Deserts are dry or arid areas that receive less than 250 mm of rain each year. Deserts can be hot or cold. They contain plants and animals that are specially adapted to these extremely dry conditions.
Most hot deserts are found between 15-30° north and south of the equator.
What causes deserts to form?
Air around the Tropics of Capricorn and Cancer is dry. This is a zone of high air pressure where the air sinks. Air at the equator rises and cools - condensation then forms rain. The air then moves north and south until it gets to about 30° north and south of the equator, where it sinks. This air is dry and no condensation can form, so there is no rain. This is known as the Hadley Cell. It shows how air moves around the atmosphere near the equator and tropics.
Some deserts are found on the western edges of continents. They are caused by cold ocean currents, which run along the coast. They cool the air and make it harder for the air to hold moisture. Most moisture falls as rain before it reaches the land, eg the Namib Desert in Africa.
Some deserts form in the rain shadow of mountains, eg the Atacama Desert is located in the rain shadow of the Andes. Air is forced to rise over mountains, air cools and condensation occurs, rain falls over the mountains, dry air sinks down the other side of the mountain.
Some deserts form in areas that lie at great distances from the sea. The air here is much drier than on the coast.