Deserts form when a region receives very little precipitation over time, leaving too little water to replace what is lost through evaporation and plant transpiration. Although deserts are often associated with extreme heat, temperature alone does not determine whether a landscape becomes a desert. Some deserts are scorching, while others are cold, with winter snow and freezing temperatures. What they share is a persistent shortage of available moisture.
Two major processes explain why many of the world’s deserts exist where they do: global atmospheric circulation and rain shadows created by mountains. Atmospheric circulation influences where moist air rises, releases precipitation, and descends as dry air. Mountain ranges, meanwhile, can force moist air to shed much of its water on one side, leaving the opposite side considerably drier. Other factors, including distance from the ocean, cold ocean currents, and seasonal wind patterns, can reinforce these effects.
Together, these processes shape the distribution of deserts across continents and help explain why some regions remain dry for thousands or millions of years.
How atmospheric circulation creates dry regions
The atmosphere constantly moves heat and moisture around Earth. Solar energy warms the planet unevenly, with the tropics receiving more direct sunlight than the polar regions. This uneven heating drives large-scale circulation patterns that influence wind direction, rainfall, and the location of many deserts.
Near the equator, intense sunlight warms the surface and the air above it. Warm, moisture-laden air rises, expands as atmospheric pressure decreases with altitude, and cools. As it cools, water vapor condenses into tiny droplets or ice crystals, forming clouds. When these particles grow large enough, they fall as rain or snow.
This process helps make equatorial regions, including parts of the Amazon Basin and central Africa, exceptionally wet. The rising air loses substantial amounts of moisture as precipitation. Eventually, much of it moves away from the equator at high altitude and begins to descend in the subtropics, roughly between 20 and 35 degrees north and south of the equator.
Descending air is central to the formation of many major deserts.
Why sinking air suppresses rainfall
As air descends through the atmosphere, it encounters increasing air pressure and is compressed. Compression warms the air, even without additional heat entering from its surroundings. This is known as adiabatic warming.
Warmer air can maintain a higher concentration of water vapor before reaching saturation. As a result, the relative humidity—the amount of water vapor in the air compared with the maximum it could contain at that temperature—decreases as the air warms.
This process makes cloud formation less likely. Existing clouds may evaporate, and the atmosphere becomes less favorable for widespread precipitation.
The result is a broad belt of generally dry conditions across the subtropics. Many of the world’s largest hot deserts lie within these latitudes, including the Sahara in northern Africa, the Arabian Desert in the Middle East, and the deserts of Australia.
These regions are not dry simply because the sun is strong or the air is hot. Their dryness is closely connected to persistent atmospheric conditions that favor sinking air rather than the upward movement needed to produce rain.
The role of global circulation cells
Earth’s large-scale atmospheric circulation is often described using three connected systems in each hemisphere: the Hadley cell, the Ferrel cell, and the Polar cell. These are simplified models of broad atmospheric movements, not rigid compartments that remain unchanged throughout the year.
The Hadley cell is especially important for understanding subtropical deserts. Warm air rises near the equator, moves poleward high in the atmosphere, and gradually descends in the subtropics. Near the surface, some of that air flows back toward the equator, helping complete the circulation.
Earth’s rotation deflects moving air through the Coriolis effect. This helps produce the prevailing trade winds that blow toward the equatorial region and the prevailing westerlies found at higher latitudes.
The descending branch of the Hadley circulation creates a large-scale environment in which clouds and rain are often limited. However, the exact boundaries of these dry zones shift with the seasons, and regional weather systems can still bring storms and substantial rainfall.
This is why subtropical deserts are widespread but not uniform. Some areas experience occasional heavy rain, while others remain extremely dry for long periods.
How mountains create rain shadows
Atmospheric circulation explains many broad desert belts, but it does not explain every desert. Some of the world’s driest regions exist because mountains interfere with the movement of moist air.
A rain shadow is a relatively dry area on the downwind side of a mountain range. It forms when moist air encounters a mountain barrier and is forced upward.
As the air rises, it expands and cools. If it cools enough, water vapor condenses, clouds develop, and precipitation falls on the windward side of the mountains. Windward means the side facing the incoming prevailing wind.
After crossing the mountain crest, the air descends on the leeward side. Having lost some of its moisture, it becomes warmer as it is compressed by increasing atmospheric pressure. Relative humidity falls, making further cloud formation and precipitation less likely.
The land beyond the mountains therefore receives less moisture than the windward slopes. If this pattern persists and other sources of precipitation are limited, a dry region can develop.
