Fog Nets: Harvesting Water from Thin Air

What it is

Fog is not rain. Fog droplets measure 1 to 40 micrometres. Too small to fall, too heavy to stay aloft.

Wind pushes them through a fine mesh. They collide, coalesce, and drip into a gutter. A pipe carries the water downhill into a tank. No pumps. No power.

Why it works

Three things govern collection: wind speed, mesh geometry, droplet size.

Aim for 50 to 60 percent porosity. Too dense and wind diverts around the panel. Too open and fog sails through untouched.

Forests have always done this. Cloud forests stay lush on modest rain because the canopy strips moisture from passing fog. Researchers in the Canary Islands measured laurel-like native trees adding several hundred extra millimetres a year through fog drip alone. A fog net replicates that canopy on a bare ridge.

Where it works

Fog harvesting is site-specific. You need a reliable fog source: cold ocean currents, high cloud belts, or persistent inversions. The geography is consistent. Coastal highlands or mountain ridges, with prevailing winds shoving marine fog up against the terrain.

Chile. The Atacama coast invented modern fog harvesting. Some stations log zero rain for years, but the Humboldt Current drives a dense fog called camanchaca. Villages between 500 and 1,000 metres sit inside it. FogQuest installed the first large systems at Chungungo in the 1980s, and they ran the community supply for over a decade.

Morocco. Dar Si Hmad operates a system on Mount Boutmezguida at 1,225 metres. It serves over 400 people across several villages, and the nets produce through the dry summer when wells fail. Similar projects run in Eritrea, South Africa, Colombia, Guatemala, Nepal, and Oman.

The non-negotiables are fog frequency, wind speed of 2 to 8 metres per second, and the right elevation.

Yields

A well-sited net yields 5 to 15 litres per square metre per day across the productive season. Dense fog events push that to 20 to 30.

That sounds small until you scale it. A standard large fog collector is 40 square metres, 10 wide by 4 tall. At 10 L/m²/day, one panel delivers 400 litres a day. Enough drinking and cooking water for 20 to 40 people.

Yields swing hard by season. Northern Chile produces mostly May to November. Moroccan and East African mountain sites run year-round but dip when the cloud base lifts above the collector. Size your storage to bridge the lean months. Never depend on fog alone if the resource can collapse for a quarter of the year.

Assess before you build

Use a Standard Fog Collector. One square metre, deployed for a minimum of 12 months.

A full year of SFC data is the price of admission. No shortcuts. Sites averaging under 3 L/m²/day across the assessment are not viable for drinking water. They can still feed supplemental irrigation or seed a reforestation effort, but do not promise a household supply.

Build it

The design is almost embarrassingly simple.

Two posts, steel pipe or treated timber, 10 to 12 metres apart. Set deep, guyed against wind. Stretch a double layer of Raschel mesh between them, with the bottom edge 1 metre off the ground and the top at 4 metres. Run a PVC or galvanised gutter along the lower edge. Pipe it to a sealed tank downhill.

The mesh. Raschel is cheap, UV-stabilised, light, and sold worldwide. A 35 percent shade rating doubled gives you that 50 to 60 percent working porosity. Replace every 5 to 10 years depending on sun and wind. The cost is trivial next to trucked water or desalination.

Run it as a community

The systems that last are the ones the community owns.

Outside groups bring engineering and seed funding. Locals run the operation. Top-down installs where the NGO leaves after the ribbon-cutting fail within a few seasons. Dar Si Hmad trained women as technicians, set up a water committee for distribution and maintenance, and the system has run for more than ten years.

Simple hardware, local ownership, and a steady fog resource. That combination beats rainwater harvesting on the sites where rain barely shows up.