In this chapter the phenomena of dew condensation is discussed. It is followed by discussion of extraction of moisture from air by passive condensers. Ancient and more recent attempts to build dew condensers are discussed. Basic properties of an efficient passive dew condenser are derived using a simple mathematical model. Construction and trial of condensers made of three different materials is presented.

Harvest of Rain and Fog

A rain harvest system needs only to intercept rainwater and channel it down to storage. Rain harvest technology is well known and necessary gear is widely available. The techniques to harvest fog are also now well established. There are reports of successful systems of fog harvest in several countries of South America. Essentially large nets of polypropylene are hung across the path of the moving clouds of fog near the ground. The water gets collected on the mesh and channeled to storage through appropriate size conduits. The detailed description of the harvest technique and systems are given in Appendix - 2.

Harvest of Dew

Dew harvesting differs from rain and fog harvesting. A dew harvest system must first make water by condensing the vapors contained in the air and then channel it down. Examples of this phenomenon can be easily seen in daily life. When a bottle of cold water is taken out of the fridge and placed on the dining table a thin film of condensate (water) is formed on its outer surface (Figure 3.1).