Desert Formation: Why Some Regions Become Dry

Deserts form where a region receives too little usable moisture over long periods. The cause may be sinking dry air, mountain barriers, cold ocean currents, great distance from moisture sources, or extreme cold. Many deserts develop through a combination of these processes working together.

Low rainfall is only part of the story. A place can also become very dry when evaporation and potential water loss remain much greater than precipitation. This water imbalance helps explain why deserts occur beside oceans, behind snowy mountains, deep inside continents, and even in polar regions.

• Subtropical circulation creates broad belts of sinking, dry air.
• Mountains can remove moisture from air before it reaches their leeward side.
• Cold ocean currents can suppress the rising air needed for substantial rainfall along some coasts.
• Continental interiors may be too far from reliable ocean moisture.
• Polar environments receive very little precipitation because cold air contains little water vapor.

What Makes a Region Dry Enough to Become a Desert?

A desert is defined mainly by persistent lack of precipitation, not by heat or sand. A commonly used descriptive threshold is around 250 millimeters (10 inches) of precipitation per year, although real desert boundaries are also identified through vegetation, climate, evaporation, and long-term water balance.

This is why both the Sahara and Antarctica can be described as deserts. Their temperatures could hardly be more different, yet both receive very little precipitation across large areas. Desert climate is therefore better understood as a moisture problem than a temperature category.

Scientists also use the aridity index, which compares precipitation with potential evapotranspiration. Potential evapotranspiration represents how much water could be transferred from the surface and plants to the atmosphere if sufficient water were available. When atmospheric demand stays far above the available precipitation, the landscape has a strong tendency toward aridity.

The Main Ways Deserts Form

Earth’s deserts do not share one origin. Their locations reflect interactions among global atmospheric circulation, mountains, oceans, latitude, elevation, and distance from moisture sources. The same dry region may be influenced by several of these controls at once.

Subtropical High Pressure Creates Broad Dry Belts

Many of the world’s large hot deserts occur near the subtropics because of large-scale atmospheric circulation. Air heated in the tropical zone rises, carries moisture upward, and produces heavy rainfall around the Intertropical Convergence Zone. Higher in the atmosphere, part of that air moves toward higher latitudes before descending again.

The descending portions of the Hadley circulation are commonly found around the subtropics. Sinking air becomes compressed and warmer, lowering its relative humidity. This stable air makes the sustained upward movement needed for deep clouds and frequent rainfall harder to develop.

The result is a broad association between subtropical high-pressure zones and deserts such as the Sahara and Arabian Desert. The pattern is not a perfect latitude line. Continents, monsoon systems, oceans, mountain ranges, and seasonal circulation shift the wet and dry zones considerably.

Research into the Sahara also shows why the simple textbook picture should not be stretched too far. The average Hadley circulation helps explain the global dry belt, but regional atmospheric dynamics and nearby monsoon circulation also influence where sinking air becomes strongest and how rainfall changes through the year.

Mountains Create Rain Shadows

A mountain range can produce dry land even when a major source of ocean moisture lies nearby. When prevailing winds push moist air toward high terrain, the air is forced upward. It expands and cools, allowing water vapor to condense. Rain or snow then falls on the windward side.

After crossing the crest, the air descends toward lower elevations. It warms as atmospheric pressure increases and its relative humidity falls. Because part of the original moisture has already been lost, the leeward side can receive far less precipitation. This dry zone is called a rain shadow.

A mountain range works somewhat like a wringer placed in the path of moving moist air: much of the water is released during the climb, and the air reaching the far side is considerably drier. The atmosphere is more complicated than a sponge, but the comparison captures why two places separated by one mountain chain can have sharply different rainfall totals.

The Patagonian Desert east of the Andes is a clear example. Moisture-bearing westerly air from the Pacific crosses the Andes and loses much of its moisture on the western side. Dry descending air then reaches the Argentine side. The Sierra Nevada produces a related pattern in parts of the western United States, including the dry Basin and Range region.

Cold Ocean Currents Can Dry a Coast

Being next to an ocean does not guarantee rain. Some of Earth’s driest landscapes lie directly beside the sea because cold ocean currents cool and stabilize the lower atmosphere.

When air near the ocean surface is chilled, vigorous upward convection may be limited. Moisture can remain trapped at low altitude as fog or low cloud rather than rising high enough to create frequent heavy rain. Coastal deserts can therefore be humid near the surface yet extremely rain-poor.

The Benguela Current contributes to the dryness of the Namib along southwestern Africa. NASA notes that the cold current suppresses rainfall while also helping produce fog that supplies moisture to parts of the desert ecosystem.

