Subtropical deserts are the large, persistently dry zones that straddle the tropics of Cancer and Capricorn, roughly between 15° and 30° latitude in both hemispheres. They form primarily because of a planet-scale atmospheric conveyor belt called the Hadley circulation, which lifts moist air near the equator, wrings out its moisture as tropical rain, and then dumps the now-dry air back down at subtropical latitudes. That sinking air suppresses cloud formation and rainfall so effectively that some of these deserts receive less than 25 millimeters of rain in an average year. The Sahara, the Arabian Desert, the Kalahari, and much of Australia’s interior all owe their existence to this process, but the details get more interesting once you look at how ocean currents, mountain ranges, and biology interact with that basic atmospheric engine.
The Hadley Cell and Why It Matters
Near the equator, intense solar heating warms the ground and ocean surface, which in turn heats the air above. That warm, humid air rises, cools as it gains altitude, and sheds its moisture as the heavy rainfall we associate with tropical rainforests. Once depleted, the now-dry air spreads poleward at high altitude, gradually losing energy and sinking back toward the surface around 20° to 30° latitude. This loop is the Hadley cell, and the descending branch is the single biggest reason subtropical deserts exist.
When air sinks through the atmosphere it compresses and warms, which lowers its relative humidity even further. The result is a persistent high-pressure zone at the surface that actively discourages clouds from forming. Rain-bearing weather systems are steered away, and whatever moisture does manage to drift in tends to evaporate before reaching the ground. This is why the world’s driest places cluster in two rough belts girdling the planet, one north and one south of the equator. The Sahara and Arabian deserts dominate the northern belt; the Namib, Atacama, and interior Australia anchor the southern one.
One way scientists quantify how dry a place is uses what’s called an aridity index: the ratio of rainfall to potential evapotranspiration (essentially how much water the atmosphere could suck out of the ground if it were available). Under a widely used classification, a ratio below 0.03 counts as hyper-arid, 0.03 to 0.2 is arid, and 0.2 to 0.5 is semi-arid.1PubMed Central. Version 3 of the Global Aridity Index and Potential Evapotranspiration Database The core of most subtropical deserts falls squarely in the arid or hyper-arid range, with potential evaporation outpacing rainfall by a factor of ten or more.
Cold Ocean Currents and Coastal Deserts
The Hadley cell sets the stage, but a second mechanism sharpens the aridity along western continental coasts. The Atacama Desert in South America and the Namib Desert in southwestern Africa are both positioned within the subtropical high-pressure belt, but they are also flanked by cold ocean currents, the Humboldt and Benguela respectively, that dramatically cool the adjacent sea surface.
Subtropical high-pressure systems drive winds that push warm surface water away from the coast. Cold, nutrient-rich water wells up from the deep to replace it. This chilled ocean cools the lowest layer of air over it, creating a cold, moist marine boundary layer capped by a temperature inversion. Warm, dry air sits on top and acts like a lid, trapping moisture near the surface and preventing it from rising high enough to form rain clouds.2ScienceDirect. A comparative study of the atmospheric water vapor in the Atacama and Namib Desert Fog banks are common along these coasts, and that fog becomes a crucial water source for organisms, but actual rainfall stays vanishingly low. Parts of the Atacama have gone decades without measurable rain.
The upshot is a coastal desert that is extremely dry yet surprisingly cool compared with inland subtropical deserts. Daytime temperatures in coastal Namib or Atacama locations can sit 10 to 15 degrees Celsius below what you’d experience at the same latitude in the Sahara. Fog and low stratus clouds block some solar radiation, adding to the coolness. It is a different sensory experience from the scorching inland deserts most people imagine, yet it is just as hostile to unassisted plant growth.
Rain Shadows and Mountain Barriers
Topography adds another layer of drying in many subtropical regions. When prevailing winds carry moist air toward a mountain range, the air is forced upward, cools, and drops its moisture as rain or snow on the windward side. By the time the air descends on the lee side, it is substantially drier. This rain-shadow effect doesn’t create subtropical deserts on its own, but it can amplify them considerably where the Hadley cell’s dry zone coincides with the lee side of a major mountain range.
The Andes provide a striking example. The central Andes, rising to over 6,000 meters in places, block moisture-laden air coming from both the Pacific and the Amazon Basin, intensifying the aridity of the Atacama to its west and the Monte Desert to its east. Research on fossil communities in the region has tied stronger rain-shadow effects in the central Andes to differences in regional moisture sources, suggesting the mountains have shaped local aridity patterns for millions of years.3PubMed. Body Size Regulates Niche Overlap Asymmetry in the Subtropical Andes Rain Shadow: Isotopic Paleoecology of Oligocene South American Ungulates Similar dynamics play out where the Atlas Mountains wall off the northern Sahara from Mediterranean moisture, and where the Western Ghats in India help create the Thar Desert’s rain shadow.
