Far from being barren wastelands, deserts are biologically rich habitats with a vast array of animals and plants that have adapted to the harsh conditions there. Some deserts are among the planet’s last remaining areas of total wilderness. Yet more than one billion people, one-sixth of the Earth’s population, actually live in desert regions. Deserts cover more than one fifth of the Earth’s land, and they are found on every continent. A place that receives less than 10 inches (25 centimeters) of ...

Far from being barren wastelands, deserts are biologically rich habitats with a vast array of animals and plants that have adapted to the harsh conditions there. Some deserts are among the planet’s last remaining areas of total wilderness. Yet more than one billion people, one-sixth of the Earth’s population, actually live in desert regions.

Deserts cover more than one fifth of the Earth’s land, and they are found on every continent. A place that receives less than 10 inches (25 centimeters) of rain per year is considered a desert. Deserts are part of a wider classification of regions called “dry lands.” These areas exist under a moisture deficit, which means they can frequently lose more moisture through evaporation than they receive from annual precipitation.

And despite the common conceptions of deserts as dry and hot, there are cold deserts as well. The largest hot desert in the world, northern Africa’s Sahara, reaches temperatures of up to 122 degrees Fahrenheit (50 degrees Celsius) during the day. But some deserts are always cold, like the Gobi desert in Asia and the desert on the continent of Antarctica. Others are mountainous. Only about 10 percent of deserts are covered by sand dunes. The driest deserts get less than half an inch (one centimeter) of precipitation each year, and that is from condensed fog not rain.

Desert animals have adapted ways to help them keep cool and use less water. Camels, for example, can go for days without food and water. Many desert animals are nocturnal, coming out only when the brutal sun has descended to hunt. Some animals, like the desert tortoise in the south-western United States, spend much of their time underground. Most desert birds are nomadic, crisscrossing the skies in search of food. Because of their very special adaptations, desert animals are extremely vulnerable to introduced predators and changes to their habitat.

Some of the world’s semi-arid regions are turning into desert at an alarming rate. This process, known as “desertification,” is not caused by drought, but usually arises from the demands of human populations that settle on the semi-arid lands to grow crops and graze animals. The pounding of the soil by the hooves of livestock may degrade the soil and encourage erosion by wind and water.

Which of the followings best replaces the word “precipitation” in the second paragraph?