As clouds move over hills and mountains, the hilltops and ridgelines are enveloped in fogs. Just as the leaves and needles of trees can collect some of the water in these fogs, large artificial collectors can produce a flow of potable water. Fog collection is a resource that should be evaluated in arid or seasonally arid areas where other traditional sources of water cannot meet the needs of the people. The project costs are small, the technology simple, the water of very good quality, and the source sustainable for periods of hundreds or thousands of years. Fog collection works with nature and complements other water supply systems.
For centuries people have known that trees collect the tiny water droplets that make up fog. Two thousand years ago Pliny the Elder wrote about the Fountain Tree, or Holy Tree, on the island of El Hierro in the Canary Islands. From that time to at least 1800, the inhabitants obtained much of their water from fog that dripped from the leaves of one or more trees. In 1776, Gilbert White in England made observations on how trees collected water from the "swimming vapours" of fog. This was called occult precipitation in the last century and the terminology persists in some literature to this day.
Because fog droplets are very small (ca 0.001-0.040 mm), they are blown by the wind. The fog collector should be a vertical surface. It cannot, however, be a solid surface or the wind will flow around the obstacle. Meshes of different sizes have proved most effective. Fog formed on a water body or nocturnal radiation fogs in low-lying areas, lack sufficient liquid water content or sufficient wind speeds for substantial water collection. Upland areas with fog produced by the advection of clouds over the terrain or, in some cases, by orographic lifting on the mountains, are ideal for fog collection. Vertical frames with double layers of polypropylene mesh, with fibres about 1 mm wide and covering around 60% of the surface areas of the collector, have been used in in the mountains of Chile. A collector 12 m long and 6 m high typically produces from 150 L/day to 750 L/day. 75 collectors measuring 48 square metres each had an average production of 11,000 litres/day, enough to provide each of the 340 villager of Chungungo with more than 30 L per person per day. Sustained production rates over periods of two and a half months in the Sultinate of Oman were as high as 70 L/m2/day, although the limited collection season makes the technique perhaps more suitable to forestry applications. Namibia is the first African country in which the possibility of using fog collection as a water supply for indigenous people is being evaluated. There have also been scientific evaluations underway for several years in both the Canary Islands and South Africa. All show positive results. Other evaluation projects are in Mexico and Nepal.