Background
All air well designs incorporate a substrate with a temperature sufficiently low so that dew forms. Dew is a form of precipitation that occurs naturally when atmospheric water vapour condenses onto a substrate. It is distinct from fog, in that fog is made of droplets of water that condense around particles in the air. Condensation releases latent heat which must be dissipated in order for water collection to continue.
An air well requires moisture from the air. Everywhere on Earth, even in the hottest climates, the surrounding atmosphere contains at least some water. According to Beysens and Milimouk: "The atmosphere contains 12,900 km3 (3,000 cubic miles) of fresh water, composed of 98 percent water vapour and 2 percent condensed water (clouds): a figure comparable to the renewable liquid water resources of inhabited lands (12,500 km3)." The quantity of water vapour contained within the air is commonly reported as a relative humidity, and this depends on temperature—warmer air can contain more water vapour than cooler air. When air is cooled to the dew point, it becomes saturated, and moisture will condense on a suitable surface. For instance, the dew temperature of air at 20 °C (68 °F) and 80 percent relative humidity is 18 °C (64 °F). The dew temperature falls to 10 °C (50 °F) if the relative humidity is only 25 percent.
A related, but quite distinct, technique of obtaining atmospheric moisture is the fog fence.
An air well should not be confused with a dew pond. A dew pond is an artificial pond intended for watering livestock. The name dew pond (sometimes cloud pond or mist pond) derives from the widely held belief that the pond was filled by moisture from the air. In fact, dew ponds are primarily filled by rainwater.
A stone mulch can significantly increase crop yields in arid areas. This is most notably the case in the Canary Islands: on the island of Lanzarote there is about 140 millimetres (5.5 in) of rain each year and there are no permanent rivers. Despite this, substantial crops can be grown by using a mulch of volcanic stones, a trick discovered after volcanic eruptions in 1730. Some credit the stone mulch with promoting dew; although the idea has inspired some thinkers, it seems unlikely that the effect is significant. Rather, plants are able to absorb dew directly from their leaves, and the main benefit of a stone mulch is to reduce water loss from the soil and to eliminate competition from weeds.
Read more about this topic: Air Well (condenser)
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