The strength of a rain shadow depends on several factors, including the amount of moisture in the incoming air, the height and orientation of the mountains, the prevailing wind direction, and the stability of the atmosphere. Not every mountain range creates a major desert, and the leeward side is not always dry throughout the year.
Real-world examples of rain shadows
The western United States offers a clear example of how mountains influence precipitation. Moist air from the Pacific Ocean moves inland and encounters the Coast Ranges and the Sierra Nevada. These mountains force air upward, producing precipitation on many western slopes.
As air crosses the Sierra Nevada and descends toward the Great Basin, it becomes warmer and drier. The Great Basin, which includes much of Nevada and parts of neighboring states, contains extensive desert and semiarid landscapes influenced by this rain-shadow effect.
Farther north, the Cascade Range also produces strong differences in precipitation between its western and eastern sides. Western slopes receive substantial moisture from Pacific weather systems, while parts of eastern Washington and Oregon lie in a pronounced rain shadow.
The Andes Mountains provide another example. Moisture from the Atlantic and the Amazon Basin contributes to rainfall on the eastern side of the mountains. The high Andes limit the movement of much of this moisture toward the west. Combined with other atmospheric and oceanic influences, this helps create extremely dry conditions in parts of western South America, including the Atacama Desert.
In Asia, the Himalayas and surrounding mountain systems influence moisture transport into Central Asia and the Tibetan Plateau. Some interior regions remain dry partly because mountains obstruct moisture-bearing winds, although continental distance, atmospheric circulation, and seasonal wind patterns also play important roles.
These examples show that rain shadows are not isolated curiosities. They are a major part of the climate system, influencing vegetation, water availability, and human settlement over large areas.
Why some deserts form near oceans
It might seem that coastal regions should always be wet because they are close to a large supply of water. Yet some of the world’s driest deserts lie beside the ocean.
Cold ocean currents help explain this apparent contradiction. When relatively cool seawater chills the air above it, evaporation is often reduced compared with warmer ocean conditions. The lower atmosphere may become stable, meaning that air near the surface resists rising freely.
This stability can favor low clouds and fog without producing much rain. Moisture may be present in the atmosphere, but the conditions needed to lift it high enough for substantial precipitation are often missing.
The Atacama Desert along the Pacific coast of South America is strongly influenced by the cold Humboldt Current and persistent atmospheric conditions that inhibit rainfall. The Namib Desert in southwestern Africa is similarly influenced by the cold Benguela Current.
Both deserts demonstrate that humidity and rainfall are not the same thing. A coastal desert can experience fog and moist air near the surface while receiving very little measurable precipitation.
Cold currents are not the sole cause of these deserts. Large-scale atmospheric circulation and, in the case of the Atacama, the rain-shadow effects of the Andes also contribute to the region’s exceptional dryness.
Why distance from the ocean matters
Oceans supply much of the water vapor that eventually falls as precipitation over land. Air moving inland can transport this moisture, but rainfall patterns depend on the path the air takes, the terrain it encounters, and the weather systems that lift it.
As air masses move across continents, they may release moisture through repeated episodes of precipitation. Areas deep within continental interiors can therefore receive less precipitation than coastal regions, particularly when prevailing winds carry limited moisture inland.
This effect is known as continentality. It is especially important when combined with mountain barriers and circulation patterns that discourage rainfall.
Central Asia contains many dry regions influenced by this combination of factors. The farther moisture-bearing air must travel across land, and the more barriers it encounters, the more difficult it can be for sufficient moisture to reach some interior locations.
Distance alone does not guarantee aridity. Large interior regions can still be wet if prevailing winds and storm systems regularly deliver moisture. Continentality is best understood as one influence among several that determine a region’s water supply.
Why deserts are not always hot
The common image of a desert is a landscape of blazing sunshine, sand dunes, and extreme daytime heat. In reality, deserts are defined primarily by their lack of precipitation, not by temperature or the presence of sand.
The Sahara and Arabian deserts are predominantly hot deserts, but the Gobi Desert in Asia experiences cold winters and can receive snow. Parts of the Great Basin in the United States have cold winters, while the high-elevation deserts of the Andes experience substantial temperature variations.
Cold deserts can form when mountain barriers block moisture, when regions lie far from major moisture sources, or when large-scale atmospheric conditions limit precipitation. High elevation can further lower temperatures even where the climate remains dry.
Dry air also allows temperatures to change rapidly. Because the atmosphere contains relatively little water vapor and deserts often have limited cloud cover, the surface can lose heat quickly at night through radiation. This helps explain why some hot deserts have warm days and surprisingly cold nights.
