The Atacama Desert is dry because it sits at the intersection of three powerful drying mechanisms that reinforce one another: a semi-permanent high-pressure system that pushes sinking air over the region, the cold Humboldt Current running along the coast that chills moisture out of the atmosphere before it can reach land, and the Andes Mountains rising sharply to the east, blocking humid air from the Amazon basin. Any one of these would create arid conditions. Together, they produce a place where some weather stations have never recorded measurable rainfall, and where parts of the landscape have been functionally rainless for millions of years.
The Subtropical High-Pressure System
The single biggest driver of Atacama aridity is its position beneath the southeastern Pacific subtropical anticyclone, a broad zone of high atmospheric pressure that sits over the ocean off South America’s western coast. Air within this system sinks from high altitude toward the surface, warming and drying as it descends. Because it is sinking, it resists being displaced by weather fronts that might carry moisture. The result is a stable atmospheric lid that blocks storms approaching from the Pacific and suppresses the kind of convective rising air that produces rain clouds elsewhere in the tropics.
This pattern is not unique to the Atacama; similar high-pressure cells create deserts in other subtropical zones around the world. But what makes the Atacama extreme is that the anticyclone here is especially persistent and powerful, reinforced by other local factors. The high-pressure system drives strong equatorward winds along the coast, which in turn set off a chain of oceanic effects that amplify the dryness even further.
The Humboldt Current and Coastal Cooling
Those equatorward winds generated by the subtropical high sweep cold water northward from the Southern Ocean along the Chilean and Peruvian coast. This is the Humboldt Current (sometimes called the Chile-Perú Current), and it makes the sea surface along the Atacama coast far colder than you would expect at these latitudes. The winds also force deeper, even colder water to rise to the surface through a process called upwelling.
Cold ocean water has a dramatic effect on the air directly above it. It chills the lowest layer of the atmosphere, creating a strong temperature inversion: a warm, dry layer sits on top of a cool, moist layer near the surface. This inversion acts like a cap, trapping moisture close to the ocean and preventing it from rising high enough to form rain clouds. Instead, the trapped moisture condenses into a persistent deck of low-lying stratus clouds and fog that hovers over the ocean but rarely penetrates far inland.
Research on fog and low cloud frequency along the Chilean coast has found a strong link between this thermal inversion and the presence of the cloud layer, with the inversion’s strength driven by sea surface temperature and atmospheric sinking from above.1Atmospheric Research. Synoptic control of the spatiotemporal variability of fog and low clouds under ENSO phenomena along the Chilean coast (17°-36° S) The cooling of the lower atmosphere by the Humboldt Current is then compensated by even more subsidence along the coast, creating a feedback loop that makes conditions drier still.2Earth and Planetary Science Letters. Andean uplift, ocean cooling and Atacama hyperaridity: A climate modeling perspective
The Andes as a Wall Against Moisture
To the east of the Atacama, the Andes rise to peaks above 6,000 meters. This massive mountain barrier intercepts nearly all moisture traveling westward from the Amazon basin and the South American interior. Humid air masses that form over the tropical lowlands are forced upward as they approach the Andes, cooling and releasing their moisture as rain or snow on the eastern slopes. By the time the air crosses the mountain crest and descends toward the Atacama, it has been wrung nearly dry. This is the classic rain shadow effect, and the Andes produce one of the most complete rain shadows on Earth.
A study comparing the Atacama to the Namib Desert in southwestern Africa illustrates just how complete this barrier is. In the Namib, easterly winds can carry moisture from the African interior over the escarpment and into the coastal desert, especially during summer. In the Atacama, the Andes block any equivalent exchange between the continent’s interior and the coastal atmosphere. The result is that the free troposphere above the Atacama stays drier than the Namib’s for most of the year.3Global and Planetary Change. A comparative study of the atmospheric water vapor in the Atacama and Namib Desert The Namib is extremely dry by any normal standard; the Atacama is drier because it lacks even this intermittent continental moisture supply.
