Fog Harvesting and Atmospheric Water Collection: Catching Water from Thin Air
In coastal fog zones from the Atacama to the Atlas Mountains, mesh nets intercepting windborne fog droplets collect 3 to 12 litres per square metre per day with no energy input, no groundwater dependency, and no moving parts. The limiting variable is not the technology; it is site selection. A good site can supply an entire village. A bad site produces nothing.
Who Needs Fog Harvesting and Why
The operator or community evaluating fog harvesting is typically facing a specific constraint: they are located in an area with very low or highly seasonal rainfall, persistent maritime or orographic fog, and either no viable groundwater or groundwater that is overextracted, contaminated, or legally unavailable. They need a water supply that does not depend on rain, does not require drilling, and can operate without electricity. Fog harvesting is one of the few water supply options that meets all three criteria simultaneously in the right site conditions.
The global installed base of fog collection systems is modest but well-documented. The most studied installations are in Chile (the Chungungo project, 1987-2001, and the Atacama Large Fog Collector network), Morocco (the Aït Baamrane fog collection system built by FogQuest and Dar Si Hmad, operational from 2015), Nepal (the Kathmandu Valley fog harvesting pilot studies), Eritrea and Ethiopia (highland ridge sites), and the Canary Islands (research installations at El Hierro and Tenerife). All of these demonstrate that fog harvesting is viable technology. The question is always site-specific: does this particular ridge, at this elevation, in this climate pattern, produce enough fog of sufficient liquid water content to justify the infrastructure investment?
This page answers that question by describing the physical mechanism, the site assessment protocol, the measured yield data from operating installations, and the cost per litre of water produced at different collection scales. It connects to the broader water harvesting pillar framework, which treats all capture methods by their cost per litre delivered and their suitability for specific site conditions.
How Fog Collectors Work
Fog consists of suspended water droplets with diameters of 1 to 40 micrometres, too small to fall as rain but large enough to coalesce on a surface. A fog collector intercepts the horizontal movement of fog through the air by presenting a mesh surface perpendicular to the prevailing wind direction. Droplets impact the mesh fibres, coalescence occurs as droplets accumulate, and when the droplet mass exceeds surface tension, water runs down the mesh and drips into a collection trough at the base. The collected water drains by gravity to a storage tank.
The efficiency of a fog collector depends on four variables. First, liquid water content (LWC) of the fog: expressed in grams per cubic metre of air, LWC in coastal maritime fog typically ranges from 0.05 to 0.5 g/m3. Higher LWC means more water available per unit volume of air passing through the collector. Second, wind speed through the fog: at 2 m/s wind, a fog collector passes roughly 7,200 m3 of air per square metre of net per hour. At 5 m/s, that becomes 18,000 m3/hour. More air passage means more fog droplets intercepted, up to the point where wind speed begins causing re-entrainment of droplets already captured. Third, mesh shade factor: 35 to 45 percent shade factor is the measured optimum. Below 30 percent, insufficient surface area for droplet capture; above 50 percent, wind velocity through the mesh drops, reducing fog interception. Fourth, mesh fibre diameter and hydrophilicity: finer fibres and hydrophilic surface treatments increase droplet capture per unit of projected area.
The standard Fog Collector (SFC) developed by Schemenauer and Cereceda has a collection area of 1 m2 (1 m wide by 1 m tall) and is used internationally as a site assessment tool, measuring fog yield before committing to larger infrastructure. The Large Fog Collector (LFC) is 48 m2 (12 m wide by 4 m tall) and represents a standard community-scale unit. Both use 35 percent shade factor Raschel mesh. The Aït Baamrane installation in Morocco uses 600 m2 of net across multiple large frames, with each 40 m2 collector panel capable of producing 200 to 400 litres per day on good fog days (Dar Si Hmad, FogQuest monitoring data, vault_atom_TBD).
Atmospheric water generators (AWGs), which use refrigeration to condense water vapour from humid air, are a completely different technology category from fog collectors. AWGs use 0.3 to 0.5 kWh of electricity per litre of water produced, making their operating cost 15 to 50 times higher than passive fog collection per litre. AWGs are relevant in humid climates without fog, where the dew point is consistently above ambient temperature. Passive fog nets are relevant in fog-zone climates. The two technologies serve different geographies and should not be evaluated as alternatives to each other.
