A desert is an area of land characterized by extremely low levels of precipitation, typically less than 250 millimeters (10 inches) annually, or where water loss to evaporation significantly exceeds precipitation. Arid conditions are shaped by natural phenomena influencing global weather patterns and local geography. This article explores the primary factors contributing to desert formation.
Global Air Circulation Patterns
Large-scale atmospheric movements play a significant role in desert distribution, particularly through Hadley Cells. These cells are massive convection currents that begin near the Earth’s equator, where intense solar heating causes warm, moist air to rise. As this air ascends, it cools, and the water vapor it carries condenses, leading to heavy rainfall and lush vegetation in tropical regions.
After releasing its moisture, the drier air flows poleward at high altitudes. This dry air descends around 30 degrees latitude in both the Northern and Southern Hemispheres. As the air sinks, it warms due to increased atmospheric pressure, reducing its relative humidity and enhancing its capacity to absorb moisture. This creates persistent belts of high atmospheric pressure, known as subtropical high-pressure belts or “horse latitudes,” which are largely cloud-free and receive minimal precipitation. Many of the world’s largest hot deserts, such as the Sahara and the Arabian Desert, are located within these zones due to this constant downward movement of dry air.
Geographic Barriers and Distance
Physical landscape features contribute to desert formation, often creating localized arid conditions. One prominent mechanism is the “rain shadow effect,” which occurs when mountain ranges block the path of moisture-laden air. As prevailing winds carry moist air towards a mountain, the air is forced upward, causing it to cool and expand. This cooling leads to the condensation of water vapor, resulting in clouds and precipitation on the windward side of the mountain.
Once the air passes over the mountain and descends on the leeward side, it has lost most moisture. As it descends, the air compresses and warms, reducing its relative humidity and increasing its ability to absorb moisture. This process creates a dry region on the leeward side, forming a rain shadow desert. Examples include deserts found east of the Sierra Nevada Mountains in California.
Beyond mountain barriers, distance from large bodies of water can also lead to desert conditions in continental interiors. Air masses originating over oceans gradually lose their moisture as they travel inland, releasing precipitation along the way. By the time these air masses reach continental interiors, they are significantly drier with little moisture left for rainfall. This remoteness from oceanic moisture sources contributes to the aridity of vast inland deserts, such as the Gobi Desert in Asia.
Influence of Ocean Currents
Cold ocean currents create coastal deserts, even near large bodies of water. These currents bring cool water from polar regions towards the equator, flowing along the western coasts of continents. As air masses move from the ocean towards the land, they pass over these cold currents. This interaction cools the air significantly, leading to stable atmospheric conditions that suppress vertical air movement necessary for cloud formation and rainfall.
Although the air above these cold currents may be humid, the stable conditions prevent the moisture from rising and condensing into rain-producing clouds. Instead, this cool, stable air often leads to coastal fog and mist, but very little rainfall. When this cool, dry air moves inland and warms, its capacity to hold moisture increases, drawing available moisture from the land. This results in arid coastal deserts, like the Atacama Desert in Chile and the Namib Desert in Africa, despite their proximity to the ocean.