Millions of Years in the Making
The Atacama is not just dry in the way that other deserts are dry. Parts of it have been hyperarid, meaning less than about one millimeter of rain per year, for an almost incomprehensible stretch of time. Isotopic analysis of ancient carbonate deposits in the Calama Basin indicates that hyperaridity began there during the middle to late Miocene, roughly 10 to 15 million years ago, driven by the rise of the Andes to elevations above two kilometers.4Earth and Planetary Science Letters. Evidence for the development of the Andean rain shadow from a Neogene isotopic record in the Atacama Desert, Chile Separate geological work confirms this timeline, showing that the uplift blocked summer monsoon moisture from the east and triggered the drastic drop in precipitation.5Geology. Neogene climate change and uplift in the Atacama Desert, Chile
One striking measure of how long the extreme dryness has persisted comes from organic chemistry. Researchers have found lipid molecules in Atacama soils that retain their original chemical structure, including fragile functional groups and unsaturated bonds, despite being deposited between 40,000 and 2 million years ago. Normally, microbial activity breaks down such molecules rapidly. The fact that they survive intact suggests that the soil has been too dry to support even basic microbial decomposition for extraordinarily long periods, a preservation process researchers call “xeropreservation.”6Organic Geochemistry. Xeropreservation of functionalized lipid biomarkers in hyperarid soils in the Atacama Desert
The extreme dryness has also allowed unusual chemical deposits to accumulate on the surface. Natural perchlorate, for example, is found in Atacama soils in concentrations far higher than anywhere else on Earth. Isotope data support the idea that this perchlorate was deposited from the atmosphere over millions of years and simply never washed away, because there has been so little rain to dissolve it.7Environmental Research. Perchlorate contamination in Chile: Legacy, challenges, and potential solutions
When It Does Rain
The Atacama is not absolutely rainless everywhere and always. Rare but sometimes intense rainfall does occur, almost exclusively tied to El Niño events. During an El Niño, the warming of eastern Pacific waters disrupts the normal atmospheric patterns that keep the desert dry. The 1982–83 El Niño, estimated at the time as the most severe of the century, pushed rain southward along the Peruvian coast and into northern Chile, with sporadic showers continuing for months. Botanical fieldwork afterward found a rich bloom of desert plants, with species diversity and density that had not been observed since a major El Niño in 1925.8Elsevier Oceanography Series. The Botanical Response of the Atacama and Peruvian Desert Floras to the 1982-83 El Niño Event
These events can produce astonishing transformations. Dormant seeds that have lain in the soil for years or decades germinate rapidly after rare rainfall, carpeting normally barren ground with wildflowers in a phenomenon Chileans call the “desierto florido” (flowering desert). But the blooms are brief, and the underlying aridity reasserts itself quickly.
Pollen records preserved in ancient rodent middens tell a longer story. Over the past 16,000 years, the Atacama has gone through periods of wetter conditions, particularly between roughly 13,000 and 9,000 years ago, followed by extreme drying around 8,000 years ago. Conditions similar to the present hyperaridity became established around 4,000 years ago.9Journal of Quaternary Science. Late Quaternary environmental dynamics in the Atacama Desert reconstructed from rodent midden pollen records
Fog and the Thin Line of Life Along the Coast
Although the rain shadow and atmospheric sinking keep rain away, the cold ocean does produce one form of moisture that reaches land: fog. The same stratus cloud deck trapped under the inversion layer sends fingers of fog, known locally as “camanchaca,” rolling into coastal hills and valleys. This fog is the primary water source for native plants and biological soil crusts in the Atacama’s coastal zone.10Journal of Geophysical Research: Biogeosciences. Relative humidity patterns and fog water precipitation in the Atacama Desert and biological implications
The biological effect is measurable. In the hyperarid core of the desert, when annual rainfall drops below about one millimeter, colonization of rocks by photosynthetic microorganisms drops to virtually zero. But along the coast at the same latitude, where fog provides moisture even though rain is essentially absent, researchers have found complex communities of cyanobacteria, archaea, and other bacteria living on the undersides of translucent quartz stones. Colonization rates in these fog-fed zones are significantly higher than in the rainless interior.11PubMed Central. Hypolithic cyanobacteria supported mainly by fog in the coastal range of the Atacama Desert Fog, then, draws a narrow biological boundary along the coast: life clings where marine moisture reaches, and drops away sharply once it no longer can.
Ancient Groundwater Beneath the Surface
The Atacama’s surface is parched, but it is not entirely without water. Beneath the desert floor, groundwater systems hold water that fell as rain thousands or even millions of years ago. Isotopic analysis of water in the Salar de Atacama watershed reveals that nearly all of the inflowing groundwater is “fossil water,” recharged during wetter periods in the distant past rather than by modern precipitation. The isotopic signatures of these ancient waters are distinctly different from water falling on the Andean plateau today.12Water Resources Research. Stable and Radioisotope Systematics Reveal Fossil Water as Fundamental Characteristic of Arid Orogenic‐Scale Groundwater Systems
Geological evidence suggests that the most significant groundwater recharge events occurred during the late Pleistocene, when streams in the hyperarid zone carried perennial flow. These deposits likely represent the most important recharge events of the last 18,000 years, with a lesser recharge episode occurring during the Medieval period, roughly 700 to 1,070 years ago, when phreatophytic trees grew in desert drainages.13PubMed Central. Perennial stream discharge in the hyperarid Atacama Desert of northern Chile during the latest Pleistocene Today, this ancient groundwater feeds salt flats like the Salar de Atacama, which holds a massive accumulation of halite spanning the late Miocene to the present along with lithium-rich brines that have become economically significant.14Geophysical Research Letters. Regional groundwater flow and accumulation of a massive evaporite deposit at the margin of the Chilean Altiplano
A Stand-In for Mars
The Atacama’s extreme aridity has made it one of the best places on Earth to study the limits of life and to rehearse the search for biosignatures on other planets, particularly Mars. The hyperarid core presents conditions remarkably similar to Martian surface soils: extremely low water availability, intense ultraviolet radiation, and high concentrations of oxidizing salts.