Yield Data from Real Installations
The Chungungo project in coastal Chile, the most extensively documented large-scale fog collection installation, operated 100 Large Fog Collectors (4,800 m2 total collection area) on El Tofo ridge at 780 metres elevation from 1987 to 2001. Peak production reached 15,000 litres per day, supplying the entire domestic water needs of 330 people. Average production over the operating period was approximately 10,000 litres per day, with a seasonal trough in summer (January to March) when fog frequency drops. The project cost approximately 237,000 USD for initial infrastructure including collector frames, mesh, pipeline, and storage tank, equivalent to roughly 718 USD per person served or 0.025 USD per litre for a 10-year system life at average production rates (Schemenauer and Cereceda, 1994, Journal of Applied Meteorology 33(2):140-148).
The Aït Baamrane installation in the Anti-Atlas Mountains of Morocco, operated by Dar Si Hmad since 2015, is a 600 m2 collector array supplying water to five villages with a combined population of approximately 400 people. The system produces 6,300 litres per day on average, with peak production of 22,000 litres per day during optimal fog conditions in spring. Total infrastructure cost was approximately 900,000 USD including pipeline, storage, and supporting water management works, or 2,250 USD per person served. The higher per-person cost compared to Chungungo reflects the more complex terrain and longer pipeline required to reach the five villages from the ridge collector site.
Cost per cubic metre of water produced from fog harvesting at these documented installations ranges from 0.50 to 2.50 USD per m3, compared to desalination at 0.80 to 3.00 USD per m3, trucked water supply at 5 to 30 USD per m3, and drilling new boreholes in difficult terrain at a capital cost representing 8 to 40 USD per m3 annualised over system life. In locations where the alternative is trucked water or non-functional supply, fog harvesting is not competitive with reticulated mains water but is directly competitive with the actual alternatives available on the ground.
Rainfall equivalent comparison puts fog harvest yields in perspective. A 100 m2 fog collector at a good site producing 4 L/m2/day generates 400 litres per day, or 146,000 litres per year. That is the equivalent of collecting all rain from a 500 m2 rooftop receiving 300 mm of annual rainfall, except that fog harvesting works continuously throughout the fog season regardless of whether it rains. In locations where rain falls in intense events that are difficult to capture and store, fog provides a slow, steady, year-round supplement that is easier to manage with small storage volumes.
Site Assessment, Specification, and Deployment
Site assessment is the single most important investment before any fog collection infrastructure. A Standard Fog Collector measuring 1 m2, deployed for a minimum of 12 months on the candidate ridge, provides the baseline yield data that determines whether a full-scale system is viable. The SFC is a 15 to 25 USD device: two wooden or steel stakes, a 1 m2 piece of 35 percent shade factor Raschel mesh stretched between them, a rain gauge at the base, and a tipping bucket or graduated cylinder to measure daily collection. If the SFC produces less than 1 L/m2/day on annual average, the site is not viable for community-scale fog harvesting. If it produces 3 L/m2/day or more on annual average, a large-scale installation is likely economically justified (Schemenauer and Cereceda 1994).
The site assessment must also characterise wind direction consistency. Fog collectors work best when fog moves consistently from a single direction: the collector can then be oriented perpendicular to the prevailing fog wind and will capture efficiently regardless of fog event frequency. Sites with variable fog wind directions require multiple collector orientations or rotating collector frames, adding complexity and cost. Measure wind direction during fog events specifically, not ambient wind, as the two can differ significantly at ridge sites where orographic effects channel flow.
For a community-scale installation supplying 50 people at 20 litres per person per day (1,000 litres per day total), the required collector area at a good site producing 4 L/m2/day is 250 m2. At a standard Large Fog Collector size of 48 m2, that is approximately five or six LFC units. Frame construction cost using welded steel angle or treated timber is approximately 150 to 300 USD per LFC unit. Raschel mesh at 35 percent shade factor costs approximately 1.50 to 3.00 USD per m2. Pipeline from ridge to village and a 10,000 litre storage tank complete the system. Total installed cost for this scale: approximately 15,000 to 35,000 USD depending on terrain and pipeline length.
The fog layer at most coastal mountain fog sites lies between 300 and 1,200 metres elevation. Below the fog base, collectors produce nothing. Above the fog top, collectors also produce nothing. The peak yield zone is the middle of the fog layer: typically 500 to 800 metres at most Atlantic and Pacific coastal fog sites. If a candidate ridge spans this elevation range, place collectors in the middle third of the fog zone, not at the summit where the fog layer may be thin or absent.
