The Atacama Desert, often cited as the driest non-polar desert on Earth, has a complex climate that defies simple categorization. It is not defined by consistently high temperatures, like the Sahara, nor by constant freezing, like the Antarctic Dry Valleys. Instead, the Atacama features dramatic thermal variability, where geography and oceanic forces combine to create an environment of extreme aridity and significant temperature fluctuation.
A Climate of Extremes
The Atacama is characterized by a massive difference between daytime and nighttime temperatures, known as the diurnal range. During the day, particularly in summer, temperatures can be warm, often reaching comfortable highs of 20°C to 25°C in central areas, and sometimes exceeding 30°C in lower-altitude regions.
However, the temperature plummets dramatically once the sun sets. Nighttime lows routinely drop to near-freezing, with high-altitude locations experiencing temperatures of -2°C to -8°C during the winter months of June and July. This extreme thermal swing means the desert is capable of both hot and cold conditions within a 24-hour cycle.
External Climate Drivers
The desert’s unique temperature profile and hyper-aridity are driven by two major external forces. The first is the massive topographic barrier created by the Andes Mountains to the east and the Chilean Coastal Range to the west. This dual barrier creates a profound two-sided rain shadow, effectively blocking moisture from reaching the desert floor from both the Pacific and the Atlantic sides.
The second primary driver is the cold, north-flowing Humboldt Current, which runs along the Pacific coastline. This current carries frigid water northward, cooling the marine air above it substantially. The cool air suppresses evaporation and creates a persistent atmospheric temperature inversion layer near the coast, preventing the formation of rain-bearing clouds.
The Impact of Extreme Aridity
The extreme aridity of the Atacama plays a direct role in creating the wide daily temperature swings. The desert is classified as hyper-arid, with some areas receiving less than 2 millimeters of rainfall annually. This near-total absence of water vapor means the air lacks the primary component necessary to hold onto heat.
Water vapor acts as a natural atmospheric blanket, trapping outgoing longwave radiation from the Earth’s surface. Because the air is so dry, this heat retention mechanism is absent. As soon as the sun dips below the horizon, the surface heat rapidly radiates back into space, causing the temperature to drop precipitously. This quick, efficient radiative cooling transforms a mild desert day into a freezing desert night.
Unique Environmental Consequences
The combination of cold, dry air and high altitude has produced several unique environmental consequences. The coastal fog, locally called Camanchaca, forms when the cold, stable air from the Humboldt Current meets the warmer landmass. This fog rolls inland and provides the only consistent source of moisture for specialized ecosystems along the coastal hills.
This persistent fog nourishes specialized life forms, such as endemic cacti, algae, and lichens, which have adapted to harvest water directly from the air. The exceptionally clear and dry air, coupled with the high altitude, also creates some of the best conditions on Earth for astronomical observation. This has led to the construction of major international observatories, such as the European Southern Observatory’s Very Large Telescope (VLT) at Cerro Paranal.