Air is rich of water. It is just a matter to get the water out.

Water condensing is the change of the physical state of water from gas phase into liquid phase, and is the reverse of vaporization. It can also be defined as the change in the state of water vapour to water/any liquid when in contact with any surface.

Condensation normally occurs when the combination of the relative humidity and a surface temperature are at a crossing point known as the dew point.

An absorption refrigerator is a device that uses heat to cool other parts. It is a commonly known technique used in gas refrigerators as well. With the heat of the gas burner the refrigerator is cooled down. 

By using this technique to cool down a metal plate that is in touch with the sky it is possible to retrieve water. The heat source is the sun.

See also: https://greenpeacechallenge.jovoto.com/ideas/31772/