The Nets That Catch Clouds

The mesh has no moving parts and uses no power. It simply stands in the fog.

Mount Boutmezguida rises to 1,225 metres in the Anti-Atlas behind Sidi Ifni, on the Atlantic coast of southern Morocco. It is dry country. The villages of Aït Baamrane below the mountain depended on wells whose water table was sinking. For the women and girls who fetched it, collecting water took three hours or more every day.

Nets have stood along the ridge since 2015, and since 2018 there have been 31 of them, covering 1,686 square metres. Droplets gather on the mesh, run down into gutters, collect in cisterns and then travel through 26 kilometres of pipe to taps in sixteen villages on the slopes below. The system supplies around 1,300 people and a school, and waters around 7,000 head of livestock. Wasserstiftung, the German foundation that built the current array, describes it as the largest fog-collection installation in the world. The whole system had a budget of €600,000.

The mountain does not make the water. It stands in the way of it.

Fog harvesting cannot be deployed anywhere. It works only in a small number of very specific places. The reason this coast is one of them has nothing to do with Morocco in particular. Four things have to happen in sequence, and the figure below adds them one at a time.

Off this shore, the sea is cold, chilled by a current that runs down past the Canary Islands. Low cloud forms over cold water like this, and regional winds carry it towards the coastal mountains. The cold current works together with the Azores anticyclone to produce a deck of low stratocumulus, while the prevailing wind pushes the cloud inland. Where a ridge rises into that layer, the cloud reaches ground level as fog. Boutmezguida gets fog on about 143 days a year, mostly between December and June.

Schematic rather than to scale. The same sequence produces the world’s other great fog coasts, the Atacama in Chile and the Namib in Namibia, and Morocco’s Atlantic coast down to the Saharan shore. They are broadly the same phenomenon, repeated three times.

You cannot collect fog. You can only intercept it.

The reason a rain gauge left out in fog stays empty is a matter of size. A fog droplet is between about one and a few tens of microns across. A raindrop is vastly larger. A droplet that small falls so slowly that moving air matters more than gravity. It therefore does not fall to the ground. Instead, it travels sideways with the wind until it hits something.

So the system gives the droplet something to hit. A vertical panel of mesh sits in the passing air. Droplets strike the fibres, merge into larger drops and run down into a gutter and a tank. How much water reaches the gutter depends on the water content of the fog, the size of the droplets, the mesh and the wind speed. If the mesh is woven too tightly, the air goes around it instead of through it. If it is woven too loosely, the droplets pass straight through.

The mesh itself is unglamorous. The standard is Raschel, a polyethylene shade netting from a Chilean manufacturer. It is hung in a double layer so that the water runs off quickly. It works, and it is cheap. It also tears. The wind that delivers the fog is the same wind that damages the collector, and unresolved wind problems helped end at least one earlier project, in Yemen.

Chungungo worked for years. Then nobody was left who could repair it.

In the 1980s, a Canadian cloud physicist, Robert Schemenauer, and a Chilean climatologist, Pilar Cereceda, were among the researchers who began systematic fog work on the ridge at El Tofo, above the village of Chungungo in northern Chile. Fifty large collectors went up, and 41 more followed in the 1990s. Water ran down the mountain to Chungungo, a parched former mining town, at an average of about 15,000 litres a day for some three hundred people.

By every technical measure, it was a success. The international media reported on it for years, and it became the prototype for fog projects elsewhere. Morocco’s first nets followed the design principle of FogQuest, the NGO Schemenauer went on to lead.

More than ten years later, the nets were in disrepair. By the summer of 2002, only nine collectors were still hanging. The reasons were not technical. The system had been handed to the local population in the mid-1990s, and the local committee meant to run it could pay for routine maintenance but not for major repairs. The village had tripled in size, and many people had come to see fog water as second-rate, a stopgap until the state brought a real pipe. When local politicians began lobbying for a pipeline from a river 20 kilometres away, the mesh on the ridge stopped being an achievement and became a reminder of what they had been given instead.

Morocco spent about ten years measuring before a village got any water.

The Moroccan project was designed by people who had read the Chilean post-mortem. Dar Si Hmad, the NGO that runs it, spent about a decade researching fog in the area before its first nets supplied a village. Measurements began in 2006 at a coastal site and on Boutmezguida. After two years, the mountain was yielding more than 7 litres a square metre a day, against under 2 on the coast. That difference is unusual. Many fog projects are built on an assumption about the site and discover the truth only afterwards.

The water is metered through pre-paid household meters, so the people who draw it pay for it. Women, who had done the fetching, were trained to monitor the system and report problems by mobile phone. The organisation ran literacy and income-generation training alongside the water supply, so that people living under the system would understand it rather than see it as a piece of foreign equipment on their mountain.

The collector itself was also redesigned. The CloudFisher, developed by Wasserstiftung with the Munich designer Peter Trautwein, hangs a three-dimensional mesh in a steel frame on rubber expanders. This allows the panel to give in a gust instead of tearing. Its makers rate it for winds of up to 120 kilometres an hour. It is a direct answer to the wind damage that ended earlier projects.

• 1980s

  El Tofo

  Researchers begin systematic fog work on a ridge in northern Chile. Fifty large collectors go up, and 41 more follow in the 1990s.