The Atacama Desert presents an even more layered case. The cold Humboldt Current along the Pacific coast, subtropical atmospheric conditions, high terrain, and the Andes all contribute to its extreme aridity. Its dryness cannot be assigned to one cause alone.

Continental Interiors Lose Access to Moisture

Some dry regions lie so far from reliable ocean moisture that air masses reaching them have already crossed enormous stretches of land and mountain terrain. This effect is known as continentality.

Ocean water is the main source of atmospheric moisture, but that moisture does not travel evenly across continents. Storm systems may release precipitation along coasts and mountain slopes long before reaching an inland basin. Farther inland, the remaining air can be much drier.

Central Asia contains some of the clearest examples. The Gobi and Taklamakan are separated from major moisture sources by vast distances and high mountain systems. Rain-shadow effects reinforce that inland position, producing broad areas with sparse precipitation and large seasonal temperature changes.

Interior basins can further reinforce visible desert conditions. Water that does arrive may flow toward a low basin without reaching the sea. After it evaporates, salts remain behind. Repeated cycles can create playas, salt flats, saline lakes, and dry lake beds.

Polar Regions Can Be Deserts Too

Desert formation does not require hot temperatures. Very cold air carries little water vapor, which limits the amount of precipitation available in polar environments.

Large areas of Antarctica receive remarkably little snow each year. Snow and ice remain because temperatures keep melting and evaporation low, not because precipitation is abundant. This produces a landscape that is simultaneously icy and extremely dry.

There is an important classification detail here. Polar deserts can be called deserts under precipitation-based geographic definitions, while some international dryland classifications exclude polar and subpolar regions when calculating aridity-based dryland statistics. The exact area described as “desert” therefore depends on the definition being used.

Why Several Causes Often Work Together

The driest places tend to be easier to understand when several processes are considered together. Atmospheric circulation sets the broad climate pattern, while terrain and oceans reshape it regionally. Local elevation, groundwater, soil, and basin geometry then determine what the landscape looks like on the ground.

How Desert Landscapes Develop After the Climate Turns Dry

Dry climate alone does not automatically produce dunes. Once moisture becomes scarce, the appearance of a desert depends heavily on geology, sediment supply, wind, slope, and occasional runoff.

Where loose sand is abundant and winds repeatedly move it, dunes can develop. Other deserts expose broad gravel plains or bedrock because fine particles have been removed. Closed basins may accumulate salts and clay after temporary lakes evaporate.

• Sand seas develop where enough movable sand and persistent wind are available.
• Desert pavement can form where finer sediment is removed or redistributed, leaving closely packed stones at the surface.
• Alluvial fans build where short-lived flows carry sediment out of mountains and spread it across basin margins.
• Playas and salt flats occupy low areas where water temporarily collects and later evaporates.
• Dry valleys and channels record rainfall events that may be rare but locally powerful.

This explains why identifying a desert from satellite imagery is not as simple as searching for yellow sand. Some of the world’s driest terrain is rocky, salty, icy, or covered by sparse shrubs rather than dunes.

Desert Formation and Desertification Are Different Processes

Natural desert formation describes the climatic and geographic processes that create persistently arid environments. Desertification has a different meaning.

The United Nations Convention to Combat Desertification defines desertification as land degradation in arid, semi-arid, and dry sub-humid areas resulting from climatic variations and human activities. It does not simply mean that an existing desert is spreading outward like a moving wall of sand.

Poor land management, vegetation loss, soil erosion, unsuitable irrigation, and prolonged climatic stress can reduce the productivity of vulnerable drylands. A degraded dryland may become more desert-like in appearance, but that process should not be confused with the geological and atmospheric origins of natural deserts such as the Sahara, Namib, Gobi, or Atacama.

Drying Trends Do Not Mean Every Drying Region Is Becoming a Desert

Long-term aridity is changing in many regions. A 2024 UNCCD assessment reported that 77.6% of Earth’s land experienced a drier climate during 1991–2020 than during 1961–1990. Over those periods, drylands expanded from 37.5% to 40.6% of global land area excluding Antarctica, an increase of about 4.3 million square kilometers.

Those numbers describe shifts in long-term aridity, not the instant creation of new sand deserts. A region can cross an aridity threshold while still supporting farms, grasslands, shrublands, towns, or forests adapted to relatively dry conditions. The ecological response depends on how rainfall, temperature, evaporation, soils, vegetation, and water management change together.

Aridity also differs from drought. A drought is a temporary period when precipitation falls below the normal level for a location. Aridity is a persistent climatic condition. A normally humid region can experience drought without becoming a desert, while an established desert can experience an unusually wet year without losing its desert climate.

