Deserts are landscapes defined by extremely low precipitation, typically receiving less than 250 millimeters (10 inches) of rainfall annually. While often associated with scorching heat and vast sand dunes, deserts result from specific natural processes that shape Earth’s climate and geography. Understanding these processes reveals how diverse environments, from arid plains to ice-covered poles, can share the common characteristic of aridity.
Global Air Circulation Patterns
Many of the world’s major hot deserts form due to large-scale atmospheric movements, specifically Hadley cells. These circulation patterns begin at the equator, where intense solar radiation warms the surface and causes moist air to rise. As this warm, humid air ascends, it cools and expands, leading to condensation and abundant rainfall. This process accounts for the lush rainforests found near the equator.
Once the air releases its moisture, it flows horizontally away from the equator, both north and south. Around 30 degrees latitude in both hemispheres, this dry, cool air begins to descend. As the air sinks, it warms due to compression, further reducing its relative humidity and making it highly stable.
This descending dry air creates persistent high-pressure zones at these subtropical latitudes. The stable atmospheric conditions within these zones inhibit cloud formation and precipitation, leading to extensive arid regions. The Sahara Desert, for instance, exemplifies a subtropical desert formed by Hadley cell dynamics.
Mountain Barriers and Ocean Influence
Geographic features like mountain ranges and ocean currents also play a significant role in creating arid conditions. When moist air encounters a mountain barrier, it is forced to rise, a process known as orographic lift. As the air ascends, it cools, and its moisture condenses, falling as rain or snow on the windward side. This results in lush vegetation on the side facing prevailing winds.
After shedding its moisture, the dry air descends on the leeward side of the mountain range, warming as it moves downwards. This warming further reduces the air’s humidity, creating a dry region known as a rain shadow. Deserts like the Mojave, situated in the rain shadow of the Sierra Nevada mountains, are prime examples.
Cold ocean currents contribute to coastal desert formation by cooling the air directly above them. This cool air is very stable, preventing the vertical air movement necessary for cloud formation and precipitation. While fog might be common, significant rainfall is rare. This mechanism is responsible for some of the driest places on Earth, such as the Atacama Desert and the Namib Desert. Additionally, areas deep within continental landmasses, far from oceanic moisture sources, can experience arid conditions due to a lack of atmospheric moisture, known as continentality.
The Unique Case of Polar Deserts
The Earth’s polar regions, despite vast ice sheets, are classified as deserts due to extremely low precipitation. These areas experience intensely cold temperatures, which significantly limit the amount of moisture the air can hold. Even though ice and snow are abundant, new precipitation, often fine snow or ice crystals, is minimal.
Much of the existing ice and snow accumulates over long periods or results from sublimation, where ice directly turns into water vapor. Similar to hot deserts, high-pressure systems are often present over the poles. These systems feature descending dry air, further suppressing significant precipitation. This combination makes the Arctic and Antarctic regions some of the driest places on Earth.