A desert is an extremely dry area of land with extreme environments and sparse vegetation. They are likely to develop in areas where annual precipitation is less than 250 mm. It is one of Earth’s major types of ecosystems, supporting a community of distinctive flora and fauna specially adapted to the harsh environment. Geologists group deserts into five categories:

• Subtropical deserts: They are found along the Tropic of Cancer, between 15 and 30 degrees north of the Equator, or along the Tropic of Capricorn, between 15 and 30 degrees south of the Equator, where convection cells diverge. They are caused by the circulation patterns of air masses. Hot, moist air rises into the atmosphere near the Equator. As the air rises, it cools and drops its moisture as heavy tropical rains. The resulting cooler, drier air mass moves away from the Equator. As it approaches the tropics, the air descends and warms up again. The descending air hinders the formation of clouds, so very little rain falls on the land below. Subtropical deserts include the Sahara Desert in Northern Africa, the Kalahari Desert in Southern Africa, and the Tanami Desert in Northern Australia.
• Coastal Deserts: Cold Ocean currents contribute to the formation of coastal deserts. They are generally found on the western edges of continents near the Tropics of Cancer and Capricorn. Winter fog, produced by upwelling cold currents, frequently blankets coastal deserts and blocks solar radiation. The heavy fog drifts onto land. Although humidity is high, the atmospheric changes that normally cause rainfall are not present. For example, the cold Humboldt Current flowing from Antarctica to Southern Chile creates the Atacama Desert of South America, while the cold Benguela Current is critical to the formation of the Namib Desert.
• Rain Shadow Deserts: These deserts exist near the leeward slopes of some mountain ranges. When moisture-laden air hits a mountain range, it is forced to rise. The air then cools and forms clouds that drop moisture on the windward (wind-facing) slopes. When the air moves over the mountaintop and begins to descend the leeward slopes, there is little moisture left. The descending air warms up, making it difficult for clouds to form. For example, the Death Valley, in the U.S. is in the rain shadow of the Sierra Nevada mountains. Similarly, the Patagonian Desert lies in the rain shadow position on the leeward side of the Andes.
• Interior Deserts: They are found in the heart of continents. By the time air masses from coastal areas reach the interior, they lose all their moisture. They are sometimes called inland deserts. For example, the Gobi Desert in China and Mongolia.
• Polar Deserts: Parts of the Arctic and the Antarctic are classified as deserts. They are very dry, but not in the traditional sense. Unlike other types of deserts, they are cold all year round with frigid winters. Instead of sand, the surrounding surface is covered in layers of ice and snow. They experience little precipitation because, above 66°N and S latitude, there is very little moisture in the air due to extreme cold temperatures.

Deserts are found on every continent and cover about one-fifth of Earth’s land area. They are home to around 1 billion people-one-sixth of the Earth’s population. It has been observed that in semi-arid areas that border deserts, climate change and human activities have led to increased desertification.