Dew Harvesting is simply taking advantage of water vapor in the atmosphere to harvest clean and potable water through condensation, a passive process that allows water particles to return to the earth in a pure form.
Dew harvesting has been practiced by humanity as far back as ancient times, in areas where rainfall and groundwater resources are scarce. Technically, the process of dew harvesting can be understood by simply analyzing how the water cycle occurs (any fourth grade drawing will do).
When there is any humidity at all in the air and there is a surface that is cool enough to provoke condensation, dew will condense on that surface until the humidity is gone or the surface has absorbed so much heat from the water molecules that the surface is then no longer cool enough to provide the condensing action.
Surface water will evaporate into the atmosphere as soon as enough sunlight heats the molecules enough for them to take gaseous form and these molecules will eventually collect in the atmosphere to create humidity, which will later condense on cold surfaces as dew and thus returning the lost thermal energy to the planet surface.
The water cycle is how our planet keeps its water clean. Warm and humid air with large amounts of humidity take cloud form, and when they hit a cold front from the proper angle, are forced to condense into rain droplets.
This same principle follows for all forms of dew harvesting, to create a cold enough surface that water particles in gaseous form will condense enough to form dew droplets at an angle for collection.
Vegetation in desert regions have developed modifications that allow them to collect their own humidity from the air for example, and through efforts of reforestation in desert regions this technology has advanced abundantly around the world.
Do-it-yourselfers looking to create zero energy homes are the most trendy examples of dew harvesters to date, with their golf-ball like cardboard and metal roofs that use geothermally cooled water pumped through them at night to bring the roof temperature down enough to harvest dew through their traditional rooftop rainwater harvesting systems.
The biggest advantage to the metal roof technique for dew harvesting is that it simultaneously serves as a passive water heater during sunlight hours as well as a rainwater catchment system and dew harvesting system.
Earlier historical examples include small-scale drinking pools of condensation at the base of plant stems to large-scale natural irrigation practices in areas without rain (like the Namib desert).
Some of the most famous human-made dew harvesting sites include; the stone piles in the Ukraine, dew ponds from southern England and even volcanic stone in the fields of Lanzarote. Collecting dew in a passive manner is an old practice.
Rediscovery of human influence over natural condensation occurred throughout history, from ancient times to medieval to the 20thcentury since which time it has been studied with some interest until recently with renewed interest in sustainability.
Metal roofs, tile roofs with geothermal water-cooling for example can collect enough water to take a bath in most cases. Solar-powered air-moisture harvesting and wind-powered air-moisture harvesting can complement dew harvesting and are currently being tested in Australia.
The International Organization for Dew Utilization uses foil-based condensers for regions where rain or fog cannot are not efficient enough. The secret to dew harvesting in general seems to be the thickness of materials, the thinner materials are, the harder it is for them to retain heat, so thatas soon as the dew has passed down to the collector, most of the leftover heat is dispersed allowing for more dew to condense more rapidly.
The method that will work best might just be a plastic tarp suspended over a barrel with a clothes line for some people, while for others, a fancy first-class metal roof with technological advancements that make all other roofs look old-fashion is the thing.
But dew harvesting is simply the collection of water condensation from cold surfaces either artificially cooled or naturally occurring, and is ideal for conditions where rainfall and groundwater sources are scarce.