The atmosphere contains a significant amount of water in the form of invisible vapor. This atmospheric moisture is measured as humidity, and its availability for collection is governed by the dew point. The dew point is the temperature at which air becomes fully saturated, causing water vapor to condense into liquid water. Exploiting this physics—by cooling air to its saturation point or using solar energy to accelerate evaporation—allows for water production without mechanical power.
Passive Dew and Condensation Collection
Collecting dew involves creating a surface that cools below the air’s dew point temperature, typically overnight, using radiative cooling. The surface radiates heat into the clear night sky faster than the air or ground can replenish it, causing the necessary temperature drop for condensation. Simple dew collectors use non-porous materials like plastic or metal sheets, laid out in an open area with a clear view of the sky to maximize heat loss. A slightly sloped surface directs the resulting water droplets to a collection trough or container at the lowest point.
A more primitive method, sometimes called an air well, uses a large pile of rocks or stones to achieve a similar effect. The thermal mass of the rocks cools slowly during the night, remaining cooler than the surrounding air as the sun rises. Humid air passing through the rock pile meets the cold surfaces, causing water vapor to condense on the stones, which then trickles down to a collection basin beneath the pile. To maximize collection, site selection is important, favoring locations with high overnight humidity and minimal obstruction that might block the cooling effect of the clear sky.
Building and Using a Solar Still
A solar still utilizes the sun’s energy for distillation, a process that both collects and purifies water from a damp source like soil or vegetation. To construct a standard ground still, locate a sunny area with damp ground, and dig a pit approximately 3 feet wide and 2 feet deep. Place a clean collection container, such as a cup or bowl, in the center of the pit.
Next, add fresh-cut green vegetation, like leaves or grass, around the collection container to increase available moisture, ensuring no plants are placed inside the container itself. Cover the entire pit opening with a sheet of clear plastic film, such as polyethylene, making sure the edges extend well beyond the pit’s perimeter. Secure the edges by piling soil or rocks over them, creating an airtight seal to prevent water vapor escape.
Place a small rock or weight onto the center of the plastic film, positioning it directly over the collection container. This weight creates a downward-sloping cone. The sun’s heat passes through the plastic, causing ground moisture and plant water to evaporate. The resulting water vapor condenses on the cooler underside of the plastic and runs down the slope until it drips into the collection vessel. A well-constructed still can yield water within a few hours, though production rates vary based on soil moisture and sunlight intensity.
Fog and Mist Interception Systems
Fog and mist interception is a collection technique distinct from dew harvesting, as it targets suspended liquid water droplets rather than condensing water vapor. These systems physically filter water from moving air, functioning most effectively in geographically limited areas where fog is frequent, such as coastal or mountainous regions. The mechanism relies on the tiny fog droplets colliding with and clinging to a fine mesh material.
The most common material for a fog collector is a vertical net or screen made from a finely woven mesh, often polypropylene or nylon. For a simple setup, stretch a section of this mesh between two vertical posts, orienting the net perpendicular to the prevailing wind direction. As the wind pushes the fog through the mesh, the water droplets are intercepted, coalescing into larger drops that gravity pulls down the fibers. A gutter or trough beneath the net catches the liquid water, directing it into a storage tank. This passive technique requires no power, providing a reliable water source where atmospheric conditions are right.