• mid-1990s

  Handed to the village

  The system passes to the local population. The committee meant to run it can pay for routine maintenance, but not for major repairs.

• 2002

  Nine collectors left

  By that summer only nine are still hanging, and the village is lobbying for a conventional pipeline instead.

• 2006–15

  A decade of measurement

  Fog yields are measured on Boutmezguida and at a coastal site, showing the mountain is by far the better location.

• 2013

  The engineering partner arrives

  Wasserstiftung installs its first CloudFisher on the mountain with Dar Si Hmad, testing mesh types with the Technical University of Munich.

• 2015

  First water

  The first nets are inaugurated, supplying about 400 people. Pre-paid meters connect homes in five villages.

• 2016–18

  The build-out

  German development funding takes the array to 1,686 square metres across 31 CloudFisher units, 26 kilometres of pipe and five cisterns holding 860 cubic metres.

• 2019

  The largest in the world

  Wasserstiftung describes the completed site as its largest CloudFisher installation, serving 16 villages.

In Dhofar a forest already does this, and the trees are the mesh.

The region has a second fog zone, and it operates according to completely different physics. With some annual variation, the khareef lasts from about mid-June to mid-September, when the monsoon brings moist air onto the coastal mountains of Dhofar, in southern Oman. The mountains sit inside cloud for three months, and a band of green vegetation runs along a coast surrounded by desert.

The remarkable thing is that this is already a working fog-harvesting system, and nobody built it. The Dhofar cloud forest, where Anogeissus dhofarica is the dominant tree, intercepts cloud on its leaves and drips the water onto the ground beneath. The tree is endemic to the region. Measurements under the canopy found about twice as much water reaching the ground as fell as rain. The trees are the mesh, and the seasonal woodland depends on the water it collects for itself.

The engineered version has been tested there too. Researchers at Sultan Qaboos University collected 995 litres per square metre over 77 days of one monsoon season, roughly thirteen litres per square metre per day. Earlier tests in the Dhofar mountains found fog water that met WHO standards for ions, and yields of 30 litres a square metre a day at 900 to 1,000 metres. An Omani trial with Mitsubishi put large nets at around 700 metres in the Al Qara mountains, feeding a 400-cubic-metre reservoir. The yields are among the highest recorded anywhere. What Dhofar has not had is a community supply on Morocco’s scale. In 2026, the government was still mapping sites, with 12 fog stations across nine locations.

Hassyan makes a whole fog day’s yield in about six seconds.

This is the point at which fog harvesting is usually oversold, so it is worth being exact about its scale. The largest fog installation in the world produces, on an average fog day, about 37,000 litres. Dubai’s Hassyan desalination plant, rated at 120 million imperial gallons a day, would produce that amount in about six seconds. Fog harvesting will never supply a city, will never irrigate a commercial farm and will never appear as a line in a national water balance. Anyone presenting it as an answer to regional scarcity is not being serious.

Per cubic metre, it is also not cheap. Spread the €600,000 budget across twenty years, with 143 fog days each year and about 37 cubic metres collected on each of those days, and the capital alone comes to more than €5 a cubic metre. Hassyan’s record tariff was US$0.277 a cubic metre. Judged as a supply option on a spreadsheet, fog loses.

But that is the wrong comparison, and the villages of Aït Baamrane are the reason. Nobody was ever going to run a pipe 26 kilometres up into the Anti-Atlas for 1,300 people. The realistic alternatives there were a sinking well, a water truck at truck prices or three hours of someone’s day. Against those alternatives, the mesh wins comfortably. It keeps winning every year because the collectors need no energy and no grid connection.

Seasonality is the real operational constraint. Both belts deliver water for part of the year and then largely stop. A fog system is therefore mostly a storage system.

The mesh is not the constraint. Maintenance is.

Four conditions have to hold at once for fog harvesting to be worth building, and they are restrictive enough to rule out most of the region immediately. There has to be persistent fog rather than occasional mist. In practice, that means a cold-water coast or a monsoon escarpment. There also has to be a ridge or slope that rises into the fog layer. Measured sites range from about 300 metres near a coast to 3,300 metres in the Guatemalan highlands. The site has to face the prevailing wind, with nothing upwind to strip the moisture first. Finally, there has to be a settlement below it, small enough that nobody will ever run a conventional pipe there and close enough for gravity to deliver the water.

In MENA, that describes Morocco’s Atlantic strip down to the Saharan shore, the Dhofar escarpment in Oman and its continuation into eastern Yemen, and parts of the Yemeni highlands, where test collectors above 2,000 metres averaged about 4.5 litres a square metre a day. It does not describe the Gulf, the Nile valley, Mesopotamia or the interior of the Maghreb. No amount of enthusiasm will change that.

Where the conditions do hold, the constraint is not the technology. The mesh works, the physics is settled and the CloudFisher is built to survive the wind that tears simpler nets. What ended Chungungo was that nobody was left with the money and the standing to repair it, and that is still the failure mode. A fog collector is a piece of public infrastructure with a maintenance schedule. It survives on exactly the same terms as any other infrastructure: someone has to own it, someone has to be paid to look after it and the people drinking from it have to be charged enough to fund that work. The Moroccan project metered and charged for its water from the start, and it still serves 16 villages. The high cost of the new collectors remains a concern for its funding.