By Jay Lund
In some of the world’s driest places, atmospheric moisture is a major source of water for native ecosystems. Some algae, plants and insects in the Israeli and Namibian deserts get much of their water from fog, dew and humidity. The spines of some cacti species have evolved to collect fog droplets. California’s redwood forests derive a significant amount of their moisture from fog.
Some drought-minded California residents along the coast, perhaps yearning for a clear ocean view, have suggested harvesting fog as a water supply.
Globally, few places get drinking water from coastal fog. They are mostly rural areas with abundant fog but little other available water. Communities along the parched northern coast of Chile have captured fog for some of its water supply by erecting large fences of synthetic fiber cross-wise to the coastal wind. The condensate on the netting is channelled for collection and use.
Fog harvesting yields from 1 quart to 3 gallons of water daily per square yard of fog mesh .
What would this mean for a typical coastal household?
A household of three that uses 300 gallons a day would need 1,030 to 12,300 square feet of fog mesh . To fit on a typical single-family lot, the length of the fog fence would be limited to about 50 feet. That means the fence would need to stand 21 to 250 feet tall, about the height of the State Capitol dome.
Building such a fence would cost a household thousands of dollars and require cleaning (algae tends to grow on the mesh) and repair (the mesh becomes a big sail in a storm). Homeowners probably also would want a sizable water tank to fill for periods of clear weather.
For virtually all homeowners, a fog water supply would almost always be costly and inconvenient. Some households might use fog as a supplemental supply, but it usually will be at a steep additional cost.
If these numbers were scaled up for San Francisco, population 800,000, the fog fence would need to cover 10 to 120 square miles, or 20 percent to 2.5 times the area of the city (47 square miles). Fog will unlikely be a major water supply for California.
But this is only for atmospheric fog. More petty forms of fog frequently blur discussions of water in California. If we could demistify some of this haze, we might condense our discussions and diminish our droughts.
Jay Lund is a professor of civil and environmental engineering and director of the Center for Watershed Sciences at UC Davis.
May 2016 update:
Oddly enough, apparently a SF Bay Area vodka maker is experimenting using fog-collected water in its distilling: $125 per bottle. At that price, I expect it is economical.
 The density of liquid water is 1,000 kg per cubic meter. The density of water in fog might range from 0.05 to 0.5 grams per cubic meter. If the fog mesh can wring 10 to 50 percent of the water from a coastal breeze blowing 2 miles per hour for half the day, then 1 cubic meter of fog mesh would produce roughly 1 cup to 2.5 gallons of water a day. Not a bad agreement between theory and practice.
 To meet a daily water demand of 300 gallons, the coastal household would need a giant square fog mesh of 32 to 111 feet on each side for a total area of 1,030 to 12,300 square feet, which is larger than most California houses.
Dawson, T. E. (1998), “Fog in the California redwood forest: ecosystem inputs and use by plants,” Oecologia, Volume 117, Issue 4, December, pp 476-485
Estrela, M.J., J.A. Valiente, D. Corell, M.M. Millán. 2008. “Fog collection in the western Mediterranean basin (Valencia region, Spain)”. Atmospheric Research, Volume 87, pp. 324–337
Friedmann, I., Y. Lipkin, and R. Ocampo-Paus. 1967. “Desert Algae of the Negev (Israel)”. Phycologia, 6:4, 185-200
Goodman, J. 1985. “The collection of fog drip”. Water Resources Research, Vol. 21, No. 3, pp. 392-394. A very small field experiment on coastal Montara Mountain south of San Francisco, Calif.
Henschel, J.R. and M.K. Seely. 2008. “Ecophysiology of atmospheric moisture in the Namib Desert”. Atmospheric Research, Volume 87, Issues 3–4, March 2008, Pages 362–368
Ju, J., H. Bai, Y. Zheng, T. Zhao, R. Fang, and L. Jiang. 2012. “A multi-structural and multi-functional integrated fog collection system in cactus”. Nature Communications, 4 Dec. 2012
Klemm, O. et al. 2012. “Fog as a Fresh-Water Resource: Overview and Perspectives”. Ambio. Mar 2012; 41(3): 221–234
Snyder, R.L. 1992. “Fog contribution to crop water use”. Drought tips, No. 92-40, UC Davis
Victoria, M. and M. Jaen. 2002. “Fog water collection in a rural park in the Canary Islands (Spain)”. Atmospheric Research, Volume 64, pp. 239–250