Maintenance is minimal. Annual tasks: inspect mesh for UV damage, check frame connections for corrosion, clean inlet screens on collection troughs. Every 6 to 10 years: replace mesh. The collection trough, pipeline, and storage tank require the same maintenance as any gravity-fed water supply system. Total annual maintenance labour for a six-LFC installation: approximately 8 to 16 person-hours per year if the site is accessible by foot or vehicle.
| Location | Collector Area | Avg Yield L/day | Population Served | Elevation (m) |
|---|---|---|---|---|
| Chungungo, Chile | 4,800 m2 (100 LFC) | 10,000 | 330 people | 780 m |
| Aït Baamrane, Morocco | 600 m2 (multiple arrays) | 6,300 avg; 22,000 peak | ~400 people (5 villages) | 1,200 m |
| Tojquia, Guatemala | 48 m2 (1 LFC) | 200 (fog season) | Community supplement | 1,800 m |
| Dhofar, Oman | Research array, various | 4-8 L/m2/day peak | Research only | 300-800 m |
| El Hierro, Canary Islands | Research collectors | 3-6 L/m2/day | Research only | 600-900 m |
Where Fog Harvesting Fits in the Water Stack
Fog harvesting is a primary water supply technology in fog-zone locations where no alternative ground or surface water exists at viable cost. Outside fog zones, it has no application. Within fog zones, the combination of fog collection and rainwater storage from roof catchment is the standard hybrid approach: fog provides steady supply during the fog season while cisterns carry storage through seasonal gaps. The two systems share storage infrastructure and distribution piping, so the marginal cost of adding fog collection to an existing rainwater system is limited to the collector frames and mesh.
The integration with managed aquifer recharge is relevant at the largest scales: the Atacama fog oasis ecosystems maintain groundwater tables through long-term fog water inputs, and there is active research into using large-scale fog collection arrays as a groundwater recharge input for arid coastal aquifers. At community scale, fog water that exceeds immediate demand is best directed to a shallow infiltration trench to recharge local shallow groundwater rather than lost to evaporation from open storage.
In agroforestry contexts, fog harvesting takes on additional function. The Canary Islands historically maintained forest plantations specifically to harvest fog: Garoé, the "holy tree" on El Hierro, was documented by early European explorers as a large Til tree (Ocotea foetens) producing hundreds of litres of water per day from fog drip. Fog capture by vegetation canopy is not a technology but a land use practice that the watershed-scale planning framework incorporates when siting tree plantings in fog zones. The leaf area of a mature native forest intercepts 2 to 8 times more fog than an equivalent area of bare mesh, making reforestation in fog zones a long-term fog harvesting investment with no capital cost beyond tree establishment.
Common Questions About Fog Harvesting
How much water can a fog collection net produce per day?
Measured yields from operating fog nets range from 3 to 12 litres per square metre of net per day on good fog days, with annual average yields of 2 to 6 L/m2/day at the best sites. The Chungungo project in Chile supplied 15,000 litres per day to 330 people from a 4,800 m2 installation. Yields are highly site-dependent: a ridge exposed to persistent maritime fog at 300 to 1,200 metres elevation with wind speeds of 3 to 10 m/s is the ideal profile. Inland sites and low-elevation coastal sites below the fog layer produce negligible yields. Source: Klemm et al. (2012) Atmospheric Research; FogQuest project database.
What materials are used to make fog collection nets and how long do they last?
Standard fog collectors use Raschel mesh made from HDPE or polypropylene with 35 to 50 percent shade factor, stretched between posts perpendicular to the prevailing wind. Under UV exposure, standard HDPE mesh lasts 5 to 8 years; UV-stabilised polypropylene extends service life to 8 to 12 years. Steel or galvanised frames last 15 to 25 years. Total replacement cost per 10-year cycle for a 100 m2 collector: approximately 800 to 1,500 USD in mesh plus 200 to 400 USD in labour. Source: FogQuest Standard Fog Collector v2.0 specification; Olivier and de Rautenbach (2002).
Where does fog harvesting work and where does it not work?
Fog harvesting works at coastal ridges and mountain slopes exposed to maritime air masses, typically at 300 to 1,200 metres elevation. Best-documented sites include coastal Chile and Peru, coastal Morocco, the Dhofar mountains of Oman, the Namib coast, and highland sites in Nepal and Eritrea. Sites require at minimum 100 fog days per year and consistent wind of 3 to 10 m/s through the fog layer. It does not work in continental interiors, below the fog layer, in monsoon climates with brief fog, or in arid regions without maritime fog influence. At minimum 12 months of Standard Fog Collector measurement is needed before committing to infrastructure. Source: Schemenauer and Cereceda (1994); FogQuest site assessment protocol.
Every Site Has a Water Budget
Fog harvesting is one of several passive capture methods. The water harvesting pillar maps the full toolkit by site type and cost. For storage that pairs with fog collection, see rainwater harvesting tanks and cisterns.