Fog Provides Water in the World's Driest Zones


Chilean fog collectors on the Spanish Canary Island of Tenerife use large canvas nets to trap the condensation contained in the fog. Water droplets are then collected after flowing down the net. DESIREE MARTIN/AFP/Getty Images

On the whole, the Namib Desert in Southern Africa doesn't see much rainfall. To stay alive, a few species there rely on a different source for moisture. In the Namib, fog is way more common than rain. Some of it gets blown over on Atlantic winds; some of it radiates up from the ground. Altogether, there are usually between 60 and 200 foggy days in the desert each year.

The Namib grass Stipagrostis sabulicola exploits those foggy days. Airborne water droplets get caught on its leaves and tiny vertical grooves then redirect the liquid down to its roots. Small black insects in the Namib do something similar. On cool mornings, Namib beetles (Stenocara gracilipes) use their forewings to collect fog droplets and channel them into their mouths.

It's an ingenious hydration trick. So could mankind ever harvest fog? You bet. In fact, people all over the world are doing it right now.

Moisture and Gravity

Historians don't know exactly how old the practice of fog collection is. Modern engineers have been tinkering with the idea since at least 1901, when fog capture experiments were carried out on Table Mountain in South Africa.

The country took a huge leap forward in 1969. That year, South Africa's government was looking to find a suitable source of water for its Air Force personnel at the Mariepskop radar station. Two plastic nets — measuring 91.8 feet (28 meters) long by 11.8 feet (3.6 meters) high — were set up near the facility. Placed at right angles to each other, their purpose was to gather and store windblown moisture on foggy days. The screens stayed up for 15 months. In that time, they yielded a daily average of 11 liters (2.9 gallons) of water per square meter (3.28 square feet) of collection surface area. Not too shabby.

Think of fog banks like grounded clouds. The product of condensation, fog is made up of visible water droplets that congregate in large clusters near Earth's surface. Fog nets — like the ones built for the Mariepskop radar station — are made of fine mesh and typically held up by poles in the ground.

When it's foggy outside, tiny water droplets cling to the mesh. As they accumulate, some of the droplets merge together into beads. When they increase in size, the beads grow heavy and gravity pulls them down. At the base of a fog net is a gutter that collects these descending water droplets that can then be diverted into a reservoir for storage.

Ultimately, the Mariepskop nets were dismantled once a different water source presented itself. But the world hadn't seen the last of fog harvesting. Halfway across the planet, a renaissance in this field was set to unfold.

A Global Harvest

The Chilean mountain village of Chungungo — like the Namib Desert — is arid and rain is hard to come by. Typically, the area experiences less than 6 centimeters (2.36 inches) of precipitation per year. Generations of residents have had to import drinking water by the truckload from Chile's wetter regions. And yet, Chungungo itself is not some moisture-free wasteland. Regular, heavy fogs blow in from the Pacific. So late in the 20th century, scientists decided to try and harness this resource.

In 1992, The New York Times reported on a collaboration between Chile's National Catholic University and the International Development Research Centre of Canada. Researchers from both organizations descended on a hill overlooking Chungungo where they tested out different net materials. Eventually, the teams settled on a design. Some 50 nets were made, with each containing two layers of tight, polypropylene mesh.

Though the harvest yields varied seasonally, a 2001 report from Scientific American found that the nets gathered an average total of 11,000 liters (2,905 gallons) of water every day. That was enough to provide each one of Chungungo's 300 residents with 33 liters apiece (8 gallons) daily.

The feel-good story garnered lots of headlines. Inspired by the success at Chungungo, other countries began to set up their own fog-harvesting net systems. Today, these contraptions can be found in more than a dozen nations — including Peru, Morocco and Nepal. In areas where lakes, rivers or water wells aren't options for drinking water, they can be highly useful.

Netting the Future

Fog collection technology continues to evolve. In 2013, scientists from the Pontifical Catholic University of Chile and the Massachusetts Institute of Technology announced a vastly improved mesh screen design. Through rigorous testing, they learned that thin fibers made for more effective nets. So, they created a net out of stainless steel filaments with three to four times the thickness of an isolated human hair. Then, a specially made coating was applied in order to help water droplets slide down the mesh more easily. According to the researchers, this new design represents a 500 percent improvement in fog-collection efficiency.


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