Researchers have found that even in this punishing environment, specialized bacterial communities survive in subsurface sediments down to at least 800 millimeters deep. These communities are shaped by depth-related moisture and by geochemistry, particularly salt concentrations that affect how much water is biologically available. Patches of completely uncolonized sediment occur where osmotic stress is too extreme. A rover-mounted drill has been deployed in the Atacama to test autonomous sample-recovery techniques relevant to future Mars missions.15PubMed Central. Subsurface Microbial Habitats in an Extreme Desert Mars-Analog Environment
Deeper investigation has uncovered another potential refuge for life. Clay-rich layers about 40 centimeters below the surface maintain a constant relative humidity around 78%, far above the minimum water activity threshold needed for microbial survival, and far higher than surface soils, which can have water activity values as low as 0.01. These subsurface clay layers also stay thermally stable, hovering around 17°C regardless of the wild surface temperature swings between near-freezing at night and above 50°C at midday.16Scientific Reports. Inhabited subsurface wet smectites in the hyperarid core of the Atacama Desert as an analog for the search for life on Mars If life persists in hidden clay pockets beneath the driest desert on Earth, similar refugia could conceivably exist beneath the Martian surface.
Why the Atacama Is an Astronomer’s Paradise
The same atmospheric dryness that makes the Atacama nearly lifeless makes it one of the best places on the planet to observe the sky. Water vapor in the atmosphere absorbs and scatters incoming light, particularly at infrared and millimeter wavelengths. With almost no moisture overhead, the high-altitude plateaus of the Atacama offer extraordinary atmospheric transparency. The Llano de Chajnantor, a plateau at roughly 5,000 meters elevation in the Atacama, was selected as the site for the Atacama Large Millimeter Array precisely because it combines high elevation with extremely low atmospheric water content.17Publications of the Astronomical Society of the Pacific. The Optical/Infrared Astronomical Quality of High Atacama Sites. I. Preliminary Results of Optical Seeing
Measurements of the solar spectrum across Atacama elevations show that surface irradiance increases substantially with altitude because of the thinner atmosphere and reduced water vapor. Between sea level and 5,100 meters, surface irradiance rises by about 27% in the infrared range and 20% in the ultraviolet range.18Scientific Reports. The Solar Spectrum in the Atacama Desert The clarity is not just useful for radio telescopes. Some of the world’s most productive optical observatories also operate in the Atacama, taking advantage of steady atmospheric conditions and skies that are dark and cloud-free for the vast majority of the year.
What Climate Change Could Do to the Driest Desert
It might seem paradoxical to worry about climate change in a place that barely has a climate to change, but projections suggest the Atacama will not escape the effects of a warming planet. Climate models point to a poleward and westward expansion of the South Pacific Subtropical High as global temperatures rise, pushing its southern edge to higher latitudes regardless of season.19Climate of the Past. South Pacific Subtropical High from the late Holocene to the end of the 21st century: insights from climate proxies and general circulation models A stronger and more expansive subtropical high would mean more atmospheric sinking over a wider area, potentially extending hyperarid conditions into regions that currently receive at least some rain.
At the same time, projections for the southern edge of the Atacama suggest a reduction in annual precipitation of 15 to 30% during this century, paired with an intensification of individual storm events. In other words, rain becomes both rarer and more violent when it does come. The alluvial disaster of March 2015, when sudden torrential flooding struck Atacama communities, may foreshadow this pattern.20Global and Planetary Change. Extreme ENSO-driven torrential rainfalls at the southern edge of the Atacama Desert during the Late Holocene and their projection into the 21th century For a region that has almost no drainage infrastructure because it almost never needs it, intensified flash floods pose a serious risk to the mining communities and small towns scattered across the desert.
The fossil groundwater systems are also vulnerable. Because essentially no modern recharge is replacing the ancient water beneath the Atacama, any increase in extraction, driven by lithium mining, copper mining, or growing urban demand, permanently depletes a resource that took thousands of years to accumulate. The desert’s aridity is ancient and self-reinforcing, and there is no mechanism in current or projected climate to reverse it.