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.

Three principal designs are used for air wells, designated as high mass, radiative, and active:

All air well designs incorporate a substrate with a temperature sufficiently low 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, this depends on temperature, warmer air contains 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 point temperature of air at 20 °C (68 °F) and 80 percent relative humidity is 16 °C (61 °F). The dew point temperature falls to 9 °C (48 °F) if the relative humidity is 50 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 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.

Beginning in the early 20th century, a number of inventors experimented with high-mass collectors. Notable investigators were the Russian engineer Friedrich Zibold (sometimes given as Friedrich Siebold), the French bioclimatologist Leon Chaptal, the German-Australian researcher Wolf Klaphake, and the Belgian inventor Achille Knapen [fr].