The Atacama Desert, located primarily in northern Chile, is a plateau stretching across the Pacific coast of South America. Covering approximately 105,000 square kilometers, it is the driest non-polar desert on Earth. Its extreme aridity is often studied as a proxy for the surface of Mars.
Quantifying the Aridity
The average annual rainfall in the hyper-arid core of the Atacama is often recorded as less than one millimeter. In some interior valleys, the mean annual precipitation registers near zero, with specific stations logging figures as minimal as 0.5 millimeters over decades. The coastal strip experiences slightly higher, though still minimal, amounts of moisture. Some coastal regions can receive up to 30 millimeters of precipitation annually, but this rain tends to be sporadic and highly variable, mostly falling during winter months.
Geological and Atmospheric Causes
The extreme aridity of the Atacama results from a combination of two major climate drivers. To the east, the towering Andes Mountains create an immense rain shadow effect. Moist air masses from the Amazon basin and the Atlantic Ocean are forced to rise, cool, and release their moisture as precipitation on the eastern slopes. This process leaves the air descending on the desert’s western side warm and dry, blocking moisture from the continent’s interior.
The desert’s proximity to the Pacific Ocean does not bring relief due to the cold Humboldt Current, also known as the Peruvian Current. This current flows northward from Antarctica, maintaining a low sea-surface temperature along the coast. The cold water cools the air immediately above it, leading to a stable atmospheric inversion layer that caps the atmosphere close to the ground. This inversion prevents the air from rising high enough to form rain-producing clouds, suppressing rainfall from the west.
Water Sources Beyond Rainfall
Despite the lack of rainfall, moisture is introduced into the coastal environment through camanchaca, or coastal fog. This thick, marine stratocumulus fog forms when moisture-laden air from the Pacific is cooled by the Humboldt Current but prevented from rising by the atmospheric inversion layer. The camanchaca regularly rolls inland, typically between 600 and 1,000 meters in elevation, providing a persistent source of atmospheric water for coastal life.
Communities utilize this fog through fog catchers, or trapanieblas. These installations consist of large, fine-mesh nets that harvest water droplets directly from the moving fog. These systems can yield a significant amount of potable water, with some designs collecting between two to ten liters per square meter of mesh daily. Deep groundwater reserves, accumulated over thousands of years from historical climate periods or snowmelt from the high Andes, also provide a crucial water source for towns and mining operations.
Ecological Adaptations
Life in the Atacama survives by deploying specialized biological strategies to capture and retain minimal moisture. One resilient life form is endolithic microbial communities, which inhabit the interior of translucent rocks like quartz and halite (salt). The rock provides a shield against intense ultraviolet radiation and offers thermal buffering. These organisms, including certain cyanobacteria, use the salt within the rock to draw moisture directly from the atmosphere through deliquescence.
Plants have also evolved unique mechanisms. The bromeliad Tillandsia landbeckii, a rootless plant, relies exclusively on the camanchaca for survival. Its leaves are covered in specialized, absorbent hairs called trichomes that trap and absorb fog droplets directly from the air. Fauna, such as the Chilean desert darkling beetle, harvest fog by climbing dunes and assuming a head-down posture, allowing moisture to condense on their grooved exoskeletons and channel directly to their mouths.
The desert also features a rare exception known as the desierto florido, or flowering desert. This event occurs when anomalous, heavy rainfall, often triggered by El Niño events, penetrates the dry ground. The sudden influx of moisture causes millions of dormant seeds to germinate, transforming the arid landscape into a temporary blanket of vibrant flowers.