Fog Net

Fog is cloud sitting on the ground, made of droplets too small to fall as rain, so they drift past on the wind. A fog net, also called a fog collector, is a panel of fine mesh held upright in that moving air. Droplets catch on the threads, run together until they are heavy enough to fall, and trickle into a gutter that leads to a tank.

Where the panel stands decides almost everything. It needs fog that arrives often and a wind that holds one direction, and it is set across that wind rather than along it: on a ridge line or the upwind edge of a flat-topped rise, with nothing tall breaking the airflow for a few kilometres (a couple of miles) upwind. Behind a hill, or facing away from the wind, it collects almost nothing. Collectors work best between 400 and 1200 m (1300 to 3900 ft), about two-thirds of the way up the fog layer, where the air holds the most water. Catches run from about 5 to 13 litres per square metre of mesh a day (0.1 to 0.3 gallons per square foot) and peak in whatever season brings the strongest fog and wind, so a small trial panel beats any calculation.

Collection comes in bursts while daily use does not, so the design includes a tank holding at least half the largest expected day's catch, with filtering and disinfection between it and anything drunk: fog water has met drinking standards at some sites and carried too much nitrate or dissolved mineral at others. Building it is ordinary carpentry and plumbing. The wind is the real strain, since the same wind that fills the gutter presses on a wide sail of cloth, and gusts or wet snow are what usually bring a collector down. Fouling is the quieter failure: dust, debris and algae coat the mesh and pipes, and a neglected net keeps delivering water that is slowly getting worse. The mesh wears out, rated at up to about ten years on a large panel and sold by only a few suppliers worldwide.