in ice, it is a desert. NASA says so too: Antarctica counts as a desert because it gets very little rain or snow.

The USGS recognizes polar deserts as areas with annual precipitation below 250 mm and a mean temperature in the warmest month below 10 °C; they cover nearly 5 million square kilometers, mostly bedrock or gravel plains, with snow dunes in places and freeze-thaw patterned ground up to 5 m across. Antarctica's ice-free Dry Valleys have been free of ice for thousands of years.

The Australian Antarctic Division calls Antarctica the coldest, windiest and driest continent. Snowfall is hard to measure in the wind, but average accumulation over the continent is estimated at about 150 mm of water a year, less than 50 mm over the high plateau, above 200 mm near most coasts and over 1,000 mm in one area near the Bellingshausen Sea. Dry, sinking air over the interior produces little cloud. By the 250 mm arid threshold, Antarctica's continental average of about 150 mm places it firmly among the deserts, despite its vast ice sheet; NASA and National Geographic both describe Antarctica as a desert for exactly this reason.

Some places are dry because of the wind.

Some are dry because tall mountains block the rain clouds. Some are next to cold seas. All of them keep the rain away.

Deserts form where something keeps rain away. The USGS describes several kinds.

Trade wind deserts, like the Sahara, sit where warm, dry winds clear away the clouds, so the sun beats down. Rain shadow deserts lie behind tall mountains: as air rises over the mountains, it drops its rain on one side, leaving the other side dry. Coastal deserts, like the Atacama in South America, lie beside cold ocean currents and are often covered in fog, yet get almost no rain. Other deserts lie far inside a continent, a long way from any ocean.

The USGS classifies deserts by location and dominant weather. Trade-wind deserts lie where trade winds, warming as they move toward the Equator, dissipate cloud; most major deserts, including the Sahara, which has reached 57 °C, are of this type. Midlatitude deserts, between 30° and 50° N and S, sit in interior basins far from oceans, like the Sonoran Desert. Rain-shadow deserts form on the lee side of tall ranges, where descending air has already lost its moisture, as behind the Tian Shan in China.

Coastal deserts on the western edges of continents near the tropics lie beside cold currents; upwelling produces winter fogs but little rain. The USGS calls the Atacama Earth's driest desert, where 1 mm or more of rain may fall as rarely as once every 5 to 20 years. Monsoon deserts, such as the Thar, lie downwind of ranges that strip moisture from monsoon winds, and polar deserts are the cold extreme.

In many hot deserts, days are very hot.

But nights can be very cold! The dry air cannot hold the heat. It escapes into the sky.

In many hot deserts, the temperature swings a long way between day and night. NASA's biome guide gives an average daytime temperature of about 38 °C, a little over 100 °F, but at night some deserts fall to about −4 °C, below freezing.

Why such a big change? NASA explains that the air is so dry that heat escapes quickly at night. The dry air cannot hold on to the warmth of the day, so it slips away into the sky. Temperatures also vary a great deal from one desert to another.

Dry air and clear skies make desert temperatures swing widely. NASA's desert biome page gives average daytime temperatures of about 38 °C and nighttime lows that in some deserts reach about −3.9 °C, because the air is so dry that heat escapes rapidly after sunset; temperatures also vary greatly with location.

At the extremes, the USGS notes that the Sahara has recorded temperatures as high as 57 °C, while the Australian Antarctic Division reports winter temperatures below −80 °C over Antarctica's elevated interior. Both are deserts: what they share is not temperature but a lack of precipitation.

Desert plants are good at saving water.

A cactus stores water inside its thick stem. Some plants grow, bloom and make seeds quickly, right after it rains.

Plants and animals that live in deserts have clever ways to cope with so little water. NASA describes some of them. Cacti store water in their thick stems and use it very slowly. Some bushes grow only a few leaves, so they lose less water, or have huge root systems to soak up every drop. Some desert plants live their whole life in just a few weeks, sprouting, flowering and making seeds while the rain lasts.

The USGS points out that deserts are fragile places, and that misuse of dry lands is a serious and growing problem in parts of the world.

Desert organisms are adapted to scarce and unpredictable water. NASA lists plant strategies: succulents such as cacti store water in their stems and use it slowly; shrubs conserve water with few leaves or gather it with extensive root systems; ephemeral annuals complete their life cycle in a few weeks during rainy periods.

The USGS emphasizes that deserts are fragile environments and that their misuse is a serious and growing problem in parts of the world. They are also valuable: deserts preserve fossils and human artifacts well because they are dry, hold mineral deposits, and serve as natural laboratories for studying wind on arid surfaces; the USGS notes that, apart from its poles, the entire surface of Mars is a desert.