Examples That Show Desert Formation in Real Places

Individual deserts show how the main drying mechanisms overlap rather than operating as isolated categories.

• Sahara: Subtropical atmospheric circulation promotes persistent dryness across much of North Africa, while regional circulation and monsoon behavior help shape rainfall patterns within and around the desert.
• Namib: The cold Benguela Current helps suppress rainfall along the southwestern African coast, even though fog can provide local moisture.
• Atacama: Cold Pacific water, atmospheric subsidence, the Andes, and limited access to moisture combine to produce extreme dryness in northern Chile.
• Patagonian Desert: Pacific air loses much of its moisture while crossing the Andes, leaving broad dry country on the eastern side of the range.
• Great Basin: The Sierra Nevada blocks much Pacific moisture, while interior location and enclosed drainage contribute to dry valleys and salt basins.
• Gobi: Great distance from oceans and surrounding mountain systems limit moisture, producing a cold continental desert with large seasonal temperature changes.
• Antarctica: Very low precipitation produces polar-desert conditions even though much of the continent is covered by ice.

How to Recognize Desert-Forming Conditions on a Map

Several geographic clues can reveal why a dry region exists before annual rainfall figures are even examined. They do not prove desert climate by themselves, but together they make the controlling processes easier to identify.

• A broad desert belt near 20°–35° latitude may point toward subtropical high-pressure influence.
• A dry basin immediately behind a high mountain chain may indicate a rain shadow.
• An arid western coastline beside a cold eastern-boundary ocean current may indicate coastal-current influence.
• A dry region thousands of kilometers from an ocean often shows a strong continental interior effect.
• Salt lakes, playas, and internal drainage commonly indicate that water reaches a basin but does not regularly escape to the sea.
• A dry landscape at very high latitude may result from low precipitation in a cold atmosphere rather than strong heat.

What Scientists Still Measure Carefully

Desert boundaries are not always sharp. Rainfall can vary greatly from year to year, vegetation may respond slowly, and different scientific classifications do not use exactly the same thresholds. A climatic desert boundary can therefore differ from a vegetation map or an international dryland classification.

The causes of individual deserts can also be difficult to separate quantitatively. The Atacama, for example, reflects interactions among ocean circulation, atmospheric pressure, Andean topography, elevation, and long-term geological change. Researchers continue to examine how much each process contributed during different periods of its history.

Future changes contain similar regional uncertainty. Atmospheric circulation can shift, ocean temperatures change, mountains remain fixed on human timescales, and warming raises evaporative demand. That means a global drying trend will not produce the same response everywhere.

Questions About Desert Formation

Sources

1. U.S. Geological Survey – Types of Deserts. The USGS explains trade-wind, midlatitude, rain-shadow, coastal, and polar desert types and connects each type with the atmospheric or geographic process that produces dryness.
2. U.S. Geological Survey – How the Atmosphere Influences Aridity. This federal science source explains descending subtropical air and the relationship between global atmospheric circulation and the location of many nonpolar deserts.
3. Met Office – Global Circulation Patterns. The UK’s national weather service describes Hadley cells, the ITCZ, and the sinking subtropical air associated with high-pressure regions and many hot deserts.
4. Deserts of the World – How Do Deserts Form? 5 Ways a Desert Is Created. This specialist desert reference compares major formation pathways, including subtropical high pressure, rain shadows, cold-current coasts, continental interiors, and polar dryness, with examples from deserts around the world.
5. NASA – Namibia’s Coastal Desert. NASA identifies the cold Benguela Current as a major influence on the Namib’s low rainfall and explains why fog can occur along an exceptionally dry coast.
6. United Nations Convention to Combat Desertification – UNCCD FAQ. UNCCD provides the formal distinction between desertification, natural desert environments, drought, climatic variation, and human-driven land degradation.
7. UNCCD – The Global Threat of Drying Lands. This 2024 scientific assessment reports recent global changes in aridity, including the expansion of drylands between the 1961–1990 and 1991–2020 climate periods.
8. Quarterly Journal of the Royal Meteorological Society – Monsoons and the Dynamics of Deserts. This peer-reviewed study examines why subtropical desert climates cannot always be explained by a simple annual-average Hadley-cell model and explores regional atmospheric controls on desert dryness.
9. International Journal of Climatology – The Central Andean West-Slope Rainshadow and Its Potential Contribution to the Origin of Hyper-Aridity in the Atacama Desert. This peer-reviewed research examines the Andean rain-shadow contribution to the extreme aridity of the Atacama region.
10. National Geographic Education – Rain Shadow. This reference explains the windward-to-leeward sequence of rising, cooling, precipitation, descent, and warming that produces dry terrain behind mountain ranges.