Fog catchers of the Atacama, Chile

The ocean next to the Atacama is cold.

The cold water makes fog and low clouds. But the clouds do not make rain. The fog has tiny drops of water that stick to things they touch.

Why is there fog but no rain? The ocean along this coast is cold, because of the Peru Current, also called the Humboldt Current, which brings cold water up from the deep. Cold air sits near the sea, with warmer air above it like a lid. That makes fog and flat gray clouds, but no rain.

In Chile and Peru, these coastal fogs have a traditional name: camanchaca. Their tiny water droplets drift in the air and collect when they touch something, such as a net.

Britannica explains the aridity in two parts. High in the atmosphere, the South Pacific high-pressure cell causes dry, sinking air. Along the coast, the Peru (Humboldt) Current brings up cold water from the depths, so the sea surface chills the air above it. The result is a thermal inversion, cold air at sea level beneath stable, warmer air, which produces fog and stratus clouds but no rain. Rain reaches Iquique or Antofagasta only when strong southern fronts break through.

The frequent fogs of the arid coasts of Peru and Chile are traditionally called camanchacas (Organization of American States and UNEP source book). A UC Chile study describes low stratocumulus clouds regularly covering the southeastern Pacific between 18 and 30 degrees south and reaching the coastal mountains, so marine fog is constantly present along the desert coast. UC researchers trace this cloud from central Peru to Pichilemu, at about 35 degrees south.

A fog catcher is a big net.

It stands on a hill, between two tall poles. Fog blows through it. Tiny drops stick to the net. They join into big drops and drip into a gutter. Then the water flows down a pipe to a tank.

A fog catcher is a flat, rectangular net held up by a post at each end. It faces the wind, so the fog blows straight through it. One early collector in Chile was a single panel measuring 2 by 24 meters.

The net is a fine black plastic mesh, like shade cloth. When fog droplets touch it, they cling and join into bigger drops. These grow heavy and slide down into a gutter at the bottom. From there, gravity carries the water through pipes to a storage tank. The catcher needs no electricity at all.

Full-scale collectors are flat, rectangular nets supported by posts at either end and set across the prevailing wind. The pilot collector at El Tofo was a single panel 2 m by 24 m, giving 48 m² of mesh. After testing several mesh densities, the El Tofo team used Raschel netting, black polypropylene filaments 1.0 mm wide in a triangular weave, with 35 percent coverage, mounted in a double layer. That proportion of mesh to opening extracts about 30 percent of the water in the fog that passes through.

Captured droplets coalesce into drops that fall into a trough, and the water runs by gravity through pipes to a closed cistern. The system is passive, but not maintenance-free: the source book lists checking cable tension, mending tears, brushing algae off the mesh after a year or two and cleaning tanks. Yields vary with place and season; Chilean experiments harvested between 5.3 and 13.4 liters per square meter of mesh per day.

Some years ago, a village called Chungungo got its water from fog.

Nets on a mountain called El Tofo caught the fog. Pipes carried the water down to the homes. People grew gardens with it.

In the late 1980s, researchers from Chile and Canada set up rows of fog catchers on El Tofo, a mountain in northern Chile. At first the water was for growing young trees. Then the village of Chungungo below asked for it, and a pipeline was built down the mountainside.

By 1992 the village had a steady supply of clean water: on average about 15,000 liters a day. In 1995 a magazine reporter described gardens thriving on land that had once been bare. Later the nets were not looked after, and by 2002 only 9 of the 94 collectors were still hanging. Most of the village's water now came by truck.

The El Tofo project began as a collaboration between Canada's International Development Research Centre (IDRC), Chile's National Forestry Corporation (CONAF) and UC Chile. From 1987 to 1990, 50 Raschel collectors there averaged 3.0 liters per square meter per day. The original goals were to perfect the technology and water seedlings for reforestation; only after lobbying by the community of Chungungo was the water piped down to the village. Its collectors supplied an average of 15,000 liters a day, and by 1992 residents had a steady supply of clean water for the first time.

IDRC's later review is a lesson in how technology depends on people. The town grew from about 300 to 900 inhabitants, so each household got less, and fogless spells emptied the reservoir. Up to eight different organizations shared responsibility, and the local committee could not pay for major repairs. By the summer of 2002 only nine of 94 collectors still hung, and most water was hauled in by truck. Robert Schemenauer, one of the designers, argued the simplest fix would have been more collectors and a bigger reservoir; Jorge Nef of the University of Guelph reported that villagers had not been involved in any significant way in the project's origins.

Today, scientists study fog at a place called Alto Patache.

Their station is made of round domes. It uses sunshine for power and fog for water. School children come to visit and learn.

In 1997, UC Chile researchers found a "fog oasis" south of the port city of Iquique: a patch of desert where thick fog supports its own unique ecosystem. It is called Alto Patache, and the university looks after 1,114 hectares of it.

The research station there has six domes with bedrooms, bathrooms, storage, teaching and dining areas, and an office. It gets its electricity from the Sun and its water from fog catchers. About 500 children from nearby schools have visited to see the scientists at work. Near the station, a big dome called the Desertario now teaches visitors how fog catchers work.

UC Chile's Institute of Geography began studying fog as a water source in the 1980s; Pilar Cereceda, a professor there, is one of its fog researchers. In 1997 researchers identified a fog oasis in the Tarapacá Region, 7 km north of the fishing village of Chanavaya; UC reports its distance from Iquique as 56 km in one article and 65 km in another. In 2007 the Ministry of National Assets granted the university a 25-year concession of 1,114 hectares for conservation, research and education. The station, whose activities are coordinated by Professor Pablo Osses of the Institute of Geography, is self-sufficient on solar energy and fog water, and researchers there found a lichen, Santessonia cervicornis, known only from that area.

By February 2024 UC had opened the Desertario, an interpretive center funded through the Ministry of Science's 2022 public science competition, so people from Alto Hospicio and Iquique can learn about 25 years of fog research. UC's stated future goal is for communities to have their own fog catchers. It notes that people in Tarapacá use 118 liters of water a day on average, against a national average of 200, and that a 32 m² fog catcher can produce an average of 220 liters a day.

Chile's fog catchers gave other places an idea.

Now people in other dry lands try them too. Scientists are also studying how fog water could grow vegetables in greenhouses.

The fog catchers tested in Chile have been taken to many other countries, including Namibia, Yemen, Haiti, Nepal, Peru and Guatemala, according to UC Chile.

Scientists are still finding new uses. A 2023 study measured how much water fog nets collected at two desert sites about 1,000 kilometers apart: on average about 3 liters for every square meter of net each day. The team worked out that about 2 square meters of net could water 1 square meter of vegetables growing in a greenhouse, even in the thirstiest season.

An IDRC article published in 2011 reported that the technology refined at El Tofo had been adopted or was under study in 25 countries. UC Chile names Namibia, Yemen, Haiti, Nepal, Peru and Guatemala among the countries it has reached.

Current research looks at food as well as drinking water. A 2023 paper in the journal Sustainability by UC Chile researchers compared Alto Patache with Falda Verde, about 1,000 km apart. Annual average collection was 2.9 liters per square meter per day at Alto Patache and 3.3 at Falda Verde. Modeling greenhouse hydroponics at both sites, the authors found that 2.13 m² of fog collector could supply 1 m² of greenhouse crops in the season of highest water demand, with pumps run from solar panels. UC researchers also monitor the fog itself to see how climate change affects the cloud.