What the Land Surface Looks Like
People often picture subtropical deserts as endless sand dunes, but sand seas actually cover a relatively small fraction of most of these landscapes. Vast areas are covered by rocky plateaus, gravel plains, salt flats, and a feature called desert pavement: a tightly interlocking layer of pebbles and stones that armors the surface.
Desert pavements have long fascinated geologists. A detailed study in the central Sahara showed that many of the surface stones previously attributed to human activity were actually produced by in-situ weathering and wind abrasion over long timescales.4Journal of Arid Environments. Formation of desert pavements and the interpretation of lithic-strewn landscapes of the central Sahara Repeated cycles of erosion during wetter and drier periods throughout the Quaternary concentrated rock fragments at the surface, mixing them with occasional human artifacts but overwhelmingly reflecting natural geomorphic processes. The result is a hard, wind-polished surface that can persist for thousands of years, protecting the finer sediments beneath it from further erosion.
Elsewhere, the interplay of wind and scarce water creates distinctive landforms. Ephemeral streams called wadis (or arroyos, depending on the continent) carve channels that remain dry for most of the year but can flash-flood violently after rare storms. Alluvial fans spread out at the base of mountain ranges where these streams lose energy on flatter ground. Salt pans mark places where occasional standing water evaporated completely, leaving bright mineral deposits behind. Each of these features reflects the same underlying reality: water is the sculptor, but it visits only briefly.
Biological Soil Crusts
One of the least appreciated features of subtropical deserts is what covers the bare ground between plants. In many areas, the soil surface is bound together by a living community of cyanobacteria, algae, fungi, lichens, and mosses collectively called a biological soil crust (or biocrust). These organisms form a thin but remarkably functional layer that stabilizes the soil against wind erosion, fixes atmospheric nitrogen, redistributes water during rare rainfall events, and contributes to nutrient cycling.5Geological Society of America. Military Geosciences in the Twenty-First Century – Section: Role of biological soil crusts in desert hydrology and geomorphology
Biocrusts are fragile. A footprint, a tire track, or livestock hooves can break the crust and expose the underlying soil to wind and water erosion. Recovery times vary from years to decades depending on climate and composition. In some subtropical deserts, particularly where off-road vehicle traffic or overgrazing is common, biocrust destruction accelerates dust emission and degrades soil fertility in ways that compound over time. If you have ever noticed dark, bumpy patches on desert soil that look slightly different from the surrounding sand, you were probably looking at biocrust, and walking on it was doing more damage than it seemed.
How Plants Survive Extreme Aridity
Plants in subtropical deserts face a fundamental dilemma: photosynthesis requires opening tiny pores called stomata to absorb carbon dioxide, but every second those pores are open, water escapes. In a place where water may not arrive for months, that trade-off can be lethal. Many desert plants have evolved a workaround called Crassulacean acid metabolism, or CAM, which flips the schedule. CAM plants open their stomata at night, when temperatures are lower and humidity is higher, absorb carbon dioxide, and store it as organic acids. During the day, they close up and use the stored carbon for photosynthesis without losing water to the hot, dry air.6Aliso: A Journal of Systematic and Floristic Botany. Photosynthesis of Arid and Subtropical Succulent Plants
Many CAM plants are also succulents, meaning they store water in fleshy tissues. The water content of succulent desert plants can reach 90 to 95 percent, held in specialized water-storage cells called hydrenchyma. These plants tend to have a low surface-area-to-volume ratio, minimizing the skin through which water can escape while maximizing internal storage. The large cell vacuoles that help store water also happen to be useful for storing the organic acids central to CAM, so the two adaptations reinforce each other.7PubMed Central. Physiology, genomics, and evolutionary aspects of desert plants – Section: Development of succulent structure Some succulents are flexible enough to switch from CAM back to a more conventional photosynthetic mode when conditions improve, such as after a rare heavy rainfall.
Not every desert plant is a succulent, of course. Many shrubs have tiny, waxy, or reflective leaves that cut water loss. Some drop their leaves entirely during drought and photosynthesize through green stems. Others, like the desert flowering plant Cistanthe longiscapa in the Atacama, show considerable variation in their physiology across sites with different moisture levels, adjusting traits such as succulence and acid metabolism depending on local conditions.8PubMed. Cistanthe longiscapa exhibits ecophysiological and molecular adaptations to the arid environments of the Atacama Desert Annual wildflowers take a different approach altogether: their seeds can lie dormant in the soil for years, germinating only when rainfall exceeds a threshold, racing through their entire life cycle in weeks, and leaving behind the next generation of drought-proof seeds.
How Animals Handle the Heat
Desert animals face the same water and heat constraints as plants, but they can move. Many avoid the worst of the daytime heat simply by being nocturnal or crepuscular, retreating to burrows, rock crevices, or shade during peak sun. Burrows just a short distance underground can be 15 to 20 degrees cooler than the surface, making them effective refuges without any physiological specialization at all.
