Fog Nets: Harvesting Water from Thin Air

How Fog Nets Work

Fog is not rain. Rain falls as discrete droplets large enough to be pulled earthward by gravity. Fog consists of vastly smaller water droplets, typically 1 to 40 micrometres in diameter, suspended in moving air. These droplets are too small to fall but too heavy to remain airborne indefinitely. When fog-laden wind passes through a fine mesh, the droplets collide with the mesh fibres and coalesce into larger drops that gravity can pull downward. The drops trickle to the bottom of the mesh, drip into a gutter or trough, and flow by gravity into a storage tank. No energy input is required beyond the wind that carries the fog.

The physics governing collection efficiency depend on three factors: wind speed, mesh geometry, and droplet size. Faster wind pushes more fog through the mesh per unit time but also tends to re-entrain collected droplets if the mesh is too dense. Mesh that is too open lets fog pass through without interception. The optimal porosity, the fraction of open space in the mesh, is approximately 50 to 60 percent. At this density, the mesh intercepts enough droplets to be productive while allowing sufficient airflow to avoid creating a pressure barrier that diverts wind around the panel rather than through it.

The concept is not new. Forests on fog-prone coastlines and mountains have always practised fog harvesting, which is why cloud forests are so lush despite receiving modest rainfall. The canopy of a large tree acts as a biological fog net, intercepting moisture that drips to the forest floor and recharges the soil. Researchers studying fog deposition in the Canary Islands found that laurel forests there intercept fog equivalent to several hundred additional millimetres of rainfall per year. Artificial fog nets replicate this process on barren hillsides where deforestation or natural aridity has removed the vegetative fog-catching surface.

Where Fog Nets Work

Fog harvesting is site-specific. It requires a reliable fog source, which typically means proximity to cold ocean currents, high-altitude cloud belts, or persistent temperature inversions. The most productive sites share a common geography: coastal highlands or mountain ridges where prevailing winds carry marine fog or orographic cloud inland and upward against elevated terrain.

The Atacama Desert of Chile and Peru is the birthplace of modern fog harvesting. Despite being one of the driest places on Earth, with some weather stations recording zero rainfall for years, the coast receives dense fog called "camanchaca" driven by the cold Humboldt Current. Villages at elevations of 500 to 1,000 metres above sea level sit directly in the fog belt and have used fog nets since the 1980s. The FogQuest organisation and its predecessors installed some of the first large-scale fog collection systems in the Chilean village of Chungungo, supplying the community's drinking water for over a decade.

Morocco's Anti-Atlas mountains host another major fog harvesting project. The Dar Si Hmad foundation installed a fog collection system on Mount Boutmezguida at 1,225 metres elevation that supplies clean water to over 400 people in several villages. The site receives persistent fog driven by Atlantic trade winds, and the nets produce water even during the dry summer months when conventional water sources fail. Similar projects operate in Eritrea, South Africa, Colombia, Guatemala, Nepal, and Oman. In each case, the critical success factor is a thorough site assessment confirming consistent fog frequency, adequate wind speed (ideally 2 to 8 metres per second), and appropriate elevation.

Yields and Performance

A well-sited fog net produces 5 to 15 litres of water per square metre of mesh per day, averaged over productive fog seasons. Peak collection during dense fog events can reach 20 to 30 litres per square metre per day. These figures may sound modest, but they add up. A standard large fog collector (SFC) measures 40 square metres (typically 10 metres wide by 4 metres tall). At an average yield of 10 litres per square metre per day, one collector produces 400 litres daily, which is enough to supply drinking and cooking water for 20 to 40 people.

Yields vary enormously with site conditions and season. Coastal sites in northern Chile produce primarily during the southern winter (May to November), when fog frequency peaks. Mountain sites in Morocco and East Africa may produce year-round, with a dip during the hottest months when the cloud base lifts above the collection elevation. Seasonal variation must be factored into system design: storage tanks need to bridge low-production periods, and communities should not rely solely on fog water if production drops to near zero for months at a time.

Standard fog collectors, the small-scale research units used for site assessment, measure one square metre and are deployed for a minimum of 12 months to characterise a site's fog water potential. Any community project should begin with at least one year of SFC data collection before investing in large-scale infrastructure. Sites that consistently produce less than 3 litres per square metre per day averaged over the assessment period are generally not viable for community water supply, though they may still be useful for supplemental irrigation or reforestation support.

Construction and Community Projects

The dominant fog net design is remarkably simple. Two vertical posts, typically steel pipes or treated timber, are set into the ground or anchored to rock at a spacing of 10 to 12 metres. A double layer of Raschel mesh, the same UV-stabilised polyethylene shade cloth used in agriculture, is stretched between the posts at a height of 1 to 4 metres above the ground. A PVC or galvanised steel gutter runs along the bottom edge of the mesh, collecting the water that drips down, and a pipe conveys it to a sealed storage tank downhill.

Raschel mesh is the material of choice because it is inexpensive, UV-resistant, lightweight, and available worldwide. A 35 percent shade rating (which corresponds to roughly 50 to 60 percent effective porosity when doubled) provides optimal fog collection efficiency. The mesh is replaced every 5 to 10 years depending on UV exposure and wind stress, at a cost that is trivial compared to alternatives like desalination or trucked water.

The most successful fog harvesting projects are community-driven. External organisations provide technical expertise and initial funding, but the community owns, operates, and maintains the system. This model has proven far more sustainable than top-down installations where an NGO builds the infrastructure and then departs. The Dar Si Hmad project in Morocco is a model of this approach: local women were trained as technicians, a community water committee manages distribution and maintenance, and the project has operated continuously for over a decade. The combination of simple technology, local ownership, and a reliable fog resource makes fog nets one of the most appropriate water supply solutions for remote, elevated communities in fog-prone regions where conventional rainwater harvesting cannot meet demand.

Fog Nets: Harvesting Water from Thin Air

How Fog Nets Work

Where Fog Nets Work

Yields and Performance

Construction and Community Projects

See Also