give it more time to heat. Additives When distilling brine or other polluted water, adding a dye can increase the amount of solar radiation absorbed. Reverse still A reverse still uses the temperature difference between solar-heated ambient air and the device to condense ambient water vapour. One such device produces water without external power. It features an inverted cone on top to deflect ambient heat in the air, and to keep sunlight off the upper surface of the box. This surface is a sheet of glass coated with multiple layers of a polymer and silver. It reflects sunlight to reduce surface heating. Residual heat that is not reflected is reemitted in a specific (infrared) wavelength so that it passes through the atmosphere into space. The box can be as much as 15 °C (27 °F) cooler than the ambient temperature. That stimulates condensation, which gathers on the ceiling. This ceiling is coated in a superhydrophobic material, so that the condensate forms into droplets and falls into a collector. A test system yielded of water per day, using a surface or approximately 1.3 L/m2 (0.28 gal/ft2) per day. Efficiency Condensation traps are sources for extending or supplementing existing water sources or supplies. A trap measuring in diameter by deep yields around per day. Urinating into the pit before adding the receptacle allows some of the urine's water content to be recovered. A pit still may be too inefficient as a survival still, because of the energy/water required for construction. In desert environments water needs can exceed per day for a person at rest, while still production may average only . Several days of water collection may be required to equal the water lost during construction. Applications Remote sites Solar stills are used in cases where rain, piped, or well water is impractical, such as in remote homes or during power outages. In subtropical hurricane target areas that can lose power for days, solar distillation can provide an alternative source of clean water. Survival Solar stills have been used by ocean-stranded pilots and included in life raft emergency kits. Using a condensation trap to distill urine will remove the urea and salt, recycling the body's water. Wastewater Treatment Solar stills have also been used for the treatment of municipal wastewater, the dewatering of sewage sludge as well as for olive mill wastewater management. See also Concentrated solar still Desalination Freshwater Mária Telkes