Larger mammals that cannot hide underground use a different suite of strategies. Research on free-living large mammals in arid zones has shown that many allow their body temperature to fluctuate more widely than their counterparts in milder climates, absorbing heat during the day rather than spending water to sweat it away, and then dumping the stored heat at night through radiation.9PubMed. Adaptation to heat and water shortage in large, arid-zone mammals Some species also employ selective brain cooling, shunting cooled venous blood from nasal passages toward the brain to keep it a few degrees below core body temperature. This lets the animal tolerate a higher overall body temperature without risking brain damage.
Reptiles, being ectotherms, regulate body temperature through behavior rather than metabolism, shuttling between sun and shade to stay within their functional range. But some have surprising physiological tricks as well. The Gila monster, a large lizard native to the Sonoran Desert, uses evaporative water loss through its cloaca as a cooling mechanism. Researchers found that cloacal water loss was negligible at moderate temperatures but spiked dramatically above 35°C, coinciding with a measurable suppression of body temperature below ambient levels.10PubMed. Cloacal evaporative cooling: a previously undescribed means of increasing evaporative water loss at higher temperatures in a desert ectotherm, the Gila monster Heloderma suspectum When the animals were dehydrated, this cooling response was delayed and weakened, underscoring how tightly water balance and thermoregulation are linked in desert life.
Why Some Subtropical Regions Aren’t Deserts
If the Hadley cell dumps dry air across the subtropics uniformly, you might expect an unbroken belt of desert at 25° latitude. That obviously isn’t what happens. Southeastern China, the southeastern United States, and parts of southeastern Brazil all sit at subtropical latitudes yet receive heavy rainfall. The difference comes down to moisture delivery from warm ocean currents and monsoon circulation patterns that override the Hadley cell’s drying influence.
Eastern sides of continents at subtropical latitudes often receive moist air from warm western boundary currents (like the Gulf Stream off the U.S. coast or the Kuroshio off East Asia). That moisture-laden air is drawn inland by seasonal pressure shifts, producing summer monsoons or convective storms that deliver far more water than the Hadley cell can suppress. Western continental coasts at the same latitudes, as we saw earlier, get the opposite deal: cold upwelling currents, stable high pressure, and almost no rain. The result is that subtropical deserts are heavily skewed toward the western and interior portions of continents, while eastern margins are often lush.
Subtropical Deserts in a Warming Climate
Climate models consistently project that the Hadley cell will expand poleward as greenhouse gas concentrations rise, pushing the subtropical dry zones into areas that currently receive enough rain to support agriculture and ranching.11Geophysical Research Letters. Annular modes and Hadley cell expansion under global warming Roughly half of that projected expansion is linked to shifts in large-scale atmospheric circulation patterns at higher latitudes, meaning the tropical and extratropical parts of the atmosphere are changing together in ways that reinforce drying.
The practical consequences are already visible in some regions. Parts of the Mediterranean basin, southern Australia, and the southwestern United States have experienced declining rainfall trends over recent decades that are consistent with poleward expansion of the dry zone. For communities living at the edges of existing subtropical deserts, this means aquifers recharge more slowly, growing seasons shorten, and the margin of error for rain-fed agriculture shrinks. Desertification, the process by which previously marginal land tips into desert conditions, is accelerated when the atmospheric circulation itself shifts the rainfall boundary.
How far and how fast the dry zones expand depends heavily on emission trajectories. Under higher-emission scenarios, the shift could push dryland boundaries several degrees of latitude poleward by the end of this century, potentially affecting hundreds of millions of people who currently live in semi-arid transitional zones. Under lower-emission paths, the expansion is smaller but still meaningful. Either way, understanding how subtropical deserts form is increasingly relevant to understanding where future water stress will hit hardest.
Common Misconceptions About Subtropical Deserts
A few widespread misunderstandings are worth clearing up. First, subtropical deserts are not the same as all deserts. Polar deserts (Antarctica, parts of the Arctic) and rain-shadow deserts (the Gobi, Patagonia) form through entirely different mechanisms. A desert defined by low precipitation alone can sit at any latitude. What makes subtropical deserts distinctive is their cause: the descending branch of the Hadley circulation and the persistent high-pressure systems it produces.
Second, “subtropical” does not mean mild. Some subtropical deserts record surface temperatures above 50°C in summer and can drop below freezing on clear winter nights. The subtropical label refers to latitude, not to a comfortable climate. Interior deserts like the Sahara and the central Australian outback experience some of the most extreme temperature swings on Earth, precisely because dry air and clear skies allow rapid heating during the day and rapid radiative cooling after dark.
Third, subtropical deserts are not biologically barren. As the sections on biocrusts, CAM plants, and animal thermoregulation illustrate, these environments support a highly specialized community of organisms. Biodiversity per square kilometer is low compared with a rainforest, but the adaptations on display are among the most inventive in the living world, and the ecosystems that result are surprisingly sensitive to disturbance. Walking off-trail across what looks like empty gravel may crush decades of biocrust growth, and a single badly timed flash flood can reshape a wadi channel that local wildlife depends on.