One of the more innovative methods of gathering freshwater is harvesting it from clouds, but hasn’t been unproven. When the base of the cloud is in contact with the ground, it is called fog. Fog collectors work best in coastal areas where the water can be harvested as the fog moves inland driven by the wind.
What is Fog?
Fog can form at lower humidity, and sometimes it does not form with the relative humidity at 100%. The air will be supersaturated if moisture is added beyond this point, which presents conditions favorable for condensation in the form of rain or fog.
Fog formation requires all of the elements that normal cloud formation requires, the most important being condensation nuclei, in the form of dust, aerosols, pollutants, etc., for the water to condense upon. When there are exceptional amounts of condensation nuclei present, especially hygroscopic (water seeking) particles such as salt, the water vapor may condense below 100% humidity.
Sea fog is a fog caused by the peculiar effect of salt. Sea fog is common along coastlines where airborne salt particles are generated from the salt spray produced by breaking waves. These salt particles act as the nuclei for water to condense. Unusual to sea fog is that, due to the hygroscopic nature of salt, condensation can occur with humidity as low as 70%.
Fog typically contains about 0.2 grams of water per cubic meter; the amount of water that can be harvested depends on surface area of the collector, efficiency of collection, and wind speed.
The keys to the feasibility of artificial fog catchers are meteorology and topography, as well as a community base to maintain the system. First, moisture-laden air must be driven by wind to a geographic barrier, such as a mountain range that intercepts the clouds. These conditions occur most commonly in a place where there is onshore winds air mass, which creates appropriate conditions. The same combination can occur with interior mountain chains. Secondly, modest in quantity, the more cost-effective fog-collecting site is best located near the point of use.
In order to implement a fog-harvesting program, the potential for extracting water from fog first must be investigated. The occurrence of fog can be assessed from reports compiled by government meteorological agencies. To be successful, this technology should be located in regions where favorable climatic conditions exist. Since fog and clouds are carried to the harvesting site by the wind, the interaction of the topography and the wind will be influential in determining the success of the site chosen. The following factors affect the volume of water that can be extracted from fog and the frequency with which the water can be harvest:
- Frequency of fog occurrence is a function of atmospheric pressure and circulation, oceanic water temperature, and the presence of thermal inversions.
- Fog water content is a function of altitude, seasons, and terrain features
- Design of fog water collection systems takes into account wind velocity and direction, topographic conditions, and the materials used in the construction of the fog collector.