The distinction matters because desert ecosystems must cope with both water scarcity and the temperatures associated with their particular location.
How scientists distinguish deserts from other dry landscapes
Deserts do not all receive the same amount of rainfall, and there is no single precipitation threshold used universally to define every desert. Scientists commonly describe aridity by comparing the water supplied through precipitation with the water that could potentially be lost through evaporation and plant transpiration.
This potential loss is called potential evapotranspiration. It reflects the atmospheric demand for water, which depends on factors such as temperature, sunlight, wind, and humidity.
When potential evapotranspiration greatly exceeds precipitation, the climate has a strong water deficit. Moisture becomes scarce for plants, streams may remain dry for long periods, and soils can accumulate salts where evaporation removes water but leaves dissolved minerals behind.
Semiarid regions occupy an intermediate position. They receive more moisture than the driest deserts but may still experience frequent droughts and limited plant growth.
The difference between deserts and semiarid environments is often gradual rather than sharply defined. Vegetation, soil characteristics, and seasonal rainfall patterns help reveal how water scarcity shapes the landscape.
How desert conditions shape ecosystems
A persistent shortage of water places strong limits on plant growth and animal activity. Desert organisms have evolved a variety of ways to survive, reproduce, and obtain moisture under these conditions.
Some plants develop extensive root systems that capture water from a broad area, while others grow deep roots that reach moisture stored below the surface. Certain plants store water in specialized tissues, and many reduce water loss through small leaves, protective coatings, or the timing of their growth and reproduction.
Some desert plants remain dormant during dry periods and grow rapidly after rainfall. Seeds may survive in the soil until sufficient moisture becomes available for germination.
Animals also show adaptations to water scarcity. Many are active during cooler nighttime hours, reducing water loss and exposure to daytime heat. Others obtain a substantial portion of their water from food or conserve moisture through highly efficient kidneys and other physiological mechanisms.
Desert landscapes are not necessarily lifeless. Their biological communities are shaped by the timing and reliability of water, and brief periods of rain can trigger rapid changes in plant growth and animal activity.
Water scarcity also influences soil formation, river systems, and the movement of sediment. Sparse vegetation leaves some surfaces vulnerable to wind erosion, while occasional intense storms can cause flash floods because dry or crusted ground may absorb water slowly.
Why desert boundaries can change
Although many deserts have persisted for long periods, their boundaries and levels of aridity can shift as climate conditions change.
Atmospheric circulation patterns vary over time, and the position of storm tracks can determine whether a region receives regular precipitation or prolonged drought. Changes in sea-surface temperatures can influence moisture transport, while changes in the strength or seasonal position of prevailing winds can alter rainfall.
Mountain ranges also affect climate differently when the dominant wind direction changes. A region that receives moisture under one circulation pattern may become substantially drier under another.
Over longer timescales, changes in Earth’s orbit, the distribution of continents, the height of mountain ranges, and the composition of the atmosphere have influenced global and regional climate. Some areas that are now desert have supported lakes, rivers, or more extensive vegetation during wetter periods.
Human activities can intensify land degradation in dry environments. Excessive groundwater extraction, poorly managed irrigation, and the removal of vegetation can reduce water availability or damage soils. However, these processes do not explain the original formation of major climatic deserts, which are primarily associated with atmospheric and geographic conditions.
Desertification—the degradation of land in dry regions—is also not identical to desert formation. It describes the loss of biological productivity and land quality, often through interacting climatic and human pressures, rather than the simple expansion of natural desert climates.
How the main causes work together
No single mechanism explains every desert. Large-scale atmospheric circulation creates broad zones where rainfall is naturally limited. Mountains intensify dryness in many locations by removing moisture from air before it reaches the leeward side. Cold ocean currents can stabilize the lower atmosphere and suppress rainfall, while distance from the ocean limits the moisture available to continental interiors.
These influences frequently overlap. The Atacama Desert, for example, is shaped by subtropical atmospheric conditions, the cold Humboldt Current, and the rain shadow of the Andes. The Great Basin reflects both the influence of Pacific mountain barriers and the broader circulation patterns of western North America.
Understanding these interactions explains why deserts occur in distinct geographic patterns rather than being distributed randomly across the planet.
Ultimately, deserts form where the long-term supply of precipitation is too small to meet the water demands of the landscape. Atmospheric circulation helps determine where air rises and rains and where it sinks and dries. Mountains redirect moisture, oceans influence the properties of the air above them, and continental geography shapes how far water vapor can travel inland. Together, these processes create the diverse dry environments found across Earth, from hot coastal deserts to cold interior basins and high mountain plateaus.