FOG contains anywhere from 0.05 gram to as much as 3 grams of water per cubic meter. Collecting those droplets can provide clean drinking water¿often in suprisingly large amounts.

To most people, fog is a bad thing. It delays flights, endangers drivers, runs ships aground, ruins the view from mountaintops and generally makes for gray, damp weather. But to some communities in the developing world, fog could mean getting enough clean water to drink. Indeed, a small group of scientists and researchers is creating techniques for wringing the water from fog. And their success suggests that fog collection may be among the simplest, cheapest and most environmentally friendly solutions to the water supply problems in certain remote regions.

Collecting fog may sound a bit like trying to grab actual fistfuls of air. But the task is far less daunting when you consider that fog is hardly different from rain. What differentiates the two is the size of the water droplets and the speed at which they fall. Raindrops range from five millimeters to 0.5 millimeter in diameter and shoot toward the ground at speeds between two to nine meters per second. Fog droplets, on the other hand, are a mere 40 to one micron in diameter (1,000 microns are 1 millimeter); they fall at only about one to five centimeters per second. Because they are so light and drop slowly, fog droplets travel almost horizontally, even in the lightest breeze.

Consequently, you can't catch fog in a bucket. Instead a good fog collector is typically a vertical or almost vertical surface that fog droplets can drift onto and then run down. Following this basic design, trees, in fact, make great natural fog collectors. "There is a history in the southeastern part of the Arabian peninsula, where some people had built sort of mud walls around the trunk of a tree so that the fog that was collected by the tree would be restrained around the trunk when it dripped down in bigger drops," explains Robert Schemenauer, a fog collection consultant in Canada. "You would expect that in any arid part of the world. Trees were collecting fog water and as it was dripping down, people would look at it and say, 'Hey, I need water,' and they would stick a skin underneath or a gourd or a piece of cloth to get the water."

Image: KEITH MAC QUARRIE MESH collects fog when water droplets land on it and run down to a container.

Scientists in Chile have experimented with man-made fog collectors for several decades. The northern coastal areas of the country are extremely dry but get a lot of fog at the same time. One such place is the fishing village Chungungo, home of one of the first fog water projects and still the largest project to date. "There are extensive layers of low clouds over the ocean, so if you're standing in Chungungo, you look up and there is the bottom of a cloud and it's a few hundred meters above your head," Schemenauer says, describing the site. "The wind pushes that cloud against these coastal hills. So where the cloud is touching the hill, you have fog. And that fog will flow through passes in the hills, it may cover the hilltops or it may just push against the hillside."

Despite all that fog, the land gets virtually no precipitation. The water droplets in the fog are carried over the slope by the wind, and eventually evaporate farther inland. "You loose the droplets unless you have something sticking up from the ground that's an efficient collector of the fog droplets," Schemenauer explains. "That could be vegetation, but the problem is that there is no vegetation to start with, so you can't get enough water on the ground to irrigate whatever seeds might be there."