Why are deserts so dry?

A desert is defined by one thing above all: dryness. Whether it is a sea of sand dunes, a rocky plateau, or even a frozen polar plain, what makes a place a desert is that it receives very little precipitation, usually less than about 25 centimeters of rain a year. The dramatic landscapes and tough, specialized plants and animals of deserts are all shaped by this scarcity of water. But why do some regions get so little rain in the first place?

One of the biggest reasons is found in the way air circulates around the planet. Near the equator, the Sun's intense heat warms the air, which rises, cools, and drops heavy rainfall over the tropics. That air, now stripped of its moisture, travels away from the equator high in the atmosphere. At around 30 degrees latitude north and south, this dry air sinks back down toward the ground. As it sinks, it warms, and warm sinking air does not form clouds or rain. Many of the world's great deserts sit in these bands of descending dry air.

A second major cause is mountains. When moist air from the ocean is pushed up against a mountain range, it is forced to rise. As it rises it cools, and the moisture condenses and falls as rain on the side of the mountains facing the wind. By the time the air crosses over to the other side, it has lost most of its moisture. The dry region on the far side is said to lie in a rain shadow, and many deserts form in exactly these rain-shadow zones behind mountains.

There are still other ways deserts form. Some regions are simply very far inland, deep in the interior of a continent, so far from any ocean that the air reaching them has already lost its moisture along the long journey over land. In other places, cold ocean currents flowing along a coastline chill the air above them, and this cool, stable air resists rising, so it rarely forms rain. In every case, the result is the same: a land that receives little rain and becomes a desert.

This article was written using information from Wikipedia.