An air well or aerial well is a structure or device that collects water by promoting the condensation of moisture from air. Designs for air wells are many and varied, but the simplest designs are completely passive, require no external energy source and have few, if any, moving parts.
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 deserts, the surrounding atmosphere contains at least some water. According to Beysens and Milimouk: "The atmosphere contains 12,900 cubic kilometres (3,100 cu mi) 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 16 °C (61 °F). The dew temperature falls to 9 °C (48 °F) if the relative humidity is 50 percent.
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.