The desert biome covers about one-fifth of Earth’s surface. This biome has a layer of soil that can either be sandy, gravelly or stony, depending on the type of desert. Deserts get less than 25 centimeters (10 inches) of rainfall a year, and the organisms that live in deserts are adapted to this extremely dry climate.

The four main types of desert include hot and dry deserts, semi-arid deserts, coastal deserts and cold deserts. In hot and dry deserts, also known as arid deserts, the temperatures are warm and dry year round. Some famous arid deserts include the Sahara Desert that covers much of the African continent and the Mojave Desert located in the southwest of the United States. Semi-arid deserts are a bit cooler than hot and dry deserts. The long, dry summers in semi-arid deserts are followed by winters with some rain. Semi-arid deserts are found in North America, Greenland, Europe and Asia. Coastal deserts are a bit more humid than other types of deserts. Although heavy fogs blow in from the coast, rainfall is still rare. The Atacama Desert of Chile in South America is an example of a coastal desert. Cold deserts are still dry but have extremely low temperatures in comparison to the other types of deserts. The Antarctic is an example of a cold desert.

Plants in deserts have adaptations to conserve water. For example, cacti have enlarged stems to store water, as well as spines to protect these water reserves from thirsty animals. Animals such as the black-tailed jackrabbit (Lepus californicus) are also adapted to life in the deserts of North America, such as the Sonoran Desert, Mojave Desert and Chihuahuan Desert: Their extra-long ears help to transfer out excess heat from their body into the air.

Many people also live in deserts around the world. The Arabic-speaking Bedouin tribes of the Middle East and North Africa and the Tamazight-speaking Amazigh peoples of North Africa have historically practiced nomadic pastoralism in the deserts of these regions, but many people of both groups live settled lifestyles. Densely populated metropolitan cities can also flourish in deserts. Cairo, the capital of Egypt, is located in the Sahara, the world’s largest hot desert. It is one of the largest cities in Africa and the largest desert city, with a population of over 22 million people. The second-largest desert city is Lima, the capital of Peru. As a result of its geographic features, Lima is a coastal desert in a tropical region. The Andes Mountains to the east of the city block moist trade winds from the Pacific and create a rain shadow effect, keeping Lima from getting much rainfall despite its tropical latitude. Meanwhile, the cold Humboldt Current along the western coast cools the air, preventing typical tropical temperatures. Rain is very rare in Lima, yet the cool air and ocean mist make the city humid and foggy.